PRECAST CONCRETE HANDBOOK
PRECAST CONCRETE HANDBOOK First published 2002 Second edition 2009 ISBN 978-0-9577467-4-9
Publishers National Precast Concrete Association Australia www.nationalprecast.com.au
and Concrete Institute of Australia www.concreteinstitute.com.au Cover Design
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© 2009 National Precast Concrete Assoc iation Australia and Concrete Institute of Australia. All rights reserved. Except where the Copyright Act and the 'Limited-licence Agreement' with these files allows otherwise, no part of this publication may be reproduced, stored in a retrievalsystem, or transmitted in any form, or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the National Precast Concrete Association Australia or the Concrete Institute of Australia. Every effort has been made to trace and acknowledge copyright but in some cases this has not been possible. The publishers apologise for any accidental infringements and would welcome any information to redress the situation.
Disclaimer Since the information provided in this publication is intended for guidance only and in no way replaces the services of professional consultants on particular projects, no legal liability can be accepted by National Precast Concrete Association Australia or Concrete Institute of Australia for its use. The Precast Concrete Handbook is intended for use by professional personnel competent to evaluate the significance and limitations of its contents and able to accept responsibility for the application of the material it contains.
Preface
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Chapter 9 has major revisions – The Thermal Properties section has been rewritten to take account of new thermal performance provisions for all classes of buildings required under the BCA. The Acoustic Properties section has been rewritten to take account of new sound insulation provisions for residential buildings as required under the BCA.
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Updating of Chapter 11 to reflect the current erection procedures of AS 3850 and other
Introduction In 2002 the National Precast Concrete Association Australia (NPCAA) and the Concrete Institute of Australia (CIA) jointly published the Precast Concrete Handbook in order to advance the knowledge of design, manufacture and use of precast and prestressed concrete throughout Australia. The Precast Concrete Handbook is neither a standard nor a textbook but rather a reference document recommending good practice in precast construction to designers, engineers, architects, builders and students. It provides guidance for those involved in the design, specification, manufacture and installation of precast concrete. The information provided accords with Australian Standards and sound industry practice. This second edition of the Precast Concrete Handbook is not a substantial rewrite of the first edition but is rather an upgrade to reflect the changes in the Building Code of Australia (BCA) and in Australian Standards over the past six years. As well, it attends to error s and omissions which have been drawn to our attention by readers. The significant changes in this edition are: •
Updating to Building Code of Australia 2007 edition.
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Updating to Standards Australia AS 3600 Concrete structures, published in 2009.
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Update to Standards Australia loading codes AS/NZS 1170.0, AS/NZS 1170.1, AS/NZS 1170.2 published in 2002 and AS 1170.4 published in 2007.
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Updating to Standards Australia AS 3850 published in 2003 as it applies to flat precast panels.
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Updating to current NPCAA and CIA publications and publications of other technical associations where relevant.
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Chapter 2 includes updated and new technical data on some precast products.
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Chapter 5 has changes to the Analysis and Design sections to reflect the changes to AS 1170.4.
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A rewrite in Chapter 6 of the section on Vibration Control.
publications. Substantial effort has been made to ensure that this Handbook is accurate. However, neither the National Precast Concrete Association Australia nor the Concrete Institute of Australia can accept responsibility for any errors or oversights in the use of material in the preparation of engineering designs or in the design of precast concrete. The designer must recognise that no handbook or code can substitute for experience and good engineering judgement. This publication is intended for use by professional persons competent to evaluate the significance and limitations of its contents and able to accept responsibility for the application of the material it contains. Acknowledgements We gratefully acknowledge those individuals responsible for developing the first edition of the Precast Concrete Handbook. Readers should refer to this edition for the names of the contributors who laid the foundation for this second edition. A wide range of consultants, academics and industry professionals gave generously of their time to prepare material and review draft copies and final proofs of this second edition. We acknowledge their contribution with sincere gratitude.
The National Precast Concrete Association Australia, formed in 1990, is the recognised agency of the Australian precast concrete industry. It promotes and represents manufacturers of high-quality, factoryproduced precast concrete components. As well, it promotes precast concrete as material of choice to the building and civil construction industries. Membership of the Association comprises precast manufacturers together with suppliers to the industry of equipment, facilities, materials and services. Membership also includes industry professionals such as architects, engineers and accountants.
The Concrete Institute of Australia is an independent, non-profit organisation made up of many members who share a common interest in staying at the forefront of concrete technology, design and construction in Australia. The mission of the Concrete Institute is to promote and develop excellence in concrete technology, application, design and construction throughout Australia. The main aims of the Concrete Institute are to: •
Provide a forum for the sharing of knowledge and experience between members and to disseminate this information for the benefit of the concrete and construction industry.
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Provide industry representation through the promotion of good concrete construction, and to establish and maintain relations with appropriate local, national and international bodies where this will further the vision and mission of the Institute.
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Facilitate and manage knowledge governance through publishing, education and training strategies. Engage in higher-level professional development activities such as those which satisfy the Continuing Professional Development requirements maintained by accreditation bodies such as Engineers Australia.
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Facilitate and manage knowledge development through the identification and recognition of challenges facing the industry and the encouragement of solutions through investigation, research and other scientific or technological development.
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Raise the profile of the Institute through a clearlydefined image which increases public awareness and defines its place in the construction industry, and establishes its magazine Concrete in Australia as a primary communication vehicle for the concrete industry.
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Facilitate the recognition of excellence through strategies which include the recognition of concrete technology application and construction excellence throughout Australia.
The aims and activities of the National Precast Concrete Association Australia include: •
Promoting Members’ products through the National Precast website, publications and exhibitions.
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Representing the precast concrete industry to government and other authorities.
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Participating in precast-related technical activities, such as developing and improving standards and
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Providing technical advice to specifiers and potential clients.
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Referring Members to specifiers and potential clients.
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Conducting training and information events.
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Providing resources to tertiary educational institutions.
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Promoting best-practice in occupational health and safety in the workplace.
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Sourcing and promoting best-practice in product design and manufacturing processes.
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Sourcing and disseminating new and relevant industry information.
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Creating opportunities for Members to network
specifications.
among themselves and with others in the construction industry. The successful use of precast concrete requires an understanding of the design, detailing, manufacture and installation of precast elements. Its good performance depends on the environment and its relationship with other building materials as well as on the quality of the elements themselves.
What you will find in this Chapter •
A brief history of precast concrete in Australia.
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Typical applications of precast concrete in building, illustrating how the product can contribute to the aesthetics and to the structural efficiency of structures.
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Typical applications of precast concrete in civil engineering works from bridges to drainage.
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Inspiration which will convince you to design in precast concrete.
1.1 Brief History 1.1.1 Introduction 1.1.2 The beginnings 1.1.3 Precast in buildings 1.1.4 Hollowcore construction 1.1.5 Civil infrastructure 1.1.6 Commercial and other influences 1.2 Building Applications 1.2.1 Office buildings/hotels 1.2.2 Institutional/public buildings 1.2.3 Shopping centres 1.2.4 Industrial buildings 1.2.5 Medium- and high-rise residential buildings 1.2.6 Multi-level parking structures 1.2.7 Correctional facilities 1.3 Civil Applications 1.3.1 Stadia 1.3.2 Tunnels 1.3.3 Marine structures 1.3.4 Bridges and culverts 1.3.5 Towers and poles 1.3.6 Mining infrastructure 1.3.7 Noise walls 1.3.8 Retaining walls, storage bins 1.3.9 Drainage and environmental products 1.3.10 Landscaping and municipal products 1.3.11 Other products 1.4 Precast Advantages 1.5 Sustainability 1.6 Bibliography
1.1.1
Introduction
The history and development of precast concrete in Australia is linked with the development of the building and construction industries. Since World War II, precast concrete has played a large part in improvement in construction productivity, in improvement in the quality of structures and in the production of architectural finishes impossible to achieve with insitu concrete methods. The evidence of the pre-eminence of precast concrete is all around us. The Sydney Opera House could not have been constructed any other way.The Wooloomooloo Railway Viaduct and the Gladesville Bridge in NSW and the O-Bahn track system in Adelaide are innovative infrastructure uses. Public buildings such as Parliament Houses in Sydney and Canberra, the Adelaide Convention Centre, the Depar tment of Defence Lavarack Barracks in Townsville, a myriad of high-rise hotel and office buildings, most modern hospitals and the majority of modern drainage structures and industrial buildings are all testament to the huge contribution
that precast concrete has made and is making to Australian construction.
1.1.2
The beginnings
The first use of precast concrete may well have been by W H Lascelles who introduced an innovative housing system to the UK in 1875. The Lascelles system consisted of precast cladding panels fixed to a structural frame.
Figure 1.1 Bradleys Head Lighthouse, Constructed 1907–10
In Australia, the full history of the early days of precast concrete has not been documented but some of the early firms and personalities are known. The first known use was in 1904 for the Sydney Harbour’s Bradleys Head lighthouse which is still in use today. Precast cluster piles support four precast shell sections which were filled with mass concrete (Figure 1.1). The Australian precast industry owes its beginnings in part to the need for water and sewage pipes, the deficiencies in Australian hardwoods and to Sydney’s rat plague at the beginning of the century. At the turn of the 20th century, commercial and shipping areas of Sydney Harbour were redesigned and along with this a precast sea wall was built. It was given an especially smooth surface on the tidal side which rats coming off ships found too slippery to climb. Although much of the srcinal sea wall has been demolished during later reconstruction, sections can be still seen at low water in the area south of Pyrmont Bridge and in Walsh Bay. In 1908 a fullyprecast trestle-wall system was built at Millers Point Wharf (Figure 1.2). When built, the pontoon for wharfs 6 and 7, Circular Quay, was the largest of its type in Australia and measured 33 m long x 20 m wide x 2.4 m deep. What is thought to have been the first application of precast formwork was in Jones Bay wharf in 1915. The piers here were over 15 m long, and consisted of pipes as formwork for insitu concrete. In 1910, the centrifugal-spun reinforced-concrete pipe – a world-first – was invented by two brothers, W R Hume and E J Hume. The brothers patented it under the Hume’s Patent Cement Iron Syndicate Ltd, a company which later became Hume Brothers Cement Iron Company Ltd, Hume Pipe Company (Australia) Limited in 1920 (Figure 1.3), and eventually CSR Humes Ltd. This company is still one of the leading precast manufacturers in Australia. The process and practice Australia, of pipe spinning adopted throughout Europe,was Asiawidely and the Americas.
Figure 1.2 Millers Point Wharf, Constructed in 1908
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E G Stone, a man prominent in many aspects of early Australian concrete construction, was responsible for designing and building in 1910 the remarkable Denny Lascelles Austin Wool Store at Geelong (Figure 1.4). Cement Linings Limited was formed in 1936 and had its early business in areas other than precast but soon expanded its activities in this field. In 1946 it changed from being a single-product company (cement linings) into a pipe and precast company by acquiring Monier Industries Ltd, and thereafter was known as Monier.
Figure 1.3
At the outbreak of World War II, all concentration turned to the war effor t. Very little building took place and severe restrictions were placed on the building industr y with many materials being unavailable. The activities of the Monier group were largely defence work. After the war, house bricks were among the products in short supply and Monier took advantage of this in 1946 by developing a system called Monocrete which incorporated 100-mm-thick walls of hollow precast panels slotted into grooved columns. This was used initially in houses and subsequently in schools and other buildings (Figure 1.5). Similarly, prestressed and reinforced precast concrete was used for the fabrication of members as a substitute for structural steel. The precast post-tensioned frame for the 1952 Warragamba Ice Tower is an example (Figure 1.6).
Hume Pipe Company (Australia) Limited, 1920
Figure 1.5 Monocrete System used in Chatswood High School, NSW
Figure 1.4 Denny Lascelles Austin Wool Store, Geelong, 1910 Figure 1.6 Warragamba Ice Tower, 1952
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Figure 1.7 St James Building, part of AMP Centre, Corner Bourke and William Street, Melbourne, 1971
Figure 1.8 Fremantle Passenger Terminal
Figure 1.9 30-Storey Block of Flats Built by Housing Commission, Victoria, 1969
Photograph courtesy of Cement Concrete and Aggregates Australia
In Adelaide, precast concrete had its beginnings with rivals and cousins Albert and Victor Del Fabbro, manufacturing toilet partitions and wash troughs. In 1955, Victor went on to build the large Mosaic Flooring Co factory which was bought by Pioneer Concrete in 1959 and which is still in operation as SA Precast Pty Ltd. In 1935, Nap Floreani started Floreani Bros and later Constress, while the Dalle Nogare family started Marble and Cement in the early 1950s. With GMH and Chrysler establishing
has been successfully exported from Queensland to NSW as well as to New Zealand and beyond.
facilities in stone South masons Australia, work expanded rapidly and many and concrete workers were brought from Italy as immigrants. This provided Adelaide with a very skilled workforce and factory capacity in excess of its long-term needs, so export to Melbourne, the ACT and other states began in 1962. Thereafter, Adelaide supplied over half of the architectural precast being used in Victoria (Figure 1.7) and the ACT. Excellent examples of early precast concrete in Adelaide include the loadbearing walls in the IMFC building which was completed in 1970. Projects in New Zealand and New Guinea have also been supplied from Adelaide.
30 storeys and detached dwellings in precast loadbearing panels (Figure 1.9).
In Western Australia, the use of precast was restricted to drainage and civil works until the late 1950s when architectural cladding began to be used. Between 1950 and 1956, structural floor units of precast reinforced inverted U-sections were used in WA with spans of up to 6.5 m in structures such as the 1950 Brockman River bridge near Bindoon. Early architectural projects such as the Fremantle Port Authority Passenger Terminal (Figure 1.8) and the Narrows Bridge were carried out by Monier. Other firms including Humes, Clough, Marble and Cement Works, Delta Concrete and Unit Concrete were engaged in precast manufacture. During the 1980s, precast structural frames were introduced to Perth buildings and have been a major feature of construction since. Flooring products also enjoy wide acceptance in Western Australia. The development of precast in Queensland paralleled Western Australia with civil products produced by Humes, Monier and Rocla from early times. The use of architectural precast commenced under the influence of Dowsett, Monier and Humes in the 1960s. Precast Concrete Pty Ltd commenced business in 1968 and soon became Queensland's major architectural precaster, through projects such as the Parliamentar y Annexe in George Street, Brisbane. Queensland’s first precast concrete bridge is believed to be that at Tenthill Creek near Gatton. It was also one of the earliest local examples of prestressing. Today, a variety of architectural, infrastructure and drainage products are produced by a number of precasters throughout Queensland. Architectural precast, especially with polished finishes,
In Victoria, the early post-war years also saw the birth of a precast activity which was to become one of Australia’s biggest – the Concrete House Project, the building arm of the Housing Commission, Victoria. With a huge factory in Holmesglen in the south-east of Melbourne (previously used for manufacturing Centurion tanks during the war) through to the 1970s it was building apartment blocks of up to
In terms of innovation in complex architectural and structural work, few companies could match EPM. EPM (srcinally short for England Pipe and Marlite) began in Melbourne in 1951 manufacturing smalldiameter unreinforced concrete pipes and wash tubs. In 1953, the company began unit mould prestressing and produced the first steam-cured concrete products in Australia. It, along with a competitor, High Strength Concrete, introduced the use of precast columns, beams and pretensioned floor planks and double-Ts to Melbourne. The first recorded use in Australia of precast concrete exposed aggregate wall panels was in construction of the Melbourne Grammar School’s boatshed on the banks of the Yarra River in 1953. These panels were made by EPM. This company went on to establish in Sydney and carry out projects such as the Sydney Opera House, the Commonwealth Parliament House, the Eastern Suburbs Railway viaducts as well as thousands of other high- and low-profile jobs.
Other early Victorian precasters included SVC (who carried out early prestressed and architectural work but specialised in drainage products), Mays Vibrated Concrete (which specialised in structural work and had a plant in Tasmania), Buchans, High Strength Concrete, Monier, Humes and Rocla. Melocco Bros established in the 1960s and Fabbrostone and Associated Precast Concrete in the 1970s. The nature of the Victorian precast market changed in the 1980s and 1990s as precast claimed the bulk of industrial and high-density residential buildings. As the use of precast in buildings escalated in Sydney there were many new firms to supply the need. In 1970 the major suppliers of architectural precast concrete in NSW were EPM, Humes, the BMI company Melocco Bros, the Pioneer company Anslow Marble, Pebblecrete Precast, Fabbrostone, Prestige Precast and Gosford Quarries. Monier had just withdrawn from the field after carr ying out projects such as Australia Square. The structural market was supplied by EPM, Humes, Monier, Peter Verhuel and a number of regional precasters. The drainage product market was led by Monier, Rocla and CI&D with many smaller producers around the state.
The severe recession of the mid 1970s took a heavy toll on these firms, however, and by 1980 the only architectural producer left from the above list was EPM. Rescrete, which developed into a firm with a diverse product range, started as a small precaster in 1968 and Beresford Concrete Products entered the drainage-product market in 1971. Structural Concrete Industries (SCI), specialising in infrastructure projects, commenced business in 1979. By the end of 2000, the NSW industry had evolved, along with the construction industry and architectural and construction fashions, with older firms closing or adapting and new firms commencing operations. 1.1.3
Precast in buildings
The building boom of the late 1960s and early 1970s caused huge growth in precast manufacturing capacity throughout Australia. Unfortunately the boom-bust nature of our small economy continued true to form and many firms born in those times did not survive. Precast panels were used as permanent formwork on the MLC Centre (Figure 1.10) and Northpoint towers in Sydney and on many other buildings throughout Australia. The former’s complex building facade followed a Nervi Curve so that the column units on every floor had different dimensions and different curvature. Precast was used to clad the new Commonwealth and NSW Parliament Houses. However, it was the spectacular use of precast concrete in the construction of the Sydney Opera House which best demonstrated the versatility of the medium. The sail-like shells incorporated posttensioned complex segmental precast components while the paving and facade were honed and etched precast incorporating Tarana granite (Figure 1.11). Australian buildings acquired a different appearance in the 1960s and 1970s. There was a sudden upsurge in the number of multi-storey buildings erected and – instead of expanses of glass – buildings of the 1960s showed the concentration of designers on form, texture and colour. Precast concrete finishes became better utilised, many multi-storey buildings used loadbearing precast, leading to a trend towards totally-precast structures. One such building was the then IBM building at the corner of Coventry and Sturt Streets, South Melbourne (Figure 1.12), designed by Joshua and Mary Pila, where prestressed double-Ts, core walls and stair flights were used in conjunction with a sandblasted loadbearing facade. Exposed-aggregate and reconstructed-granite facades were increasingly used. Apart from their aesthetic quality, such precast panels were also durable, waterproof, fire resistant and structurally efficient. Off-site manufacture also gave benefits of savings in construction costs and eliminated congestion on construction sites.
The use of precast concrete as a cladding for office buildings has fluctuated with architectural fashions but precast has always held the major share of top hotels and civic buildings. The loadbearing polished facade of the Westin Hotel at No. 1 Mar tin Place in Sydney, carried out at the end of the 1990s, is an excellent example (Figure 1.13). From the 1990s, particularly in Melbourne, there was a dramatic increase in the number of building facades being constructed from precast concrete with painted finishes. These were mostly residential and commercial medium-rise buildings. As precast used in this way became more popular, it was increasingly designed to be loadbearing – often with internal walls and floors also being precast. Factory buildings came to be clad in precast concrete by different processes in different parts of Australia. In Sydney, EPM, followed by Spancrete, installed hollowcore machines in the mid 1970s and hollowcore became the dominant cladding (see Clause 1.1.4). In other areas of Australia, it was tiltup which dislodged traditional brick and block walls but most tilt-up contractors changed over to factory precast construction and in those areas reinforced concrete precast panels dominate. A variety of precast flooring systems have led to increasing penetration of the flooring market. Hollowcore, partially-reinforced permanent formwork (Figure 1.14) and prestressed beam and infill systems (Figure 1.15) have been the main products. At the end of the 1990s it was estimated that precast concrete supplied some 5% of the suspended flooring market.
Figure 1.10 MLC Tower, Sydney
Figure 1.11 Sydney Opera House
Figure 1.13 Westin Hotel, No. 1 Martin Place, Sydney
Figure 1.12 Former IBM Building, South Melbourne
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Figure 1.14 Precast Permanent Formwork and Hollowcore
Figure 1.15 Beam and Infill System (Ultrafloor) in a Commercial Development
Figure 1.16 Hollowcore Banded Architectural Wall Panels on Factory Building
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Figure 1.17 Narrows Bridge over Swan River, Perth
Figure 1.19 Subiaco Football Stadium. Perth
Figure 1.18 The Rip Bridge near Gosford, NSW
1.1.4
Hollowcore construction
The first hollowcore production in Australia was probably carried out by Mays Vibrated Concrete, which produced floor ing, in Victoria in the 1950s. It was not until EPM, with an American Stressdeck system, and then Spancrete, with an American Spancrete machine, began manufacturing in the mid 1970s that hollowcore made a major impact. Hollowcore machines were being used in the US and Europe almost exclusively to produce floor planks. In Sydney, however, they were also adapted to make exposed-aggregate or plain-finish wall panels, and some five million m2 of walling was manufactured between 1974 and 2000 (Figure 1.16). It is thought that more hollowcore walls are manufactured in Sydney than anywhere else in the world. Victoria and Western Australia also have strong hollowcore industries. Hollowcore has great advantages in weight and, being prestressed, in durability. Its low consumption of raw materials makes it eminently suited to a more environmentally-conscious future. 1.1.5
Civil infrastructure
Large civic structures are the visible face of precast concrete but less visible forms have been crucial in the development of Australia. The steady expansion of the road and rail transport systems, the sewerage systems and the electric power distribution and communication systems would not have been possible without the precasting industry. Bridge beams, railway sleepers, culverts, pipes, tunnel liners, piles, utility poles and septic tanks are typical of the products which have been widely used throughout the countr y. These products were pioneered by firms such as Monier, Humes and Rocla throughout Australia as well as by firms such as SVC and Mays Vibrated Concrete, EPM, CI&D, Dowsett and many others in each state. In the 50s, comparatively few large concrete bridges were built in Australia. Mainly they were built of steel and to fairly orthodox and not particularly inspiring designs. Then in the late 1950s and early 1960s came a series of beautiful concrete structures which were acknowledged as superb examples of the use of structural precast concrete combining appearance with utility. Examples include such bridges as the Narrows, Perth (Figure 1.17); San Remo, Melbourne; the segmentally-constructed Port Augusta bridge in South Australia; the vehicular ramps at Tullarmarine Airport, Melbourne; Alfords Point and De Burghs bridges in Sydney; the Rip Bridge near Gosford, NSW (Figure 1.18) and the Gateway bridge in Brisbane.
Following the publication in 1967, by the Cement and Concrete Association of Australia, of a Technical Study on concrete sleepers for use in heavy-duty track, tenders were called in 1970 for concrete sleepers for a 75-km standard gauge spur line between Port Augusta and Whyalla in SA. Sleepers for this line were supplied by Monier from a factory set up at Port Augusta. Since then, by building on the experience of railway systems overseas and developing systems suited to its own environment, Australia has become a world leader in the design and manufacture of prestressed concrete sleepers. Many large sporting complexes have been built with precast components since the days when Humes manufactured the prestressed grandstand for Sandown Park, Melbourne in 1962. Other projects include, the MCG Great Southern Stand, the National Tennis Centre and Colonial Stadium in Melbourne, the Subiaco Football Stadium in Perth (Figure 1.19) and the Hindmarsh Soccer Stadium in Adelaide. In the Homebush Olympic precinct in Sydney the seating and associated walling for the Showground, Superdome, Tennis Centre, Aquatic Centre, Olympic Railway Station and much of Stadium Australia, was precast. 1.1.6
Commercial and other influences
The development of the precast concrete industry in Australia has been influenced by many factors. Perhaps the most important has been the evolution of cranes, especially mobile cranes, which are now more manouverable and have large lifting capacities. Better roads and more sophisticated road transport rigs have made larger loads and greater delivery distances economical. Another influence has been the course of industrial relations in Australia. The details of these and other such important factors are, however, outside the scope of this brief history. Firms manufacturing precast concrete are primarily suppliers to the building industry and the civil construction industries. The commercial environment which has prevailed in the Australian construction industry is very adversarial. Many precasters have not been as adept commercially as they have been technically and this has led to business failures over the years. Despite many initiatives and considerable effort, the industr y, in common with all other specialist subcontractor sectors, has never been able to achieve standard and equitable contract documents. Archaic provisions such as retentions and liquidated damages still bedevil the industry. Unrealistic schedules and bid shopping have been the norm rather than the exception and seldom has the precast industry earned returns commensurate with the investment
1.2.1
Office buildings/ hotels
and risk involved. Much enthusiasm, innovation and capital has been wasted by needless disputes and poor contracting practices. Successful firms are those who have learned that work carried out within a poor contractual framework is work which is not worth doing.
Precasting has always been a high-risk industry for its participants. Those firms which manage the risks best are those who will continue to lead the precast industry to realise its full potential in contributing to the productivity of Australia.
The combination of high-quality architectural loadbearing or non-loadbearing exterior walls with precast columns and beams and mass produced structural precast floor and roof components can produce open, attractive, fire-resistant, economical low-rise and high-rise office or hotel buildings. Precast hollowcore and composite flooring systems reduce interior framing, providing large column-free areas while single- and double-Ts are ideal for very long spans. Interior or exterior shear-wall systems and rigid-frame column/beam jointing have all been successfully used to resist lateral forces. For the exterior of the office building or hotel, architectural precast offers the opportunity to meet aesthetic and practical requirements through a range of colours, forms and textures. Significant time savings usually result from the choice of a total precast concrete structure with the superstructure being prefabricated while the on-site footings are being built. 1.2.2
Institutional/ public buildings
Designers strive to create institutional or public buildings (airports, theatres, courts, museums, libraries, convention centres, universities) which are open, functional and inviting. The use of precast concrete, both prestressed and reinforced, contributes in a number of ways:
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Architectural precast provides such visual expressions as strength and massiveness, or grace and openness.
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Exposing the concrete in the interior of public buildings can produce a dramatic effect.
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Rugged exterior and interior surface finishes showing different colours and textures are visually stimulating and are durable.
Figure 1.20 Office Building – Santos Building, Adelaide
Figure 1.21 Hotel – Shangri La Hotel, Circular Quay, Sydney
Figure 1.22 Public Building – ACT Magistrates Court, Canberra
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Figure 1.23 Shopping Centre – Westfield, Hornsby, NSW
Figure 1.24 Industrial Building – Typical Precast Industrial Building
1.2.3
Shopping centres
Shopping centres are invariably constructed to meet a firm and often optimistic occupancy date. The use of precast wall panels allows an important part of the construction process to be carried out off-site prior to critical dates. Concrete is fire-resistant, tamperproof and insulates from both heat/cold and noise. All these qualities are required for this type of building. The use of precast in suburban shopping centres has remained strong for decades. Precast concrete makes shopping-centre expansion more efficient because it allows construction with a minimum of disruption. Shopping centres often need large areas of suspended slabs for which precast construction is ideal. In most cases, floor-to-ceiling heights in shopping centres are over four metres, making conventional formwork and falsework expensive and slow. The use of precast in these applications is costeffective, especially with the use of long-span precast prestressed band beams and hollowcore floor panels tied together with an insitu concrete topping. This method of construction eliminates the need for temporary propping of any kind. The use of precast columns enhances the speed and efficiency of construction. Depending on the grid layout, other precast elements, beam-infill composites or permanent formwork panels may be used. The precast concrete frame can be clad with precast wall panels with varying architectural finishes or with hollowcore panels. A distinctive appearance can be achieved through a choice of finish – profiled, finished in natural colours, tinted with colourfast oxides or painted. For further information on wall panel finishes refer to Chapter 10 Architectural Elements.
1.2.4
Industrial buildings
Industrial buildings require speed of construction with the walls providing a durable, weatherproof enclosure and a choice of exterior finishes to reflect the activity of the occupant. The external walls may also be required by regulation to meet fire ratings or to contain fire hazard. In some circumstances they may need to insulate adjacent properties from noise. Security for the occupant is always a consideration. These requirements are met when the walls are constructed of precast panels. The panels are often cladding attached to a structural frame of concrete or steel and carry only nominal vertical loads. A steel portal frame is commonly used to provide a clear span with non-loadbearing exterior walls. The panels stand vertically on strip footings or span between columns. Pretensioned hollowcore panels are often used in this way and these are a modular 1200 mm wide. Thickness is determined by structural or firerating requirements. With larger modules or where non-regular shapes or specific architectural detailing are required, reinforced wet-cast panels are often more suitable.
Loadbearing panels eliminate columns. The lateral stability of the building is usually provided by a braced roof system that transfers horizontal loads into the plane of the panels, which act as shear walls, and then to the footings. Precast components are ideal for mezzanine floors, and other suspended flooring, where they occur in industrial buildings.
In summary, the use of precast structural and architectural elements for this type of project minimises construction time and produces a highquality finished product. For expansion projects it offers minimal disruption to the existing facility.
1.2.5
Medium- and high-rise residential buildings
The trend towards inner-city living has resulted in an increased demand for high-density, medium- and high-rise residential accommodation in most capital cities in Australia. The need to meet this demand has resulted in an increased use of precast in these projects. Unlike insitu construction, the use of precast has in most cases eliminated the need for a structural frame by utilising loadbearing precast wall panels. Precast wall panels used for the external walls can be profiled to architectural requirements and can be supplied ready to receive an on-site painted finish or can be pre-finished in the factory. Precast wall panels are also used for loadbearing party walls, lift and stairwell shafts. Stair flights can also be provided in precast concrete. To further enhance the use of precast walling, the use of precast flooring makes possible a complete precast structural system which provides speed and efficiency of construction. A total precast structure provides major advantages including:
1.2.6
Multi-level parking structures
Multi-level parking structures offer an ideal application for precast and prestressed concrete. Both architect and engineer have the opportunity to exploit the inherent qualities and flexibility of precast concrete. The architect can incorporate important architectural features in most of the precast elements as they remain visible in the completed structure. For the engineer, the many advantages include large clear spans affording easy parking access to motorists. The structural frame consists of precast columns, precast or prestressed perimeter beams, with either hollowcore or double-T floor panels. To complete the structure, precast concrete balustrading, planter boxes and end-wall panels are often incorporated. Preplanning and detail documentation are essential to ensure efficient and speedy construction. As elements are large and heavy, crane size and its positioning is critical. Particular attention must be given to weathering requirements as in most cases the interior of the structure is exposed to the weather. (For details, see Chapter 7 Connections and Fixings.)
•
Speed of construction;
1.2.7
•
Dramatically reduced labour force on site;
Precast construction is an ideal solution for correctional facilities by virtue of its advantages in security, maintenance and durability. The basic cell units may be assembled from individual panels or any combination of panels and integrally-cast wall, roof or floor members. Typical integral shapes are inverted Ls and Us and open boxes. The partially-completed cells can be fitted out with their permanent furniture prior to deliver y. The cells are loadbearing when stacked in the usual two-high arrangement and can support loads from other parts of the complex. In addition to cell units, precast is often used for perimeter and division walls.
•
High quality finished product;
•
A more durable building.
Correctional facilities
Figure 1.25 Medium- and High-Rise Residential – ‘The Wave’, QLD
Figure 1.26 Multi-Level Parking Structure – Brisbane Airport, QLD
Figure 1.27 Correctional Facility – Correction Centre, Junee, NSW
Figure 1.28 Stadia – Hindmarsh Soccer Stadium, Adelaide
Figure 1.29 Tunnels – Gold Coast Desalination Project, Queensland, Incorporating some 21,000 Precast Segments
Figure 1.30 Tunnels – City West Cable Tunnel, Sydney
Figure 1.31 Marine Structures – Precast Wharf on the Brisbane River, Queensland
Large sporting stadia are ideal structures for the use of precast concrete components. The seating requirements of thousands of spectators can be met
The segments are manufactured in very high-quality moulds with tight tolerances. Production of the segments may be via a simple static process where the moulds are laid out within a manufacturing area and concrete is brought to each mould. More typically, however, the segments are produced in purpose-built semi-automatic carousel productionline plants where the moulds travel within a closed circuit. The circuit will provide for the filling and finishing activities, a large curing chamber and then
using a combination of precast concrete seating platts which are supported on raked beams of steel or concrete.
a product-removal station. A cleaning station and a reinforcement/cast-in fitting placement station complete the circuit.
In addition, precast concrete can be used in the flooring systems of the catering and entertaining areas as well as wall units and spandrel or fascia units.
Typical uses for precast segmentally-lined tunnels include, road and rail tunnels, sewers, water supply, high-voltage electricity supply cable tunnels and intake/outfall tunnels for desalination plants.
1.3.1
Stadia
As stadia are in the public spotlight and construction times are ver y tight so as to minimise disruption to programmed spor ting events, the use of precast concrete can be instrumental in the success of a project. 1.3.2
Tunnels
As Australian cities become increasingly congested and the environmental and disruptive impact of providing road, rail, power and other utility services become critical factorssointoo theiscontinued of the urban centres, the value functioning of tunnelling being recognised as the appropriate vehicle for delivering such utilities. Once considered prohibitively expensive, the disruption cost and environmental risk of opentrench construction methods are now often balanced by the cost benefits of tunnelling. Where it is necessary for tunnels to be lined, segmental precast tunnel liners now provide a universal solution to this need where the tunnel is excavated using a purpose-built tunnel boring machine (TBM).
1.3.3
Marine structures
Marine structures vary in size from simple piers or jetties to major shipping wharfs such as for the delivery of materials from major mining projects. Structures can be designed to carry a variety of load combinations covering vehicular access and conveyor or pipeline loading systems. Precast concrete is an obvious material for marine structures due to the following distinct advantages: • Factory-produced precast concrete is better able to satisfy the very rigid specifications required for the durability of marine structures. •
Use of precast minimises delays to construction by eliminating time losses due to weather and wave conditions.
•
Components can be made to many configurations and structural capacities.
Typically, a lined tunnel comprises rings, the outside diameter of which is in the order of 100 mm less than the diameter of the bored tunnel. These rings may be between 1 and 2 m in length and will be divided into 6 or more trapezoidal precast segments.
1.3.4
Bridges and culverts
Precast concrete is used extensively in the construction of road and rail bridges. Its use provides the designer and contractor with a large range of options for bridges and culverts spanning from 600 mm through to 200 m in segmental construction.The development of prestressed concrete with standard pretensioned and posttensioned elements has provided many different alternatives for the designer faced with particular site conditions, durability requirements and the need for acceptable aesthetics of the structure. Types of structures range from rectangular culverts for small spans from 600 mm to 6 m, arch structures from 6 to 21 m and prestressed plank and girder options that range from 5 m through to 40 m. In addition to these, precast post-tensioned segmental girders can be used to bridge spans up to 200 m. (Refer to Chapter 2, Section 2.3.1.1 for summary of span ranges.) The various state road and rail authorities utilise a number of standard sections for various bridge spans. These girder types include deck and plank units, I-Beams, T-Beams, and open- and closed-flange Super-tees. Advantages of precast concrete in the various bridge applications include: •
Simple and quick erection
•
The tops of decks and tees can receive ballast or pavement materials directly
•
Units may be customised to meet specific load configurations
•
Suitable for use in remote locations
•
Minimising on-site construction time.
1.3.5
Towers and poles
Towers Precast concrete towers essentially consist of a shaft made of precast elements supported on insitu concrete footings. Precast concrete construction is a very flexible method and a wide variety of plan shapes may be produced, including circular, square, rectangular, oval and egg-shaped. The segments can also be designed to incorporate water tanks, lifts, access ladders, service penetrations and the like. The tower segments may change section as height increases. This concept becomes more economical when several towers are to be constructed. Towers using single elements up to 4.3 m in diameter can be transported and from which towers up to approximately 80 m tall can be constructed. At present, precast towers are used for heights of up to approximately 60 m. They have the advantage of permitting construction in a large range of shapes
and a wide variety of surface finishes. Another important advantage is the high speed of erection possible. Typically, a 25-m high tower can be constructed on site in approximately five working days. In the past decade there has been an increase in tower construction due to the development of high-frequency radio communication networks. Other towers have been constructed for television broadcasting, air traffic control, stadium lighting and for general public access. Poles Precast concrete poles, either prestressed-spun or steel-reinforced are available from Australian manufacturers in some states. Applications for precast poles include: •
Lighting for streets and highways
•
Power transmission and distribution
•
Substation poles
•
Lighting towers for sports arenas, parking areas, etc
•
Radio masts
•
Support columns for elevated signs
•
Railway power distribution.
Figure 1.32 Culvert – Precast Twin-Cell box Culver t with Wing Walls and Apron
Figure 1.34 Bridge – Erection of 1500-mm depth by 30-m span Super-Tee Bridge Girder
Figure 1.33 Bridge – Precast Pedestrian Suspension Bridge, Woy Woy, NSW
Figure 1.35 Bridge – 1150-mm depth I-Beams used on Pacific Highway Project, Central Coast, NSW
Figure 1.36 Tower – Traffic Control Tower at Sydney’s Kingsford Smith Airport
Figure 1.37 Mining Infrastructure – Prestressed Concrete Sleepers used for Transportation by Heavy-Haul Locomotives
Figure 1.39 Noise Wall – Variegated Coloured Panels in Noise Wall, Tugun Bypass, Gold Coast
Figure 1.41 Retaining Wall – Hollowcore Panels used as Retaining Walls
Figure 1.38 Noise Wall – Use of Form Liners to Achieve an Attractive Stone Finish
Figure 1.40 Retaining Wall – Precast Units in Reinforced-Soil Wall to Over-Bridge Approach, Adelaide
1.3.6
Mining infrastructure
Precast concrete has many uses within Australian mining infrastructure projects by reason of the following advantages: •
The ability to produce units off-site for remote projects
•
Enabling major structural work to be taken off the project critical path
•
Vehicle loadings used in mining are very large and precast prestressed concrete offers an ideal solution
•
Infrastructure to mining projects such as railways and wharfs lend themselves to the use of precast concrete because of the size and repetitive nature of their components.
Typical uses of precast concrete are heavy-duty railway sleepers for hauling of ore from mines, supports for conveyors used to carry ore to the treatment plants, large arch structures at mine entrances and at ore loading stations, tunnel liners and prestressed wharf units at shipping terminals. 1.3.7
Noise Walls
Precast concrete wall systems to control traffic noise are used extensively on road projects in Australia. Design options include panels curved in two directions, a wide range of surface textures, colours and sculptured surfaces. Different types of noise walls may be used for the one project for an effective and economical design solution. There are three types of noise walls to reduce traffic-noise problems – reflective, dispersive and absorptive – and advice may be sought from an acoustic consultant before choosing the relevant type.
1.3.8
Retaining walls, storage bins
Precast elements are frequently used to provide stability in retaining wall structures and for storage bins for aggregates, grain and other materials. Common systems are crib walls, L-shaped wall panels, hollowcore wall panels and reinforced-soil walls. L-shaped wall panels are interlocking units with heights generally available in the 1.2- to 4-m range. They can be provided with mastic-sealed interlocking joints and made into a monolithic structure using galvanised steel straps. Hollowcore panels are often applicable for storage and retaining walls. They have the advantages of longer spans for horizontal applications. Reinforced-soil walls are composite structures formed by the interaction of earth backfill with reinforcement of steel strips or geosynthetics. The earth mass behind the facing panels tends to act as a cohesive monolithic body, supporting its own weight as well as the external loads for which it has been designed.
Reinforced-soil walls are typically faced with precast concrete panels; the backs of the panels have connections to distribute the soil reinforcement within the earth backfill. The panels themselves do not hold up the wall but act as architectural facing and protect the wall from erosion. Panels can be supplied for this system in a range of shapes and surface textures from cruciform shape to large rectangular panels.
Reflective barriers are located at the edge of the road and reflect traffic noise. They are less effective when they reflect towards buildings built on the high side of the road. Dispersive barriers give diffuse reflection which avoids concentrations of reflected noise. Absorptive barriers absorb sound by forcing the sound pressure waves to move in and around many tiny fibres or passages to dissipate the sound energy. A combination of barrier types may be suitable for particular sites and topographical conditions. Reflective and dispersive noise walls can be supplied in precast units either as solid reinforced concrete panels, or hollowcore prestressed panels, with a wide range of surface finishes. Absoptive wall panels offer fewer aesthetic opportunities.
Figure 1.40 Drainage – Precast Silt Arrestor Chambers
Figure 1.41 Environmental – 3000-Litre Precast Septic Tanks
Figure 1.42 Landscaping – Precast Polished Planter Boxes at Entrance to Shopping Centre
s n ri ie S ic r E
y b h p a r g to o h P
Figure 1.43 Municipal – Precast Multi-Opening Headwall with Integral Wingwalls and Energy Dissipator
1.3.9
Drainage and environmental products
The introduction of precast concrete drainage pits and associated products into the market in the early 1980s, revolutionised the industry by making the installation of pits much simpler and substantially reducing construction times. Previously, all drainage and junction pits had been built insitu. Precast pits not only reduce the installation time, but minimise the problems associated with wet weather. Today, precast pits are accepted by most local government and state road authorities as alternatives to insitu construction. Precast pits come in a range of standard sizes from 450 to 1200 mm square, while non-standard pits are also made to order to suit specific requirements. Domestic septic tanks are manufactured in capacities up to 7500 litres from precast concrete using either conventional reinforcement or fibresteel reinforcement.
1.3.11
Other products
The precast industry has developed innovative solutions to allow prefabrication of many specialised building components and products. These include: •
Cell-type units – bathrooms, pontoons, transpor table sheds and telecommunication units
•
Burial units – multi-section crypts, vaults and memorials
•
Rural products – fence posts, water tanks and cattle grids.
Other trade-waste products such as grease arrestors, oil and silt arrestors, general purpose and dilution pits have also followed the path of the drainage pits from insitu to precast. Most modern environmental products such as gross-pollutant traps, designed to prevent water-borne rubbish and silt from fouling waterways are also generally supplied in precast, designed and cast to suit specific site and inlet requirements. 1.3.10 Landscaping and municipal products The requirements for landscaping and municipal products are functionality, durability and aesthetics. All three of these can be met by using precast concrete. There is no limit to the types of municipal and landscaping products which traditionally are supplied in precast instead of insitu, with the added features of precision moulded products and wide range of colours and textures. Examples of these include: •
Monuments, signage, etc
•
One-piece kerb entry units
•
New Jersey safety barrier s
•
Headwalls for culverts.
Precast concrete offers numerous advantages over and above the desirable features inherent in good insitu concrete construction. Important advantages include:
Fire Resistance Precast concrete has inherent fire resistance and is a material ideally suited for structural and architectural elements in residential and commercial buildings prone to fire attack, particularly by bushfires. Precast offers inherent fire protection because its noncombustible composition inhibits the spread of fire. Precast concrete floor and wall panels perform better in fire than other materials such as wood and steel, both of which must be treated, coated or covered to meet fire requirements thus increasing costs and creating the possibility for errors and missed details during installation.
The BCA provides regulations setting out the methods of providing fire resistance levels which involve particular arrangements of non-combustible building elements to prevent the spread of fire and provide safe escape routes for building occupants. A structural concrete element is designed to have a fire resistance period (FRP) for structural adequacy, integrity and insulation to be not less than the required fire resistance level (FRL). For further information on designing precast concrete structures for fire resistance, reference should be made to Section 5 of AS 3600.
Faster Construction Manufacturing of precast components can begin as soon as drawings are approved.This ensures they are ready for erection as soon as foundation work and other site preparation is completed. Once precast erection commences, on-site construction and off-site manufacture can be overlapped, thereby reducing overall site construction times. The continuous, uninterrupted erection of precast structural components lends itself perfectly to fasttrack construction schedules. Installation incorporates the latest in connection technology, and can proceed swiftly and safely in almost any weather by experienced erectors. This results in construction times that can be up to 75% less than for traditional construction methods.
Enhanced durability Durability is defined as the ability of a structure to resist the ravages of its environment. Precast concrete offers a very durable, low-maintenance product, benefiting from fabrication in a controlled factory environment, use of quality materials and proven manufacturing techniques. Sophisticated mix designs characterised by low water-cement ratios, good compaction and adequate curing (all associated with durable concrete) are synonymous with precast products.
Design Freedom The initial plasticity of concrete allows the casting of complex shapes.This, together with the available colours and textures allows the designer scope to express mass or space, simplicity or grace. With advances in design, manufacture and installation, many structures thought to be impossible to construct with precast are now being realised, and the design flexibility offered by precast is unparalleled.
Precast’s ability to enclose the structure much sooner than traditional types of construction enables ear lier access for follow-on trades. The long clear-spans provide an instant work platform, while minimal propping further enhances access and improves project construction times. For example, over 50 hollowcore floor planks or 20 wall panels can be placed in a day with one erection crew.
Moulds can be created to suit any requirement, giving the capacity to produce both structural and architectural elements. Structural elements can be tailor-made to suit the project requirements. A myriad of exterior architectural facades can be achieved using different colours, textures and finishes, from a grey unpainted off-form finish to decorative polished and highly-detailed finishes. As such, the use of precast is limited only by the designer's imagination.
Lower Cost Precast provides the owner, developer and contractor with a firm budget and scope of work for the building, early in the project. Whilst the face value cost of precast may not always appear lower than traditional construction methods, significant cost savings are realised from areas such as: •
initial design for precast, eliminating the need for conversion from traditional construction methods;
•
manufacture of precast elements concurrent with commencement of early site works;
•
expedited construction;
•
reduced time on site;
•
reduced site defects;
•
reduced propping and scaffolding costs;
•
lower site labour costs;
•
reduced plant, amenities, tools and materials storage requirements;
•
economies from specifying fewer larger elements;
•
re-use of moulds;
•
lower costs of finance resulting from reduced time on site;
•
earlier revenue receipts because of shorter project times.
y ile a B ic N : h p a r g to o h P
It is only when there is an early understanding and recognition of these cost savings, that the maximum benefits of precast can be realised in the project, when compared with traditional construction methods. Factoring in the cost savings makes it obvious that precast is the more economical choice.
High-Quality Finishes
Precast provides architects and designers with a variety of aesthetic options. Precast can be grey and off-form, whereby the use of state-of-the-art steel casting beds and forming equipment result in a quality of finish which is far superior to that which can be achieved on-site. A variety of architectural finishes can also be achieved by varying: •
the colour, with different cements, aggregates, pigments, paints or stains;
•
the form, with moulds which can be made especially for a project by the precaster, or form liners, or by embedding thin brick, stone or other materials into the precast;
•
the finish, by grit-blasting, acid-etching, honing and polishing.
More information is provided in Chapter 10 Architectural Elements.
Environmental Benefits Precast concrete has many environmental benefits during construction and for the life of the structure. The manufacture of precast uses less energy than that required for either str uctural steel frame components or glass curtain walling. Recycled supplementary cementitious materials such as fly ash and blast furnace slag, silica fume, recycled aggregates and grey water can be incorporated into precast concrete. This diverts materials from landfill, reducing use of virgin materials and the overall environmental burden. On site, precast construction creates less air pollution, noise and debris. The high-quality finish of precast concrete means that it can be left untreated and exposed in order to maximise concrete’s thermal mass benefits and to contribute to green energy-management solutions.
Health and Safety Precast manufacturers employ safe work practices both during production and during erection of elements on site. If the precast is being installed by the manufacturer, it is customar y to provide to the customer work method statements and proof of compliance with safety standards. Once precast floors are installed, they provide a safe working platform for the erection crew. With precast construction, site safety is improved because on-site trades and their associated activities are minimised or even eliminated. Finished components are delivered to site and lifted directly from the vehicle into position on site, often without the need for scaffolding.
Less waste is created with production of precast concrete. Tight control of quantities of constituent materials and precise mix proportions mean optimum use of materials. Standard precast products such as beams, columns, floors, walls, decks, road barriers and drainage products are manufactured in moulds that are re-used many times. Any waste materials are more readily recycled because production is in one location. Site waste is also reduced as only the finished elements are delivered to the construction site.
•
Sustainability is defined as development that meets the needs of the present without compromising the ability of future generations to meet their own needs. It allows the protection of the environment and prudent use of natural resources. Sustainable development challenges the design and construction industry to create buildings that acknowledge the life cycle of a building. Recognising that operating a building over time is far more energy intensive than developing it, demand for durability and energy performance is growing. Greenhouse gas emissions in buildings are due to both embodied energy and operating energy. Architects, engineers and builders are choosing concrete for its durability, reduced maintenance and energy performance; properties not found in other building materials like steel or timber. When compared with other building materials, concrete is a responsible choice for sustainable development. The underlying properties of concrete make a strong contribution to sustainable construction. The ingredients of concrete are locally sourced, while concrete itself is durable, versatile and can be completely recycled. Although concrete has a high level of embodied energy, designers and builders can adopt the following options to reduce embodied energy and make it more sustainable. •
Recycling of concrete waste The Australian Greenhouse Office encourages and rewards builders and designers to give due attention to the use a significant recycled content in building construction or refurbishment. Concrete waste can be processed to produce roadbase/fill material, recycled concrete aggregate and recycled concrete fines. Extensive research has been undertaken to increase the use of recycled concrete worldwide. The primary use of recycled concrete in Australia is for roadbase material, which not only reduces the need for natural fill but is also commercially viable.
Use of Supplementary Cementitious Materials The quality and properties of concrete can be improved by replacing a portion of the cement with industrial by-products known as supplementary cementitious materials (SCM) such as fly ash, blast furnace slag and silica fume. Use of these materials also reduces both mining of natural resources and greenhouse emissions associated with cement production while disposing of a waste material previously destined for landfill. Fly ash is commonly used to replace between 20–25% of portland cement in a blended cement, although higher percentages are possible and could be adopted where appropriate for a greater impact.
•
•
Increase the use of recycled water in concrete Recycled water has been successfully used in concrete for many years. Its use, quality and limits are assessed under AS 1379. In addition, finishing processes such as polishing and honing can use recycled water.
Improving building design and specifications This involves: developing low-energy, long-lasting yet flexible buildings and structures; exploiting the thermal mass of concrete in a structure to reduce energy demand; considering innovative or alternative design that incorporates de-materialisation such as using materials that have undergone an energy-saving process or action during manufacture or sourcing such as a filler component in cement manufacture.
Precast concrete is the predominant construction technique in Australia for industrial, commercial and residential development. Precast concrete is manufactured in a controlled environment allowing more efficient use of materials with very little waste compared with insitu concrete. Formwork is reduced or eliminated, as is its associated waste, and buildings can be erected quickly. The advantage of controlled concrete manufacture becomes apparent as each part of the process can be easily monitored and controlled due to the operations being repetitive. Specific examples of where sustainable designs, using precast construction, can make a considerable environmental impact are given in Table 1.1.
Table 1.1 Specific Examples where Sustainable Designs using Precast Construction can make a Considerable Environmental Impact
Larson, T D, Portland Cement and Asphalt Concretes McGraw Hill, 1963.
The First Fifty Years , Monier, 1986.
Ritter, P,Concrete Fit for People Down-to-Earth Bookshop Press, 1980.
Taylor, W H,Concrete Technology and Practice 3rd ed. Angus & Robertson, 1965. Anderson, A, ‘From Roman Era to the Atomic Age: Concrete Masonr y has Withstood the Test of Time’ Journal of Industry , Vol 38, pp 5–10, Dec 1970.
‘Cement Industry Well Placed’ Journal of Industry and Commerce, No. 23, pp 6–8, 1981. Cameron, H S, Profile and History of the Australian Industry in United Nations Industrial Development Organisation Workshop on Cement and Concrete Products, Brisbane, 1981.
Constructional Review Vol 50, No. 4, Nov. 1977. (Special issue devoted to the history and use of concrete in Australia). Fraser, D J, ‘Early Reinforced Concrete in New South Wales (1895–1915)’, Multi-Disciplinar y Engineering, Vol 9, No.2. pp 82–91, Oct 1985. Jumikis, T, ‘Exposed Precast Concrete in Buildings’, Architecture in Australia, Vol 57, pp 323–327, April 1968. Lewis, M, 200 Years of Concrete in Australia, Concrete Institute of Australia, 1988.
What you will find in this Chapter •
Technical information on standard precast units to assist designers in choice of precast elements for building, bridge and civil components.
•
Data sheets setting out profiles, dimensions, section properties and structural capacity of most proprietary products and composite systems.
•
Comparative span selection charts for floor systems and vehicular, railway and pedestrian bridges.
•
Information on the process of precast manufacturing, featuring explanation on shop drawings, moulds and other matters pertinent to precast construction.
2.1
Introduction
2.2 Building Components 2.2.1 Floors 2.2.1.1 Comparative spans for floor systems 2.2.1.2 Hollowcore planks 2.2.1.3 Composite flooring 2.2.1.4 Solid slabs 2.2.1.5 T-beams (single-tee/double-tee) 2.2.1.6 Beams (rectangular/spandrel/ledger/tee) 2.2.1.7 Beam shells 2.2.1.8 Grandstand seating units 2.2.2 Walls 2.2.2.1 Low-rise wall panels 2.2.2.2 Architectural wall panels 2.2.2.3 Hollowcore wall panels 2.2.2.4 Sandwich panels 2.2.3 Service Cores 2.2.4 Columns 2.2.5 Stairways 2.3
Bridge Components 2.3.1 Highway Bridges 2.3.1.1 Comparative spans for vehicular bridges 2.3.1.2 Bulb-tees 2.3.1.3 Super-tees (open-flange/closed-flange) 2.3.1.4 I-girders (AUSTROADS) 2.3.1.5 Broad-flange girders 2.3.1.6 Deck units (shear key/transversely stressed) 2.3.2 Rail Bridges 2.3.2.1 Comparative spans for rail bridges 2.3.2.2 Rectangular voided beams 2.3.2.3 T-beams 2.3.3 Pedestrian and Cycleway Bridges 2.3.3.1 Comparative spans for pedestrian and cycleway bridges 2.3.3.2 Walk-thru (U-section) 2.3.3.3 Walk-over (single-tee/double-tee/super-tee)
2.4 Civil Components 2.4.1 Substructures 2.4.1.1 Box culverts (small-size range/large-size range) 2.4.1.2 Arch elements 2.4.1.3 Piles (square/octagonal) 2.4.2 Other Structures 2.4.2.1 Retaining walls – General and using Standard Components 2.4.2.2 Retaining walls – Purpose-designed 2.4.2.3 Reinforced-soil retaining walls 2.4.2.4 Noise walls
2.5 Component Manufacture and Production Facilities 2.5.1 Introduction 2.5.2 Quality assurance 2.5.3 Environmental aspects 2.5.4 Shop drawings 2.5.5 Moulds 2.5.6 Concrete 2.5.7 Reinforcement and prestressing strand 2.5.8 Finishing 2.5.9 Curing 2.5.10 Handling, transport and erection 2.5.11 Repairs
This Chapter covers technical data on the elements that have become standard in the Australian precast concrete industry. Standard products provide a basis for the designer to choose the appropriate element
Building components are defined here in the context of elements that form the skeletal structure of a building, in three broad markets: •
Residential (houses, apartments, townhouses)
to the structural requirements for building and civilmeet applications.
•
Commercial (offices, hotels, shopping centres, factories)
•
Services (hospitals, schools, parking stations, sports stadia).
The following pages set out profiles, dimensions, section properties and structural capacity of the elements commonly used in the Australian building and construction industry, divided into three categories: •
Building Components;
•
Bridge Components; and
•
Civil and Environmental Components.
Many of these elements are available in the market under various proprietary names. Designers should refer to the NPCAA and its website (www. nationalprecast.com.au) for advice on manufacturers, their proprietar y products and the geographic area where they operate. The following information is indicative and should be used for estimating purposes and preliminary sizing purposes by persons experienced in engineering design or construction. The information provided does not replace the need for detailed design by a qualified engineer once a size or member is chosen; no legal liability can be accepted by the National Precast Concrete Association Australia or the Concrete Institute of Australia for the information in these tables or notes. No allowance has been made for additional permanent loadings (such as those from ceilings, services and partitions) for the flooring units, except for topping loads used with hollowcore planks. It is conservative to add the additional permanent action to the imposed action. When the additional permanent action equals the imposed action it is about 10% conservative. When additional permanent actions exceed the imposed action it is recommended a suitable adjustment be made to the additional permanent actions. Where possible, the manufacturer's design information for actual action (load) capacities and other specific design information should be referred to.
The general advantages of precast building systems over other systems are: Design Freedom The initial plastic property of concrete allows the most elaborate shapes to be cast. Coupled with the structural properties of concrete and the extensive range of texture and colour possible, the designer has significant scope to express mass or space, simplicity or grace. Quality Control Precast concrete is manufactured in a factory under controlled conditions which ensures high quality. Structural Capability Loadbearing precast wall panels and columns serve as an important part of a structural frame. They form the supporting structure for floors and roof at the building perimeter. Thermal and Acoustic Properties The mass of concrete provides for good acoustic and thermal insulation. Precast concrete can provide any required fire resistance level. Speed of Construction Precasting combined with speed of erection saves valuable construction time. Floor and wall elements are manufactured while footings and other preliminary site work proceed, allowing delivery and erection from truck to structure on precise and predetermined construction schedules. Durability Precast concrete has a proven track record in relation to durability, offering excellent weathering and corrosion-resistant qualities. Elimination of Formwork The absence of conventional formwork reduces on-site labour and allows other trades to work below the main work platform unimpeded.
LEFT: Hollowcore floor planks BELOW: Precast permanent formwork
ABOVE: Grandstand seating platt RIGHT: Precast beam shell
RIGHT: Architectural wall panel
BELOW: Hollowcore wall panels
RIGHT: Precast column
Precast prestressed concrete is the dominant structural material for short- to medium-span bridges for vehicular and pedestrian applications. With its inherent durability, low-maintenance and assured
quality, prestressed concrete is atological product bridge construction. The ability quickly erect for precast concrete components under all weather conditions with minimum disruption to traffic, adds to the economy of the project.
The following pages set out profiles, dimensions, section properties and structural capacity of the elements commonly used in Australian bridge construction. These are divided into three groups: •
Highway bridges
•
Rail bridges
•
Pedestrian and cycleway bridges.
ABOVE: Precast I-girders BELOW: Open-top super-tees
The various standard sections covered under Highway Bridges suit a range of spans from 7 to 40 m (see 2.3.1.1 Comparative selection chart for vehicular bridges). The standard sections covered under Rail Bridges suit a range of spans from 4 to 25 m (see 2.3.2.1 Comparative selection chart for rail bridges). The sections for Pedestrian and Cycleway Bridges cover a range of spans from 9 to 45 m (see 2.3.3.1 Comparative selection chart for pedestrian bridges ).
ABOVE: Closed-top
super-tees in twin viaduct RIGHT: Precast
deck units
There are considerable infrastructure applications, other than conventional building and bridge structures, where precast construction offers the advantage of repetition and standardisation. This section features a small range of products which fall into this category and which are commonly supplied by the Australian precast industr y. They include substructure elements, such as box culverts, arches and piles, but not the vast range of drainage and similar products available. There are also other components, such as retaining walls and noise walls included in this section, but not items like street furniture or poles which tend to be unique to each manufacturer. For details and manufacturers of items not covered in this section, visit the NPCAA website at: www.nationalprecast.com.au.
TOP: Precast arch elements MIDDLE: Box culverts BOTTOM: Precast noise wall
2.5.1
Introduction
The potential to be designed for very efficient use of raw materials. This is particularly true of prestressed products such as bridge beams and hollowcore walls and floors.
•
Manufacture in specialised factories allowsnoise, air and water pollution to be controlled and minimised.
•
Crushing and recyclingof concrete waste and concrete that has reached the end of its service life has become routine.
Construction using precast concrete elements has major advantages over alternative forms of construction. Precast elements are manufactured in purpose-built factories where high quality as well as environmental constraints with regard to noise and both airborne and waterborne pollution areeasier to achieve. Precast construction reduces the need for on-site labour and delays due to weather and other site-specific disruption. Construction times are reduced because critical activities can occur off-site concurrent with on-site works rather than being carried out sequentially. 2.5.2
Quality assurance
The Australian precast concrete industry has the facilities, people and commitment to achieve its customers’ quality objectives.These objectives are usually reflected within the framework of quality management systems designed to comply with the AS/NZ ISO 9000 series of standards. The Quality Plans and Inspection and Test Plans which are generated under these standards address the following: •
How requirements of the specification are to be met.
•
Process control.
•
Inspections and tests to be carried out.
•
Allocation of responsibilities.
•
Control of non-conformance.
•
Corrective action.
•
What records are to be kept and how they are to be distributed.
2.5.3
Environmental aspects
Concrete, and in particular precast concrete, has low contained energy and has the least impact on the environment of the building materials commonly available for major works. It is directly derived from natural materials – water, sand, limestone, shale (to form cement) and gravel or crushed rock and thus consumes less energy in its manufacture than competing building materials. Environmental strengths of precast concrete include the following:
•
•
Long life with minimal maintenance cost.
•
The potential to provide facade shading and heatsink characteristics to reduce heating and cooling costs.
2.5.4
Shop drawings
Shop drawings are generally required to describe full construction details of the precast concrete elements and consist of: •
Layout and fixing drawingswhich show the general arrangements, fixings, fittings and erection details.
•
Product drawings which detail for each unit: – dimensions; – finishes; – reinforcement size, location and cover; – fixings, fittings and ferrules; – lifting inserts; – unit volume and mass.
These drawings often incorporate changes from the srcinal architectural and structural designs to improve buildability and to accommodate the manufacturer’s preferred processes and should be approved by the customer’s designers before manufacture starts.
Figure 2.1 Precast Concrete is Manufactured in Specialised Factories Minimising Noise, Air and Water Pollution
2.5.5
Moulds
Experienced manufacturers in Australia design their own moulds and it is not usually necessary for the customer to define anything except the shape and expected performance of the precast concrete products. Moulds may be constructed of steel, timber, concrete, plastic or other appropriate material or from a combination of these. They are often built up on casting beds which can be reused for a number of projects. Because of the effect of high-intensity, external mould vibration, repetitive-use moulds usually need to be designed for higher loads induced by fresh concrete than those specified in AS 3610. Joints and seals in moulds must be designed to prevent mortar and grout loss during compaction of the concrete. In some instances it is necessary for moulds to be near-watertight to ensure the specified surfaces are to be achieved. Job-specific moulds designed for repetitive use can take many weeks to manufacture whereas singleuse timber moulds can usually be produced in a matter of days. Moulds for standard sections such as bridge beams will normally need no more than minor modification for each new project and can be brought into production relatively quickly.
a project. Stripping tapers allow blockout formers to strip without becoming locked within the concrete. In the design of long moulds, the manufacturer will make provision for differential movement between the mould and concrete, especially during steam curing. Moulds for prestressed units must allow for shortening as well as local loads caused by hogging when prestress loads are transferred to the concrete. Prestressed precast units are usually made on long-line stressing beds but can sometimes be economically made in moulds where the prestress load is carried by the mould itself; prestressed hollowcore units are made on long-line stressing beds but usually without any edge formers. Void formers in prestressed units are subject to hydrostatic uplift forces during vibration of the concrete and need to be restrained independently of the reinforcement and prestressing strand.
Figure 2.2 Setting up a Mould for Prestressed Bridge Planks which Need only Minor Modifications for each New Project
Good off-form concrete can be very difficult to produce and requires particular attention to mould surfaces, stiffness, sealing against slurry and grout loss and have design details which simplify changes for different unit types. Mould liners made from polystyrene, silicone or polyurethane can yield complex and even undercut shapes. Masters for these shapes can be produced by CAD/CAM-driven equipment. Good precast unit design permits moulds to be made with a minimum of loose pieces and to ensure a minimal number of mould changes in the course of Figure 2.3 Complex Mould Liner Produced from CAD/CAM-Driven Equipment
2.5.6
Concrete
While some precast concrete manufacturers batch their own concrete others use premixed concrete. In either case the provisions of AS 3600 and AS 1379 apply.
A very wide variety of concrete strengths and characteristics is used in the manufacture of precast concrete. Strengths typically vary from 30 to 80 MPa and slumps vary from zero for hollowcore manufacture to the high slumps of 250 mm produced by superplasticisers. Concretes that are designed to produce an architectural effect will sometimes utilise unconventional mix designs and in those cases the relevant characteristics of the aggregates and the concrete will be tested to ensure that strength, durability and other criteria are met.
2.5.7
Reinforcement and prestressing strand
Mesh, bar and strand within precast units must be clean and placed accurately and securely. Designers of structural members, sometimes detail conjested reinforcement which can affect reliable achievement of cover as well as compaction of the concrete. Manufacturers will seek to simplify reinforcement layouts wherever possible and will ensure accuracy by prefabricating rigid reinforcing cages. The relevant standards are AS/NZS 4671 and AS/NZS 4672.
Manufacture within precast factories allows accurate reinforcement placement and excellent compaction. These factors, combined with mixes designed for durability, ensure that stringent strength and durability criteria can be met.
Figure 2.4 A Wide Variety of Concrete Mixes are used in the Manufacture of Precast Concrete
Figure 2.5 Great Care is Taken to Ensure Reinforcement, Prestressing Strand and Inserts are Clean, Placed Accurately and Held Securely
2.5.8
Finishing
Precast elements are manufactured for a large range of uses as diverse as drainage pits to polished reconstructed stone facades. It is important that the finish appropriate for each use is specified and that over-specification is avoided. Finishes can be divided into those which occur before the precast unit is stripped from the mould and those which occur after stripping. In the former category are included machine and hand-trowelled finishes, water-washed exposed aggregate, form-liner finishes and broomed and off-form finishes. The latter category includes retarded, sandblasted, polished, acid-etched and painted finishes. The key criteria for finishes are colour, colour variation, size and number of blowholes and texture. Specifying finishes by description is seldom satisfactory where the standard of finish is important. Specifying by reference to samples and to existing uses of the same finish in earlier buildings and structures is a safer and more realistic method.
2.5.9
Curing
Curing regimes vary between manufacturers and are affected by the nature of the product being manufactured, the climate at the plant and other criteria. As a general rule, precast concrete units are cured until the compressive strength is high enough to ensure that stripping does not have an effect on the performance or the appearance of the final product. Initial curing of precast concrete takes place in the mould, usually by covering to prevent loss of moisture and, especially in structural prestressed products, by the application of radiant heat or steam. Additional curing has been shown not to be necessary to attain the specified strength. For further guidance on this matter, refer to Recommended Practice – Curing of Concrete (Z9), Concrete Institute of Australia, 1999.
Architectural finishes are discussed in much more detail in Chapter 10 Architectural Elements.
Figure 2.6 Precast Concrete has a Diverse Range of Applications Requiring Appropriate Finishes
2.5.10
Handling, transport and erection
It is important that precast units are not overstressed and that damage is minimised during handling, transpor t and erection.This subject is covered fully under Chapter 11 Handling, Transport and Erection.
Figure 2.7
Handling, Transport and Erection of Precast Concrete is an Important Consideration of the Manufacturing Process
2.5.11
Repairs
Precast concrete units will sometimes need to be repaired. This need can arise from damage during handling, transport and erection, from design changes on site such as where holes must be drilled for services or from accidental impact or staining damage. Concrete is a very versatile and forgiving material and effective structural repairs can be made in such a way that strength and durability are not adversely affected. Excellent repair materials such as polymermodified cements, epoxies and crystal-forming chemicals are often used as is appropriate. Achieving an effective architectural repair is often much more difficult and is dependent on the skill of the person the repair. Exposedfinishes. aggregate surfaces areunder easiertaking to match than off-form Nevertheless the possible need for some repair work must be accepted on any project. All repairs should be carried out in accordance with the manufacturer’s Quality Plan, Work Method Statements and Instructions as agreed with the customer.
What you will find in this Chapter • An appreciation and understanding of the materials and their properties commonly used in the manufacture of precast reinforced and prestressed concrete. • Tables of values of material constants and engineering properties used in design. • Material compliance criteria for use in specifications. • Application and the effect of the various admixtures used in concrete. • Information on concrete durability and protective coatings.
3.1 Definitions and Notation 3.1.1 Definitions 3.1.2 Notation 3.2 Introduction 3.3 Materials 3.3.1 Cement 3.3.2 Supplementary cementitious materials (SCMs) 3.3.3 Aggregates 3.3.4 Water 3.3.5 Chemical admixtures 3.3.6 Pigments (oxides) 3.3.7 Reinforcement 3.3.8 Reinforcing bars 3.3.9 Reinforcing mesh 3.3.10 Steel fibres and other types of reinforcement 3.3.11 Prestressing tendons 3.3.12 Prestressing hardware
3.3.13 of reinforcement 3.3.14 Welding Mechanical splicing of reinforcing bars 3.3.15 Durability considerations for reinforcement, tendons and cast-in items Example 3.1 Calculation of coating thickness for given service-life 3.4 Concrete and Concrete Properties 3.4.1 General 3.4.2 Workability 3.4.3 Compressive strength 3.4.4 Tensile strength 3.4.5 Modulus of elasticity 3.4.6 Poisson’s ratio 3.4.7 Coefficient of thermal expansion 3.4.8 Shrinkage and creep
3.4.9 Permeability and absorption 3.5 Grouts and Mortars 3.5.1 General 3.5.2 Portland and blended cement grouts and mortars 3.5.3 Non-shrink grouts 3.5.4 Epoxy grouts 3.6 References
3.1.1
Definitions
For the purpose of this chapter the following definitions are used: High alumina cement (calcium aluminate cement)
The product obtained by grinding clinker resulting from fusing a mixture of aluminous and calcareous materials.
Reinforcement
Steel bar, wire or fabric but not tendons. Tendon
A wire, strand or bar or any discrete group of such wires, strands or bars that is intended to be pretensioned or post-tensioned.
3.1.2
Notation
The following notation is used in this chapter: d = the nominal diameter of bar Ecj = the mean value of the modulus of elasticity at the relevant age f ’c = the characteristic compressive strength of concrete at 28 days f ’ct.f = the characteristic flexural tensile strength of concrete f ’ct = the characteristic principal tensile strength of concrete fcmi = the mean value of the compressive strength of concrete at the relevant age R e = the value of the yield stress (or 0.2% proof stress) determined from a single tensile test in accordance with AS 13913.1 R ek.L = the lower characteristic value of the yield stress determined from a series of tensile tests
R ek.U = the upper characteristic of of thetensile yield stress determined from value a series tests R m = the value of the maximum tensile strength determined from a single tensile test in accordance with AS 13913.1 esu = the uniform strain at maximum stress, corresponding to the onset of necking 3.14 (called A gt in AS/NZS 4671 ) r = the density of concrete
The behaviour of reinforced and prestressed concrete under service loads and at the ultimate limit is to a large extent determined by the properties of the material components as well as by the
3.3.1
Cement
manufacturing process. There simple of rules which, if followed, lead toarethesome production precast concrete with a service life of 100 years or more. This versatile structural and architectural product is enhanced by being produced under the factory conditions maintained by competent precast concrete manfacturers. It is important that specifiers understand the nature of the materials used in reinforced and prestressed concrete; this Chapter will assist in achieving such understanding. It is assumed that the reader is familiar with the requirements of the referenced Standards.
•
Cement should comply with AS 39723.2. The Standard classifies cements into two broad classes: General Purpose and Special Purpose. General Purpose Portland cement (type GP) or
Blended cement (type GB) is normally used in precast concrete construction. • Special Purpose These cements are most frequently specified for elements in aggressive environments, eg tidal and splash zones and in sulfate-bearing soils. High alumina cement should not be used in precast concrete because it suffers reversion and loses a large proportion of its strength in warm, humid conditions3.3. It may also cause ‘flash set’ if mixed with portland cement. Shrinkage-limited cement (type SL) is used where high-early strength is required or where there is a need to reduce concrete shrinkage. Where uniformity of colour is required, all cement for a project of the same typegenerally and be from the sameshould plant.be Off-white cement produces better off-form colour consistency than grey cements. The type of cement used will affect the fresh and hardened properties of the concrete. Therefore, it is better to specify the desired properties for the concrete rather than use a prescriptive specification setting out cement type, mix proportions, etc. Precast manufacturers can provide advice on the implications of the various mix designs, including the choice of the cement type (particularly as they affect the precasting process). Table 3.1 sets out the major technical and commercial implications of the various cements.
3.3.2
Figure 3.1 Comparison of Colour of Exposed Aggregate Panels using GP Cement and Off-White Cement. TOP: GP Cement BOTTOM: Off-White Cement
Table 3.1 Technical and Commercial Implications of Various Typical Cements
Supplementary cementitious materials (SCMs)
Supplementary cementitious materials (SCMs) are fly ash, ground granulated iron blast-furnace slag and silica fume. They should comply with the requirements of the appropriate parts of AS 35823.4–3.6. SCMs are discussed in the Concrete Institute of Australia, Current Practice Notes CPN 253.40, CPN 263.41 and CPN 273.42. The benefits arising from the use of SCMs include: reduced cost, improved workability, lower heat of hydration, improved durability (as a result of reduced permeability), improved chemical resistance and increased strength. A full discussion of these is beyond the scope of this Handbook. Blended cements are produced by adding SCMs at the cement plant or at the concrete batch plant. SCMs of the same type but from different sources, each complying with the requirements of the appropriate Australian Standard, may affect significantly concrete performance, in either or both the fresh and hardened states. Thus, any change in source of supply of cement or SCM needs to be evaluated in terms of both fresh and hardened properties. The use of appropriate levels of fly ash or slag has been found to effectively control the expansion due to alkali aggregate reaction (AAR)3.7. High slag blends are also used to improve the durability of elements in contact with sea water. Hydration of SCMs is improved with steam curing. The PCI3.8 gives guidance on practices to be adopted when using silica fume. SCMs in general and fly ash in particular increase the difficulty of maintaining colour control. They should not be used if large areas of adjacent elements can be viewed concurrently and a uniform colour is required. Designers should be aware that the use of GB cement and high proportions of SCMs could result in low early-strengths of concrete. This may mean precast units need to stay in the mould for two days before demoulding.
3.3.3
Aggregates
General Aggregates should comply with the requirements of AS 2758.13.9. It contains clauses
that deal with a number of properties required to be known for mix design, eg par ticle density, water absorption and particle size distribution, and for which the project specification should provide choices or limits. The specification should also set out which of the three alternative clauses for specifying requirements for aggregate durability is to be adopted. Methods of test for aggregates are covered by AS 11413.10 and these are summarised in Table 3.2. Not all the tests listed in Table 3.2 will be either appropriate or necessary for aggregate for a given project. Only those tests required for the project should be specified. Over-specifying will add to the cost and may incur time delays while testing is completed. If an aggregate does not comply with the criteria for a particular test it may still be satisfactory. This can be confirmed by carrying out appropriate tests on concrete made using the aggregate. Figure 3.2 shows the preferred aggregate shapes to be adopted. Guidance on the use of potentially alkali reactive aggregates is given in Alkali Aggregate Reaction: Guidelines on Minimising the Risk of Damage to Concrete Structures in Australia3.7.
The maximum particle size of aggregate has an effect on whether the concrete can be easily compacted around the reinforcement, tendons or ducts. Section 4.10 in AS 3600 3.11 suggests that the maximum nominal size of aggregate should not be greater than the specified cover to reinforcement and tendons, but the configuration of all the items should also be taken into account. Special Aggregates may be required to give desired architectural features such as colour and texture for exposed aggregate surfaces. Preferably, the chosen aggregate should have a proven service record. If not, it should be tested for compliance with the relevant requirements of AS 2758.13.9. Note that some testing programmes, eg those for alkali aggregate reactivity, may up tomay six months to complete. Choiceleadof a new take aggregate thus require a considerable time before manufacture of the elements can begin. Where special aggregates are to be used it may be desirable to stockpile them at the beginning of the project to minimise colour and/or other variations in supply during the project. Gap-graded aggregates will give the most uniform exposed aggregate surface.
Figure 3.2 Categorisation of Aggregate Particles by Shape and Surface Texture
Table 3.2 Aggregate Properties – Test Methods [After Guide to Concrete Construction3.28]
3 3
Table 3.2 continued Aggregate Properties – Test Methods [After Guide to Concrete Construction3.29]
3.7
3.3.4
Water
Water should be free from matter which in kind and quantity will reduce the strength and/or the durability of the concrete or lead to corrosion of reinforcement, prestressing strand, or other cast-in hardware. Harmful materials include sugar, chlorides, industrial wastes, and acids. AS 13793.12 sets out performance criteria for water if the quality is unknown and allows the use of recycled water subject to certain provisions. The use of recycled water is an important part of satisfying environmental regulations and minimising the impact of precast concrete production on the environment. Using recycled water may lead to a rise in the proportion of soluble salts and alkalis in the concrete. Testing should be carried out to ensure the limits referred to in Table 3.2 are not exceeded, and where potentially alkali-reactive aggregates are being used, the limits suggested in the guidelines3.7 should be adopted. 3.3.5
Chemical admixtures
Chemical admixtures should comply with the requirements of AS 1478.13.13 and must not contain chlorides. The various types are listed in Table 3.3. The use of admixtures must not compromise the limits on soluble salt contents referred to in Table 3.2. Specifiers and manufacturers should know the basic ingredients of any admixture being specified or used. Where two or more admixtures are to be used in combination they should be checked for compatibility. It is also desirable to trial admixtures in mixes using the specific materials to be employed on the project to ascertain the dosage for the desired performance. Calcium Chloride should never be used as a direct additive in reinforced or prestressed concrete as it can lead to reinforcement corrosion and thus pose an extreme durability risk.
3.3.6
Pigments (oxides)
Pigments are ultra-fine particles (approximately one-tenth the size of cement particles). They do not dissolve and stain the concrete materials but are dispersed as fine solids throughout the concrete matrix and are bound in the same manner as the other aggregates. Pigments should be: • chemically inert to avoid affecting the chemical reaction between the cement and water; • alkaline resistant since cement, and hence concrete, is highly alkaline; • insoluble to prevent leaching-out by weather; and • light-fast, to eliminate fading. Pigments may be either natural or synthetic. The latter offer a superior product due to their controlled chemical composition and extremely fine particle size. They produce strong colours and colour saturation. By contrast, natural oxides have low tinting strength. Mineral-oxide pigments are the most widely used materials, fulfilling all the above requirements and providing the base colours: yellow, brown, red and black. By blending these colours, manufacturers can offer a wide range of colour shades. See Figure 10.11 in Chapter 10 Architectural Elements for samples of oxide colours.
Table 3.3 Chemical Admixtures for Concrete [After Guide to Concrete Construction3.28]
3.3.7
Reinforcement
The term reinforcement is frequently applied to any material used to reinforce a concrete member. It is commonly used to cover steel reinforcing bars, prestressing tendons, steel fibres and non-metallic reinforcement. However, in AS 3600 it has a specific definition, ie steel bar, wire, or fabric but not tendons. This definition has been adopted in this Handbook. When placed into the mould after fabrication, reinforcement should be clean and free from harmful matter likely to impair the bond with the concrete, eg loose mill scale, loose rust, oil, grease and retarders. 3.3.8
Reinforcing bars
Reinforcing materials, ie bars (including wire sizes) and mesh are covered by AS/NZS 4671 3.14. Bars are classified by: shape, ductility class, strength grade and size. Shape Bars range from 4 mm in diameter upwards and may be plain, deformed or indented. The geometrical requirements for ribs and indentations are given in the Standard. The rib pattern may be used to identify the bar. Indentations, where used, are required to be between 0.03d and 0.10d where d is the nominal bar diameter. It is anticipated this will limit the use of indentations to small bar diameters, eg < 10 mm. AS 3600 requires that main reinforcement, ie other than fitments, be either deformed bars or mesh. Ductility Class There are three ductility classes, ranging from low through normal to seismic, designated L, N and E respectively. The requirements for L and N are set out in Table 3.4 adapted from AS/NZS 4671. Class L reflects the ductility of cold-worked reinforcement, eg mesh, and AS 3600 imposes limitations on its use, ie shall not be used in any situation where the reinforcement is expected to undergo large elastic deformation under strength limitstate conditions.
Class N reflects the ductility of hot-rolled bars and the rules in AS 3600 are based on its ductility. Class E hasimposed been specified to suit loading the ductility demands by the seismic in New Zealand and is not available in Australia.
There are three strength grades for reinforcement in the Standard but only two – 250 and 500 – are available in Australia. The other, 300E, is a seismic grade especially formulated for New Zealand requirements and has a lower weldability than 500N. Thus it should not be used in Australian projects. The numerical value represents the lower characteristic yield strength expressed in megapascals. Strength is now also controlled by the inclusion of an upper yield strength Table 3.4. Strength Grade
The size of the bar is the numerical value of the nominal diameter expressed in millimetres. Commonly-available bar sizes in Australia are shown in Tables 3.5a, band c. The Standard sets out requirements for weldability by setting limits on carbon equivalent, and bendability by including requirements for bend and rebend tests. Bar sizes for fitments will depend on local practice and availability from manufacturers. They are produced from coiled rod of grade 250 MPa, or more common, from coiled wire of grade 500 MPa. Designers should specify their required strength for fitments. D500L bars should be used only as fitments or trimmer bars in precast concrete elements. Size 12N bars are more likely to be used than 12L, depending on local availability. Size
Table 3.4 Mechanical Properties of Reinforcing Steels [After AS/NZS 46713.14]
(1)
(2)
(2)
e
(2)
3.15
Table 3.5a
Figure 3.3
Nominal Values for Hot-Rolled Deformed Bars of Grade D500N
Identification of Various Grades and Ductility Classes of Deformed Reinforcement [After AS/NZS 46713.14]
Table 3.5b
Nominal Values for High-Strength Deformed Bars of Grade D500L
Table 3.5c Nominal Values for Hot-Rolled Plain Round Bars of Grade R250N
3.3.9
Reinforcing mesh
Welded wire mesh is a prefabricated reinforcement consisting of parallel cold-rolled wires welded together in square or rectangular grids. Each wire intersection is electrically resistance-welded by a continuous automatic welder. Pressure and heat fuse the intersecting wires together and fix all wires in their proper position. Mesh is commonly Ductility Class L reinforcement and subject to the limitations noted in Clause 3.3.8. Information on stock mesh sizes in Australia is given in Table 3.6. It should be noted that different sizes of mesh may be available from certain manufacturers, and for large projects, special meshes can be made.
Table 3.6 Information on Mesh Sizes Commonly Available in Australia [Based on AS/NZS 46713.14]
*
Rectangular
Square, with edge side-lapping bars
Square, without edge side-lapping bars
3.3.10
Steel fibres and other types of reinforcement
The use of steel fibres as reinforcement is not covered by AS 3600. Although not prohibited, a designer using it has to demonstrate that the design will comply with the performance requirements in the Building Code of Australia (BCA)3.16. Steel fibres
Figure 3.4 Section of Steel Fibre Reinforced Concrete
Non-metallic reinforcement is commercially available, however, it is not covered by AS 3600 and any application of it has to be justified by use of alternative procedures as provided for in the BCA. Non-metallic reinforcement
For aggressive environments (for example ‘C’ and higher exposure classifications, as defined in AS 3600) or where an extended design life is required, eg 100 years, or where the minimum covers specified in Clause 4.10 of AS 3600 cannot be achieved, using stainless steel reinforcement provides a solution that minimises the risk of corrosion3.17. To be used as reinforcement, stainless steel bars will need to be deformed (see Clause 19.2.1.1 in AS 3600). They should be of Type 316 or Duplex grade 2205 stainless steel. Designers will need to check the mechanical properties, eg Es and ductility, of the chosen stainless steel and apply the design rules from AS 3600 as appropriate. Despite the cost being of the order of Stainless steel reinforcement
five to seven times thathave of ordinary reinforcement, this increased cost will only a small effect on the total construction costs of the structure3.17. Correct welding of stainless steel reinforcement will ensure no reduction to its corrosion resistance, (refer AS 1554 Part 63.18). Proper handling and storage of stainless steel reinforcement on site will eliminate the possibility of carbon steel contamination. Rostam 3.17 notes that stainless steel reinforcement can be combined with ordinary (black) steel reinforcement without risk of corrosion due to bi-metallic action.
Table 3.7 Properties of Common Seven-Wire Stress-Relieved Ordinary Strand to AS/NZS 4672.13.19 3.3.11 Prestressing tendons
Steel tendons for prestressed concrete may be wires, strands or bars. Wires and strands tend to be used for pretensioned members. 7-wire strand systems are the most common systems used for post-tensioned members. In precast prestressed structural concrete members, nearly all tendons are 7-wire, stress-relieved, high tensile steel and strand conforming to AS/NZS 4672.13.19. Prestressing tendons should be clean and free from General
harmful as other loose coating mill scale, loose rust, mud, oil,matter grease such or any which could reduce the bond between the concrete and the steel. A slight film of rust is acceptable, but there should be no pitting of the surface. Prestressing wire Wires are manufactured from high carbon steel by cold drawing. This wire is then normally stress relieved by a process of straightening and low temperature heat treatment in order to increase its ductility. A further process of stabilisation by stretching and heat treatment is often used to improve the stress relaxation properties of the steel. Minimum breaking loads and other properties are listed in Table 3.7. Prestressing strand 7-wire, stress-relieved strand comes in a variety of sizes, the most common being 9.5-, 12.7- and 15.2-mm diameter. Furthermore, strand may be normal relaxation (Relax 1) or low relaxation (Relax 2). Minimum breaking loads and other properties are listed in Table 3.7.
The most common strand type is stress-relieved, Relax 2 (formerly Low Relaxation). The apparent elastic modulus of strand is usually lower than that for a single wire because of the tendency for the individual wires to move relative to each other and straighten very slightly when tensioned. Figure 3.5 shows a typical stress-strain curve for 12.7-mm strand. Because of the absence of a definite yield point, the 0.2% proof stress is used as a nominal measure for yield stress. Also, AS 3600 nominates the yield strength to be 82% of the ultimate tensile strength and AS/NZS 4672.13.19 requires that the proof load be at least 82% of the minimum breaking load for 7-wire, stress-relieved strand. High strength bar High-strength, hotrolled, steel bars are required to comply with 3.19. The 0.1% proof stress is taken AS/NZS at 81% of4672.1 the minimum tensile strength. Minimum breaking loads and other properties are listed in Table 3.8.
Hot-Rolled Ribbed Bars
Hot-Rolled Round Bars
Table 3.8
Properties of Common High-Strength Prestressing Bars [After AS/NZS 4672.13.19]
Figure 3.5 Typical Stress-strain Curve for 12.7-mm, 7-wire, Stress-relieved (Relax 2) Strand
3.3.12 Prestressing hardware
The hardware, ie ducts, anchorages, etc, varies for each prestressing system. However, the hardware for any system should comply with the requirements of AS 3600. Ducts Ducts may be fabricated from either steel or plastic. In bonded, post-tensioned construction where a bond between the concrete outside the duct and the grout inside the duct is required, steel sheathing is formed into a corrugated, helical tube, or the duct is formed from smooth, thin-walled steel tube. Corrugated ducts from other materials are also available. Anchorages The anchorages for post-tensioning tendons are specially designed for the type of tendon that they are anchoring. Designers should consult the suppliers of the various post-tensioning systems for details of the available system and required ancillary reinforcement. The most versatile are 7-wire strand systems, with various anchorages for multistrand (two or more strands per tendon), live and dead-end conditions, as well as monostrand systems (one strand per tendon). Special attention should be given to the inclusion of adequate reinforcement in anchorage zones. These should contain sufficient horizontal and vertical stirrups or grillage reinforcement placed in the plane parallel to the end surface to control the induced tensile forces. General
Figure 3.6 Prestressing Ducts and Anchorages with Associated Reinforcement
3.3.13
Welding of reinforcement
Reinforcing steel complying with AS/NZS 4671 may have a carbon-equivalent value of up to 0.44. Any structural welding of reinforcing steel should comply with AS/NZS 1554.33.20 and be carried out by qualified operators. For all welds, low-hydrogen welding rods are required and reference should be made to the manufacturers’ data for specific advice regarding the various weld types being considered. Locational tack-welds are widely used throughout the precast industry for pre-assembly of reinforcement cages in lieu of tying at bar intersections. They may be smaller than tack welds as defined in AS 1554.4 and are (currently) not covered by it. They should be performed by trained personnel and should be executed in a manner that does not cause notching or reduce the cross sectional area of the main bar. Reinforcement cages could be pre-assembled in a jig rather than in the mould to optimise the accuracy of their location and to eliminate the risk of damage to the mould (and the soiling of it – with consequent undesirable effects on exposed concrete surfaces). Prestressing tendons must not be welded and should be protected from damage by stray electric currents or earthing currents from any welding processes being carried out in their vicinity. This is to prevent the possibility of electric arcing and consequential notching of the strand wires.
3.3.14
Mechanical splicing of reinforcing bars
Mechanical splicing of reinforcement is the quickest method of developing continuity of the reinforcing bar across a construction joint without the need to damage the formwork, or where lap splicing is not appropriate. There are a number of different splicing systems on the market. The majority involve some kind of threaded coupler that can be used to join two threaded bar ends together. These are generally processed in the factory and sent to site ready to assemble. Other systems involve clamping devices that can be used to connect or continue insitu bars on site. There are a number of situations and structures in which the designer requires a more rigorous performance from a mechanical splice. Couplers are available that can meet the following requirements that do occur in practice: • The need to transfer the full-bar-strength under tension or compression without loss due to the presence of the coupler, ie a bar break requirement. If the structure is likely to sustain impact load, the splice should be able to outperform the bar in strength and ductility. •
Figure 3.7 Welded Beam Cage and Locational Tack-Welds (inset)
•
•
The acceptable performance of the splicea load in is permanent set or slip conditions. When applied to any mechanical splice it will elongate. Following the removal of the load, any permanent elongation that remains is referred to as slip. If this value is excessive, the ser viceability of the structural member may not achieve the design limits in regard to crack size or deflection. There are proprietary systems available that exhibit low-slip performance. The need to withstand cyclic loading. In certain conditions the splice could be subject to a load that is being constantly applied and removed throughout the life of the structure. This could result in failure if the splice is not able to withstand such fatigue loading. Restrictions in cross-sectional area of the splice.
There be occasions when a lap splice will and take up toowill much room in the structural element a smaller coupler system will be required to allow the proper flow of concrete around the coupler. For further information on design and detailing of mechanical splices reference should be made to the product manufacturers.
3.3.15 Durability considerations for reinforcement, tendons and cast-in items Concrete cover
Embedment in concrete protects reinforcing steel, tendons and steel hardware from corrosion. A protective iron oxide film forms on the surface of the bar, wire or strand as a result of the high alkalinity of the cement paste. The alkalinity of the cement paste may be reduced due to reaction with carbon dioxide in the atmosphere or from the presence of chlorides. Chlorides may be found in concrete aggregates, water, cementitious materials or chemical admixtures and hence may be present in the concrete when cast. AS 13793.12 restricts the acid-soluble chloride ion content of concrete as placed to less than or equal to 0.8 kg/m3. Concrete of low penetrability and of sufficient cover over the steel will provide the necessary protection against chloride penetration. To provide corrosion protection to reinforcing steel, wire and tendons, concrete cover should conform to Section 4 of AS 3600. Tables 3.9 and 3.10 show the minimum cover required for the durability exposure classifications nominated in AS 3600 and the associated characteristic compressive strength of concrete to be used. Table 3.9 implies the use of steel moulds and external form or table vibration whereas Table 3.10 applies to forms made of other materials and where the concrete is compacted by using immersion vibrators (poker vibrators). Design for fire resistance in accordance with Section 5 of AS 3600 may require greater concrete covers than nominated in these tables. For marine exposure environments, (exposure classification C1 or C2 in AS-3600), the cover concrete should have low penetrability, low chloride diffusion characteristics and sufficient cover should be provided. See Performance Criteria for Concrete in Marine Environments3.21 for a detailed discussion of these points. Values for cover should be in accordance with AS 3600, see Tables 3.9 and 3.10. Achieving low levels of chloride diffusion will typically require a concrete containing SCMs, with a watercement ratio ! 0.36 and a cementitious content " 450 kg/m3. Also, the concrete will need to be cured either by heat-accelerated methods to achieve a maturity factor " 350 °C .h, or moist-cured for seven days.
Figure 3.8 Reinforcement in Perfect Condition After Being Cut Out of an Acid-Etched, Veneered Precast Unit in a Marine Environment for more than 25 Years
Table 3.9 Required Cover (mm) with Rigid Formwork and Intense Compaction
Table 3.10 Required Cover (mm) with Standard Formwork and Standard Compaction
In its Recommended Practice3.21, the Concrete Institute has reviewed performance criteria for concrete in marine environments. It evaluated the relative merits of a number of commonly-used methods in terms of use as a tool for design, pre-qualification and quality control. None of the methods was rated acceptable in all three areas. The specification of minimum strength and cover, as in AS 3600, was the only method to gain an acceptable rating in any area (pre-qualification and quality
Protective coatings on reinforcement
control). Major deficiencies were noted for both the rapid chloride-ion penetrability method and the chloride diffusion method. Both were rated unacceptable as a design tool, poor as a pre-qualification tool and as poor and unacceptable, respectively, as a quality control tool. On this basis, neither should be used in specifications for precast concrete. The Recommended Practice lists a number of tests under development, eg a modified ASTM C12023.22 method, that show promise. An alternative strategy that can be employed is the correlation of the ASTM C1202 test along with long-term data (one to two years) from salt-water immersion or ponding tests. Cover is the minimum clear distance from the reinforcement to the surface of the concrete. It applies to all bars, eg stirrups, not just main bars.
necessarygalvanising) to dip the to item bath of molten zinc (hot-dip getina athickness of practical use; that provided by electroplating is insufficient. Galvanised reinforcement should not be coupled directly to dissimilar metals such as aluminium, copper or stainless steel. Polyethylene and similar tapes can be used to provide local insulation to these materials. Zinc-coated wire should be used to fix the reinforcement. Hydrogen gas may be liberated when fresh concrete comes into contact with zinc, or galvanised reinforcement, if the chromate content of the cement is low. The normal process of galvanising finishes with a dip in chromate solution, prevents this reaction. If there is any evidence of hydrogen bubbles forming due to this reaction, potassium or sodium dichromate should be added to the water used for the concrete at a rate of 0.3 g per litre. Concrete mixes which include chromate inhibitors are subject to special handling precautions due to the toxicity of the chromate. See CIA Current Practice Note 17 3.23. Zinc coatings are ineffective in the presence of high concentrations of chloride ions. A physical barrier rather than a sacrificial coating would be required for this. Fusion-bonded epoxy is effective but coated reinforcement is not readily available in Australia. Epoxy is difficult to protect from damage during construction and small breaks or pinholes in the coating drastically reduce its effectivenes. Concrete technology has been developed for these highchloride environments and provides a better solution.
For aggregate the concrete is notexposed measured to the surfaces, srcinal surface; instead,cover the depth of the matrix removed from between the pieces of coarse aggregate (depth of exposure) is subtracted. Attention must also be given to scoring, false joints, shade grooves and drips, as these may reduce cover. Cracking may allow oxygen and moisture to reach the embedded steel, providing conditions where rusting of the steel and staining of the surface may occur. A sufficient amount of closely spaced reinforcement limits the width of cracks, hence minimising the intrusion of water, and maintaining the protection of the steel. Prestressing may also be used to limit, or eliminate, cracking. In some situations, lowshrinkage concrete may be specified High-quality concrete provides adequate corrosion protection for reinforcement for most conditions. Even in moderate to severe aggressive environments, concrete can provide adequate protection with proper attention to mix design, steel stress level, the extent of cracking under service loads, and the depth of concrete cover. Only when these protection measures are not feasible, will it be necessary to consider other ways of protecting reinforcement, such as galvanising or epoxy coating. as described below. Alternatively, the use of stainless steel reinforcement can be considered.
Reinforcement can be coated to enhance its corrosion resistance, though AS 3600 makes no concession for its use. Appropriate quality of concrete and amount of cover in combination with proper detailing is considered sufficient to protect reinforcing for the normal life of a structure. The most commonly-used coating is zinc metal, which provides sacrificial electrolytic protection of the steel and tolerates breaks in the coating. It is
Protective coatings for cast-in items
Connections and their coatings should be designed to withstand the natural degradation caused by the environment for a defined maintenance-free period for theindesign life of the structure. They to may beorlocated enclosed locations or exposed aggressive environments such as, damp conditions, wet ground, aggressive soils or marine and other salt-laden atmspheres. The usual practice for fixings inside an enclosed building is to use mild-steel fixings, zinc-plated ferrules and hot-dip galvanised bolts, nuts, brackets, etc. Zinc plating of ferrules cast in concrete provides some protection for the thread and is adequate for the outside, which is protected by the concrete. In Australia, galvanised bolts are increased in size by the galvanising process and threads in ferrules
and nuts must be cut to over-size to suit. If standard nuts and ferrules are galvanised, the threads need to be re-cut after galvanising which will remove most of the coating. Evidence of reduced durability is often marked by corrosion of exposed steel components, or by cracking and spalling of concrete in the vicinity of cast-in metal fitments.
The use of non-corroding materials may be required in conditions of exposure classifications B1, B2, C1, C2 and U, as defined in AS 3600. In areas of winter frost or in refrigeration structures, the effects of frost action, freezing and condensation must be considered. Table 3.11 provides guidance for typical material types and coatings for applications in various exposure environments. Comments on the various types of coatings are given on the following pages.
Table 3.11 Materials and Coating for Cast-in Items in Various Environments
3.11
•
Zinc Plating This is an economical, versatile
and effective method of applying a protective coating to small steel fitments. It is a process where, by electrolysis, zinc is plated to steel. It is the most widely used method of applying metallic zinc coatings to small fasteners including ferrules, inserts, bolts and washers. The process produces relatively light, uniform coatings of excellent appearance which are, however, generally unsuitable for outdoor exposure. Zinc plating provides a coatingwith 5 tothe 12hot µmdip thick and should not be confused galvanising process, which applies much heavier coatings providing correspondingly longer service lives. AS 1897 3.24 specifies plating thicknesses that can be accommodated on external threads to various tolerance classes. • Hot-Dip Galvanising This produces a heavy coating of zinc. The coating is applied by the immersion of clean, prepared steel components in molten zinc. This results in a zinc coating which is metallurgically bonded to the base steel and consists of a succession of zinc-iron alloy layers and an outer zinc layer. The period of corrosion protection provided by a galvanised coating in a given environment is proportional to the mass of zinc in the coating. The protective life of the coating is therefore directly determined by the environment to which it is exposed. The anticipated life of a 600-g/m 2 (85-µm) galvanised coating in various environments, measured in years, is shown in Table 3.12. AS/NZS 4680 3.25 and AS 12143.26 provide further guidance on coating requirements and measurement of thickness. • Zinc painting or spraying Zinc painting covers zinc coatings up to 1500 g/m2, equivalent to 250 µm, applied by either manual or mechanised methods. The steel surface must be prepared by grit blasting. (Note that, unlike Hot-dip galvanised coatings, zinc paints tend to run off edges and hence leave these areas relatively unprotected.) The resulting provides protection forzinc the coating underlying steel incathodic the same way as a galvanised coating. This method of coating is ideal for repair or reinstatement of damaged coatings. AS/NZS 3750.153.27 provides some guidance on their use. There are no generally-recognised corrosion rates for zinc coatings and mild steel, but orders of magnitude for various exposure conditions are shown in Table 3.12.
Figure 3.9 Example of Coated Cast-in Items
•
Stainless Steel – Grade 304 and 304L These are
general-purpose grades, frequently referred to as 18-8 stainless steel. Grade 304 is an austenitic stainless steel, which restricts carbide precipitation during welding and is therefore suitable for most applications where built-up connection components are fabricated by welding. The material may be welded under normal conditions with an E308 electrode. Corrosion-resistance life is
•
very 304L long. grade has better machining properties The and is therefore appropriate for inserts and ferrules. Stainless Steel – Grade 316 and 316L These grades are referred to as molybdenum-alloyed, austenitic, chromium-nickel steels. They are considered to have higher corrosion resistance than the 304 grades and are therefore often used in marine environments. The additional resistance to pitting and crevice corrosion is afforded by the higher (2% – 3%) molybdenum content. The 316L grade has better machining properties and is therefore appropriate for inserts and ferrules.
Table 3.12 Service Life of Zinc-Coated, Mild-Steel fixings [After EN ISO 1473:1999]
Example 3.1 Calculation of coating thickness for given service-life
Calculate thickness of coating and sacrificial steel for fixing in Exposure Classification A2 and B1 to give a design life of 100 years. Fixing is hot-dip galvanised with a coating thickness of 85 mm.
Exposure Classification A2 Rate of corrosion of zinc coating = 0.7 m/yr Table 3. 12
Time to commence corrosion of steel = 85/0.7 = 121 yrs \
OK
Exposure Classification B1 Rate of corrosion of zinc coating = 2 m/yr Table 3. 12
Time to commence corrosion of steel = 85/2 = 43 yrs Rate of corrosion of steel = 40 m/yr Table 3.1 2
Required additional steel thickness for 100 years = (100 - 43) x 40/1000 = 2.28 mm Thickness to be increased by 2.3 mm for each exposed face
3.4.2
3.4.1
General
Concrete should be specified in accordance with Clause 1.6 of AS 13793.12. The recommended minimum characteristic compressive strength for precast concrete elements is 32 MPa. However, durability requirements in Section 4 of AS 3600, orthe other design criteria, may require higher compressive strengths to be specified. Where it is necessary to nominate specific cement types, particular aggregates or grading, the use of pigments, and/or specific criteria such as shrinkage limit, minimum cement content or a water-cement ratio, special-class concrete will have to be specified. In cases where specific criteria are nominated, the specification should set out the criteria, the method of test, the testing regime, and the acceptance/ rejection criteria. Generally, concrete for precast elements will be special-class for one reason or another. AS 1379 provides rules for the assessment of concrete specified by strength grade. The characteristic 28-day compressive strength of concrete used in precast construction is usually in the 32 MPa to 50 MPa range. This enables elements to be stripped at an early age and to be handled within 24 hours after casting – maximising the use of the forms and the casting space. In prestressed work, the concrete strength at which the pre-tensioned strands are released and the force transferred to the concrete or the strength at which post-tensioned cables are stressed should be specified by the designer. Generally, it is in the range 25 to 35 MPa. The strength of concrete is commonly considered to be its most impor tant characteristic because properties such as stress-strain relationship, tensile strength, shear strength and bond strength are frequently expressed in terms of the compressive strength. While in many cases, other characteristics such as durability and volume stability may be more important, strength gives a good indication of concrete quality and consistency. In general, design values of the hardened state properties are set out in Section 6 of AS 3600.
Workability
Workability is the property of freshly mixed concrete that determines the ease with which it can be mixed, placed, consolidated and finished. In most cases this is measured by the slump test. This test, however, is only an indirect measure of workability. The use of chemical admixtures to provide higher workability concretes that achieve the specified compressive strength and don’t segregate, removes the necessity for designers to specify limits on slump for concrete in most precast elements. This is particularly true in the case of high-range waterreducing admixtures (superplasticisers). 3.4.3
Compressive strength
The compressive strength of concrete made with aggregate of adequate strength is governed by either the strength of the cement paste or the bond between the paste and the aggregate particles. At early ages, the bond strength is lower than the paste strength. At later ages, the reverse may be the case. For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregate, and water (under the same mixing, curing, and testing conditions) is influenced by: •
the ratio;to cement; the water-cement ratio of aggregate • the grading, surface texture, shape, and strength of aggregate particles; and • the maximum size of the aggregate. Mix factors, partially or totally independent of the water-cement ratio, which affect the strength are: • Type and brand of cement and use of SCMs. • Amount and type of admixture, in particular the use of air-entraining agents. • Mineral composition of the aggregate. As noted above, concrete tends to be specified by compressive strength. The specified strength should be the highest value of the required strengths from each of the relevant design criteria, ie durability, structural performance and construction •
requirements (eg strength at stressing).
3.4.4
Tensile strength
A measure of the performance of precast concrete is its resistance to cracking, which is a function of the tensile strength of concrete. Reinforcement does not prevent cracking, but controls crack width after cracking has occurred. Section 3 of AS 3600 refers to two tensile strength values. The first, the characteristic flexural tensile strength f’ct.f, is sometimes referred to as the modulus of rupture. It is measured by breaking a standard beam in flexure. Although these tests are carried out on some concretes, for example in pavements, the flexural tensile strength is generally estimated from an equation relating it to compressive strength. The relationship given in Section 3 of AS 3600 is: f ’ct.f = 0.6• f ’c Note that this relationship is a lower-bound value. The other tensile strength is the characteristic principal tensile strength, f’ct, and is a measure of the tensile strength in pure tension, ie not induced by bending. Again, Section 3 of AS 3600 suggests a lower-bound relationship to f’c, ie f ’ct = 0.36•f’c 3.4.5
Modulus of elasticity
The modulus of elasticity, Ec, is the ratio of normal stress to corresponding strain for tensile or compressive stress. It is the material property which determines the deformability under load and creep deformation. The modulus of elasticity of concrete is not as well defined as that for steel. It is defined by an approximate slope, such as the ‘secant modulus’. Calculations which involve its use have an inherent imprecision, but this seldom affects practical performance. While it may be desirable in rare instances to determine the modulus of elasticity by test, the equation given in AS 3600 (shown below) is usually satisfactory. Ecj = r1.5 x 0.043•fcmi for fcmi ! 40 MPa or Ecj = r1.5 (0.024•fcmi + 0.12) for fcmi > 40 MPa where:
3.4.6
Ecj = modulus of elasticity at the time tested r = density of concrete fcmi = the mean value of the compressive strength at the time tested.
3.4.7
Coefficient of thermal expansion
The coefficient of thermal expansion of concrete varies with the aggregate used (see Table 3.13). Ranges for normal density concrete are 9 to 13 x 10-6/°C. Section 3 of AS 3600 suggests a value of 10 x 10-6/°C and this will be satisfactory for most projects. If greater accuracy is needed, tests should be conducted on the specific concrete. Since the coefficient of thermal expansion for steel is also about 11 x 10-6/°C, the differential movement between steel and concrete when a member is heated or cooled will not produce significant stresses in the concrete. However, steel exposed to direct sun will expand more quickly than the surrounding concrete due to its higher conductivity.
Table 3.13 Average Coefficients of Linear Thermal Expansion of Rock (Aggregate) and Concrete
(10/°C)
(1)
(2) (2)
(2)
Poisson’s ratio
Poisson’s ratio is the ratio of transverse strain to axial strain resulting from uniformly distributed axial load. Values generally range between 0.11 and 0.27. AS 3600 gives the value of 0.20.
3.4.8
Shrinkage and creep
Precast concrete elements are subject to air-drying as soon as they are removed from the moulds. As a result of this drying, the concrete slowly loses some of its srcinal water, causing shrinkage to occur. When concrete is subjected to a sustained load, the deformation may be divided into two parts: • an elastic deformation which occurs immediately; and •
abegins time-dependent called immediatelydeformation, and continues overcreep, time. which Shrinkage and creep strains vary with relative humidity, hypothetical thickness (as defined in AS 3600), level of sustained load including prestress, concrete strength at time of load application, amount and location of steel reinforcement, and other characteristics of the material and design. Concrete will creep more the earlier load is applied. Thus, in general, precast concrete elements will tend to creep less than cast-insitu elements because loads will be applied to them later. 3.4.9
Permeability and absorption
The permeability of concrete in water-retaining structures is rarely of any consequence. Typical water permeability is in the order of 25 to 50 x 10 -12 metres/second (see Khatri and Sirivivatnanon3.29). The actual watertightness of the structure is usually controlled by leakage through cracks and joints. However, the water permeability and the rate of absorption of cover concrete are measures of its resistance to chemicals that may be damaging to reinforcement and are occasionally specified when using precast concrete for special conditions. A number of laboratory tests have been developed to measure these properties but usually lack correlation with actual durability in the field. Before specifying this type of test, the designer should be convinced that measurement of the property is required and that it can be measured in an accurate and meaningful way. They need to be aware that testing is expensive, can take considerable time and may need to be carried out on trial mixes well before production can commence.
3.5.4
3.5.1
General
When water, sand and a cementitious material are mixed together without coarse aggregate, the result is called grout, mortar or dry-pack, depending on consistency. These materials numerous applications in precast concrete have construction. Sometimes they are used for fire or corrosion protection treatment and at other times to transfer loads in horizontal and vertical joints. 3.5.2
Portland and blended cement grouts and mortars
Most grout is a simple mixture of cement, sand, and water. Proportions are generally one part cement to 2 to 3 parts sand. The amount of water depends on the method of placement. Dry-pack is the name used for very stiff, sand-cement mixes. It is used if a relatively high strength is needed, eg under column base plates. Compaction is by hand tamping, using a rod or stick.
Epoxy grouts
Epoxy grouts are used when very high strength is desired, or positive bonding to the concrete is necessary. They are mixtures of epoxy resins and a filler material, usually sand. The physical properties of epoxy compounds vary widely. The user should be familiar with the compound to be used, either through experience or test. Of particular impor tance in some applications is the thermal expansion, which can be up to seven times that of concrete and may creep under sustained load. This can result in interface debonding.
Flowable andbut pumpable grouts can produced no additives have problems withbelow strengthwith due to high water-cement ratios and a tendency for solids to settle out. With proper attention to cement content, sand gradings and additives, flowable grouts up to 60 MPa can be produced. 3.5.3
Non-shrink grouts
When complete filling of spaces or bond to reinforcement in core holes is essential, non-shrink grouts are used. These are usually proprietary products of the following basic types: • Grouts which expand in a plastic state (a) Aluminium powder based grouts which release hydrogen gas. There is some evidence that the hydrogen gas may reduce the fatigue capacity of reinforcement. (b) Ammonium salt based grouts which release nitrogen gas. (c) Lime based grouts which expand as lime is formed in the plastic state. • Grouts which expand in the hardened state are usually based on iron powder which corrodes with time and expands as iron oxides are formed. This type of grout is not suitable in contact with reinforcement and is usually used in underpinningtype applications.
3.14 AS/NZS 4671 Steel reinforcing materials, Standards Australia, 2001.
3.15 AS/NZS 3679.1 Structural steel Part 1: Hot-rolled bars and sections, Standards Australia, 1996 (including Amendments 1–1997 and 2–2000). 3.1 AS 1391 Methods for tensile testing of metals, Standards Australia, 2007. 3.2 AS 3972 Portland and blended cements, Standards Australia, 1997. 3.3 Neville A A “New” Look at High-Alumina Cement. Concrete International, August 1998, pp 51–55. 3.4 AS 3582.1 Supplementary cementitious materials for use with portland and blended cement – Fly ash. Standards Australia, 1998 (including
Amendment 1–1999). 3.5 AS 3582.2 Supplementary cementitious materials for use with portland and blended cement – Slag – Ground granulated iron blast-furnace, Standards
Australia, 2001. 3.6 AS 3582.3 Supplementary cementitious materials for use with portland and blended cement – Amorphous silica, Standards Australia, 2002.
3.16 Building Code of Australia Australian Building Codes Board, 2008. 3.17 Rostam, S. Maintenance and repair of concrete structures – and feedback to design of durable concrete structures. Paper presented at Concrete
Remediation, Design and Long-Life Solutions seminar, Sydney, May 2000. 3.18 AS/NZS 1554.6 Structural steel welding – Welding stainless steels for structural purposes , Standards Australia, 1994. 3.19 AS/NZS 4672.1 Steel prestressing materals Part 1: General requirements Standards Australia, 2007. 3.20 AS/NZS 1554.3 Structural steel welding – Welding of reinforcing steel, Standards Australia, 2008. 3.21 Performance Criteria for Concrete in Marine Environments (Z 13) Concrete Institute of Australia, 2001.
3.7 Alkali Aggregate Reaction: Guidelines on Minimising the Risk of Damage to Concrete Structures in Australia (CCAA T47/SAA HB79) Cement
Concrete & Aggregates Australia and Standards Australia, 1996. 3.8 PCI Committee on Durability, Guide to Using Silica Fume in Precast/Prestressed Concrete Products PCI Journal September/October 1994,
pp36–45. 3.9 AS 2758.1 Aggregates and rock for engineering purposes Part 1: Concrete aggregates Standards Australia, 1998. 3.10 AS 1141 Methods for sampling and testing aggregates, Standards Australia, 1999. 3.11 AS 3600 Concrete structures, Standards Australia, 2009. 3.12 AS 1379 Specification and supply of concrete, Standards Australia, 2007. 3.13 AS 1478.1 Chemical admixtures for concrete, mortar and grout Part 1: Admixtures for concrete ,
Standards Australia, 2000.
3.22 American Society for Testing Materials, ASTM C1202–91 Standard test method for electrical indication of concrete’s ability to resist chloride ion penetration.
3.23 Use of Galvanised Reinforcement in Concrete (CPN 17) Concrete Institute of Australia, 2002. 3.24 AS 1897 Electroplated coatings on threaded components (metric coarse series) , Standards Australia, 1976. 3.25 AS/NZS 4680 Hot-dipped galvanized (zinc) coatings on fabricated ferrous articles , Standards Australia, 2006. 3.26 AS 1214 Hot-dip galvanized coatings on threaded fasteners (ISO metric coarse thread series) , Standards Australia, 1983. 3.27 AS/NZS 3750.15 Paints for steel structures Part 15: Inorganic zinc silicate paint, Standards Australia, 1998. 3.28 Guide to Concrete Construction, 2nd Edition, (CCAA T41/SAA HB64) Cement Concrete & Aggregates Australia, Standards Australia, 2002.
3.39 Khatri, R P and Sirivivatnanon, V ‘Methods for the determination of water permeability of concrete’ ACI Materials Jour nal, May-June 1997, pp 257–261. 3.40 Fly Ash and its Use in Concrete (CPN 25) Concrete Institute of Australia, 2003. 3.41 Ground Granulated Iron Blast Furnace Slag and its Use in Concrete (CPN 26) Concrete Institute of Australia, 2003. 3.42 Amorphous Silica and its Use in Concrete (CPN 27) Concrete Institute of Australia, 2006.
There are some simple rules which, if followed, lead to the production of concrete with a service life of 100 years or more. This versatile structural and architectural product is enhanced by being produced under the factory conditions, maintained by competent precast concrete manfacturers.
Precast manufacturers can provide advice on the implications of the various mix designs, including the choice of the cement type (particularly as they affect the precasting process).
Concrete should be specified in accordance with AS 1379.The recommended minimum characteristic compressive strength for precast concrete elements is 32 MPa. To allow for next-day demoulding, 40 MPa is commonly used. AS 3600 has a specific definition of reinforcement, ie steel bar, wire, or mesh but not tendons. This definition has been adopted in this Handbook.
Embedment in concrete protects reinforcing steel, tendons and steel hardware from corrosion. A protective iron-oxide film forms on the surface of the bar, wire or strand as a result of the high alkalinity of the cement paste.
What you will find in this Chapter •
The types of variation from true dimension you can expect to find in precast building construction.
•
Acceptable reinforcement tolerances.
•
Acceptable manufacturing tolerances for individual precast units.
•
Methods to measure these variations in dimensions for compliance.
•
Acceptable variations in dimensions of a structure into which precast units are to be placed.
4.1 Definitions 4.2 General 4.3 Reinforcement Tolerances 4.4 Manuf acturing Tolerances 4.4.1 General 4.4.2 Checking elements for compliance 4.4.3 Values for manufacturing tolerances 4.5 Building Tolerances 4.5.1 General 4.5.2 Tolerances on a completed structure 4.5.3 Tolerances within a structure 4.6 References
For the purpose of this chapter the following definitions are used: Tolerance The permitted deviation from a specified linear, angular or profile dimension or shape. Reinforcement tolerances Tolerances on reinforcement length, shape, dimensions, and cover on tendon location in a precast element. Manufacturing tolerances Tolerances on the dimensions and shape of a precast element. Building tolerances Tolerances on the overall building, the building structure, and parts of the building that interface with precast members.
Precast construction requires an appreciation of the deviations that naturally occur during the fabrication and construction process. The variation in insitu building dimensions can usually be accommodated as construction proceeds, whereas variations have to be estimated andprecast provision made for them in dimensions and joint details on the workshop drawings. The connection system has to accommodate the variation of the suppor ting structure from its true position by simple adjustment. Slotted and oversize boltholes in brackets, shimming at supports, welding and grouting are used as appropriate to do this. The variation from true dimension of a precast member can be controlled during manufacture to limits defined in this chapter. As a rule, tighter tolerances than the industry norm will incur greater cost and should be considered only after consultation with an experienced precaster.
Dimensions that require definition of tolerance are grouped here as: •
Reinforcement dimensions and location and
•
tendon location. Member dimensions and shape.
•
The structure, for example the tolerance on the position of the erected building and the position of the parts of the building that affect the positioning and support of precast members.
Generally, it is preferable to specify tolerances as ± ‘x’ about the true value. Where a dimension can be allowed to vary only in one direction the total deviation is specified as - 0 + ‘x’, or - ‘x’ + 0. The procedure for measuring the actual dimensions of a member must be agreed prior to any work commencing to avoid misunderstandings, delays or disputes (see Chapter 12). A suggested method is set out in Clause 4.4.2.
General tolerances on the fabrication of reinforcement are set out in Clause 19.2.1 of AS 36004.1 and are reproduced below. Frequently, tighter tolerances are required for precast members
because morereinforcement precise dimensions or for the assemblyof oftheir complex cages. These are agreed between the precaster and the reinforcement fabricator to suit the circumstances.
(a) Reinforcement shall be fabricated to the shape and dimensions shown in the drawings and within the following tolerances – (i) On any overall dimension for bars and mesh except where used as a fitment: -25, +0 mm (A) For lengths up to 600 mm (B) For lengths over 600 mm -40, +0 mm (ii) On any overall dimension of bars or mesh used as a fitment: (A) For deformed bars and mesh -15, +0 mm (B) For plain round bars and wire. -10, +0 mm (iii) On the overall offset dimension of a cranked column bar
-0, +10 mm
(iv) For the sawn or machined end of a straight bar intended for use as an end-bearing splice, the angular deviation from square, measured in relation to the end 300 mm, shall be within
2°
Tolerances on the position of reinforcement and tendons (strands) are set out in Clause 19.5.3 of AS 3600, ie:
The deviation from the specified position of reinforcement and tendons shall not exceed the following: (a) For positions controlled by cover in beams, slabs, columns and walls -5, +10 mm (Where a positive value indicates the amount cover may increase and a negative value the amount cover may decrease.) (b) For positions not controlled by cover, (i) the location of tendons on a profile
5 mm
(ii) the position of the ends of reinforcement 50 mm (iii) the spacing of bars in walls and slabs and of fitments in beams and columns 10% of the specified spacing or 15 mm, whichever is greater
Note that the above tolerances on cover are not symmetrical and reflect the requirement that cover be maintained. If designers are concerned with the effect of tolerance on cover it is preferable to increase the cover than to change the tolerance limits. Designers should note that covers given in AS 3600 for corrosion protection are minimum values.
Figure 4.1 Types of Manufacturing Tolerances
4.4.1 General
The actual dimensions of a precast member may be slightly different to its nominal dimensions and the differences will vary from member to member even though they may come from the same mould. Tolerance limits define the envelope within which the precast element must lie. Random deviations may be plus or minus; variations in measurement can arise in mould construction and modification. Mould dimensions can vary slightly during assembly due to clearance in the parts, from wear during repetitive use and from minor distortion under load. Variations in length, width and thickness dimensions usually affect assembly on site. Out-ofplane deviations as measured by warp, twist and bow, mainly affect appearance but can also have an effect on connections and joints. Movement will occur after casting due to shrinkage and creep under self-weight or prestress. Creep and shrinkage are time-dependant dimensional changes that are not manufacturing tolerances as such and are usually beyond the control of the manufacturer. The tolerances on the fabrication of cast-in metal items such as connection brackets are primarily the concern of the precaster and are taken up in the manufacturing tolerances of the member. Guidance on the deviations that can be expected can be found in Section 14 of AS 4100 Steel structures4.4.
For the purpose of this Handbook, manufacturing tolerances are classified as: •
Linear (Type L) those covering the linear dimensions of the member and location of cast-in features and hardware
•
Angular (Type A) those covering squareness and trueness
•
Profile (Type P)
those covering the shape of the member. For consistency, precast members should be checked for dimensional acceptability in the sequence linear, angular and profile, see Clause 4.4.2. Tolerances should be measured with the member in the same attitude and supported in the same manner as in the completed structure. The effects of temperature and differential temperature should be taken into account if they are likely to affect the dimensions of the member during checking.
4.4.2
Checking elements for compliance
Linear tolerances (Type L) Compliance with tolerances in linear dimensions should be checked first. These cover the basic thickness, width, length and cross-section of the member. Measurements should be made in the direction specified, usually at right angles to an edge, as in Figure 4.2.
Figure 4.2 Checking Linear and Angular Tolerances
Angular tolerances (Type A) If linear tolerances have been met then the element
should be checked for compliance with tolerances in angular dimensions, see Figure 4.2. Angular tolerances cover squareness or trueness to a specific angle. The squareness tolerance is the distance by which the shorter side of the member deviates from a straight line perpendicular to the longer side and passing through the corner of the member where the two sides meet. Trueness to an angle other than 90° is similar but with the reference line at the true angle. Any error in straightness of the sides is ignored. Profile tolerances (Type P) If both linear and angular tolerances have been met then the member should be checked for compliance with tolerances in profile. Profile tolerances cover flatness, straightness, warp and twist. These terms are defined below and illustrated in Figure 4.3. The importance of careful selection of reference lines and planes for measurement of profile tolerances should be noted. •
flatness tolerance, Pf, is the maximum distance by which any point on a nominally-plane surface of a member may be from a 3-metre straightedge placed anywhere on the surface and parallel to the nominally-plane surface.
•
straightness tolerance, Ps, is the maximum distance by which any point on an edge of the element may be from a straight line drawn through the extremities of that particular edge.
•
warp tolerance, Pw, is the maximum acceptable distance of any point on a surface from a plane containing any three corners of the surface or points on the perimeter of the member. If the surface is not a rectangle, these three corners are those points on the surface which are the corners of a rectangle covering the greatest possible surface area of the member. Where there is any doubt as to the location of these points, the method of measurement should be documented. A warp tolerance is usually applied to planar elements and may not be relevant for all shapes.
•
twist tolerance, Pt, is the rotation of one end of the member relative to the other end or to some other line or surface specified by the project documents. A twist tolerance is usually applied to elongated and beam-type elements.
Figure 4.3 Checking Profile Tolerances
Hog or camber of a member is the deflection due to the application of a prestressing force and is usually upwards, Figure 4.4. It can be estimated by calculation and an acceptable range determined by using appropriate combinations of the lower and upper bounds of the prime variables in the calculation. Hog should be measured with the element orientated in the same attitude and supported in the same manner as in the completed structure. The measurement should be made immediately on application of the prestress force to avoid the effects of shrinkage and creep. Alternatively, the time at which the measurement is to be made should be specified (typically 28 days after casting). The hog should fall within the specified range for the particular element.
Figure 4.4
Definition of Hog
4.4.3
Values for manufacturing tolerances
Tolerances for various elements are set out in Table 4.2. The interpretation of these deviations is illustrated in Figures 4.1 to 4.3. The tolerances generally are the same as those in AS 36104.2 and can be readily achieved in normal circumstances. They have proved to be adequate to allow the assembly of elements on site and to allow elements and building joints to function correctly in service. The precast
manufacturer details member clearances, connections and joints on the workshop drawings taking these limits into account. There is usually no tangible benefit to the construction in specifying tighter tolerances; it only increases the cost to reflect the increased risk of non-compliance. Hollowcore units are pretensioned and cast by machine on a long-line bed in a continuous length. They are cut to length by a diamond saw. The passage of the machine, slight slumping of the concrete and inaccuracy in positioning the cutting equipment result in higher deviations from nominal dimensions than for wet-cast elements. These are shown in Table 4.1.
Table 4.1 Tolerances for Hollowcore Units
Table 4.2 Manufacturing Tolerances for Precast Elements
Precast piles, bridge planks and girders manufactured for highway and other bridges are subject to product specifications issued by the relevant authority. Typical dimensional tolerances of these members are set out in Table 4.3. The elements must comply with these tolerances at 28 days after casting.
For irregular, curved or unusual-shaped units, the necessary tolerances should be clearly defined in the specification.
Table 4.3 Dimensional Tolerances for Bridge Elements at 28 Days after Casting 4.3] [After Table B110.1 RTA Specification B110
Table 4.4 Tolerances for a Completed Structure
4.5.1
General
Difficulties sometimes arise during erection of precast elements where a structure has been built out-oftolerance with little or no consideration given in the design for the subsequent fixing of the precast. There are two aspects to be considered: •
Tolerances on the completed structure. These determine that the structure is built in accordance with the drawings and documentation
•
Tolerances on the dimensions between parts of the structure and their relationship to each other. These determine, in part, that the structure will fit together, perform adequately in ser vice and have a satisfactory appearance.
AS 3600 sets out in Clause 19.5 some general tolerances for points on the surface of buildings. However, these give the limits beyond which the design rules in the Standard no longer apply. Morestringent tolerances are required in the actual construction to ensure the proper fit of the precast elements, serviceability and an acceptable appearance for the structure. Suggested tolerances for the completed structure are given in Table 4.4. These apply to both precast and insitu structures. 4.5.3
Table 4.5
Tolerances Within a Structure
4.5.2 Tolerances on a completed structure
Tolerances within a structure
Inaccuracies on the dimensions between parts of the structure , eg the dimension between columns, may affect the erection of precast elements into the frame. The out-of-position tolerances in AS 3600 are also limits beyond which the design rules in that Standard do not apply. They are not intended as building tolerances for fit of components or for serviceability. Usually, tighter limits will need to be specified to ensure correct fit of precast members. The tolerances in Table 4.5 are suggested limits for some building parts.
4.1 AS 3600 Concrete structures, Standards Austral ia, 2009. 4.2 AS 3610 Formwork for concrete, Standards Australia, 1995.
4.3 RTA QA Specification DCM B110 Manufacture of pretensioned precast concrete members, Ed 1/Rev 0, Roads and Traffic Authority NSW, October 2006.
Building with precast concrete demands a clear appreciation of the tolerances of each aspect of construction.
4.4 AS 4100 Steel structures, Standards Australi a, 1998.
The various tolerances that need to be considered are – cast-in items (including reinforcement), manufacturing and building.
What you will find in this Chapter •
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•
•
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An understanding of the design process for precast buildings and how to design a total precast building. An explanation of the differences between commoncastinsitu design and the approach adopted for precast design. Alternative design methods to ensure lateral stability is achieved. Provision for time-dependent deformations and volumetric movements. A discussion of the effects of seismic forces (actions) on precast buildings and guidance on suitable detailing. A design example of a single-storey industrial building, (a) as a portal frame clad with precast hollowcore panels and (b) as precast loadbearing panels supporting a braced roof. A design example of a four-storey precast building with shear walls, examined for lateral load resistance.
5.1 Definitions and Notation 5.1.1 Definitions 5.1.2 Notation 5.2 Introduction 5.3 General Design Considerations and the Design Process 5.3.1 Building Code of Australia and Australian Standards requirements 5.3.2 The design process 5.3.3 Lateral load-resisting systems 5.3.4 Framing dimensions 5.3.5 Span-to-depth ratio 5.4 Applied Actions 5.4.1 Action combinations 5.4.2 Earthquake actions and detailing 5.5 Control of Volume Change Deformations and Restraint Forces 5.5.1 General 5.5.2 Shrinkage 5.5.3 Creep 5.5.4 Temperature strains 5.5.5 Thermal bowing 5.5.6 Influence on non-structural components 5.5.7 Volume change effects in moment-resisting frames 5.5.8 Isolation joints 5.5.9 Spacing and width of isolation joints 5.6 Diaphragm Action 5.6.1 Basis of action 5.6.2 Rigid and flexible diaphragms 5.6.3 Shear transfer between elements 5.6.4 Chord forces 5.7 Shear Walls 5.8 Base Fixity of Columns 5.9 Analysis of Precast Buildings for Horizontal Actions 5.9.1 Single-storey industrial building Example 5.1a One-storey Building with Hollowcore Cladding to Portal Frame Example 5.1b One-storey Building with Loadbearing Panels and Braced Roof 5.9.2 Four-storey building Example 5.2 Four-storey Building 5.10 References
5.1.2
Notation
The following notation is used in this chapter (Note, that it is generally not the same as in AS 1170.45.1): Aw = the cross-sectional area of a shear wall a = the acceleration coefficient Ec = the modulus of elasticity of concrete 5.1.1 Definitions For the purpose of this chapter the following e = the static eccentricity of the centre of rigidity definitions are used (where possible these agree with (shear centre) to centroid of applied actions those in the relevant Australian Standard): Fi = the force resisted by an individual shear wall, i = the height of the building = the storey height = the moment of inertia of a section = the polar moment of inertia Ductility (of a structure) = the equivalent moment of inertia The ability of the structure or element to sustain its load-carrying capacity and dissipate energy = the sum of ly 2 of east-west walls when responding to cyclic displacements in the = the sum of lx 2 of north-south walls inelastic range, such as earthquakes. = the coefficient of thermal expansion Ductility in precast concrete structures can be = the shrinkage strain coefficient influenced by individual components wedging or levering during load displacement. = the creep factor coefficient Robustness = the maturity coefficient used in calculating Structures shall be designed, tied together both the design creep factor vertically and horizontally and detailed so that the structure can withstand events like fire, explosion, L = the distance between supports impact or consequences of human error without M = the bending moment being damaged to an extent disproportionate to P = tensile force or action the srcinal These usuallyapproach only effect = the lateral wind or earthquake action part of the cause. structure andevents the design and response is therefore different from other actions. Q = the first moment of area about the neutral axis Structural systems r = the rigidity of a shear wall (1/D) Bearing-wall system Structural systems in which loadbearing walls ri = the rigidity of wall, i provide support for all or most of the vertical S = the site factor actions while shear walls (or braced frames) T = the torsional moment provide the resistance to horizontal actions. This system is very common in precast. TD = the maximum differential temperature Shear-wall system TE = the average temperature causing Structural systems in which loadbearing elements axial extension such as columns and walls provide support for all t = the panel thickness or most of the vertical actions while shear walls provide the resistance to horizontal actions. This th = the hypothetical thickness system is very common in precast and is often a W = the total lateral load bearing wall system. x¯ = the distance to the centre of rigidity Moment-resisting-frame system y¯ = the distance to the centre of rigidity A structural system with an essentially complete space frame providing support for all the vertical y = the distance from the neutral axis to and horizontal actions by both flexural and axial fibre under consideration resistance of the members and connections (Not D = the theoretical magnitude of bowing that common in precast). = the sum of flexure and shear deflections Space-frame system ecs = the design shrinkage strain of the concrete A two or three-dimensional structural system composed of interconnected members (other ecs.b = the basic shrinkage strain of the concrete than load-bearing walls) which is capable of fcc = the design creep factor supporting vertical loads and may also provide fcc.b = the basic creep factor of the concrete horizontal resistance to horizontal forces. Often lateral actions can be carried by shear walls also. me = the coefficient of shear friction This type of system is also not that common in Sr = the sum of the rigidities of all shear walls precast. Z = the earthquake hazard factor Diaphragm
A horizontal or nearly horizontal, structural system acting to transmit horizontal actions to the structural system resisting the horizontal actions.
H h I Ip Ieq Ixx Iyy kth k1 k2 k3
This chapter provides guidelines for the analysis and design of buildings and structures that are constructed either wholly or partly of precast concrete elements. These elements may be designed as either reinforced or prestressed units. Precast concrete offers numerous advantages over and above the desirable features inherent in good insitu concrete construction. Important advantages include: • Increased construction speed • Plant-controlled, quality-assured component manufacture and off-site manufacture • Enhanced durability • Reduced congestion on the construction site by offsite manufacture • A wider variety of architectural shapes • A wider variety of surface finishes (textures and colours) and greater flexibility in the choice of finishes for a given surface as the orientation of the unit in the casting position need not be the same as that in the final location Minimisation of the cost of environmental management of dust, noise, water, etc as they are removed from the site to a closely-controlled factory environment • When prestressing is used, greater span-todepth ratios can be achieved and these may result in reduced storey heights, larger columnfree spacemore controllable performance, and minimisation of material usage. A clear understanding of the difference between precast and insitu construction will help the designer to focus on the essential issues and ensure that the inevitable compromises which arise in all designs are confined to secondary aspects. Good design in either system involves understanding the method of construction, the implicit constraints and the aspects that facilitate buildability. •
In Australia, precast elements are typically projectspecific, not standard components. Many of the 5.2. benefits of standardisation are, however, retained This is achieved by standardising the component type and method of connection rather than trying to produce repetitive elements. This results in precast structures that do not need to be repetitive or modular and allows greater architectural freedom. Because of small production runs, the manufacturing and erection process of precast structures imposes a number of considerations on design and construction. The following factors should be considered when developing design concepts. • High on-site labour costsmean the saving in construction time for a precast structure will frequently have a more significant influence on cost than the quantities of materials. • The labour content of the manufacturing process can be significant and elements should be detailed to ensure minimisation of labour even at the cost of extra material. • The structure should incorporate as small a number as possible of different types of elements to minimise the number of moulds required. Elements should be standardised so that variations of a basic type can be produced in the same mould. Connections should be simple and quick to make so that speedy and continuous erection can be maintained. It should not be necessary for the crane to support an element after placing and during alignment. • Setting up and adjusting the precast elements should require no fixed scaffolding, only mobile scaffolding or an extendable mobile access platform that can be quickly moved to new working positions. When first erected, precast concrete elements are usually unstable until connections providing moment transfer or other bracing elements are incorporated. The construction sequence involves erecting discrete elements that may be braced and/or propped for temporary stability and then connecting them together to form a monolithic structural system that resists the applied loads. Generally, the elements will be in one-way bending only or carry axial load. •
Factors that need to be considered for bracing and propping include: • Bracing of wall panels will usually be required and consideration will include their location, whether they will penetrate floors and what bottom anchors will be used. • Propping and bracing of precast floor units and precast beams need to allow for out-of-balance forces due to erection or construction on one side of their suppor t. • The ground or structure on which props to precast beams are supported must be capable of carrying these construction loads. • Where precast is to be supported on steel beams it is vital to ensure the beams cannot rotate or twist due to out-of-balance forces. • Precast flooring systems needs to be designed to construction loads in the formwork code AS 36105.3 as well AS/NZS 11705.4. Precast, prestressed concrete beam and slab elements are usually most economical when they can be designed and connected into a structure as simple-span elements. This is because: • positive moment capacity is much easier and less expensive to attain with both reinforced and pretensioned elements than negative moment capacity at supports; • connections to provide continuity at the supports are sometimes complicated and costly. It is therefore simpler when designing precast concrete structures to have connections which allow lateral movement and rotation, ie pinned ends, and achieve lateral stability using the floor and roof diaphragms in conjunction with shear walls. This form of construction is frequently referred to as ‘skeletal frame construction’ or b‘ raced frame construction ’. In such construction, most of the structural elements are of precast concrete including: the columns, spandrel and edge beams, internal beams, floor and roof units, stair cases and walls. The structure depends on shear walls (from the stair well, lift shaft and other walls) for lateral stability and
Single-storey buildings
These usually are of three types as follows. • Steel portal frames or steel-braced frames with precast cladding for industrial buildings. • A loadbearing panel building or“concrete box”. This is typically a box-like structure and utilises a stiffened steel roof structure as a diaphragm to transmit the lateral actions due to wind or earthquake to the transverse walls and then to the footings. • Precast walls or columns cantilevering from the ground. Carrying lateral loads by cantilever action of precast columns or walls can be an economical option on one- or two-storey buildings where the BCA requires post-fire stability of external walls. Columns or walls, in these cases, are designed to cantilever from the foundations with a momentresisting connection at the base which will require insitu structure to resist such overturning moments. This, however, is not a very common system.
Low-rise buildings
These cover the range of buildings from 2 storeys to about 8 storeys for which three precast systems have evolved in Australia5.2. Panelised Precast Structural Frame (Bearing Wall or Shear Wall Systems)
This is a structure incorporating structural precast walling and/or exposed spandrel panels with a precast floor system spanning between walls, Figure 5.1. The floor systems act as horizontal diaphragms to transfer horizontal actions to the shear walls. The temporary bracing should be designed where possible so that it does not penetrate the floor above. Figure 5.1 Panalised Precast Structural Frame (Bearing Wall System)
has pin-joint connections. bracing will be required for lateral stabilityTemporary during erection. Designers are reminded of the need for a positive connection, both vertically and horizontally, between all elements for structural robustness in accordance with Section 6 of AS/NZS 1170.0 5.5. Precast buildings tend to fall into three categories as follows. • Single storey buildings • Low- and medium-rise buildings of say 2–8 storey • High-rise greater than say 8 storeys.
Structural Precast Skeletal Frame (Shear Wall or SpaceFrame System)
This is a building incorporating a structural precast frame of columns and beams with a precast floor system, Figure 5.2. The frame is usually pin-jointed but can be moment-resisting. Lateral actions are carried by shear walls for pin-jointed frames or by frame action for moment-resisting (space) frames. The floor systems act as horizontal diaphragms to transfer horizontal actions to the walls or frame. Figure 5.2 Structural Precast Skeletal Frame
Mixed (Hybrid) Precast Structural Frame (Shear Wall or Space-Frame System)
As the name suggests, a building combining more than one of the above structural framing systems, Figure 5.3. Figure 5.3 Mixed Precast Structural Frame
General
It is vital that designers appreciate that the most appropriate structural solution is always project related. This optimum solution is usually on a rectangular grid. For example, unlike traditional insitu concrete construction, the most economical solution when hollowcore is used is with the floors spanning the longer dimension.This is because for hollowcore slabs the additional cost of increasing the span from, say, 8 m to 12m, or from 12 m to 17 m is small. Framed structures supporting beams with hollowcore floors should therefore span the shorter dimension to allow the beam depth to be optimized. Other precast floor systems may dictate other solutions. Shear walls are often the most effective and economical method of providing stability for low- to medium-rise buildings. All buildings have a lift and/or stair shaft that can readily be used as shear walls. They are designed either as individual walls connected to form a boxed shaft or as box structures (when the shaft is cast as a complete box) cantilevering from the footings. This in turn will require an adequate footing capable of resisting the resulting overturning moments. Providing full-capacity moment or torsional connections between structural components to generate frame action can be expensive and often not warranted where requirements for earthquake loading are low,the such as in Australia. However, in New Zealand this will not be the case. A mix of precast and insitu concrete should be avoided, particularly if the insitu is a small component and where it is not possible to obtain an economical work cycle. High-rise buildings
Miscellaneous Construction (Bearing Wall, Shear Wall or Space-Frame System)
This term is used to describe buildings that incorporate precast concrete combined with other structural materials. For example a structure with precast flooring supported on insitu concrete beams or walls, a steel frame or masonry walls, or a panellised structural envelope with insitu concrete floors.
For taller structures, precast concrete can also be used. The structure can be ‘space frames’ in conjunction with insitu concrete lift shafts, stair walls and wall-only elements which will act to brace the structure and provide lateral stability. Precast elements that can be used in such buildings include walls, columns, beams and cladding. Clause 5.2.3 of AS 1170.45.1 requires stiff components such as precast concrete walls, stairs and ramps that are not part of the seismic-forceresisting system, to be separated from the structure such that interaction does not take place as the structure undergoes deflections due to earthquake effects. Also, for such a structural system, all horizontal precast members except those required to provide lateral stability may be ‘pin-ended’ at their supports and therefore do not transfer significant moments to the columns. Connections can be completed by bolting, dowelling or welding together the connection hardware.
General design principles
To fully realise the benefits, and thereby gain the most economical and effective use of precast construction, it is recommended that the following general design principles be adopted: • Design the building as a precast structure from the outset and preferably the complete building if possible. This avoids the difficult compromises inherent in trying to adapt an insitu design to a precast solution. • Use shear walls to resist lateral actions where possible in low-rise and medium-rise buildings. • Use design concepts that ensure maximum repetition of units in manufacture and, whenever possible, use standardised elements and sections. Formwork can be very expensive and it is therefore beneficial if the design uses the minimum number of element types and standardises crosssection details and connection types. • Consult a recognised local precast manufacturer to confirm sizes and shapes of elements locally available as limitations of size and mass are often a function of production, transport and erection considerations. The National Precast Concrete Association Australia can assist with this process. • Design elements as‘simple-span’ members, and provide for continuity and structural redundancy only between those members intended to provide the necessary load resistance, including lateral resistance, for the building structure. This is effectively achieved with properly conceived connection details and adequate recognition of vertical action transfer paths through the members to the footing of the structure. • Simplify the support of the erected elements and the connection details. This makes for a good design and will help minimise the time for the individual units to be incorporated into the structure. • Remember that the successful design of precast buildings is largely dependent on carefully conceived and simple details, connections and associated precast elements. •
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•
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Consider the effect of restraint of dimensional change in precast members on the structure and its elements. Dimensional changes are caused by shrinkage, creep and temperature change and the restraint of the movement can subject members to significant forces. Where possible, restraint should be relieved at the ends of members by appropriate connection design. Do not overlook the inherent capacity of architectural elements (which are often used only as cladding) to resist lateral actions. Remember that while prestressing improves the economy and serviceability performance of precast members, it is usually viable only when elements are of standard shape and capable of being cast in ‘long-line’ beds.
Recognise that concrete is afor solid material. This wind can provide a design advantage stability under and other lateral actions (except earthquake), acoustical and vibration control, reduction in heating and cooling loads, and fire resistance. In addition, the high permanent-to-imposed action ratio will provide a greater factor of safety against gravity overloads. However, framing details, or loading conditions such as earthquake, which result in eccentrically loaded supports, need careful attention.
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5.3.1
Building Code of Australia and Australian Standards requirements
The Building Code of Australia (BCA)5.6 specifies
Construction sequence and temporary stability if it is an unusual or complicated structure. Approximate member sizes for alternative designs are costed to get the optimum solutions.
the performance requirements for buildings and structures. These performance requirements and the associated deemed-to-comply solutions make it clear that a designer is not constrained to use the referenced standards, eg AS 36005.7. However, the designer generally will find it simpler and more straightforward to follow the deemed-to-satisfy path and use the referenced standard. Thus this handbook has been written to comply with AS 3600. Other standards not called up directly by the BCA but referenced in a standard called up by the BCA, must also be complied with. Other standards not referenced or called up at all within the BCA are like a handbook or manual and the designer has an option to use them or not. Where the local authority has mandated the use of a given standard, eg AS 38505.8, then the design and
Development (planning) application is lodged and, if required, a further budget costing is carried out to confirm the project is on budget. Final Design – where the optimum preliminary design is fully analysed and the project fully designed and detailed for the design life covering all limit states including the preparation of project documentation, plans and specifications. It is important that the designer recognises that the documentation is the means of communicating the design intentions to the contractor/builder and it should be reviewed from this viewpoint before being issued. Guidance should be provided on how the structure is stabilised during erection of the precast elements for complex or unusual structures until lateral stability is achieved by the completed structure. It may be necessary that the designer describes the sequence for construction
construction must follow this standard.
to ensure the design concept is not compromised and the structure remains stable during erection. It is important to understand that in order for a precaster to prepare shop drawings, complete and final, fullydimensioned drawings are required, otherwise delays may occur. For a precast structure, the following must be considered during the design process: • The design of each member during handling, transport and erection • The design of the structure during construction (sequence, support of individual members, propping, bracing, etc) • The design of the completed structure. The effect of loads, forces and deformations on the joints and the behaviour of the total structure under
5.3.2
The design process
The process of design encompasses several identifiable stages commencing with a conceptual design of the building form and ending in its completed design approved for construction. Sufficient structural engineering input at the conceptual and preliminary stages will avoid unexpected difficulties when final design is carried out. These stages may be summarised as: Conceptual Design – where the broad principles of the design are developed, the likely structural form, the extent and use of precast, the space and usage requirements, how it is going to be built, and a preliminary budget estimate to confirm that the project appears economically viable, are carried out. Preliminary Design – where the client requirements
for the project are detailed. The primary considerations in the preliminary design of the total structure are: • Whether the building will be totally precast or for which parts or sections precast is to be used • Lateral load-resisting systems including shear and core walls and moment frames • Vertical load resisting systems • Likely structural members including footings, columns, walls, floors and which members will be precast, etc
Framing dimensions Span-to-depth ratios Control of volume change deformations and restraint forces Connection concepts
•
the various design fordesign durability, fire resistance and loads. other Design relevant loadings should also be carefully considered.
5.3.3
Figure 5.4 Bearing-Wall Systems
Lateral load-resisting systems
Precast concrete elements can be assembled and integrated with insitu concrete construction in a wide variety of structural systems. Brief notes on broad types of structural systems are given below.The nomenclature used is similar to that adopted in AS 1170.45.1. Bearing-wall system – defined as a structural system with loadbearing walls providing support for all or most of the vertical actions and shear walls or braced frames providing the horizontal resistance to imposed actions, Figure 5.4. Figure 5.5 shows a shear wall system of mixed construction with shear walls and columns. These types of construction are suited to residential type occupancies as well as offices, etc and are commonly used for many buildings in Australia. Space-frame system – defined as a structural system with interconnecting members, other than loadbearing walls, which is capable of providing support for vertical actions and may provide the horizontal load resistance by flexural action of the members, Figure 5.6, 5.7 and 5.8. This type of system is generally not economical in Australia because of the cost and time of on-site work. Cantilevered columns or wall panels will usually be
feasible only in low-rise buildings. Basefooting, fixity can be achieved by fixing the column to the eg by using an encastré detail. Only the minimum number of frames to establish stability should be used. It is sometimes feasible to provide a moment connection at only one end of an element, or a connection that will resist moments with lateral forces in one direction but not in the other, to reduce the build-up of restraint forces, Figure 5.8.
Figure 5.5 Shear Wall System (Mixed Precast Structural Frame)
Figure 5.8
Figure 5.6 Beam-Column Joints for Moment-Resisting Frames
Base Fixity Providing Stability for Low- and Medium-Rise Moment-Resisting Frames
5.3.4
Figure 5.7 Space-Frame System Showing the Required Vertical and Lateral Load Resistance of the Members
Framing dimensions
When possible, bays sizes or dimensions should fit the module of the components selected. Dimensions of common unit types and shapes are shown in Chapter 2 Products and Processes, but local precasters should be consulted about economical shapes and sizes available in the particular geographical area. It is often feasible to combine wall panels and column elements in multi-storey units, thereby achieving an economy because there are fewer components to handle and fewer joints. Height, width, length and mass of units may be limited by transport regulations and lifting limits. The limitations imposed by these factors on the erection procedure and the stability of units during erection should also be considered, seeChapter 11 Handling, Transport and Erection. Columns can be either single-storey or multi-storey in height. The choice is a trade off between using the low cost of manufacture and higher cost of erecting columns floor-to-floor as against multi-storey columns. Multi-storey have been traditionally used in the US and Europe. Experience suggests that single-storey columns are often the most economical in Australia.
5.3.5
5.4.1
Action combinations
Span-to-depth ratio
Selection of floor-to-floor dimensions should take into account the practical span-to-depth ratios of the horizontal framing elements, allowing adequate space for air conditioning ducts and other services. The values shown in Table 5.1 are intended for initial sizing, not limits. The required depth of a beam or slab is influenced by the ratio of imposed action to total actions. Where this ratio is high, deeper sections may be required. Prestressed beams suppor ting floors with wide load widths will often be at the lower end of the span-to-depth range to allow for the partial continuity and the ledges to support the floors.
Table 5.1 Typical Span-to-Depth Ratios for Precast Elements
In Australia, the nomenclature for loads has been changed to conform to that used by ISO. Generally, loads are now referred to as actions, though text books, computer programs and overseas standards may still use the old nomenclature, for example referring to dead and live loads instead of permanent and imposed actions. In this edition of the Handbook, ‘actions’ has generally been used. Actions and action combinations generally are set out in the AS/NZS 1170 suite of standards5.4. AS 36005.7 specifies that, where applicable, the prestressing force, P, is to be included in any combination with a load factor of 1.0, except for the ultimate limit state case at transfer when the load factor is 1.15. 5.4.2
Earthquake actions and detailing
Earthquake (seismic) action needs to be considered for all buildings and structures in Australia. Depending on the geographic location, the importance level, the probability factor, the hazard factor, the sub-soil conditions at the site and the earthquake design category (EDC), the design requirements for this action may be met by: • a simple static check and specific earthquake detailing; or • static analysis and design for action combinations including earthquake actions and the incorporation of specific ear thquake detailing, or • dynamic analysis and design for action combinations including earthquake actions and the incorporation of specific earthquake detailing. There is a critical difference between actions due to earthquake and actions due to wind. Actions due to gravity and wind tend to be independent of the material used and the structural form of the building. Earthquake actions are caused by the vibration of the foundation material and are generated by the inertial forces in the building as it responds to the imposed ground motions. The seismic actions are therefore dependent not only on the earthquake intensity but also on the mass, strength, stiffness and ductility of the structure. The earthquake forces are dynamic and reverse in direction over a short time (typically 15 to 45 seconds in Australia but can be much longer in higher-risk areas overseas); elements and connections have to be designed to accommodate this cyclic loading.
Because the actions are inertial by nature, they will be generated in all building elements, both structural and non-structural. Thus fixtures, mechanical equipment, architectural cladding, partitions, etc all have to be designed for earthquake actions. Because the actual seismic actions during an earthquake may exceed the design actions, proper detailing for seismic actions must not be ignored, even when the design actions for wind exceed the design seismic actions. Designers should be aware that in Australia the risk
5.5.1 General
The strains resulting from shrinkage, creep, and temperature change, and the forces caused by resisting these strains have important effects on
from earthquakes is lower than in many other parts of the world and therefore design approaches and detailing specified in AS 1170.4 and AS 3600 may not be sufficient for other geographic areas. In New Zealand, seismic loadings are considerably higher and specific details will apply for that country. To limit the damage to non-structural or architectural elements, the elements are usually uncoupled from the structural system so that they are not forced to undergo as much deformation as the supporting structure. However, this means that the joints between the element and the structure must be wide enough to permit the relative deformations to take place, while the fixings used to hold the element to the structure must be both strong and ductile enough to carry the induced loading and allow the relative movement.
connections, service load behaviour, and the ultimate capacity of precast structures. Consequently, these strains and the resulting forces must be considered in the design. Moisture differences between the inside and outside of an enclosed building can also cause dimensional change, but the effect is relatively small and difficult to calculate and is therefore usually ignored. Shrinkage and creep are additive and lead to a shortening of the element, so the usual critical design condition is shrinkage, creep and a temperature drop. Volume change is usually designed for by: limiting the overall size and number of spans in a building; splitting the total structure into a series of separate structures (by using isolation joints); utilising appropriate connection and joint details; or by a combination of all three.
The energy dissipation characteristics of precast walls will depend to a great extent upon the behaviour of the connections. Precast walls with large openings can behave in a ductile manner in flexure, similar to a beam-column system. However, unless comprehensive test data is available, precast wall-type structures should be treated as shear-wall structures. At the design stage, the direction of the ground motion is unknown, therefore a structure shaped so as to be equally resistant in any direction is the optimum solution. Experience has shown that structures that are symmetrical in plan, with minimum torsional eccentricity, generally behave better in earthquakes than structures that are asymmetrical and have their centres of mass and rigidity well separated. Precast members may be detailed to overcome the difficulties inherent in achieving the earthquake detailing required to give ductile behaviour for structures subject to high risk of ear thquakes, eg in New Zealand. See the New Zealand Concrete Society guidelines5.9 for designing connection and fixing details to provide ductile behaviour in these high-risk areas.
Approximate values of volume change deformations for preliminary design are: • Shrinkage strain for structural wall panels (also columns) – 200 x 10-6 mm/mm. • Creep strains of loadbearing exterior walls, for panels supporting floors (also columns) – 120 x 10-6 mm/mm for panels supporting only other wall panels – 30 x 10-6 mm/mm. • Thermal movements should be determined using the procedures outlined inClause 5.5.4. In concrete elements, particularly beams, there is a certain amount of thermal lag that reduces the temperature effect. In addition, it is apparent that elements within air-conditioned buildings will not undergo as large a temperature differential as elements in non-air-conditioned ones. The above volume change movement values are approximate only but will be suitable for most designs. However, where volume change movements may be significant, eg in large buildings and in special structures such as cool stores, it is recommended that an in-depth determination be made to establish more exact values of strain compatible with the specific design parameters.
Estimates of building movement must be tempered with engineering judgement. Floors and interior walls attached to exterior loadbearing panels will tend to restrain vertical movement; also, heavily-loaded elements will tend to distribute load to less heavily loaded ones. For low- to medium-rise structures the major effect will be in the horizontal direction. Nevertheless, vertical elements, such as loadbearing wall panels, are also subject to volume-change strains. The effects in the vertical direction will be significant only in high-rise buildings, and then only differential movement between elements will significantly affect the performance of a structure. This can occur, for example, at the corner of a building where loadbearing and non-loadbearing panels may meet. 5.5.2
Shrinkage
Shrinkage is defined as the decrease in the volume of hardened concrete with time. It is independent of externally applied actions. The decrease in volume is mainly attributed to the moisture loss caused by drying and hydration as well as the chemical changes that result from the carbonation of cement hydration products. Shrinkage begins immediately after the concrete member is exposed to a drying environment. The rate and amount is dependent on the concrete mix design and concrete materials, the temperature and humidity of the environment, and the size and shape of the member. AS 3600 provides that the shrinkage strain, ie the shrinkage strain expected in the concrete member in its environment, can be calculated from the basic shrinkage strain of the concrete using any accepted mathematical model for shrinkage strain provided the basic shrinkage strain,ecs.b, is predicted by the model. It also provides an acceptable approximate model. McDonald et al5.10 have shown that the AS 3600 method is better than most of the overseas methods in predicting the shrinkage of Australian concretes but is not as good as that in the superseded standard, AS 14815.11. 5.5.3
Creep
Creep of concrete is the time-dependent increase in strain under sustained loading. AS 3600 provides that the creep strain at a given time can be calculated from the basic creep factor of concrete, fcc.b, using any accepted mathematical model for creep behaviour provided thatfcc.b is predicted by the model. It provides an approximate method for 5.10 calculating the design creep factor. McDonald et al have shown that the AS 3600 approximate method is better than other methods for predicting the creep of Australian concretes.
5.5.4
Temperature strains
AS/NZS 1170.15.12 includes a clause covering thermal effects. It refers to a design method proposed by Hirst5.13. However, more directly applicable papers, by him and others, are published in Concrete 855.14 and ASCE5.15. In the paper in Concrete 85 it is suggested that for walls at any given time there is a non-linear temperature profile through the thickness of the wall. This non-linear temperature profile can be split into the three components listed in the Standard and shown in Figure 5.9. Figure 5.9 Components of Temperature Profile (from Hirst 5.11)
The uniform temperature determines the expansion or contraction of the wall, the linearly varying temperature profile determines the bending (bowing) of the wall and the residual non-linear temperature profile tends to produce a set of equilibrating stresses. Teicher et al5.14 suggest that in Australia it is usually the maximum value of effective temperature that is of concern since this causes the wall to expand. Similarly, the maximum positive temperature differential, ie a hotter external face, is of interest as it causes the wall to bow outwards. For a given wall, the conditions favouring extremes occur only once a year when the orientation of the wall with respect to the sun produces maximum heating. In the paper, they give an example of the calculation of expansion of a wall panel and bow. 5.5.5
Thermal bowing
As noted in Section 5.5.4 a temperature difference between the inside and outside of a wall panel or between the top and underside of an uninsulated roof deck will cause the elements to bow.
The theoretical magnitude of bowing,D (Figure 5.10), can be determined by: D = k th TD L 2/8t where: k th = coefficient of thermal expansion TD = maximum differential temperature L = distance between supports t = panel thickness
Figure 5.11 Corner Separation Due to Thermal Bow
Figure 5.10 Thermal Bow of an Uninsulated Wall Panel
Figure 5.12 Force Required to Restrain Bowing (Members may Span Vertically or Horizontally)
D
Limited records of temperature measurements indicate that in open structures, such as the roofs of parking stations, the maximum differential temperature, TD, seldom exceeds 16 to 22°C. In an insulated sandwich-wall panel, the theoretical difference can be higher, but this is tempered by ‘thermal lag’ due to the mass of the concrete. While the magnitude of bowing is usually not very significant, in the case of wall panels it may cause unacceptable separation at the corners, see Figure 5.11, and possible damage to joint sealants. It may therefore be desirable to restrain bowing by providing one or more connectors between panels. Figure 5.12 gives equations for calculating the required restraint and the moments this would cause in the panel but designers should note the forces can be high. Design considerations for panels meeting at corners should include the influence of temperature differences between the panels on adjacent sides of the corner because of sun exposure. Depending upon the exterior panel finishes and plan orientation of the building, 5 to 8°C temperature differences may develop. Because of the likely corner separation, mitred corner joints should be avoided. Butt joints, ie oversail joints, are one solution and the use of L-shaped panels another. Experience is that spandrel panels are unlikely to be a problem.
Similarly, differential temperature can cause upward bowing in roof elements, especially in open structures such as parking stations. If these elements are restrained from rotation at the ends, positive moments (bottom tension) can develop at and near to the support, as shown inFigure 5.12(d) and (e). The bottom tension can cause severe cracking, depending on the amount of reinforcement, but once the cracks occur, the tension is relieved. Note from Figure 5.12 that if only one end is
A number of computer analysis programs are available that allow the input of the shortening strains of elements from volume changes as well as the strains from gravity and lateral loads. 5.5.8
Isolation joints
Isolation joints are provided in structures to permit movements such as the differential movement between parts of the building supported by discrete footing systems and to limit the magnitude of forces restrained, as is force, sometimes done tomoment relieve axial that result from the restraint of volume change volume change the restraint is deformations (shrinkage, creep and temperature doubled. Also note that, since thermal bow occurs changes). with daily temperature changes, the cyclical effects If the strains generated by temperature rise are could magnify the potential damage. significantly greater than the shrinkage and creep strains, an expansion joint is needed. However, in 5.5.6 Influence on non-structural most concrete structures expansion joints are seldom components required. Only joints that permit contraction of the Volume change deformations are of concern for their structure are needed to relieve the strains caused by implications on structural behaviour but of equal shrinkage, creep and temperature drop. Such joints concern is the influence of volume change movement are properly called contraction or control joints but on non-structural items such as sealants. are frequently incorrectly referred to as expansion Exterior sealants used to prevent water penetration joints. into the building must be able to accommodate It is desirable to have as few isolation joints as movements caused by volume changes. Sealants possible. Isolation joints are often located by subjected to volume change movements, either ‘rule-of-thumb’ methods without considering the horizontally or vertically at building corners, at structural framing method. The purpose of adjacent non-precast construction or be at windows not having similar movements must given special Clause 5.5.9 is to present guidelines for determining if joints are required and, if so, their spacing and consideration. width. As the height and length of a building increase, the Jürgen Ruth5.16 notes that while the basic concept of cumulative movements at the top or ends of the joints may be correct, the intended result is often not structure increase. The movements of exterior walls achieved in practice. He then suggests two strategies can affect the interior partitions on upper floors for avoiding joints, viz: resulting in distress or cracking of the partitions. Non-structural components within the building • Limit the restraint forces and deformations to interior must be detailed to allow for volume change acceptably low values movements of exterior precast structural walls. • Design the building to withstand the projected deformations and forces. 5.5.7 Volume change effects in In terms of deformations caused by shrinkage and moment-resisting frames creep, suggestions are made regarding selecting The restraint of volume changes in moment-resisting appropriate materials and construction practices, eg frames causes tension in the horizontal members timing placement to minimise temperature rise, and (beams) and deflections and moments in the structural layout. columns. The magnitude of these tensions, moments and deflections is dependent on the distance from 5.5.9 Spacing and width of isolation the centre of stiffness of the frame. It is also affected joints by the degree of fixity of the column base. Isolation joints are required between separate Since the shortening takes place gradually over a structures even where they form a single building, period of time, the effect of the shortening on the eg when parts are supported on discrete footing shears and moments of the supports is lessened systems. They are also required where it is desired because of creep and micro-cracking of the element to avoid structural plan irregularities and to separate and its support. low-rise from high-rise portions of a structure to The degree of fixity used in the volume change give better structural behaviour, including seismic analysis should be consistent with that used in the behaviour, Figure 5.13. analysis of the column for other loadings, and the determination of slenderness effects.
There is a wide divergence of opinion concerning the spacing of isolation joints. Typical practice in concrete structures, reinforced and prestressed, is to provide isolation joints at distances between 45 and 90 m. However, reinforced concrete buildings exceeding these limits have performed well without isolation joints. Recommended joint spacings for precast concrete buildings are generally based on local 5.17. experience, or those given in the CPCI Manual These latter recommendations should be reviewed
5.6.1 Basis of action
Horizontal actions from wind or ear thquake are usually transmitted to shear walls or momentresisting frames through the floors and roof acting
before they are adopted for a given structure in Australia because of the difference between the types of connections used, the column stiffness in simple-span structures, the relative stiffness between beams and columns in framed structures, and the environmental conditions, eg weather exposure. Nonair conditioned structures such as parking stations, are subjected to greater temperature changes than occupied structures, so lesser distances between isolation joints are warranted. The connection design methods inChapter 7, Connections and Fixings, can aid in determining the spacing of isolation joints. The width of the joint can be calculated using a coefficient of thermal expansion of 10 x 10-6 mm/(mm °C) for normal-density concrete.
as horizontal diaphragms in both insitu concrete and precast floors. Floors and roofs incorporating precast units, including those that do not have a topping screed, can act as horizontal diaphragms. (The design of precast units to carry floor or roof loads is covered in Chapter 6, Design of Elements.) The PCI Design Handbook5.18 notes that in many precast structures, the configuration and behaviour of the diaphragm is simple with rectangular floors or roofs, spanning between precast frames or walls. These provide connectivity and lateral load distribution and can easily be modelled as a deep horizontal beam. However, in some unusual cases, the PCI Design Handbook suggests the features of the structure may create conditions that are much more complex. The features may include excessive horizontal spans
Figure 5.13 Locations of Isolation Joints
between the vertical elements of the lateral-forceresisting system, large openings or discontinuities, large torsion effects from the eccentricity of the lateral force with respect to the centre of stiffness, or lateral transfer requirements due to vertical discontinuities and flexible diaphragms. Where the diaphragm can be analysed by considering the floor or roof as a horizontal beam, then the shear walls or structural frames form the supports for it and the lateral actions are transmitted to them. As in a beam, tension and compression are induced in the chords or flanges (as shown in Figure 5.14) and the perimeter frame must be capable of carrying the induced actions. When precast concrete floor or roof elements spanning parallel to the supporting shear walls or frames are used for the diaphragm, the shear in the diaphragm beam must be transferred between adjacent precast floor or roof elements and to the supporting structure. The web shear must also be transferred to the chord elements. Thus, the design of a diaphragm is essentially a connection design problem. Note, however, that the floor elements can span in the other direction and the floor will still act as a diaphragm.
Most major texts (eg Guidelines for the Use of Structural Precast Concrete in Buildings5.9, Multi-Storey Precast Concrete Framed Structures5.19 and the PCI Design Handbook5.18) on the topic of diaphragm
action of precast floors note the paucity of experimental data and therefore caution designers to adopt conservative values for shear resistance. However, they confirm that unscreeded floors can be used as diaphragms. Where earthquake action is a major consideration, special attention needs to be given to the robustness of the system and details. This includes checking that vertical support for the floor elements, resulting in the collapse of the floor, is not lost due to the
Figure 5.14 Analogous Beam Design of a Rigid Diaphragm
elongation of the supporting beams at plastic hinges. In these situations, reference toGuidelines for the Use of Structural Precast Concrete in Buildings5.9 and the PCI Design Handbook5.18 is recommended.
5.6.2
Rigid and flexible diaphragms
Building structures generally in the past have been designed using the assumption that the floor systems serve as rigid diaphragms between the vertical elements of the lateral-force-resisting system. A diaphragm is classified as rigid if it can distribute the horizontal forces to the vertical lateral-load-resisting elements in proportion to their relative stiffness. Close examination of the effective properties of diaphragms coupled with long-span applications suggest that precast diaphragms in these circumstances may in fact be flexible.
5.6.3
While seismicity in Australia usually not warrant designers considering the fullwill range of options for diaphragms, designers should be aware of the alternatives that they might need to consider when designing diaphragms in special or unusual circumstances. Designers should refer to thePCI Design Handbook5.18 for a full discussion on the subject. Shear transfer between elements
In floors or roofs without composite topping, the shear transfer between elements is accomplished either by grout keys or by welding between adjacent beam flanges. Such floors are not common in Australia. Eliott5.19 recommends that the average shear stress at the interface between units should not exceed 0.23 MPa at the ultimate limit state. The PCI Design Handbook5.18 recommends a shear stress of about 0.55 MPa when using grout keys which is less conservative. This shear stress should be calculated using a section depth 30 mm less than the overall precast concrete depth to allow for the fact that the bottom of the joint does not fill with grout. (It also takes account of the fact that differential camber between adjacent units may reduce the joint depth.) If this value is exceeded, the shear force should be carried by reinforcement placed across the ends of the units (see Figure 5.14).
When untopped elements are used as diaphragms, opening of the joints between adjacent floor elements must be prevented by surrounding the diaphragm with confining concrete beams. In floors or roofs with a composite topping, the topping itself can act as the diaphragm, if it is adequately reinforced. Reinforcement requirements can be determined by shear-friction. Weld plates/bars may be analysed as illustrated in Figure 5.15, which shows two examples of many satisfactory details. Designers should note that the satisfactory nature of a given detail used by a precast concrete manufacturer may be demonstrated by its record in service. It should be noted that the connections between elements often serve functions in addition to the transfer of shear for lateral loads. For example, weld plates in flanged elements are often used to adjust differential camber. Grout keys may be called upon to distribute concentrated loads. Appropriate detailing and care in connection design is necessary to ensure that diaphragm forces can be transferred to the shear walls. Connections that transfer shear from the diaphragm to the shear walls or other lateral-force-resisting systems should be analysed in the same manner as the connections between other precast members. For rigid diaphragms, the reaction forces will be determined from the storey shear with consideration of the maximum effects of torsion in the plane. Figure 5.15 Typical Flange Welded Connector Details for Untopped Floors or Roofs
5.6.4
Chord forces
Chord forces are calculated as shown inFigure 5.14. For floors and roofs with intermediate supports as shown, the shear force is carried across the intermediate beam with weld plates or bars in grout keys. The connection needs to be designed for bending and shear from the diaphragm action. The chord forces in perimeter frames and intermediate beams should be derived, based on strut-and-tie action, as in deep beams. The coupling bars holding the floor to the perimeter and/or intermediate beams are designed on the basis of shear friction. In flanged deck elements, the chord tension at the perimeter of the building is usually transferred between elements by using the same type of connection as that used for shear transfer (see Figure 5.15). When forces are high, such as in design for earthquake, transverse reinforcing bars (ie across the unit) may be placed in the flange and attached to the connection device by welding or by lapping with the connection anchorage bars. In bearing-wall and shear-wall buildings, perimeter reinforcement is required for structural integrity.
The portion of the total lateral force which each wall resists depends on the code requirements, the bending and shear resistance of the wall, the way the floors behave, and the characteristics of the foundation. It is common practice to assume that floors act as rigid elements for loads in the plane of the floor, and that the deformations of the footings and soil can be neglected. Thus, for most structures, lateral loads are distributed to each shear wall in proportion to its rigidity. Rigidity, r, is defined as: r = /D where: D = the sum of flexure and shear deflections For a structure with rectangular shear walls of the same material, flexural deflections can be neglected when the wall height-to-length ratio is less than about 0.3. The rigidity of the element is then directly proportional to its web cross-sectional area. When the wall height-to-length ratio is greater than about 3.0, shear deflections can be neglected, and the rigidity is proportional to the moment of inertia (plan dimensions). When the height-to-length ratio is between 0.3 and 3.0, an equivalent moment of inertia, Ieq, can be derived for simplifying the calculation of wall rigidity. eq I approximates the
moment of inertia that would result in a flexural deflection equal to the combined flexural and shear deflections of the wall.Figure 5.16 compares the deflections and Ieq for several load and restraint conditions. Lateral loads are distributed to each shear wall in proportion to its rigidity. It is usually considered sufficient to design for horizontal actions in only two orthogonal directions. It is important to remember that wind and earthquake actions will be such that the centre of rigidity (shear centre) of the building in the direction being considered will generally not match the line of action of wind or earthquake. This results in torsion and forces in the walls in both orthogonal directions even when the action is in one direction. In the case of earthquake actions, an extra torsion effect must also be considered. When the shear walls are symmetrical with respect to the centre of load application, the force resisted by any shear wall is given by: Fi = F ri /Sr where: F i = force resisted by an individual shear wall, i F = total force to be resisted by all shear walls ri = rigidity of wall, i
Sr = sum of the rigidities of
all shear walls If the floor is considered a rigid element, it will move or translate in a direction parallel to the applied load theoretically by an amount related to the flexural and shear rigidity of the participating shear walls, see Figure 5.17(a). If the centre of rigidity (shear centre) is not coincident with the line of action of the applied loads, the floor will tend to rotate about the
Figure 5.16 Shear Wall Deflections and Equivalent Moment of Inertia
I
I
I
I
I
I
I
I
I
I
I
Figure 5.17 Translation and Rotation of Rigid Floors
centre of rigidity, introducing additional forces, see Figure 5.17(a) and (b).The load on each shear wall will therefore be determined by combining the effects produced by rigid body translation and rotation, see AS 1170.4. A shear wall need not consist of a single element. It can be composed of independent units such as hollowcore units or other precast cladding panels or shear walls. If such units have adequate shear ties between them, they can be designed to act as a single unit, greatly increasing their shear resistance. Connecting the units can, however, result in a buildup of volume-change restraint forces. It is usually desirable to connect only as many units as necessary, near mid-length of the wall, to resist the overturning moment and thus minimise the volume-change restraint forces.
Connection of rectangular wallunits to form ‘T’ or ‘L’ shaped walls will increase their flexural rigidity, but have little effect on shear rigidity.The effective flange width that can be assumed for such walls is illustrated in Figure 5.18. Figure 5.19 shows two examples of coupled shear walls. The effect of coupling two walls is to increase the stiffness by transfer of shear through the coupling.The wall curvatures are altered from that of a cantilever because of the frame action developed.Figure 5.20 shows how the deflected shapes differ in response to lateral actions. It is important to emphasise the need to detail connections so that they can transfer the actions. Figure 5.19 Coupled Shear Walls
Figure 5.18 Effective Width of Walls Perpendicular to Shear Walls
Figure 5.20
Response to Lateral Actions
Single-storey and some low-rise buildings without shear walls may rely on the fixity of the column base to resist lateral loads. The ability of a spread footing to resist moments caused by lateral loads is dependent on the rotational characteristics of the base. The total rotation of the column base is a function of rotation between the footing and soil, bending in the base plate, and elongation of the anchor bolts, as shown inFigure 5.21. Because of the importance of this detail, care is needed when designing this connection. Figure 5.21 Assumptions Used in Derivation of Rotational Coefficients for Column Bases
The total rotation of the base is: qb = qf + qbp + qab If the axial load is large enough so that there is no tension in the anchor bolts, qbp and qab are zero, and: qb = qf Rotational characteristics can be expressed in terms of flexibility or stiffness coefficients: f= g M = M/K where: M = applied moment = Pe e = eccentricity of the applied load, P g = flexibility coefficient = gf + gbp + gab K = stiffness coefficient = /g If the axial load is large enough so that there is no tension in the anchor bolts, qbp and qab are zero, and: g = gf
The value of the rotation of a footing for a given project due to footing-soil interaction is outside the scope of this handbook and advice should be obtained from a geotechnical engineer. The of usecast-in of chemical or mechanical anchors in lieu bolts without load testing is not recommended because of the difficulty of knowing if full anchorage has been achieved. In any case chemical anchors are not permitted for erection by AS 38505.8 without load testing. As an alternative to the above,Clause 2.2.4 covers the option for developing base moment connections using dowel bars grouted in ducts, but the columns have to be temporally braced in two directions during erection until the grouting has been completed.
all the panels on the perimeter of the building will participate in carrying the applied actions. Rafters are bolted to the panels while eaves ties connect individual panels at the roof level. These provide connection points for bracing trusses in the roof plane that distribute the lateral actions. The base 5.9.1 Single-storey industrial building connections have to be able to transmit the induced Single-storey industrial and commercial buildings actions to the footings, eg by dowels into the footings require floor space with large column-free areas. Fire- and reinforcement tying into the floor slabs. resistant barriers, with ratings as set out in the BCA, Designers should note that the first option, the portal are required between tenancies and at the external walls. These requirements can be met economically using a combination of precast panels and steel-frame structure. There are two basic approaches to the design of this type of building: • A structural steel portal frame clad with precast concrete wall panels • Loadbearing precast concrete wall panels with a braced, steel-rafter roof. In the first option, the portal frame has to be designed to carry the applied actions as the wall panels act only as cladding (providing fire separation, weather protection and resisting wind actions). Wall panels may be used in either of two configurations: vertical and horizontal. In the vertical configuration the panels span from the footing to an eaves/wind beam. Generally, the base of the panel is assumed to be ‘pinned’. Usually it is restrained by a short, grouted dowel. The dowel holds the panel in position during erection and prevents lateral displacement during the life of the building. Steel clips are used to connect the panel to the eaves/wind beam. At least two connections are required at both the top and bottom. These have to be designed to carry the applied wind and ear thquake actions and also give the required behaviour in fire. Horizontal panels span between portal frames. The lowest panel is seated on the column footings. The upper panels are stacked on and are supported by the lower panels. All panels need to be restrained by the columns of the portal. The bottom panel will require restraint at the top and bottom edges, whereas upper panels will require restraint only at upper edges – provided that panels are tongued and grooved at the mating edges. The restraint fixing is usually a clip designed to carry wind action. The second option is a loadbearing panel building. This is a box-type building and utilises a stiffened roof structure to transmit lateral loads to transverse walls and thence to the footings. The panels in each wall may also suppor t intermediate floors. A number of configurations are possible. Panel size should be maximised as discussed in Clause 5.2 taking into account transport considerations. Usually,
frame with cladding panels, while not being the most cost-effective solution in material costs does allow future expansion and easy alteration compared to a box-type building. Panels usually do not act as bracing or shear walls and are clipped to the steel frame. When precast wall panels are used as loadbearing walls then it is much more difficult to alter the building in the future as the walls are shear walls and carry vertical and horizontal actions. In addition, while the panels are temporally braced and until the roof steelwork is tied to the panels and completed, no construction work can take place in the area of bracing. Future demolition of such buildings will also require careful consideration as all the wall panels will need to be re-braced, the roof removed and the panels then supported by a crane while the bottom connections are cut out. Only then can the wall panels be laid down for breaking up or removal. For both options, the controlling lateral actions generally will be the wind action but other actions such as earthquake and earth pressure must be checked. Note that earthquake considerations may affect the connection and joint design. Vertical actions on wall panels will include roof and floor actions and self-weight and, in industrial buildings, possibly crane loads. Wind actions are specified in AS/NZS 1170.25.20. The worst cases of internal pressure and external suction have to be considered and combined with other load effects. Peak pressures at eaves and ridges may control fixing design. Frequently, handling considerations will control the design of panels (see Chapter 6, Design of Elements). For the analysis of a one-storey industrial building for typical permanent, imposed and wind actions and design of the wall panels, seeExample 5.1a One-storey Building with Hollowcore Cladding to Portal Frame and Example 5.1b One-storey Building with Loadbearing Panels and Braced Roof.
5.9.2
Four-storey building
The lateral stability of the structure can be provided by shear walls, the moment-resisting capacity of the column bases, a beam-column frame, or a combination of all systems. When moment connections between beams and columns are required to resist lateral actions, it is important that the amount of beam-column moment framing is kept to a minimum and that it is located centrally so as to reduce volume-change effects. In addition, when possible, in order to reduce the size and capacity of the connections, the moment connection should be made after most of the permanent actions have been applied. This requires careful detailing, specification of the construction process, and inspection. If this is possible, the moment connections need only resist the negative moments from imposed actions, lateral actions and volume changes, and will be less complex and costly. See Example 5.2 Four-storey Building for analysis and design of a bearing wall structure for wind actions in the North-South direction.
Example 5.1 Introduction
Industrial buildings are typically portal frames clad with hollowcore or flat panels or they can have roof rafters connected directly to loadbearing panels with stability to lateral loads provided by roof bracing. The following examples will cover both arrangements for an industrial building of the overall nominal sizes and layout shown below.
The following may be adopted in the absence of other means of analysis. External pressure coefficients (cp,e ) h ! 25 m
Internal pressure coefficients (cp,i ) Depending on permeability , openings in walls, wether they are a dominant opening and wind direction, the following ranges can occur. Suction: Pressure:
- 0.3 to + 1.0
+ 0.7 to - 0.2 For the following examples, adopt cpi = + 0.2
Example 5.1a One-storey Building with Hollowcore Cladding to Portal Frame
One-storey industrial building with portal frames and hollowcore cladding panels, as shown below.
Analyse wind actions on the cladding panels and design fixings to suit.
Wind loading
AS/NZS 1170.2
Mz.cat = 0.83 M s = 1.0 M t = 1.0 M d = 1.0 z
AS/NZS 1170.2, Table 5.6
Local pressure factors Loaded area, a = 0.2 x 22 = 4.4 m Negative pressure (suction): Positive pressure:
Case SA2 Case WA1
k k
t
= 2.0 on 0.25a2
t
= 1.25 on 0.25a2
0.25a 2 = 0.5a = 2.2 m Maximum wind actions on cladding
AS/NZS 1170.2, Table 5.2
External negative pressure (suction) = - 0.65 Internal positive pressure = + 0.2 Wind pressure = 0.85 x 0.84 = 0.71 kPa Local negative (suction) factor = 2.0 acting on 2.2 m x 2.2 m Local pressure = 2 x 0.71 = 1.42 kPa External positive pressure = + 0.7 Internal negitive pressure (suction) = - 0.2 Wind pressure = 0.9 x 0.84 = 0.76 kPa Local pressure factor = 1.25 acting on 2.2 m x 2.2 m Local pressure = 1.25 x 0.71 = 0.89 kPa
cont…
Design of cladding panel fixings (long side)
NOTE:
Panels must be prevented from falling
For panels with tongue and groove
outwards during a fire in the building.
edge detail, restraint clips of every
third panel are welded to eaves tie beam. For panels with square edge detail, restraint clips of every panel are welded
to eaves tie beam.
R*A = 1.2
0.71 x 8 x 4
7
+
0.71 x 2.2 x 6.9 7
= 4.78 kN ultimate
R*B = 1.2 [(1.42 x 2.2) + (0.71 x 5.8)] - 4.18 = 3.91 kN ultimate
Outward load on clip (two per panel) =
4.78
Ultimate load on bolt and ferrule = 2.39 x
2
= 2.39 kN
175 100
= 4.2 kN
Ferrule capacity in tension (Clause 7.11.4, this Handbook )
Nbc = 0.6 x 1.0 x 1.0 x 1.25 x 22.4 = 16.8 kN > 4.78
OK
Bending moment on clip plate
b = 75 - 22 dia hole = 53 mm Required plastic modulus:
(Assume f
y
= 250 MPa)
For rectangular section, Sx = Required d =
4 x 1096
53
b d2 4
= 9 mm
Use 75-mm-wide x 12-mm-thick plate
Shear to dowel = 3.91/2 = 1.85 kN ultimate
Example 5.1b One-storey Building with Loadbearing Panels and Braced Roof
One-storey industrial building, with loadbearing wall panels, as shown below.
Analyse structure and design loadbearing wall panels including connections (assume 2-hr fire rating).
See Example 5.1 Introduction
Wind actions
= 0.84 p kPa
0.84 = p kPac
Total wind action on longitudinal walls = 0.84(0.7 + 0.5) x = 220 kN at eaves
V*t
50.4[7.5 2/(2 x 6.5)]
= 110 kN per end wall
p,e
Total wind action on end walls V*t = 0.84(0.7 + 0.285) x 21.9[7.5 2/(2 x 6.5)] = 78.4 kN at eaves = 39.2 kN per longitudinal wall
Design of end walls Wind action on longitudinal walls is transmitted by tie beam to each end-wall panel through bolted connections and cast-in ferrules Number of panels in end wall = 21.6/3.6 = 6
cont…
Inplane wind load per panel to end wall = 110/6 = 18.3 kN/panel Wind uplift on purlins at eaves beam = 0.9 x 0.84 x 3.6 x 8.4/2 = 11.4 kN
Wall, W = 3.6 x 0.15 x 8 x 24(kN/m 3) = 103.7 kN
Permanent load factor = 0.8 or 1.2
W* = 0.8 x 103.7 = 83.0 kN or
*
W = 1.2 x 103.7 = 124.4 kN
Taking moments about bottom corner of panel M = 3.6/2(83.0 - 11.4) - (18.3 x 7.5) = -8.4 kN
Tension in dowel = 8.4/3.4 = 2.5 kN
N20 dowel in tension (footing connection) or
in shear (floor slab connection) would be OK, suitably anchored to resist applied actions
Note: AS 3850 does not allow friction, so provide restraint in Option 2 below
Design of longitudinal walls Longitudinal wall panels carry rafter loading and inplane wind actions from end walls Adopt roof permanent action = 0.1 kPa, 4.5 kN imposed action at mid span of rafter Permanent action of roof = 0.1 x 8.4 = 0.84 kN/m Permanent action of rafter = 0.6 kN/m Rafter reaction = 21.6/2(0.84 + 0.6) = 15.55 kN Roof imposed action = 0.25 kPa = 0.25 x 8.4 = 2.1 kN/m Rafter reaction (at eave) = 21.6/2 x 2.1 + 4.5/2 = 24.93 kN
(Note: 4.5 kN is an additional imposed action) cont…
Total wind action from end walls = 39.2 kN/longitudinal wall Uplift at first rafter support due to longitudinal wind (rafters 21.6 span at 8.4 centres) P* = 8.4 x
21.6 2
x 0.84
0.9 + 0.5 2
= 53.3 kN
Uplift due to cross wind P* = 8.4 x
21.6 2
x 0.84
0.9 + 0.5 + 0.3 3
+ 0.2 = 58.4 kN
Loads on panels Assume roof wind actions taken by panels supporting roof rafters and end wall panels = 7 per side CASE 1 (P A + IA) :
[PA = Permanent Action; IA = Imposed Action; WA = Wind Action]
e = 150/2 + 150/2 + 20 = 170 mm (allows 20 mm tolerance)
Rafter PA = 1.2 x 15.55 = 18.66 kN Rafter IA = 1.5 x 24.93 =37.4 kN
Rafter PA + IA = 18.66 + 37.4 = 56.06 kN
Panel PA = 1.2 x 103.7 = 124.4 kN
At mid height N * = 56.06 + 124.4/2 = 118.26 kN M* = 56.06 x 0.17 = 9.53 kN.m
CASE 2 (PA + WA upwards, on longitudinal walls):
Rafter PA = 0.8 x 15.55 = 12.44 kN Rafter WA = 58.4 kN
F* = 12.44 - 58.4 = -45.96 kN Panel PA = 0.8 x 103.7 = 83 kN
Net reaction = - 45.96 + 83 = 37.0 kN downward Wind pressure at panel adjacent to roller door (1.5 panels) Wind pressure = 0.84(0.7 + 0.2) x 3.6 x 1.5 = 4.08 kN/m
At mid height N * = (1.2 x 103.7 x 1/2) - 45.96 = 16.26 kN
M* = 2
45.96 x 0.17 8
+
4.08 x 7 2 2x2
-
4.08 x 1 2
= 27.88 kN.m
CASE 3 (PA + WA upwards, on end walls):
Rafter PA = 0.8 x 15.55 = 12.44 kN Rafter WA = 53.3 kN
F* = 12.44 - 53.3 = -40.86 kN Panel PA = 0.8 x 103.7 = 83 kN
Net reaction = - 40.86 + 83 = 42.14 kN downward Wind pressure at panel adjacent to roller door (1.5 panels) Wind pressure = 0.84(0.65 + 0.2) x 3.6 x 1.5 = 3.86 kN/m
At mid height N* = (1.2 x 103.7 x 1/2) - 40.86 = 21.36 kN M* = 2
40.86 x 0.17 8
+
3.86 x 7 2 2x2
-
3.86 x 1 2
= 19.21 kN.m
cont…
CASE 3 continued: Horizontal force on longitudinal walls due to wind on end walls, V *t (Assume spread over 7 panels)
H* = 39.2/7 = 5.6 kN per panel
N* = 21.36 kN at mid height (see previous page)
Panel design Use CASE 3 for inplane shear
At mid height, panel Z = 150 x 3600 2/6 = 324 x 106 mm3
Check stress at tension edge of panel
=
AS 3600, Clause 11.6.3(b)
21360 3600 x 150
-
5600 x 7000 324 x 10
6
= - 0.08 MPa
Nominal tension only
AS 3600, Section 12; this Handbook, Clause 7.10.3
Using strut-and-tie:
Tensile force in tie =
5600 x 7000 3200
Area of tie reinforcement = 0.7 x 500
Compression force in strut =
5600 x 7697 3200
= 12 250 N
12 250
= 13 470 N
= 35 mm2
OK by inspection for strut 150 x 300 mm
For forces perpendicular to wall and Hw/tw = 7000/150 = 47 < 50
AS 3600, Clause 11.1(b)
then, 0.03 f' c Ag = 648 kN
or SL92 central Panel connections (bottom connections as for end walls)
Example 5.2 Four-storey Building
Typical four-storey residential building as shown below.
Unfactored loads are assumed as follows PERMANENT ACTIONS
Walls: 200 mm thick
Roofing, mechanical etc Hollowcore planks/60 topping TOTAL WIND ACTIONS
4.80 kN/m
2
Typical floor:
Roof :
2 0.50 kN/m Partitions
2
0.50 kN/m
4.64 kN/m2 Hollowcoreplank/60 toppings 4.64 kN/m 2 5.14 kN/m TOTAL 5.14 kN/m
0 to 9 m elevation
1.20 kN/m
2
Over 9 m elevation
2 2
1.40 kN/m 2
cont…
Analyse and design the structure for wind in the North-South direction
Design of wall elements For wind in the transverse (North-South) direction, normal practice for this structure would be to conservatively neglect the resistance provided by stairs, lifts and longitudinal walls. However, since the cross walls are rigidly connected (see 'Wall Panel to Wall Panel Connection' previous), the
flanged walls and lift units are considered in this solution. (Would also apply to earthquake actions.)
Elements resisting North-South wind are as follows:
Properties of resisting elements: A sample calculation of properties is given for element 3
The effective flange projection of the longitudinal wall, bf, is the smaller of 12t or H/10 ( see Figure 5.18 )
bf= 12t = 12 x 200 = 2400
or
bf = H/10 = 10 400/10 = 1040
governs
Sectional properties
Aw = 8200 x 200 = 1640 x 103 mm2
Af = (1040 + 1040) x 200 = 416 x 10 3 mm2
yb =
(1 640 000 x 4100) + (416 000 x 8100) 1 640 000 + 416 000
= 4910 mm
yt = 8200 - 4910 = 3290 mm
Ixx =
200 x 82003 12
+ 1 640 000(4100 - 4910) 2 + 416 000(8100 - 4910)2 = 14, 500 x 109 mm4
cont…
The equivalent stiffness is calculated using the Case 1 multi-storey formula ( see Figure 5.16)
Ieq =
Ixx 1 + 13.4 I Aw
xx h2
=
14 500 x 10 9
9
1 + 13.4 x 14 500 x 10 9 1640 x
103
x
= 780 x 10 mm
4
2600 2
Since 3, E, P and h are all constants when comparing stiffness, kr varies directly with Ieq The distribution of wind action to element 3, based on its relative stiffness is:
Summary of properties of resisting elements
Element number Element dimensions
(All walls 200 thick)
480
1640
1640
734
12 144
14 500
182
775
780
2
4
8
364
3100
6240
2.0
8.0
8.0
49.0
98.0
196.0
8.918
75.950
152.880
cont…
Permanent actions and transverse-wind actions on wall element 3:
Concentrated actions (P1, P2, P3 and PR) from the 200-wide corridor lintels have been conservatively neglected to simplfy calculations. Hence, uniform unfactored permanent actions on walls at each level, is: W1 = W2 = W3 = WR = 5.14 x 9.0 = 46.26 kN/m Wall weight = 4.8(2.6 - 0.26) = 11.23 kN/m Based on relative stiffness, 8.0% of the total wind action is to be carried by this element . Wind action, w l = 49 x 1.2 x 0.08 = 4.7 kN/m Wind action, w
u
= 49 x 1.4 x 0.08 = 5.5 kN/m
Check overturning of shear wall (permanent action), resisting moment about toe of wall (see above) 8.2
Mo = 8.2 x
2
(4 x 46.26) + (4 x 11.23) = 7731 kN.m
Factor of safety =
Mo Mwind
=
7731 265
= 29.1
> 2.0
OK
Check for tension using factored loads Permanent action on wall, P = 8.2 x 4(42.26 + 11.23) = 1885.6 kN Maximum wind moment at base, M wind = 265 kN.m
f=
0.85 P L
-
1.5 Mwind L2/6
=
0.85 x 1885.6 1.5 x 265 = 195.5 - 35.5 = + 160 kN/m (compression) 8.2 8.22/6
No tension connections are required between the panels and the footing. Thus the building is stable under wind loads in the North-South direction. When tension exists, see Example 5.1b . NOTE: Other design considerations may dictate the use of minimum vertical ties and it is recommended minimum tie-downs for erection and earthquake loads be provided.
cont…
Design of floor diaphragm Diaphragm analysis for wind from North or South This represents the temporary
( see Clause 5.6, this Handbook, for general basis )
condition when joints have
been grouted but topping
slab has not been placed (Assume rigid diaphragm)
Factored wind action for a typical floor
wf = 1.5 x 1.2 x 2.6 = 4.68 kN/m For wind from the North or South Vf R =
4.68 x 9 2
= 21 kN
Mf 4.68 x 92 Cf = T f = = = 2.82 kN L 8 x 16.8
PA PA Total PA
(as recommended in Clause 7.7.4, this Handbook)
The chord tension, T f, is resisted by the tensile resistance of the concrete of the floor slab. However, provide a tension tie, say N16 bar, for ductility. The shear key between slabs must also resist approximately the same force. Assume area of exterior hollowcore plank = 150 840 mm 2 Grout key = 75 mm deep Concrete f' c = 32 MPa
Shear resistance of grouted key
(Clause 5.6.3, this Handbook)
Average shear stress at the interface = 0.23 MPa Vr = 0.23A = 0.23 x 9000 x 75/10 3= 155 kN
> 2.82 kN
OK
NOTE: In this example, only the resistance to wind actions has been analysed. Any other required actions, such as earthquake and 'abnormal' actions must be reviewed for a complete analysis.
5.14 Teicher M A, Trenerry J M and Hirst M J S, ‘Thermal loading of concrete walls’, Proceedings: Concrete 85 Conference, Institution of Engineers, Australia, Brisbane, 1985, pp10–14.
5.1 AS 1170.4 Structural design actions, Part 4: Earthquake actions in Australia, Standards Australia, 2007. 5.2 Hughes and Crisp, 'Structural Precast Concrete in Melbourne, Australia'Concrete 07 Proceedings, Concrete Institute of Australia, 2007. 5.3 AS 3610 Formwork for concrete, Standards Australia, 1985. 5.4 AS/NZS 1170 [Set] Structural design actions, Standards Australia, 2007. 5.5 AS/NZS 1170.0 Structural design actions, Part 0: General principles, Standards Australia, 2002. 5.6 Building Code of Australia, Australian Building Codes Board and CCH Australia, Sydney, 2007. 5.7 AS 3600 Concrete structures, Standards Australia, 2009. 5.8 AS 3850 Tilt-up concrete construction, Standards Australia, 2003. 5.9 Guidelines for the Use of Structural Precast Concrete in Buildings, New Zealand Concrete Society, New Zealand National Society for Earthquake Engineering and Centre for Advanced Engineering, The University of Canterbury, Christchurch, New Zealand, 1992. 5.10 Mc Donald D, Roper H, and Samarin A, ‘Prediction accuracy of creep and shrinkage models for Australian concrete’, Proceedings: Fourteenth Australian Road Research Board Conference Vol. 7, 1988, pp 66–78.
5.11 AS 1481 SAA Prestressed concrete code [superseded] Standards Australia, 1978. 5.12 AS/NZS 1170.1 Structural design actions, Part 1: Permanent, imposed and other actions, Standards Australia, 2002. 5.13 Hirst M J S, ‘Design values for thermal loading of concrete roofs’ American Concrete Institute Journal November/December, 1984, pp 594–600.
5.15 Hirst M J S, ‘Thermal loading of concrete roofs’ Journal of Structural Engineering American Society of Civil Engineers, Vol. 110, No. 8,August, 1984, pp 1847–1860. 5.16 Ruth J, Movement joints: a necessary evil, or avoidable?, Large Concrete Buildings, edited by Rangan, B V and Warner, R F, Longman, UK, 1996. 5.17 Design Manual Precast and Prestressed Concrete, 4rd edition, Canadian Prestressed Concrete Institute, Ottawa, Ontario, 2007. 5.18 PCI Design Handbook, 6th Edition, Precast/ Prestressed Concrete Institute (PCI) Chicago, 2004. 5.19 Elliott K S, Multi-Storey Precast Concrete Framed Structures, Blackwall Science, UK, 1996. 5.20 AS/NZS 1170.2 Structural design actions, Part 2: Wind actions, Standards Australia, 2002.
What you will find in this Chapter •
Strength design principles that are of particular relevance to precast members, including prestress loss and strength at transfer, with worked examples.
•
Prestress anchorage design and development length.
•
Design for deflection and camber.
•
Design for vibration control.
•
Guidance on lateral distribution of concentrated loads between precast floor members.
•
Design considerations for handling, storage and transport of precast members, including lateral stability.
6.1 Definitions and Notation 6.1.1 Definitions 6.1.2 Notations 6.2 Introduction 6.3 Design Principles and Design of Elements 6.4 Flexure and Shear 6.4.1 Design procedure 6.4.2 Design procedure – strength at transfer 6.4.3 Design procedure – longitudinal shear 6.5 Prestress Loss, Development Length and Anchorage Zones 6.5.1 Loss of prestress 6.5.2 Development length for prestressing tendons 6.5.3 Design of anchorage zones and end blocks 6.6 Design for Serviceability Limit States – Deflection Control 6.6.1 General 6.6.2 Camber and deflection 6.6.3 Initial camber 6.6.4 Elastic deflection 6.6.5 Long-term camber and deflection 6.7 Design for Serviceability Limit States – Crack Control 6.8 Design for Serviceability Limit States – Vibration Control 6.8.1 General 6.8.2 Natural frequency of floor systems 6.8.3 Vibration due to walking 6.8.4 Vibration due to rhythmic activities
6.9 Design of Columns and Compression Elements 6.10 Specific Design Considerations for Precast Floors and Roofs 6.10.1 General 6.10.2 Distribution of concentrated loads 6.10.3 Openings 6.10.4 Cantilevers 6.10.5 Composite topping 6.11 Handling Considerations 6.11.1 Suction and impact factors 6.11.2 Flexure 6.11.3 Lateral stability 6.11.4 Storage 6.11.5 Transportation 6.11.6 Erection 6.12 References 6.13 Appendix 6A – Design Examples 6A.1 Design of a precast beam for strength at transfer 6A.2 Loss of prestress 6A.3 Debonding of strands 6A.4 Deflection of a prestressed beam 6A.5a Design of gymnasium floor for vibrations 6A.5b Design of stadium seating for vibrations 6A.6 Design of office floor for walking vibrations 6A.7 Load distribution for precast hollowcore floor 6A.8 Design for handling 6A.9 Lateral stability of a beam during handling and transport
6.1.2
Notation
The following notation is used in this chapter: Ag = the gross cross-sectional area of a member Apc = the gross area of the precast section 6.1.1
Definitions
The following definitions are used in this chapter. Where possible these agree with those in the relevant Australian Standard. Diaphragm A horizontal or nearly-horizontal system, including a horizontal bracing system, acting to transmit horizontal forces to the vertical elements resisting the horizontal forces. Pretensioning A method of prestressing in which the tendons are tensioned before the concrete is placed. Post-tensioning A method of prestressing in which the tendons are tensioned after the concrete has reached a predetermined strength.
Apb = the area of prestressing strand Ast = the area of non-tensioned reinforcement in tensile zone Asc = the area of non-tensioned reinforcement in compression zone As = the total area of reinforcement in cross-section Ast = the total area of fully-anchored reinforcement crossing the interface a
= the acceleration, or the width of bearing plate , or length of overhang
b
= the width of the cross-section
bf
= the width of the shear plane (mm)
bt
= the width of the tension zone
Cj
= a continuity factor for a vibrating member
ca
= applied overturning moment arm
cr
= resisting moment arm
d
= depth of primary prism
d
= the nominal diameter of the strand or tendon b
Ec
= the modulus of elasticity of the concrete at 28 days
Ecd = dynamic modulus of elasticity of concrete Ecj = the mean value of the modulus of elasticity of concrete at the relevant age Ep
= the modulus of elasticity of tendons
e
= eccentricity of the strand at the critical section
ei
= initial lateral eccentricity of the CG of beam (measured from supports)
FR
= the force resisting
FS
= factor of safety against cracking
FS’ = factor of safety against failure or roll-over fi
= the forcing frequency of the i th harmonic (i times the step frequency)
f ’c
= the characteristic compressive strength of concrete
fcmi = the mean insitu compressive strength of concrete at the relevant age fcp = the mean compressive strength of concrete at the time of stressing f'cf
= the flexural tensile strength of concrete
f'ct = the characteristic uniaxial strength of concrete, determined in accordance with Section 3 of AS 3600
fp
= the tensile strength of the tendon
fn
= the natural frequency of the element
fpe = the stress in the strand after all losses fpy = the yield stress of the strand fsy
= the yield stress of reinforcement
g
= the acceleration due to gravity
h
= the overall depth of the anchorage zone
hr
= the height of the roll axis of the vehicle above road
Ixx = the moment of inertia of the section Ko = sum of rotational spring constants of supports k
= a coefficient used in calculating vibrations
kcs = multiplier for calculating long-term deflection k4 = see Table 6.2 k5 = see Table 6.3 k6
= T/20 but not less than 1.0
L
= span or overall length
Ld = the development length L1 = length between supports Msw = moment due to mass of the element Msd = moment due to all sustained loads except the element mass Mlat = lateral bending moment at cracking P
= the tendon force
Pf
= the final tendon force
Pi
= the prestressing force immediately after transfer
Po = see Table 6.9 Pu
= the compressive failure load at transfer of prestress
R
= the design relaxation of the tendon
Rj
= the design relaxation of the tendon at a particular time
Rb = the basic relaxation of the tendon Ru = the ultimate strength of the section r
= radius of stability = Ko/W
S*
= the design action effect
T
= the average annual temperature in degrees Celsius
W
= total weight of beam
w
= weight per unit length of beam
wp = the weight of participants as an equivalent UDL over the floor span
Yr
= height of the roll axis above the CG of beam (adjusted for camber)
ybp = the distance of tendon centroid from bottom of cross-section Z
= total bursting force
Zo = theoretical lateral deflection of the CG of beam with full load applied laterally Z’o = Z o adjusted for cracked section at tilt angle under consideration zmax = horizontal distance from centre of vehicle to centre of dual tyres
a ai
= superelevation or tilt angle of support
b m
= modal damping ratio
= dynamic coefficient for the i th harmonic of the step or jumping frequency (Table 6.10) = longitudinal shear plane surface coefficient for reinforcement (Refer Table 8.4.3 of AS 3600)
kco = longitudinal shear plane surface coefficient for concrete (Refer Table 8.4.3 of AS 3600)
D
= the maximum deflection (in mm) of the floor structure under the mass weight supported.
ecc
= the strain due to concrete creep at tendon level (AS 3600 Section 3)
ecs
= the design shrinkage strain of the concrete (AS 3600 Section 3)
ecsb ecc
= the basic shrinkage strain of the concrete = the design creep factor, calculated in accordance with AS 3600 Section 3
f
= the strength reduction factor
qi
= initial roll angle of rigid beam = ei/Yr
qmax = tilt angle at which cracking begins (considering lateral bending) q’max = tilt angle at the maximum factor of safety against failure
r sci
= the density of the concrete
spi
= the stress in the tendon immediately after transfer
= the sustained stress in the concrete at the level of the centroid of the tendons
the final relaxation loss modified for creep Dsr = and shrinkage Dsri = the initial relaxation loss prior to transfer of prestress
Dse = the elastic loss Dss = the shrinkage loss Dsc = the creep loss
wt = the total weight supported by the floor structure expressed as a UDL Y
= height of the CG of beam above the roll axis (adjusted for camber)
This chapter provides a summary of procedures for the design of individual precast concrete members, covering reinforced, prestressed pretensioned and prestressed post-tensioned members. Unless
The design of a member or element of a building has to conform to the requirements of the Building Code of Australia6.1 and the principles of design as contained in AS 11706.2 and AS 36006.3. For bridges,
otherwise stated, in members this chapter is assumed that precast prestressed areit pretensioned not post-tensioned.
the elements areessence, designed accordance 6.36 . In AS 5100 theindesign of the with elements of a structure follow limit-state design principles and require that:
No attempt has been made in this chapter to differentiate between the design of architectural and structural members or between the design of loadbearing and non-loadbearing members, as the design approach is common to all cases, although non-loadbearing members carry only their own vertical load and sometimes lateral loads. The design of precast elements differs, however, from the design of insitu concrete elements in that one has to understand the construction process that forms the complete structure and design the precast elements accordingly. Precast design is not about taking an insitu concrete structure and breaking it up into small pieces (elements), then making the pieces in a factory, transpor ting the pieces to site, erecting and then joining the pieces together to form the final structure. Because of the erection process, precast elements will have two distinct design criteria (excluding temporary loads due to lifting and handling, etc). The first criteria is as a simply-supported element and the second criteria where the piece may be still a simply-supported member or it may be a composite or continuous member as part of the complete structure carrying a variety of loads. Further design guidance on specific structural building elements is referred to in Clauses 2.2.1 to 2.2.5, Chapter 2
•
actions for each limit state are determined;
•
the structure and its parts are analysed for the appropriate actions using the specified combinations of factored actions; and
•
the structural responses under the above actions do not exceed the appropriate member or section capacity.
For example, for the design for the strength ultimate limit state the design strength of the section shall not be less than the design action effect (derived from the combination of factored actions), ie fRu ! S* Loads and other actions and load combinations for a building are set out in AS 1170. It is specified in AS 3600 that, where applicable, the prestressing force, P, is to be included in any combination with a load factor of 1.0, except for the case at transfer when a value of 1.15 is to be used. Generally, section dimensions and properties are estimated and the member analysed for the applied actions. Choosing appropriate dimensions is a matter of experience and using general sizing rules such as span-to-depth ratio. The dimensions of the member are adjusted if the section is either under-strength or significantly over-strength, or if the serviceability or any other limit state is exceeded.
a combination of the two (Clause 5.3.3, Chapter 5). In multi-storey frames the stub-ends of the beam are usually cast integrally with their columns. Transfer of shear and control of rotation are prime considerations in detailing the joint.
6.4.1
Design procedure
The design provisions for flexure, transverse shear, and torsion of a precast element are given in AS 36006.3. Theory and design procedures are set 6.7, 6.33
out in texts or the NPCAA Hollowcore Floor – Technical Manual . The design of a prestressed member has particular aspects that must be taken into account and these are discussed below. Generally it is more economical to fully prestress a member than to use partial prestressing wherein a proportion of the tensile force is provided by normal reinforcement. Partial prestressing is used for serviceability reasons, such as reducing the creep hog of a beam subject to transient or partial live load; eg a bridge beam, or where the hog of a member with a high span-to-depth ratio must be limited. The presence of a substantial amount of reinforcement in the pre-compressed zone will reduce both the prestress loss and the cracking moment. Composite construction is an efficient use of precast and absorbs construction tolerances on site. A composite member is made up of a precast element with an insitu concrete compression zone. This increases the structural depth and the ultimate capacity. I-girders acting compositely with a bridge deck and topped hollowcore planks are common examples. The longitudinal shear at the interface must be investigated and reinforcement across the interface provided if necessar y. The decompression and cracking moments are used in the calculation of shear capacity and minimum reinforcement. Both have to take account of the proportionately greater reduction in prestress caused by the self-weight and insitu components acting on the precast section alone before it becomes composite. The composite section can be made to resist the weight of the insitu concrete by propping the precast member until the insitu concrete reaches design strength. Precast beams are usually designed as simply supported. Full beam-to-beam continuity can be achieved but detailing can be complex and may be uneconomical. It is mostly used where the continuity reinforcement can be placed in an insitu concrete topping independently of the precast element. A beam-shell structure is an example (Clause 2.2.1.7, Chapter 2). Placing a beam joint at or near a point of contraflexure in a framed structure can also achieve the effect of continuity while keeping the joint simple. The connection can be a halved joint (Clause 7.10.4, Chapter 7), a length of insitu concrete, or
Precast slabs, such as hollowcore units, can be simply supported for permanent action and continuous for imposed actions with the continuity reinforcement placed in an insitu concrete topping. The critical section for shear in shallow or slab members will often be in the transmission zone of pretensioned strand. Both the flexure-shear and the web-shear capacities are a function of the amount of prestress acting at the section. The reduced prestress in the transmission zone must be taken into account in determining the minimum shear capacity. The length required to develop the tensile capacity of the strand in flexure is much greater than the transmission length, see Clause 6.5.2. The possibility of cracks in this region at ultimate, which may affect bond length, should be checked, particularly for members that have debonded strands. 6.4.2
Design procedure – strength at transfer
At transfer of prestress to a precast element, the force in the tendons will be a maximum having been reduced only by elastic strain in the member and some relaxation in the tendons. The strength of the concrete is still developing at this stage and generally the only load acting on the element will be its selfweight. AS 3600 requires the strength of the section to be checked using a strength reduction factor, f, of 0.6 and load factors of 1.15 for the prestress and 1.15 or 0.9 for the permanent actions, depending on whether they diminish or add to the effect of the prestress. This requirement is deemed to be satisfied if the maximum compressive stress at transfer does not exceed 0.5 fcp for a rectangular stress distribution and 0.6f cp for a triangular distribution, where fcp is the mean concrete strength at transfer. The maximum tensile stress also needs to be checked. A suggested limit for this is the mean flexural tensile
strength, 0.84!fcp. It is good practice to provide nominal reinforcing in the maximum tensile zone, even ifstrength it is not required. The normal concrete at release is 35 MPa. This value of the strength reduction factor is the same as for columns and is considered to be too low for a diminishing force produced by bonded tendons where the stress distribution on the critical crosssection is essentially triangular. Experience suggests that a f factor of 0.75 is more realistic. Alternatively the actual compression stress at transfer can be limited to 60% of the strength of the concrete at release as specified in the Austroads Bridge Design Specification, clause 5.8.1.4 (b).
For a simple rectangular stress block centred on the prestress tendons and no reinforcement with gravity loads acting to reduce the effect of the initial prestress, Pi, the compressive failure load Pu is given by Warner et al(6.7) : Pu = 1.7 fcp b(ybp + M sw / Pu) Where: f cp = compressive strength of the concrete at transfer
b
= width of the element at the tendon location ybp = distance of tendon centroid from bottom of element Msw = moment due to gravity loads acting at the section
The concrete strength at transfer, fcp, in the above equation must satisfy the requirement that:
f Pu >
1.15 Pi
It is preferable to use a parallel strand profile for pretensioned members, particularly for long line work. This profile results in the maximum transfer stresses occurring near the ends of the member, at the end of the transmission length (60 diameter s for strand). Excessive stresses can be accommodated by adding reinforcement or by debonding some tendons for an appropriate distance. Debonding reduces the amount of prestress and raises its centroid. When this is used, the end section should be checked for reduced shear capacity. A check should also be carried out to ensure the available development length is sufficient to develop the required tensile capacity of the tendon at the critical location, taking account of the debonded length. For this reason, it is usually more practical to provide supplemental reinforcement in short, heavily-loaded elements than to use debonding. The centroid of the prestress can also be raised by hold-down restraints attached to the casting bed at one or two points along the member so that the centre of the strand group can be deflected upwards at each end while maintaining the required eccentricity at critical sections. It is preferable to use only one deflection point. This will place the critical design section at 0.4 x span under uniform loading. Members with two hold-down points should be checked for transfer capacity at the deflection points. The hold-downs are usually located symmetrically about the centre of the member, 0.3 x span apart. See Example 6A.1 (Appendix 6A) for Design of a precast beam for strength at transfer.
6.4.3
Design procedure – longitudinal shear
Composite construction is the combination of precast units and insitu concrete to form a single flexural entity. It requires the transfer of longitudinal shear across the interface between the precast and the insitu. The design procedure assumes a degree of roughness of the hardened surface that must be met in practice. Section 8 of AS 3600, sets out values for shear plane coefficients m and kco corresponding to degrees of roughness for use in the design. These follow the recommendations given by the FIP 6.27. Figure 6.1 illustrates m and kco values for typical shear-plane finishes of precast units. Smooth off-form surfaces, not shown in Figure 6.1, would have m and kco values of 0.6 and 0.1 respectively. There are two basic design cases: •
the insitu concrete is in uniform contact over the entire area in the form of a topping to the precast unit;
•
the insitu concrete is wider than the precast member so that the precast engages only a strip of the insitu slab.
In the first case, the longitudinal shear stress is low and no reinforcement is required across the interface. Hollowcore and single-tee floors are typical of this type of construction. Recommended minimum average thickness of the topping is 50 mm, with a minimum local value of 30 mm. The required cover to reinforcement may determine the topping thickness. The design interface capacity relies on bond and is given in Clause 8.4 in AS 3600 as:
ftu = f kco bf f ct’ ! min. of 0.2f'c or 10 MPa where: f = 0.7 tu = unit shear strength kco = 0.1, 0.2 or 0.4 depending on surface texture bf = width of shear interface (mm) f ct ’ = 0.36 Öf ’c In the second type of composite member, the shear stress at the interface is usually high and reinforcement is required across the interface. The insitu portion is often a slab spanning transversely with its thickness determined by that function. Pretensioned I-girder bridge decks are typical of this type of construction. The design interface capacity has a shear-friction component and a bond component.
ftu = fm
Asf fsy s bf !
+
gp bf
+ kco bf f ct ’
min. of 0.2f'c or 10 MPa
6.5.1
Loss of prestress
where, additionally:
m
= 0.6 or 0.9 depending on surface texture
Asf = area of fully-anchored interface reinforcement at spacing ‘s’ fsy = yield strength of interface reinforcement (! 500 MPa) gp = permanent distributed load, normal to shear interface, per unit length (N/mm) Sufficient anchorage of the reinforcement must be provided each side of the interface to develop the required stress in it, usually the yield stress. Figure 6.1 Examples of Values of Shear-Plane Surface Coefficients, Precast Units
m and kco, for Typical Finishes to
m 0.6 and k = 0.2
Methods for calculating prestress losses Two methods for calculating prestress losses can be used as appropriate for the particular situation: •
The AS 3600 Method where each component of loss is calculated separately.
•
The PCI Simplified Method, which is applicable only to pretensioned members.
The AS 3600 Method takes into account all the major variables affecting creep and shrinkage and includes the effect of non-prestressed reinforcement located in the tension zone of the element. In partiallyprestressed elements the presence of reinforcement significantly affects the losses and must not be neglected in the loss calculations.
The Simplified Method is an empirical equation that takes into account the level of concrete stress, type of prestress and volume-to-surface ratio. Prestress loss – AS 3600 method The loss of force in a prestressing tendon
m = 0.6 and k = 0.2
m = 0.9 and k = 0.4
commences from the time it is anchored at jacking and continues for the life of the member. The loss is rapid at first, diminishing exponentially with time. Total loss of prestress is typically 18 to 28% of the initial jacking load for a pretensioned member, about 250 to 400 MPa. This loss is due to shortening of the concrete at the level of the tendons, relaxation of the tendons, and any external factors which reduce the total initial force before it is applied to the concrete. Section 3 of AS 3600 identifies the following sources of loss of prestress. Immediate losses: •
Elastic shortening of concrete, net of self-weight effects.
•
The relaxation of tendons prior to transfer.
Deferred losses: •
m = 0.9 and k = 0.4
•
Shrinkage of concrete. Creep of concrete including the effects of external loads.
•
Remaining relaxation of tendons.
m 0.9 and k = 0.4
Losses applicable to post-tensioning only:
•
Friction loss due to intended or unintended curvature in post-tensioning tendons.
•
Anchorage loss in post-tensioned tendons.
Accurate determination of losses is important in lightly-prestressed elements and where control of deformation in service is a consideration. Losses have little effect on the ultimate flexural resistance of an element unless the tendons are unbonded or the final stress is less than 0.50 f . Inaccurate estimation py of the final prestress force can significantly affect service behaviour such as camber and cracking. Relaxation of tendons Relaxation of the stress in a tendon commences immediately it is stressed. The amount a tendon will relax is determined by its metallurgical properties and is also a function of the temperature at which the tendon is maintained. It is recommended that only low-relaxation (Relax 2) wire or strand is used in pretensioned members. The basic relaxation, R b, determined from the standard 1000-hour laboratory test is modified to provide an estimate of actual relaxation with time. The effect of normal heat curing can be allowed for by increasing the basic relaxation by 0.5%.
Table 6.2 Values of k [After AS 3600 Section 3]
Table 6.3 Values of k [after AS 3600 Section 3]
For a pretensioned member, the relaxation is calculated at transfer (1 day) and at finality (30 years). The 1-day loss is used with the elastic loss for determination of the concrete strength at release. Relaxation loss at transfer
srf = R 30 spi spi = the stress in the tendon
The design relaxation percentage, Rj, at a particular
Total relaxation at 30 years
time in days is determined from Rb as follows: Rj = k 4k5k6Rb
where
where R
b=
immediately after transfer
see Table 6.1
k4 = see Table 6.2 k5 = see Table 6.3 k6 = T/20 but not less than 1.0 T = average annual temperature in degrees Celsius
The final loss should be modified to account for the reduction in stress in the tendon due to long-term shrinkage and creep in the concrete. In the absence of more detailed calculations it can be taken as: Relaxation loss sr = sri + ( srf -
Table 6.1 Values of R for Relaxation Class 2 Steel [after AS/NZS 4672.1]
sri = R 1 spi
and
sri) (1 – loss of stress due to shrinkage and creep/spi)
Elastic shortening of concrete The concrete, at the level of the tendons, shortens elastically as the prestressing force is applied and bonded tendons shorten by the same amount. The modulus of elasticity for the concrete at the time of stressing, Ecj, is determined from the mean cylinder strength or can be taken to be the strength specified for transfer of prestress, fcp. Ecj = r1.5 0.043 Öfcmi
if f cmi ! 40 MPa
or Ecj = r1.5 (0.024Öfcmi + 0.12) if f cmi > 40 MPa Elastic loss Dse = spi E p / Ecj
Shrinkage of concrete Loss of stress in the tendon due to shrinkage of the concrete surrounding it is proportional to that part of the shrinkage that takes place after the transfer of prestress force to the concrete. The design shrinkage strain, ecs, is determined in accordance with AS 3600 Section 3.
The creep strain is calculated using the stress in the concrete at the level of the centroid of the tendons. The sustained stress here is the initial prestressing force prior to any time-dependent losses, less the sustained portion of stresses for service loads prescribed in AS 1170.1. As 3600 Section 3 allows the strain due to the initial stress conditions to be factored by 0.8 to allow for the reduction in prestress with time. Provided the sustained stress in the concrete at the level of the
ss = ecs Ep
Shrinkage loss
Normal reinforcement will reduce the shrinkage of the concrete. Where the reinforcement is distributed throughout the cross-section such that the effect on shrinkage is mainly axial then the loss of prestress can be taken as:
tendons does not exceed 0.5 f'c, the loss due to the creep of the concrete may be taken as: Creep loss where
Dsc = 0.8 sci(Ep/Ec)jcc jcc = the design creep factor, calculated in accordance with AS 3600 Clause 3.1.8.3
ss = E pecs / (1 + 15Ast / Ag)
Shrinkage loss
sci = the sustained stress in the
ecs can be calculated from AS 3600 Section 3 or
concrete at the level of the centroid of the tendons.
estimated from Table 6.4 which has been calculated using a basic shrinkage strain of 1000 x 10-6. Creep of concrete In AS 3600 Section 3, the design creep strain of concrete, jcc, due to a sustained stress is calculated using a basic creep coefficient modified for member size, duration of loading, maturity, environment and strength.
k3 = 1.9 - 0.8(fcp/f'c) f
cp
Anchorage seating loss and friction These two sources of loss are mechanical and apply only to post-tensioned tendons. (The manufacturer of pretensioned units will make the appropriate adjustments for these losses during tensioning.) They represent the difference between the tension applied to the tendon by the jacking unit and the initial tension available for application to the concrete by the tendon. Their magnitude can be determined with reasonable accuracy and system suppliers can provide appropriate data for design. In many cases, these losses can be fully or partially compensated for by increasing the jacking force by a calculated or predetermined amount.
The maturity coefficient, k3, is defined in terms of age at time of loading, AS 3600, Figure 3.1.8.3(B). However, it does not cater for concrete which is heat-cured to allow early release of prestress, usually within a day of casting. For this type of curing, the relationship from prior editions of AS 3600 must be used, given here in algebraic form: where:
is mean concrete strength at release of prestress
fcp/f'c is valid between 0.5 and 1.0 Table 6.4 Typical Shrinkage Strains after 30 years in Various Environments [After AS 3600 Section 3] e
Prestress loss – PCI Simplified Method A PCI committee developed The PCI Simplified Method6.5 in 1975. Reader comments on the recommendations were published in the PCI Journal in 19766.6. It applies only to pretensioned members.
The stress loss is determined by computing the value of sc0 and sc1 and substituting in the appropriate empirical equations. These equations are used to compute total loss, st, in MPa. The total loss is the sum of losses due to shrinkage, elastic shortening
For typical elements it was found that the only variable that is not included in the equation and which could make an appreciable difference to the result is the volume-to-surface ratio, V/S. A correction factor,Table 6.5, is applied for that, eg for V/S = 75 reduce losses by 3.8%. Figure 6.2 shows typical volume-to-surface values for some common structural concrete elements. Table 6.5
and creep of concrete plus loss due to relaxation of tendons.
Correction Factor for Volume-to-Surface Ratio for use with PCI Simplified Method
For pretensioned elements of normal-density concrete:
st = 137 + 16.3 sc0 + 5.4 sc1 where sc0 = concrete compressive stress
at centroid of tendon at critical section immediately after transfer
sc1 = concrete stress at centroid of tendon at the critical section caused by sustained loads not included in the calculation of sc0 (tension negative)
The equation is based on the initial stress in the strand, after reduction for anchor slip, normally used in pretensioned elements, ie 0.75 fp for low-relaxation strand. The use of a higher or lower initial stress will result in an appreciable change in net losses. Use of the equation requires the calculation of the stresses sco and sci:
sco = (P i / Apc) + (Pi e2 / Ixx) – (Msw e / Ixx) sci = M sd e / Ixx where:
Figure 6.2 Volume-to-surface Ratios for Precast Structural Concrete Elements
= eccentricity of the strand at the critical section = concrete stress at centroid of tendon at the critical section caused by sustained loads not included in the calculation of Dsco (tension negative) at centroid of tendon at critical section immediately after transfer Ixx = moment of inertia of the section
Msw = moment due to mass of the
element Msd = moment due to all sustained loads except the element mass
Pi = prestress force immediately after transfer and initial loss. (It is within reasonable accuracy to assume 7.5% initial loss for low-relaxation strand).
e
= gross area of the precast section
sco = concrete compressive stress
pc
sci
A
See Example 6A.2(Appendix 6A) for typical calculation of Loss of prestress.
Figure 6.3 Development Lengths for Typical-Size Strands
6.5.2
Development length for prestressing tendons
In a pretensioned element, the prestress force is transferred to the concrete by bond and dilation of the strand along the transmission length. A further length is required to develop the steel stress at the ultimate flexural strength of the member, the total being termed the development length. Various assumptions are made as to the distance required to transfer the prestress. AS 3600 Section 13 suggests the value depends on the: •
type of tendon, eg indented wire or strand;
•
strength of the concrete;
•
position of the tendon, eg if it has a significant depth of concrete below it; and
•
rate of release of the tendon, eg sudden release can double the suggested value.
For strand, the transmission length is deemed to be 60 db with the first 10% unstressed increasing linearly from this point to maximum prestress at 60 db. The prestress is not fully effective until this point is reached as shown in Figure 6.3 based on the equation below and in Figure 6.4.
In order to keep the concrete stresses within acceptable limits in a partially-loaded member it may be necessary to adjust the eccentricity and/or the amount of prestress by deflecting or debonding a number of strands. The selection of one or the other will depend on the section shape, physical features of the member and the number of similar units to be manufactured. The decision is best made in conjunction with an experienced structural precaster. In the debond method, isolation from the concrete is achieved by placing a length of plastic tubing over the strand. It is preferable to stagger the shielding in two or three steps to give a gradual build-up to full prestress. The transfer of prestress and development of strand capacity commence at the termination of the shielding with lengths calculated as above.
Figure 6.4 Transfer Length Measurements[After Shahawy et al 6.30]
The length required to develop the full yield stress of the strand in bond is of the order of 2.5 to 3 times the transmission length, and it is suggested6.3, 6.4 that it be taken as: Ld = 0.145(fpu - 0.67fpe)db where L d = the developmentlength fpu = stress in the strand at ultimate strength fpe = the stress in the strand after all losses
AS 3600 limits the number of debonded strands to 70% of the total but a practical range is 25% to 50%. Any more suggests that the section is too small for the application. At least one State Road Authority places the limit at 50% for bridges.
Since the debonding reduces the quantity of strand and prestress in the end region of a beam, the flexure-shear cracking and web-cracking capacities, as described in AS 3600 Section 8 will be reduced and must be checked. The flexural capacity will also be reduced, increasing in step-wise fashion along the member and must be checked against the required moment capacity. It is usual, although not entirely accurate 6.30, to pro-rata the capacity of the par tially-developed strands at the critical sections. Flexural tension can occur in the concrete in the transmission length of debonded strand at factored loads as illustrated in Figure 6.5 and may affect the bonding of the strand. It is more likely to occur where all the shielding is terminated at the one location. If tension exists but is less than the cracking moment, it may be prudent to assume the development length is double that given by the equation above and reduce the tensile capacity of the debonded strands accordingly. If the cracking moment is exceeded, cracks can penetrate to the strand and anchorage will be reduced or even destroyed. In such a case, the strand pattern and shielding layout should be revised or the capacity of the partially-developed strands disregarded up to this point. See Example 6A.3 (Appendix 6A) for discussion and example of choosing appropriate Debonding of Strands.
6.5.3
Design of anchorage zones and end blocks
Pre- and post-tensioned members The anchorage of a post-tension cable generates zones of high tensile stress in the end-block concrete that require special reinforcement. Pretensioned strands or wires are more evenly distributed over the cross-section and the force is transferred gradually by bond over the transmission length. There is usually little requirement for special anchorage-zone reinforcement. However, where the tendons are separated into distinct groups at the ends of members, transverse (spalling) tension between the groups may be high enough to cause longitudinal cracking and reinforcement must be provided. The tensile force is of the order of 4% of the prestress force of a group. Similarly, there is a shape effect. For example, there is a tension zone in the flange of a double-tee, between the prestressed stems, that may lead to longitudinal cracking. Strut-and-tie modelling can be used to transform the flow of stresses, within the end-block or transmission zone, into discrete forces, see Section 7.10 of this Handbook. AS 3600 stipulates the design-strength requirements of the struts, ties and nodes that make up the assumed truss. Other sources of consistent design rules include NZS 3101.16.39 and ACI 318-086.40. Models for some standard situations have been published in literature such as CI SP-2086.41. Figure 6.6 illustrates the modelling of the spalling force in Figure 6.7.
Figure 6.5 Applied Moment v Cracking Moment for Beams with Debonded Strands [After Russell at al6.32]
Figure 6.6 Strut-and-Tie Model of an Anchorage Zone
Figure 6.7 Splitting Stress in Anchorage Zones
Anchorage Zone Reinforcement Post-tensioning forces are applied through relatively small anchorages causing high local stresses, which decrease as the forces spread through the anchorage zone to the full cross-section of the element. The Concrete Institute of Australia 6.8 has reviewed anchorage zones and the development of the design rules in AS 3600. It does not give design rules but does set out examples of good detailing of reinforcement for anchorage-zone reinforcement. Warner et al6.7 also give design information on anchorage zones and end blocks. In anchorage zones, usual bending theory does not apply; strut-and-tie models are recommended. The design of anchorage zones should incorporate the following steps: •
Determine the size of the primary prism around individual anchorages.
•
Provide primary bursting reinforcement within each primary prism.
•
Provide reinforcement close to the loaded face of the anchorage zone to resist the spalling stresses.
•
Check bearing stresses behind anchorages.
•
Consider the overall equilibrium of the anchorage zone and provide secondary reinforcement as close as possible to the loaded face.
Figure 6.7 graphically illustrates areas to be considered. When the anchorage forces act outside the centroids of their respective sx diagrams beyond the transmission length, Figure 6.7(a), tensile bursting stresses behind the anchorages and splitting stresses between the anchorages are created. When the anchorage forces act inside the centroids, Figure 6.7(b), tensile bursting stresses behind the anchorages and further down the length of the beam, and tensile spalling stresses at the end corners of the beam are created.
Determination of primary prisms Before the bursting stress behind an anchorage can be determined, the dimensions of the primary prism (the area in a particular plane where bursting stresses are greatest) must be determined. As shown in Figure 6.8, for distribution in a particular plane, the depth of the primary prism, d, is taken as the lesser of: •
•
Bursting stresses Bursting stresses vary with the ratio of a / d as shown in Figure 6.9. The area under each of the curves in Figure 6.9 yields the total bursting force, which may be approximated by the equation: Z = 0.3 P i [1 - (a / d)] where:
Z = total bursting force
the distance between centrelines of adjacent anchors, and
Pi = the initial tendon force a = width of bearing plate d = depth of primary prism
twice the distance from the centreline of an anchorage to the edge of the element.
When the extent of the prism is different on the two axes, or when the anchorage plate is rectangular, the distribution on each axis will also be different. Figure 6.8 Determination of Primary Prisms
Effectively-bonded reinforcement acting at a stress of 0.5 fsy (but not exceeding 0.001 strain) should be distributed through the depth of the primary prisms as required. In some cases, it may be appropriate to allow some unreinforced tension in the concrete (eg widely-spaced anchorages in large concrete sections or small, low-stress anchors in slabs). Spiral reinforcement of suitable diameter is commonly used for bursting reinforcement. The reinforcement should be detailed to allow for proper placement of concrete. When the length of the primary prism is different on two axes, the longer length should be used for the length of the spiral.
Spalling reinforcement
As shown in at Figure 6.7, zones tensile stress occur the loaded face of of high the concrete. Reinforcement to resist a total transverse force of 0.04 P, acting at a stress of 0.5 syf (but not greater than 0.001 strain) should be placed in both directions as close to the end face of the element as possible. Corners are also subject to high tensile stresses and should be adequately secured by reinforcement.
Figure 6.9
Transverse Tensile Bursting Stresses in Anchorage Zones
Bearing stresses behind anchors The designer of the anchorage zone is not usually concerned with the bearing stresses behind the anchorage. Proprietary anchorage designs are based on experience, tests and usage as well as theory. Anchorages that have been successfully used should be considered reliable. Although this is generally true of multi-strand anchorages, designers using single-strand tendons may need to check anchorage stresses, as spiral reinforcement is not provided.
Effectively-anchored reinforcement should be provided to carry the maximum moment on the assumption that the lower arm of the resisting couple is equal to half the length of the end block. Depending on the direction of the out-of-balancemoment, the reinforcement should be distributed over a distance of 0.5h from the loaded surface of the block or over a distance of 0.25h from the opposite end of the block (see Figure 6.11).
Special projects may require consultation with posttensioning suppliers.
less than 0.003 of the area of the horizontal crosssection calculated for the full length of the anchorage zone.
Equilibrium of the anchorage zone In most cases, there is a further spread of prestress force behind the primary prisms until the stresses become fully distributed over the entire cross section of the element. It is necessary to check the overall equilibrium of the anchorage zone.
The area of secondary reinforcement should not be
Since tendons are stressed sequentially, checks should be carried out to determine the worst combination of loads.
Consider an element of the end zone as shown in Figure 6.10. The forces acting on opposite faces of the element produce a moment and shear force on planes parallel to the longitudinal axis of the element. Figure 6.10 Freebody Diagram of End Zone
Figure 6.11 Equilibrium Forces in Anchorage Zone
6.6.1
Most precast, prestressed concrete flexural elements will have a net positive (upward) camber (hog) at the time of transfer of prestress caused by the eccentricity of the prestressing force. This camber may increase or decrease with time, depending on the stress distribution across the element under sustained loads and the distribution of non-stressed longitudinal reinforcement. In contrast, reinforced members will deflect only in the direction of the net sustained loads, usually downwards.
General
The design of precast concrete elements and structures for serviceability is the same as for insitu construction. The rules for deflection control given 6.3
in AS 3600 some Gilbert changes6.11, from 6.28 those given in incorporate previous editions. discusses the in-service behaviour of reinforced and prestressed concrete members and provides a series of calculations illustrating the rules in AS 3600. 6.6.2
Camber and deflection
There are many inherent variables that affect camber and deflection, such as concrete mix, storage method, time of release of prestress, time of erection and application of superimposed loads, and relative humidity. Because of this, calculated long-time values should never be considered any better than estimates. While detailed methods have been derived for predicting the long-term deflection of concrete members, the data on which they are based has a scatter of at least 15 to 30% using laboratorycontrolled specimens. Non-structural components that could be affected by camber variations, such as partitions or folding doors, should be placed with adequate allowance for these variations. Calculation of topping and bridge deck concrete quantities should also recognise the imprecision of camber calculations.
Limitations on instantaneous time-dependent deflections are specified in ASand 3600 and are reproduced in Table 6.6. Consideration should be given by the designer to the cumulative effect of deflections, and this should be taken into account when selecting a deflection limit. When checking the deflections of transfer members and structures allowance should be made in the design of the supported members and structure for the deflection of the supporting members. This will normally involve allowance for settling supports and may require continuous bottom reinforcement at settling columns. 6.6.3
Initial camber
Initial camber or hog can be calculated using moment area equations or similar. Usually only self-weight will be acting at release of prestress. Appendix A provides camber equations for common tendon profiles. Camber will vary from the calculated amount principally due to differences between assumed and actual values for the modulus of elasticity of the concrete, the prestress force and creep of the concrete.
Table 6.6 Limits for Calculated Deflections of Beams and Slabs [After AS 3600 Section 3] D
(1, 2)
(3)
6.6.4
Elastic deflection
Calculation of instantaneous deflections caused by superimposed service loads follows normal methods of structural mechanics. Design equations for various load conditions are given in Appendix A. If the bottom tension in a simple span element does not exceed the flexural tensile strength, the deflection is calculated using the uncracked moment of inertia of the section. The flexural tensile strength of concrete is defined in the AS 3600 as: f ’ct = 0.6Öf ’c Pretensioned sections are usually proportioned so that tensile stresses under shor t-term and long-term service loads are less than this value. Significant amounts of reinforcement in the precompression zone must be taken into account (Gilbert 6.11). The transfer of prestress force (as compression) to the reinforcement will reduce the cracking moment and deflect the member in the opposite direction. Reinforced elements are cracked to some degree and AS 3600 defines an effective moment of inertia which is part way between the gross and fully-cracked modulus. It takes into account the relationship between the service and cracking moments, the quantity of reinforcement and the shrinkage-induced tensile stress in the cross section. Gilbert 6.11 discusses the in-service behaviour of reinforced and prestressed concrete members and provides a series of calculations illustrating the rules in AS 3600. 6.6.5
Long-term camber and deflection
AS 3600 Section 8 provides a simple multiplier for estimating the additional long-time deflection of reinforced concrete beam elements:
the camber or deflection is impor tant not only at the initial and final stages, but also at erection, which usually occurs at some 30 to 60 days after casting when 40 to 60% of the ultimate shrinkage and creep will have taken place. Martin 6.23 has derived a consistent set of multipliers based on the above relationship for typical values of the principle variables that affect long-term deflection. The multipliers are set out in Table. 6.7 and these can be used as a guide in estimating values for typical elements, ie those elements that are within the spandepth ratios recommended in this Handbook. The gravity (self-weight) and prestress components of the initial camber are separated in order to take into account the effects of loss of prestress, which affects only the upward component. Martin’s paper also includes a sensitivity analysis using a range of typical precast products. For composite members, the final-stage multipliers in Table 6.7 are modified for the increased moment of inertia after the topping is bonded. The ratio of precast to composite moment of inertia ranges from about 0.5 for hollowcore units to 0.8 for Tee sections. The assumptions used in deriving multipliers are: Basic time dependent factor (AS 3600 Clause 8.5) 2.0 Initial loss of prestress (%) 8.0 Time-dependent loss of prestress (%) 15 Ultimate shrinkage and creep at erection (%) 50 Ratio of I precast / Icomposite 0.65
Long-term effects can be substantially reduced by adding non-prestressed reinforcement in the area of the tendons. The reduction effects proposed by Shaikh and Branson6.24 can be applied to the multipliers of Table. 6.6 as follows:
kcs = [2 - 1.2(Asc / Ast)] ! 0.8 The determination of long-term cambers and deflections in precast, prestressed elements is more complex because of the effect of prestress, the loss of prestress over time and the strength gain of the concrete after release of the prestress. In addition,
C2 = where:
C1 + A / Apb 1+ Ast / Apb
C
1
= multiplier from Table 6.7
C2 = revised multiplier See Example 6A.4 (Appendix 6A) for determining Deflection of a prestressed beam .
Table 6.7 Suggested Multipliers,C, for Estimating Long-term Cambers and Deflections for Typical Elements [After Martin6.23]
Concrete is a brittle material and even minor tensile strain will cause it to crack to some degree, in service. Two basic types of cracking can be expected to occur :
Crack control for flexure in prestressed beams under the shor t-term ser vice loads is based on either : restricting the tensile stress in the concrete to a maximum of 0.6 Öf ’c; or by restricting the increase in steel stress after decompression of the concrete to 200 MPa, along with adequate distribution of the strand in the tensile zone. Crack control for prestressed slabs is similar to beams except that the maximum tensile stress in the concrete is limited to 0.6 Öf ’c, or the steel
•
Plastic-shrinkage cracks which occur in the first hours after casting. They form while the concrete is still plastic and are not always evident during finishing. The cracks are usually wide but discontinuous.
stress increment after decompression to 150 MPa. The effect of temperature and shrinkage has to be considered for slabs and minimum reinforcement provided according to restraint and exposure conditions for the required degree of crack control.
•
Cracking of the hardened concrete caused by tensile strains resulting from restraint or flexure. They propagate until the tensile strain in the concrete is less than the fracture limit.
Cracks of less than 0.3 mm should not be treated as visual blemishes. Unless there is very good aesthetic reason, repair should not be attempted as it cannot be reversed. Transpiration of moisture and recrystallisation of cement compounds at the crack interface can naturally seal cracks without further rectification. An experienced precast manufacturer will have a range of repair techniques to reinstate cracked or damaged concrete.
They are known as shrinkage cracking and flexural cracking respectively. Plastic-shrinkage cracks usually do not affect structural capacity but may penetrate to a layer of reinforcement and require sealing if exposed to aggressive conditions, eg salt-laden air. This form of cracking usually occurs when the concrete surface is exposed to wind and temperature sufficient to cause rapid drying of the surface. Cracks may also be caused by restraint to vertical settlement of the concrete mass. This is known as plastic settlement cracking. The structural and durability requirements of AS 3600 are based on a nominal crack width of 0.3 mm. Cracks that are not expected to exceed that width do not need repairing. Crack control for flexure in reinforced beams is based on the provision and adequate distribution of a minimum area of reinforcement rather than problematical crack-width analysis6.10, 6.28. Restrictions are placed on either the bar diameter or the centre-to-centre spacing, depending on the tensile stress in the steel in critical tensile zones. These are zones of the beam where the flexural moment under direct loading produces tensile stresses in excess of 3.0 MPa in the concrete . Crack control is improved by:
•
using smaller diameter bars;
•
reducing bar spacing;
•
distributing bars uniformly across tension zones;
•
reducing stress in reinforcement at serviceability loads;
•
increasing the amount of reinforcement;
•
providing bars at re-entrant corners and other significant discontinuities;
•
curing.
Figure 6.12 Recommended Peak Vibration Acceleration Levels for Human Comfort6.12
6.8.2
6.8.1
General
AS 2670.26.37, Evaluation of human exposure to whole-body vibration, gives guidance on human response to building vibrations. It provides curves of frequency response for equal annoyance and is based on ISO 2631.26.38. The ISO document provides a baseline curve relating tolerance level to peak acceleration and vibration frequency. Peak vibration acceleration is usually expressed as a fraction of the acceleration due to gravity. The baseline cur ve is scaled to give the annoyance level for various occupancies and activities. Such a scaling is shown in Figure 6.126.12. Recommended acceleration6.14 for some typical occupancies is set out in Table 6.9. The relationship between human comfort, peak vibration acceleration and structural response is largely empirical and has been developed for flexible steel and concrete composite construction. However, the principles are of general application and can be extended to precast construction provided judgement is used in applying the relationships. The natural frequency, fn, in Figure 6.12 is estimated from the deflection of a member. This deflection is that due to the actual load the member supports, not the load assumed for strength design and includes self-weight. Continuity reduces the deflection so that it is conservative to assume that a floor is simply supported. The two sources of vibration for resonance effects, considered here, are walking and group rhythmic activity. For walking excitation the peak acceleration ratio, ap/g, is compared to the acceleration limit, ao/g, for the par ticular occupancy in Table 6.9. For rhythmic excitation the natural frequency, f n, is compared to the forcing frequency, fi , (Table 6.10) on the activity floor and the acceleration is checked against Table 6.11. This may be required for up to three vibration modes.
Table 6.8 Continuity Factor, Cj, for Floors with Two or Three Spans
Natural frequency of floor systems
The natural frequency of a floor is a determined from its maximum instantaneous deflection under the action supported6.12. The action is the total actual on the member.The imposed action component is about 10% to 15% of the structural design action. Typical actions would be 0.5 kPa for office floors, 0.25 kPa for residential floors, and, say, zero for footbridges, gymnasium and shopping centre floors. The simply-supported beam deflection formula is: 5
wt b L4
348 E
cd Ixx
Dss =
where:
wt = total actual imposed and permanent distributed actions (kPa) b = the loaded width of member (m) L = the span under consideration (m) Ecd = dynamic modulus of elasticity (MPa)
Ixx = moment of inertia of member cross-section The dynamic modulus of elasticity can be taken to be 1.15 times the AS 3600 value. The natural frequency, fn, for a floor on stiff supports can then be estimated from the expression6.4, 6.29, 6.12: 18 fn =
Ö Cj Dss
where: Cj = continuity factor j
= subscript denoting number of spans from 1 to 3
For a single span, C1 =1.0. For a series of equal spans the continuity factor is the same as a single span since a node occurs at each support, ie C j =1.0. As a guide Table 6.8 gives values of C j where a floor has one or two adjacent spans. It assumes that the moments of inertia of all members are the same and that the main span is the longest. For the three-span case, the side spans are equal. The span ratio is the ratio of the side span to the main span. The fundamental frequency of the floor structure, f n, is affected by the total deflection of the structure, not just the deflection of the slab itself. If the floor is supported on flexible girders, the deflection of the more flexible girder is added to obtain a total deflection6.12. In a tall building, the shortening of the supporting columns under the load they suppor t may also need to be added to the total. Column deflection is usually not significant in buildings less than 5 storeys high.
fn =
18
Ö
Cj (Dmember +
Dbeam + Dcolumn)
6.8.3
Vibration due to walking
Walking rate (step frequency) is between 1.6 and 2.4 Hz. The jogging rate is about 2.5 Hz and running rate is up to about 3 Hz. Occupied floors seldom have a natural frequency within the range 1.6–2.4 Hz. However, isolated structures, such as footbridges, may be as low as 2 Hz.
Pedestrian loading also has harmonics with frequencies at integer multiples of the step rate. A floor may therefore have a tendency to resonate if
6.8.4
Vibrations due to rhythmic activities
The objective is to ensure that the proportions of the floor under dynamic loading result in a natural frequency well in excess of the resonant condition. The following design criterion requires that the natural frequency, nf , is greater than the forcing frequency, fi6.13, 6.14.
one or more of its natural frequencies are within the ranges 3.2–4.8 Hz (2nd harmonic) and 4.8–7.2 Hz (3rd harmonic). It is unlikely that walking harmonics will produce significant resonance effects in a floor with a natural frequency greater than 9–10 Hz.
Ö where:
The fundamental frequency of a public area should be greater than 3 Hz since a relatively small number of people can produce resonance with coordinated jumping below this level, see Clause 6.8.4.
k
A floor system should be satisfactory if the peak acceleration ratio, ap/g, does not exceed the acceleration limit ao/g in Table 6.9. ap/g =
Po e-0.35f n
ai w p
k
fn ! f i 1 +
w ao/g
fi
t
= forcing frequency of the ith harmonic (i times the step frequency) as recommended in Table 6.10 = 1.3 for dancing, 1.7 for lively concert or sports event, 2.0 for aerobics
ao/g = acceleration limit of the activity floor (see Table 6.7 and Figure 6.12).
Table 6.9 Recommended Values of Walking Parameters and oa/g Limits6.13, 6.14
bW
where:
b
Po = a constant force, Table 6.9
fn = fundamental natural frequency of a floor system or combined floor and supporting beam
[1]
b
= the modal damping ratio, Table 6.9
W = BLw t where: B = effective width of floor. L = span of floor wt = effective (actual) weight of floor system per unit area. The effective width depends on the lateral stiffness of the floor. For torsionally-stiff members such as hollowcore slabs it can be taken as the lesser of the span6.12 or actual width, for wide single tees the lesser of the actual width or 0.6 of the span. The value of the damping ratio, b, depends on the type of floor construction, the occupancy, type of
Table 6.10 Recommended Dynamic Loading for Rhythmic Events6.13, 6.14
partitions, fit-out and ceiling construction, as noted in Table 6.9.
See Example 6A.6 for checking an office floor for walking vibrations.
a
ai
=
dynamic coefficient for the ith harmonic of the step or jumping frequency, as recommended in Table 6.10
wp = weight of participants as an effective distributed imposed action over the floor span (kPa) wt = total weight of the floor structure and participants expressed as a distributed action (kPa). Table 6.10 gives common forcing frequencies and dynamic coefficients for rhythmic activities. The weight, wp, is an estimate of the actual distributed imposed action of the participants, not the assumed design actions for strength. It is recommended that the acceleration ratio of group rhythmic activities does not exceed 5% of gravity to protect other more vibration-sensitive occupancies in the same building. Where there are mixed occupancies on the same floor area then the acceleration limits for the most sensitive activity should be used in the analysis of the rhythmic loading case. Only one harmonic needs to be considered for dancing, whereas three need to be considered for aerobics because of the repeated impacts. For sports events, the second harmonic in Table 6.10 takes into account the repeated foot-stamping type of loading by spectators. The acceleration due to harmonic resonance is obtained from6.13, 6.14. a g
=
1.3
ai w p
2b
wt
For design, b can be taken as 0.04 to 0.06. The more people on the floor, the greater the damping ratio. Vibration limits are suggested in Table 6.11. If the dynamic loading, ai wp, for the highest harmonic happens to be sufficiently small compared to the mass weight, wt, then the acceleration may be within Table 6.11 limits. In this case the inequality for f n is checked against the next lower harmonic. See Examples 6A.5a and 6A.5b.
Table 6.11 Recommended Acceleration Limits for Rhythmic Activities6.13, 6.14
The capacity of column and wall elements is determined by the interaction between the axial load and concurrent bending moment. The design of a particular section is a trial-and-error process and is more easily accomplished using load-moment interaction curve calculated for thea section. In its simplest form an interaction curve is constructed by calculating four points on the boundary. Two points plot the axial strength at zero moment on the vertical axis and the bending capacity at zero axial load on the horizontal axis. The other two plot the point at which the neutral axis coincides with the furthermost tension reinforcement and the point at which the tension reinforcement just begins to yield. The methods of analysis and the construction of such curves can be found in standard texts 6.7, 6.33. Complete interaction diagrams for common column and wall sections are published in the Reinforced Concrete Design Handbook6.35; some charts for prestressed columns are given in Chapter 2 of this Handbook.
If the bending moment on a column causes significant lateral deflection, the effective eccentricity of the axial load at mid-height is increased, increasing the moment and creating an iterative effect. AS 3600 Section 10 defines when a column is sufficiently slender for this to be taken into account. The design procedure applies an amplification factor to the moment acting on the column so that the shortcolumn design curves can be used for the design. Slender or tension-controlled members are likely to benefit from prestressing. Although the prestress decreases the axial capacity marginally, it increases stiffness by maintaining the gross section. It also enables long lengths to be handled. For braced walls, where the effective height-tothickness does not exceed 50 (subject to fire considerations) and the stress at the mid-height of the wall does not exceed the lesser of 0.03f'c or 2 MPa, the wall may be designed as a slab in accordance with Section 9 of AS 3600. Many low-rise walls meet this criteria.
This Section outlines solutions for special situations that may arise in the design of a precast floor or roof. Since production methods of products vary, local
The FIP method6.9 is based on an analysis of shear stress in the grouted keyways and transverse bending in the hollowcore slabs. It is more detailed and the distribution of bending moments is shown in Figures 6.14, 6.15 and 6.16 for both point and line loads. Shear distribution widths increase rapidly with increasing distance between the load and the response position, but are seldom greater than 0.125 of the span and the distribution is triangular (Elliot6.26).
precasters should be consulted. Also, test data may indicate that the guidelines presented here may be too conservative for a specific application.
Load distribution of stemmed elements may not necessarily follow the same pattern, because of different torsional resistance properties.
6.10.1
6.10.2 loads
General
See Example 6A.7 (Appendix 6A) for Load distribution for precast hollowcore floor.
Distribution of concentrated
Frequently, floors and roofs are subjected to line loads, for example from walls, and to concentrated loads. The ability of hollowcore systems to transfer or distribute loads laterally through grouted shear keys has been 6.18–6.20 and demonstrated in several published tests many unpublished tests. Research is continuing, and the recommendations here may be refined in the future. Based on tests, analysis and experience, line and concentrated loads can be resisted by an effective section as described in Figure 6.13 except that if the total deck width, perpendicular to the span, is less than the span, modification may be required. The load distribution is linear across the width and is suitable for programming.
6.10.3 Openings Large openings may be provided in precast decks by: •
saw cutting after the deck is installed and grouted;
•
forming (blocking out) or sawing in the factory; or
•
using short units with steel headers or other connections.
In hollowcore or solid slabs, structural capacity is least affected by orienting the longest dimension of an opening parallel to the span, aligning several openings parallel to the span, or by coring small holes to cut the fewest strands. Small openings (up to about 200 mm) are usually drilled on site.
Figure 6.13 Linear Load Distribution of Concentrated Loads on an Untopped Hollowcore Floor
Openings in flanges of stemmed elements should be limited to the ‘flat’ portion of the flange, ie beyond 25 mm of the edge of the stem on double-tees and 75 mm of the edge of the stem on single-tees. Steel headers, often angles, are used for framing large openings in hollowcore floors or roofs. The following are conservative guidelines regarding design of hollowcore slabs around openings. Precast manufacturers may have data to support alternative procedures: •
•
An opening located near the end of the span and extending into the span less than the lesser of 0.125 x span or 1.2 m may be neglected when designing for flexure in the mid-span region. Stress development must be considered on each side of an opening that cuts strand (see Clause 6.5.2).
•
Slabs that are adjacent to long openings (0.25 x span or more) or openings near midspan, may be considered to have a free edge for flexural design.
•
Slabs that are adjacent to openings closer to the end than 0.375 x span may be considered to have a free edge for shear design.
Figure 6.14 Load Distribution Coefficients for Linear Interior and Edge Loadings on an Untopped Hollowcore Floor
Figure 6.15
Figure 6.16
Moment Distribution Coefficients for Concentrated Interior Loads on an Untopped Hollowcore Floor
Moment Distribution Coefficients for Concentrated Edge Loads on an Untopped Hollowcore Floor
6.10.4
Many precasters prefer to design cantilevers as reinforced concrete elements. Pretensioned top strands are sometimes used in hollowcore slabs for longitudinal cantilevers extending over a support. Care must be taken in production to ensure these strands are properly bonded. The possibility of tensile stresses under design load and their effect on shear capacity in the region of negative moment must be carefully considered. Cores may need to be reinforced and filled to increase shear capacity. It is preferable that the precast section is always in net compression and tensile stresses are limited to the flexural tensile strength, 0.6Öf ’c, when reinforcement is used for negative moment resistance or in the topping of a hollowcore slab. The transmission length and total development length of top strands in cantilevers must be carefully assessed. The development length may exceed the span of the cantilever and a conservative value for stress at design load should be adopted. The transmission and development length may need to be increased if more than 300 mm of concrete is cast below the tendon, see AS 3600 Section 13 for guidance. Re-vibration to disperse any accumulated bleed water may reinstate full bond. 6.10.5
6.11.1
Suction and impact factors
Cantilevers
The most effective way to design cantilevered elements will depend on the type of product, method of production, span conditions and section properties of the element. The designer is advised to consult with local precasters to determine the most effective method.
Composite topping
During the handling process, members may be subjected to dynamic forces. For purposes of determining concrete stresses and reinforcement required, it is common practice to use multipliers on the member mass, and treat the resulting force as an equivalent static dead load. There are no fixed values which can be quantitatively derived; values which have been used in the past are generally based on the experience of the manufacturer. The safe handling of the members is the responsibility of the manufacturer. Experience will play an important role in the handling methods adopted, and thus the likely impact loads. Table 6.12 provides a recommended set of multipliers that may be used to determine equivalent static load. Impact factors on lifting devices will be satisfied if the lifting devices comply with AS 3850 6.21.
Table 6.12 Equivalent Static Load Multipliers to Account for Stripping and Dynamic Forces
(1) (2)
Precast floor and roof systems may be used untopped, or with a composite, insitu concrete topping. The composite action adds stiffness and (3) strength for gravity loads, may be required as a diaphragm to transfer lateral loads and can be used to mask hog and differences in the level between adjacent units. See FIP Guide 6.42. Tests have shown that the normal finished surface of hollowcore and stemmed deck units, provided the precast surface is thoroughly cleaned before topping is placed, will develop a shear surface coefficient, k co , of 0.2 to 0.4 (AS 3600 Section 8). In this Handbook, Clause 6.4.3 describes design procedures for horizontal shear transfer. In nearly all cases a k co of 0.2 is enough to develop the full strength of a composite precast floor member. The strength of the topping may be determined from the design requirements for the deck. Precast manufacturers’ load tables are usually based on 32 MPa for building applications and 40 MPa for bridge decks.
6.11.2
Flexure
General The location and capacity of lifting devices is generally the responsibility of the precast manufacturer, since it is a function of the manufacturing process and the strength of concrete at time of stripping. When the maximum moment at time of lifting has been determined, the stresses in the element may be evaluated based on the gross-section properties. If these are excessive, an increase in the number of lifting points may be required. The embedded devices used for stripping are usually used again for erection; however, additional devices may be required to rotate the panel from the stripped and transported position to the erection position, or turning frames used.
Figure 6.17 Moments Developed in Panels Stripped by Rotating about One Edge (Two- and four-point pic kups)
However, the geometry and practical location limitations on the pick-up points may not permit a crack-free design. In this case, provided that the in-service conditions permit, the element should be designed for the controlled cracking condition, or should be prestressed. There are two situations: (i) Elements of constant cross-section. Since the section modulus of top and bottom faces will not usually be identical, it is necessary to determine which face will control when choosing the position and number of lifting points. It is also
Elements that are stripped flat from the mould will develop moments as shown in Figure 6.18. As above, the calculated moment may be assumed to be
Ribbed elements When an element is ribbed or is of a configuration such that stripping by sliding or tilting is not practical, a system of vertical pick-up points on the top surface is used. These lift points should be located so as to minimize the tension at the face of the element.
Flat panels Panels that are stripped by being rotated about one edge, with lifting devices attached at the opposite edge, will develop moments as illustrated in Figure 6.17. For determining stresses, it may be assumed that the calculated moment is resisted by a width as shown.
resisted by the width as shown. Lifting slings that are inclined to the plane of the panel will induce in-plane compression in the member. When the point of load application is not coincident with the centroid of the cross section, additional flexure will be introduced. A tilt table will significantly reduce stripping stresses but is expensive to install and slows the production process. The slight inclination of the panel as it is lifted from the tilted position to the ver tical may be neglected when determining stripping forces and stresses, Figure 6.19.
necessary to select the controlling design limitation:
Figure 6.18 Moments Developed in Panels Stripped Flat (2 x two- and 2 x four-point pickups)
Tensile stress on one face to be less than that which would cause cracking, with controlled cracking permitted on the other face
•
Controlled cracking permitted on both faces. If only one of the faces is exposed to view, then generally this is the face that will control.
Figure 6.21. For the particular section shown, critical conditions could result between the ribs at a relatively low magnitude of moment, due to the reduced cross-section.
•
In addition to longitudinal bending moments, there could also be a transverse bending moment generated by the orientation of the pick-up points with respect to the transverse dimension,
Tensile stresses on both faces to be less than that which will cause cracking (Clause 6.6.4)
The design moments for a two-point lift, using inclined slings, are shown in Figure 6.20. When the angle of the sling to the horizontal is small, the component of force parallel to the longitudinal axis may generate a significant moment. Even though the effects of non-vertical slings can and should be accounted for, it is not recommended that this effect be allowed to dominate design moments. When this condition exists, consideration could be given to using spreader beams, two cranes or other mechanisms to reduce the angle of the sling and thus the effect of longitudinal forces. The angle of a sling to the horizontal should not be less than 45°.
•
Figure 6.19 Stripping from a Tilt Table
Figure 6.20 Pick-up Points for Equal Stresses on a Ribbed Element
equal reactions. The force in inclined slings can be determined from Figure 6.23.
(ii) Elements of varying cross-section No rules can be formulated with respect to location of lift points for elements of varying cross-section; for these cases, the location of lift points requires a trial and error process to determine the position which will result in acceptable stress. In the case of long elements of varying section, rolling blocks can be employed, Figure 6.22. The forces in the slings will be equal, and the stress analysis can proceed by considering a beam with varying load supported by
Figure 6.22 Arrangement for Equalising Lifting Loads with Elements of Varying Cross-section
Figure 6.21 Moments Caused by Eccentric Lifting
Figure 6.23
Determination of Forces in Inclined Lifting Slings
Lifting and handling devices Lifting devices for precast concrete elements exist in the form of a variety of proprietary and nonproprietary engineered systems. Systems appropriate for the precast member type and its handling application should be considered when selecting a lifting device.
Proprietary systems should be used in accordance with their manufacturer’s specifications and recommendations. Non-proprietary systems should be designed by calculation or by testing or by a combination of both to the extent applicable in AS 3850:2003 6.21. Since lifting devices are subject to dynamic loads, consideration of the ductility of materials chosen for non-proprietary engineered systems is part of the design requirements. Deformed reinforcing bars must not be used as lifting loops because the deformations will result in stress concentrations from the shackle pin. Also, they may have low ductility and low impact strength at low temperature. Round bar of 230R grade with good ductility may be used provided adequate embedment against pullout is provided as bond or by mechanical end anchorage.
In Table 6.13, the reduced ductility of the high-tensile steel in the strand has been taken into consideration for shackle pin diameters of 25, 50 and 75 mm using load reduction factors K = 0.65, 0.8 and 0.9 respectively as determined from test results 6.25, 6.44. The Working Load Limit (WLL) should be based on a limit state factor, LSF = 3.0 and a capacity reduction factor f = 0.6 against failure. Table 6.13 give the working load limit for 12.7- mm diameter strand embedded in 30 MPa (min) concrete for 25-, 50- and 75-mm shackle pin diameters and having an angle of lifting sling not less than 45° to the horizontal axis of the element. The values are derived from test results6.25. The diameter of the loop should be a minimum of 100 mm prior to lifting. Figure 6.24 Swivel Plate
Lifting devices embedded in precast concrete bridge members normally consist of strand lifting loops6.43. Loops are widely used due to their high strength and flexibility. Seven-wire prestressing strand of 9.3 mm and 12.7 mm diameter are suitable for lifting precast members where adequate embedment length is available. Strand lifting loops are particularly suitable for deep members such as beams and girders.
Table 6.13 Working Load Limit (WLL) for Strand Lifting Loops
6.25
Strand that has been previously tensioned to more than 80% of its tensile strength or has grip deformations should not be used for lifting. Where a loop incorporates multiple strands it should project a minimum of 300 mm from the element and be enclosed in steel tube to ensure equal distribution of loading. The individual strands should be sufficiently separated over the embedment length for full bond to be achieved. The angle of lifting sling should be not less than 45° to the horizontal axis of the element.
Figure 6.25 Equilibrium of Hanging Beam
Also, the bond/embedment length or embedded configuration with or without supplemental reinforcement may limit the lifting loop capacity, as may edge conditions. To ensure that an embedded insert acts primarily in tension, a swivel plate as indicated in Figure 6.24 should be used. Threaded inserts used to lift heavy members require reinforcement welded or screwed into the base to distribute the load into the member. The insert and its reinforcement must be properly anchored with the member reinforcement so that a ductile failure mode is assured. Where possible, propietary inserts should be used. Cast-in connection and fixing items (eg ferrules or J-bolts) used for final fixing should not be used for lifting or handling of any but the lightest units, and then only if approved by the designer. The shop drawings should clearly distinguish between lifting (handling) and fixing devices.
See Example 6A.8 (Appendix 6A) for Design for handling.
6.11.3
Lateral stability
Special consideration should be given to long elements with narrow compression flanges during handling, transportation and erection. Unlike the classical buckling of steel I-beams, the lateral stability of long prestressed members is dependent on the roll stiffness of the supports (ie lifting loops, truck, or bearing pads). The issue of lateral stability is actually an analysis of lateral bending and equilibrium, in which lateral stresses can be significant. A theory for evaluating the lateral stability of prestressed I-beams developed by Mast6.22 is summarised below. For a hanging beam or a beam supported from below, an equilibrium diagram is used to derive a factor of safety against cracking, FS, and a factor of safety against failure or roll-over, FS’, where in general: resisting moment factor of safety = applied moment
For a hanging beam, Figure 6.25: FS = 1/[(Z 0/Yr) + qi/qmax] FS’ = Y
q
q
r ’max/[Z’0 ’max
+ e i]
For a beam supported from below, Figure 6.26: FS = r( FS’ = r(
qmax - a)/(Z0qmax + e i + Yqmax) q’max - a)/(Z’0q’max + e i + Yq’max)
In general, reducing the distance between supports will reduce the offset of the beam centroid from the roll axis and improve stability. The distance to the centroid can be calculated using the midspan offset, D, for the full member length multiplied by the offset factor where:
Figure 6.26 Equilibrium of Beam on Elastic Supports
Offset factor = (L 1/L)2 – 1/3
1
The tilt angle at the maximum factor of safety against failure or roll-over is calculated for a hanging beam:
q’max = [e i /(2.5 Z0)]0.5 For a beam supported from below:
q’max = (z max – h r a)/r + a The initial stresses at the time of handling are significant to the factor of safety against cracking. Prior to applying the design loads, the top flange is typically very lightly stressed in compression or is in tension. In this case, very little lateral bending can be tolerated before cracking begins. The factor of safety against cracking may therefore need to be improved by adding compression to the top flange. Similarly, if the distance between supports is reduced to improve stability, the top flange compression due to self weight is reduced and additional compression may be required.
Z’0 = +Z 0(1 + 2.5 q’max)
Figure 6.27 Offset of Centroid at a Curved Arc
It is recommended to use FS • 1.0 and FS’ • 1.5. Note that the factor of safety against failure or roll-over, FS’, is based on a cracked section and therefore need not be taken as less than the calculated factor of safety against cracking, FS. The factor of safety is a function of beam geometry and support location. For a beam with curvature due to lateral sweep and ver tical camber, the centroid is moved away from the roll axis, Figure 6.27
1
12 Ec Iy L
This deflection can be adjusted for a cracked section at the tilt angle under consideration where:
w[0.1L5 – a 2 L 3 + 3a 4 L + (6/5)a5] Z0 =
The deflection of the centroid of a beam under its own weight is calculated by:
For beams supported from below, the factor of safety can be improved by increasing the rotational stiffness of the supports. Mast 6.22 provides a method for determining the stiffness of vehicles used for transpor tation and sets out guidelines for bearing pads. For safe handling of long elements, lateral stability can be improved by several methods: •
Move thebetween support supports points inward. Decreasing distance by a small amountthe can significantly increase the factor of safety against failure or roll-over. Temporary post-tensioning can be used to improve the factor of safety against cracking.
•
Increase the distance Yr for hanging beams by using a rigid yoke at the lifting points. Decrease the distance Y for beams supported from below by using lower vehicles for transportation.
•
Increase the roll stiffness of vehicles used for transpor tation.
•
•
Attach temporary lateral stiffening in the form of strong-backs, stiffening trusses or pipe frames. Sometimes two or more units can be transported together, side by side, and tied together to provide the necessar y lateral strength. Increase the lateral stiffness of the members by revising the shape to increase I y or increase the concrete strength and thus Ec.
See Example 6A.9 (Appendix 6A) for Lateral stability of beam during handling and transport. 6.11.4
Figure 6.28 Bowing due to Differential Strain
Storage
Wherever possible, an element should be stored using only two points of support located at or near those used for stripping and handling. Thus, the design for stripping and handling will usually control. Where points other than those used for stripping or handling are used for support, the storage condition must be checked. The primary causes of warpage are differential temperature, differential shrinkage, creep and storage conditions. Warping in storage can be minimized by resisting flexure about the strong axis of the element. Equalization of stresses on both faces and symmetrical reinforcement will also help to minimize deformations. When, in storage, there is a differential strain on opposite faces, it can be assumed that the panel will bend in a circular shape. From this assumption, the amount of lateral bowing can be determined as indicated in Figure 6.28. For the type of support conditions illustrated in Figure 6.29, warping can occur in both directions. By superposition, the total warpage at the maximum point can be estimated by:
Dmax = [(5wd sinq)/(384Ec)][(a4/lx) + (b4/lz)] where: Dmax = maximum deflection wd
= panel dead load per unit area
Ec
= modulus of elasticity of the concrete
a
= panel support height
b
= horizontal distance between supports lx, lz = moment of inertia of uncracked transformed section in the respective direction, per unit width of panel
Figure 6.29 Panel Warpage in Storage
6.11.6
Figure 6.30 Effect of Compression Reinforcement on Creep
When considering warpage in storage, the timedependant creep and shrinkage effects should be considered. The total deformation will also be a function of the amount of reinforcement. The total deformation at any time can be estimated as:
D = Di(1 + l) where: D = time-dependent displacement Di = instantaneous displacement l = amplification due to shrinkage and creep (Figure 6.30) p’ = A 6.11.5
sc / bd
Transportation
One of the important factors when considering transpor tation is the location of suppor ts for the elements. Panel supports (two per element) should be chosen such that the imposed tensile stresses do not exceed the value of 0.6 Öf ’c with due consideration given to the effect of dynamic loading. When an element is non-symmetrical about a bending axis of the cross-section, the location of support points to produce equal bending stresses on each face will be a function of the ratio of the distances from the bending axis to top and bottom fibre. If the tensile stresses cannot be contained within the above limiting value, auxiliary bracing (such as strong-backs or space frames) should be attached to the element prior to loading. Most precast manufacturers use either flatbed or low-bed trailers, and these undergo significant deformations while travelling. Thus, only very flexible members that can accommodate such deformations elastically can be supported at more than two points.
Erection
Generally, erection poses no particularly new problems in the design of precast elements (see Chapter 11, Handling, Transport and Erection Clause 11.4). The stress limitations and multipliers previously discussed (Clause 6.11.1) for stripping and handling are applicable during erection. The centre of gravity of the element should be computed and lifting points located so as to place the centre of gravity of the loads directly below the main hook and below the lowest point of the attachment of slings, in order that the element will hang level during erection. Walls with large openings should be carefully checked for erection stresses and braced for handling when necessary.
6.16 Timoshenko, S P and Gere Mechanics of Materials (Appendix A, pp. 485–497). Van Nostrand Reinhold, New York, 1972
6.1 Building Code of Australia, Australian Building Codes Board and CCH Australia, 2008. 6.2 AS/NZS 1170 Set Structural design actions, Standards Australia, 2007. 6.3 AS 3600 Concrete structures, Standards Australia, 2009. 6.4 Canadian Prestressed Concrete Institute, Design Manual Precast and Prestressed Concrete, 4th Edition, Ottawa, Ontario, 2007. 6.5 ‘Recommendations for Estimating Prestress Losses’ by the PCI Committee on Prestress Losses, PCI Journal July/August 1975. 6.6 ‘Readers’ Comments on the Recommendations for Estimating Prestress Losses’ PCI Journal March/April 1976. 6.7 Warner R F, Rangan B V, Hall A S and Faulkes K A Concrete Structures, Longman, Melbourne, Australia, 1998. 6.8 Current Practice Note 29 Prestressed Concrete Anchorage Zones, Concrete Institute of Australia, June 1996 6.9 Federation Internationale de la Precontrainte, FIP Recommendations, Precast Prestressed Hollow Core Floors. FIP Commission on Prefabrication, Thomas Telford, London, pp10–12 6.10 Guide to Reinforced Concrete Design, OneSteel Reinforcing, November 2000 6.11 Gilbert R I and Mickleborough Design of Prestressed Concrete, Allen & Unwin (Australia) Ltd, 1990. 6.12 Applied Technology Council,Minimizing Floor Vibration, Redwood City, California, 1999. 6.13 Allen, D E ‘Building Vibrations from Human Activities’ Concrete International 12(6), June 1990, pp 66–73. 6.14 American Institute of Steel Construction, Steel Design Guide Series 11, Floor vibrations due to human actvity, Chicago, Illinois, 2003. 6.15 Allen, D E and Murray, T M ‘Design Criterion for Floor Vibration Due to Walking’ AISC Engineering Journal Fourth Quarter (December) 1993, pp 117–129.
6.17 Harris, C M and Crede, C E Shock and Vibration Handbook Second Edition, McGraw-Hill, New York, 1976. 6.18 LaGue, D J ‘Load Distribution Tests on Precast Pre-stressed Hollow-Core Slab Construction’ PCI Journal Vol. 16, No. 6, Nov-Dec 1971. 6.19 Johnson, T and Ghadiali, Z ‘Load Distribution Test on Precast Hollow-Core Slabs with Openings’ PCI Journal Vol. 17, No. 5, Sep-Oct 1972. 6.20 Pfeifer, D W and Nelson, T A ‘Tests to Determine the Lateral Distribution of Vertical Loads in a Long-Span Hollow-Core Floor Assembly’ PCI Journal Vol.28, No. 6, Nov-Dec 1983.
6.21 AS 3850 Tilt-up concrete construction, Standards Australia, 2003. 6.22 Mast, R F ‘Lateral Stability of Long Prestressed Concrete Beams - Part 2’ PCI Journal Vol. 38, No.1, Jan-Feb 1993, pp 70–88. 6.23 Martin, L D ‘A Rational Method for Estimating Camber and Deflection of Precast Prestressed Members’ PCI Journal January-February 1977. 6.24 Shaik, A F and Branson, D E ‘Non-Tensioned Steel in Prestressed Beams’ PCI Journal Vol. 15, No. 1, Feb 1970. 6.25 Concrete Technology Associates, Tacoma, Washington State, USA,Pullout Strength of Strand Lifting Loops, Technical Bulletin 74-B5, May 1974. 6.26 Elliot, K S Multi-storey Precast Concrete Framed Structures, Blackwell Science Ltd, Oxford, 1996, p 223. 6.27 Federation Internationale de la Precontrainte, FIP Recommendations, Shear at the Interface of Precast and Insitu Concrete . FIP Commission on Prefabrication, Cement and Concrete Association, Wexham Springs, Slough, UK. 6.28 Gilbert, R I Shrinkage, Cracking and Serviceability: Where are we Headed?, Seminar Proceedings, Concrete Institute of Australia, 28 June 2000. 6.29 Mast, R F ‘Vibration of Precast Prestressed Concrete Floors’ PCI Journal NovemberDecember 2001.
6.30 Shahawy, M A, Issa, M and deV Batchelor, B ‘Strand Transfer Lengths in Full Scale AASHTO Prestressed Concrete Girders’ PCI Journal MayJune 1992. 6.31 Martin, L D and Korkosz, W J ‘Strength of Prestressed Concrete Members at Sections Where Strands Are Not Fully Developed’ PCI Journal September-October 1995.
6.32 Russell, B W, Burns, N H and ZumBrunnen, L G ‘Predicting the Bond Behaviour of Prestressed Concrete Beams Containing Debonded Strand’ PCI Journal September-October 1994.
•
6A.1
Design of a precast beam for strength at transfer
• •
6A.2 6A.3
Loss of prestress Debonding of strands Deflection of a prestressed beam
•
6A.4
6.33 Beletich, A S and Hall, D P Design Handbook for Reinforced Concrete Elements, UNSW Press, Sydney, Australia.
•
6A.5a Design of gymnasium floor for vibrations
•
6A.5b Design of stadium seating for vibrations
•
6A.6
Design of office floor for walking vibrations
6.34 Gilbert, R I Cracking, Deflection and Serviceability: AS 3600 Directions , CIA Seminar, 8 Nov. 2000.
•
6A.7
Load distribution for precast hollowcore floor
•
6A.8
Design for handling
•
6A.9
Lateral stability of a beam during handling and transport.
6.35 Reinforced Concrete Design Handbook , Cement Concrete and Aggregates Australia and Standards Australia, 4th Edition, Feb. 2002. 6.36 AS 5100 Set Bridge design, Standards Australia, 2007. 6.37 AS 2670.2 Evaluation of human exposure to whole-body vibration Part 2: Continuous and shock-induced vibration in buildings (1 to 80 Hz), Standards Australia, 1990. 6.38 ISO 2631-2:2003 Mechanical vibration and shock - Evaluation of human exposure to wholebody vibration - Part 2: Vibration in buildings (1 Hz to 80 Hz) 6.39 NZS 3101 Part 1 Concrete Structures Standard Standards New Zealand, 2006. 6.40 ACI 318-08 Building Code Requirements for Structural Concrete, American Concrete Institute, 2008.
6.41 SP-208 Examples for the Design of Structural Concrete with Strut-and-Tie Models, American Concrete Institute, 2002. 6.42 Guide to Good Practice – Composite Floor Structures, Federation Internationale de la Precontrainte (FIP), 1988. 6.43 PCI Bridge Design Manual, Chapter 3, October 1997. 6.44 AS 3850.3 Tilt-up concrete and precast concrete elements for use in buildings - Guide to erection of precast concrete members, Standards Australia, 1992 (no longer in print)
The following pages cover a variety of worked design examples, as set out below.
Example 6A.1 Design of a precast beam for strength at transfer
Standard, 300-mm deep, RTA prestressed deck unit shown below. Span is 9.5 m between supports and it is reinforced with 9–12.7-mm super low-relaxation strands plus 4–N12 corner bars. The unit is to be used as a flooring member in a
warehouse structure where it is required to carry a superimposed dead load of 5 kPa and a live load of 10 kPa. Section details are as shown in 2.3.1.6 of this Handbook.
Determine the release strength for the above straight strand configuration.
General Initial prestress
From Example 6A.2
Pi = 1136.51 kN End of beam to centre of bearing Lb = 150 mm Transmission length Lt = 60 x Dia strand = 60 x 12.7 = 762 mm Centre bearing to end of transmission length Lte = Lt - Lb = 762 - 150 = 612 mm Moment at end of transmission length due to self weight Mswt =
wsw Lte 2
(Ls - Lte) =
4.11 x 612 2
(9500 - 612)10-6 = 11.18 kN.m
Top stress due to M swt
tts =
Mswt Zt
=
11.18 8.04
= - 1.39 MPa
Bottom stress due to M swt
bts =
Mswt Zb
=
11.18 8.7
= 1.28 MPa
Stresses due to prestress at release Top fibre prestress
tpi = - 4.97 MPa
Bottom fibre prestress
bpi = 17.87 MPa
From Example 6A.2
cont…
Maximum stresses at release
Top fibre
tp = tpi - tts = - 4.97 + 1.39 = - 3.58 MPa (tension) Bottom fibre
bp = bpi - bts = 17.87 - 1.28 = 16.59 MPa (compression)
Required release strength for an essentially triangular stress distribution bp 16.59 f'cp = = = 27.65 MPa Use f'ci = 30 MPa 0.6 0.6 Maximum allowable tensile stress
tensile = 0.6f'ci = 0.630 = 3.29 MPa The 2–N12 bars in cage OK for slight excess tension
Check ultimate strength
Ultimate resistance of concrete Pu = 1.7 f'ci B ypb +
Warner et al6.7, Equation 7.75
0.8 Mswt Pu
Let A = 1.7 f'ci B = 1.7 x 30 x 600 = 30 600 kN/m
Then P = u
A ypb +
(A ypb)2 + (4 A 0.8 Mswt)
2 = 0.5[30 600 x 60 +
(30 600 x 60)2 + (4 x 30 600 x 0.8 x 11.18 x 106)] = 1974.66 kN
For = 0.75
Pu = 1480.99 kN
Reference, Clause 6.4.2, this Handbook
Factored initial prestress 1.15 P =i 1306.98 kN
<
Pu OK (but not if selfweight ignored) Release strength could be reduced by debonding two strands for a length of 1000 mm
Check effect of reduced prestress due to debonding
Eccentricity of reduced PS
Location of reduced PS from soffit
ecc = y b - ypbe = 144 - 60 = 84 mm
ypbe = 60 mm Reduced prestress (pro-rata to 9 strands) P
= 7/9 P = 7/9 x 1136.51 = 883.95 kN reduced
i
1 Stresses due to reduced prestress:
Stress due to top fibre prestress
tpr =
Preduced Apc
-
Preduced x ecc Zt
=
883.95 x 103 883.95 x 10 3 x 84 = - 3.86 MPa 164 500 8.04 x 106
Stress due to bottom fibre prestress
bpr =
Preduced Apc
+
Preduced x ecc Zb
=
883.95 x 103 883.95 x 10 3 x 84 + = 13.9 MPa 164 500 8.71 x 106
cont…
2 Stresses at end of transmission length of the 7 strands, due to reduced prestress:
Top fibre
tp = tpr - tts = - 3.86 + 1.39 = - 2.47 MPa (tension) Bottom fibre
bp = bpr - bts = 13.9 - 1.28 = 12.62 MPa (compression)
Required release strength for an essentially triangular stress distribution bp 12.62 f'cp = = = 21.03 MPa < 30 MPa OK 0.6 0.6 3 Stresses at end of transmission length of strands debonded for one metre:
Moment due to self weight Mswt =
wsw(1000 + L te) 2
[Ls - (1000 + L te)] =
4.11(1000 + 612) 2
[9500 - (1000 + 612)]10-6
= 26.15 kN.m Top stress due to M swt
tts =
Mswt Zt
=
26.15 8.04
Bottom stress due to M swt
bts =
= - 3.25 MPa
Mswt Zb
=
26.15 8.71
= 3.0 MPa
4 Stresses at release:
Top fibre tp = tpi - tts = - 4.97 + 3.25 = - 1.71 MPa (tension) Bottom fibre
bp = bpi - bts = 17.87 - 3.0 = 14.87 MPa (compression) Required release strength f'cp =
bp 0.6
=
14.87 0.6
= 24.78 MPa
Use f'ci = 25 MPa
Maximum allowable tensile stress
tensile = 0.6f'ci = 0.625 = 3.0 MPa
OK for tp = - 1.17 MPa
Bottom fibre stresses at factored moment v prestress
end
Example 6A.2 Loss of prestress
Standard, 300-mm deep, RTA prestressed deck unit shown below. Span is 9.5 m between supports and it is reinforced with 9–12.7-mm super low-relaxation strands plus 4–N12 corner bars. The unit is to be used as a flooring member in a warehouse structure where it is required to carry a superimposed dead load of 5 kPa and a live load of 10 kPa.
Determine the total loss of prestress (immediate and long-term) for the above flooring member.
Material properties 28-day concrete strength and density
f'c = 40 MPa
= 2500 kg/m3
Mean concrete strength fcm = 43 MPa
AS 3600 Table 3.1.2
28-day modulus of elasticity Ec = 24001.5(0.024 fcm + 0.12) = 24001.5(0.024 43 + 0.12) = 32 613 MPa Mean concrete strength at release fcmi = 35 MPa
Range is 25–35 MPa
1-day modulus of elasticity Eci = 0.043 x 24001.5 x fcmi = 0.043 x 2400 1.5 x 35 = 29 910 MPa Basic shrinkage strain
csb = 600 x 10 -6 Basic creep factor
cbb = 2.8
AS 3600 Table 3.1.8.2, assumed average value. See AS 3600 Commentary for regional values
Ultimate strength of 12.7-dia. strand fp = 1870 MPa
AS 3600 Table 3.3.1
Modulus of strand Ep = 195 000 MPa
AS/NZS 4672.1 cont…
Member properties Span
Ls = 9500 mm Precast section area
Bottom section modulus
Apc = 164 500 mm2
Zb =
Moment of inertia6 about X-axis Ixx = 1254 x 10 mm4
Ixx yb
=
1254 x 106 144
= 8.71 x 106 mm3
Top section modulus Zt =
CG of section from soffit
Ixx yt
=
1254 x 106 156
= 8.04 x 106 mm3
yb = 144 mm Hypothetical thickness CG of section from top
th =
yt = 300 - 144 = 156 mm
2 x A pc ue
=
2 x 164 500 2 x 600
= 274.17 mm
Member actions (loads)
AS 1170.1, load factors for storage
AS 1170.1, load factors for storage
Loaded width (soffit width) B = 600 mm Self weight wsw = Apc x 25 = 164 500 x 25 x 10-6 = 4.11 kN/m Self-weight moment Msw =
wsw x Ls2 8
=
4.11 x 9.52 8
= 46.39 kN.m
Imposed action (live load) wll = 10 kPa imposed action
Superimposed permanent action (dead load) wsdl = 5 kPa Superimposed permanent-action moment Msdl = (B x w sdl)
L s2 8
= 600 x 5 x
9.52 8
= 33.84 kN.m
cont…
Strand properties Number and location
Nt = 9 located in single layer at ypb = 60 mm from soffit Eccentricity of prestress ecc = y b - ypb = 144 - 60 = 84 mm
Diameter and area of strand db= 12.7 mm A p1 = 98.6 mm2 Total strand area Apb = Nt Ap1= 9 x 98.6 = 887.4 mm2 Jacking limit Jp = 75% of breaking load
Range is 65% to 80%
Prestress at jacking Pj = Jp Apb fp = 0.75 x 887.4 x 1870 = 1244.6 kN Area of reinforcement (4-N12 bars) As = 4 x 110 = 440 mm 2 Immediate prestress losses 1 Relaxation loss at 1-day:
Assumed basic relaxation value for Jp = 0.75 and heat curing at 80C Rb = 4% Average annual temperature T = 20°C Relaxation at 30 years j = 30 x 360 = 10,800 days k4 = 0.41
AS 3600, Clause 3.3.4.3 and Table 6.2 (this Handbook)
k5 = 1.25
AS 3600, Clause 3.3.4.3 and Table 6.3 (this Handbook) at Jp = 0.75
k6 = T/20 = 20/20 = 1 Relaxation R30 = k 4 x k5 x k6 x Rb = 1.41 x 1.25 x 1 x 0.04 = 6.15% At release: R1 = 2/3 R30 = 2/3 x 6.15 = 4.1% Prestress loss due to relaxation Plri = Pj x R1 = 1242 x 0.041 = 50.99 kN 2 Elastic loss:
AS 3600, Clause 3.4.2.3
cont…
= 13.97 MPa
Elastic loss as percentage %loss =
Plc Pj
=
62.85 1244.6
= 5.05%
Prestress force at release Pi = Pj - Plri - Ple = 1244.6 - 50.99 - 62.85 = 1130.74 kN
3 Summary of immediate losses:
Percentage loss at release due to relaxation and elastic losses lri le %loss = P + P = 50.99 + 62.85 = 9.15% Pj 1244.6
Assume temperate inland climate zone AS 3600, Clauses 3.1.2 and 3.4.3.3
Long-term prestress losses 1 Creep loss:
30 years duration t=30x360=10,800days
t
h
= 274.17 mm
Duration factor a2 = 1.0 + 1.12e -0.008t n = 1.0 + 1.12e -0.008 x 247.17 = 1.125 =
k2
a2 x t0.8 = t0.8 + 0.15t h
1.125 x 10 800 0.8 = 1.098 0.8 + 0.15 x 274.17
10 800
Strength ratio at release SR =
fcmi f'c
=
35 40
= 0.875
cont…
Maturity factor
Exposure factor
k3 = 1.9 - 0.2SR = 1.9 - 0.2 x 0.875 = 1.2
Strength factor
k4 = 0.65
k5 = 1.0 AS 3600, Clause 3.1.8.3
Design creep coefficient
fcc = k2 k3 k4 k5 fccb = 1.098 x 1.2 x 0.65 x 1.0 x 2.8 = 2.4
0.74
0.74
13.24 MPa
13.24
7.41 MPa
AS 3600, Clause 3.1.8.1
0.8
0.8 x 2.4 x 7.41 32 613
436
436
84.99
887.4 x 84.99 = 75.42 Percentage loss due to creep %loss =
Plc Pj
=
75.42 1244.6
= 6.06%
AS 3600, Clause 3.1.7 and 3.4.3.2
2 Shrinkage loss:
Basic autogenous shrinkage
e csf = (0.6f’c - 1.0)50 x 10 -6 = 0.00007 Basic drying shrinkage
e csf.b = 800 x 10 -6
Sydney aggregates
Autogenous shrinkage strain
e cse = ecsf (1.0 - e -0.1t ) = 0.00007 Drying shrinkage strain
e csd.b = ecsf.b (1.0 - 0.008f’c) = 0.000544
Shrinkage strain coefficient at 30 years a1 = 0.8 + 1.2e -0.005t n = 0.8 + 1.2e -0.005 x 247.17 = 1.1 k1 =
a1 x t0.8 t0.8 + 0.15t h
1.1 x 10 800 0.8 = = 1.08 10 800 0.8 + 0.15 x 274.17
Design shrinkage strain k4 = 0.6
AS 3600, Figure 3.1.7.2
AS 3600, Clause 3.1.7.2
Temperate inland environment
ecs = k 1 k4(ecse + ecsd.b ) = 1.08 x 0.6(0.00007 + 0.000544) = 0.000 397 cont…
77.47 MPa
397
Reduction factor for normal reinforcement in cross section Rnr = 1 +
15 x A s
=1+
Apc
15 x 440 164 500
887.4 x 77.47
= 1.04
= 66.09 kN
Percentage loss due to shrinkage %loss =
Pls
=
Pj
66.09 1244.6
= 5.31%
3 Relaxation loss after transfer: Design relaxation after transfer
D R = R30 - R1 = 6.15 - 4.1 = 2.05%
Page 6-42 this Handbook AS 3600, Clause 3.3.3.4
=1244.6x0.0205
84.99 + 77.47 =22.24kN 1274.21
Total prestress losses
Total loss of prestressing force Ploss = Plri + Ple + Plrl + Pls + Plc = 50.99 + 62.85 + 22.24 + 66.09 + 75.42 = 277.59 kN Loss of stress in a strand loss =
Ploss Apb
=
277.59 887.4
= 312.82 MPa
Final prestressing force Pf = Pj - Ploss = 1244.6 - 277.6 = 967 kN Total percentage force loss PSloss =
Ploss Pj
=
277.6 x 100 1244.6
967
967
= 22.3%
967
967
= - 4.23 MPa
= 15.21 MPa
Example 6A.3 Debonding of strands
It is usually more practical to provide supplemental reinforcement in short, heavily-loaded elements.
Elastic and relaxation loss at release is 8% of the jacking load, final loss is 18%. Strands are jacked to 70% of ultimate capacity.
Prestress details:
4–12.7-mm stress-relieved (relax 1) strands in top, 7–12.7-mm stress-relieved (relax 1) strands in bottom, fp = 1870 MPa Pi = 0.92 x 0.7 x 98.6 x 1870/10 3 = 118.7 kN/strand Pe = 118.7 x 0.82 = 105.8 kN/strand cont…
Section properties:
Ag = 240 000 mm2
yt = 300 mm
Zt = 24 000 x 10 3 mm3
I = 7200 x 10 6 mm4
yb = 300 mm
Zb = 24 000 x 103 mm3
et = - (300 - 100) = - 200 mm (for 4 top strands) ebu = 300 - 150 = 150 mm (for 3 upper strands in bottom) ebl = 300 – 100 = 200 mm (for 4 lower strands in bottom)
Loading: Load factors
Permanent loads Imposed loads
Limit state 1.2
Serviceability 1.0
1.5
0.7
Self weight
w = 400 x 600 x 10 -6 x 24 = 5.76 kN/m
Superimposed dead load wd= 35 kN/m (not including self weight of beam) Live load wl = 26 kN/m
Choose appropriate ‘debond’ lengths and determine stresses at critical sections to determine the required concrete strength, fci, at transfer and required fc at service load. Check factored flexural resistance at critical sections as limited by strand development length.
In a cantilever beam such as this, some of the bottom strands will usually be debonded because they increase the stresses produced by the cantilever moment and increase the deflection of the cantilever. A few strands should, however, continue through to the end for reinforcement and crack control during transfer, stripping, storage, transportation, erection and construction. In this example, the 4 strands in the bottom layer are debonded over the full 2-m length of the cantilever. The development length of these strands needs to be adequate so they can provide their full flexural resistance near mid span. From Figure 6.3 (this Handbook) we can see that, based on the equation for development length required to develop the full design stress of the strand, a development length of approximately 2.1 m is required for 12.7-mm strands. Debonded strands normally need a development length that is twice as long as bonded strands. Top strands are necessary along the length of the cantilever, but near the centre of the span they increase the stresses produced by the positive moment. Normally, some of the top strands should be debonded over most of the span. Caution is needed because the full flexural resistance of these strands may be needed to resist the factored negative moment over the right support. cont…
For illustration, two of the four top strands have been debonded over a length of 3.4 m from the left end of the beam. A length of 2.7 m is therefore available for development of f pr of these strands at the right support. It is preferable that this length is sufficient to develop the full strength of the strand. The variation of prestress force in each layer of strands is shown in Figure below. These forces have been labelled as Pt, Pbu and Pbl to designate their location in the beam. A transfer length of 765 mm
for the debonded strands has been assumed. Variation of prestress force in each layer of strands
Pt = Top strands Pbu = Bottom upper strands
Pbl = Bottom lower strands
To show the variation of stresses along the beam under different loads, the stresses at the top and bottom of the beam at transfer and the stresses in the beam under service loads have been plotted. To compute the stresses at transfer, the beam is assumed to be supported at the ends. The stresses at service load are based on the final prestress P e and two loading cases:
with no live load on the cantilever, and
with no live load on the 6-m span.
The following equations with appropriate variations in P and M have been used to determine the stresses:
t= Pt[(1/Ag) – (et/Zt)] + Pbu[(1/Ag) – (ebu/Zt)] + P bl[(1/Ag) – (ebl/Zt)] +M/Zt b= Pt[(1/A g) – (e t/Zb)] + Pbu[(1/Ag) + (ebu/Zb)] + Pbl[(1/A g) – (e bl/Zb)] – M/Zb Variation in Stresses along Beam
cont…
From the previous plots, it is apparent that the sections that need to be checked include:
The transfer point, 765 mm from each end
The location of maximum positive moment near mid-span
The right support.
Other locations may need to be checked, as well as the lifting and support points used during stripping, storage, transportation and erection. Strength at transfer:
At the transfer point, 765 mm from the left support Pti = 237.5 kN Pbui = 356.2 kN Pbli = 475.0 kN The moment at this point, assuming the beam is simply supported at each end on the mould
M = 16.16 kN.m
The compressive stress at transfer
b = 8.0 MPa The required concrete strength at transfer fci 8.0/0.60 = 13.3 MPa
use 25 MPa
Stresses at service loads:
The maximum positive moment occurs with no live load on the cantilever. The left reaction under this condition is 163.3 kN so that zero shear and corresponding maximum moment occur at 2.77 m from the left support. The maximum service load moment at this section is 256.0 kN.m (The use of the mid-span moment would have introduced only negligible error). Forces in the different layers of prestressed steel at this section Pt = 211.7 kN Pbu = 317.5 kN Pbl = 423.3 kN Maximum stresses at this section
t = 10.9 MPa b = - 2.97 MPa Maximum negative moment, M = - 117.9 kNm, occurs at the right support with cantilever fully loaded. Forces in the different layers of prestressed steel at this section Pt = 423.3 kN Pbu = 317.5 kN Pbl = 0 Maximum stresses at this section
t = - 0.28 Mpa b = 6.46 MPa
cont…
AS 3600, Clause 8.1.5
Adjust moment resistance based on actual development length provided. Use double development length for debonded strand. Final prestress in strand
ku =
6.50 x 98.6 x 1705 0.85 x 0.766 x 400 x 476.9 x 40
= 0.22 cont…
Mu = 0.85 f'c b d2 ku(1 -
ku 2
) = 0.8 x 0.85 x 0.766 x 40 x 400 x 476.9 2 x 0.22(1 -
0.766 x 0.22 2
)
= 382.3 kN.m 2 In negative moment region:
Mf.max = 2 2 x 0.5[(5.76 + 35.0)1.2 + 26 x 1.5] = 179 .9 kN.m
dp = 500 mm Assuming 4 strands effective 4 x 98.6 x 1870 k2 = = 0.092 400 x 500 x 40
pu = 1870(1 -
0.4 x 0.092 0.766
) = 1780 MPa
Adjust moment resistance based on actual development length provided on either side of critical section Id = 0.145(1780 - 0.67 x 1073)12.7 = 1953 mm for bonded strands Id = 1953 x 2 = 3906 mm for debonded strands From left end: lactual = 6100 mm for bonded strands lactual = 2700 mm for debonded strands Number of strands effective = 2 + (2 x 2700)/3906 = 2 + 1.38 = 3.38
k2 =
3.40 x 98.6 x 1870 400 x 500 x 40
pu = 1870(1 -
ku =
= 0.078
0.4 x 0.078 0.766
) = 1794 MPa
3.38 x 98.6 x 1794 0.85 x 0.766 x 400 x 500 x 40
= 0.115
0.766 x 0.115
Mu = 0.8 x 0.85 x 0.766 x 40 x 400 x 5002 x 0.115(1 -
2
)10 -6 = 228.7 kN.m >179.9 kN.m
From right end: lactual= 2000 mm
>1953
No adjustment to moment resistance is required end
Example 6A.4 Deflection of a prestressed beam
Standard, 300-mm deep, RTA prestressed deck unit shown below. Span is 9.5 m between supports and all loads and beam properties are as given in Example 6A.1.
From Example 6A.1
Selected properties
28-day modulus of elasticity:
Ec = 36 455 MPa
1-day modulus of elasticity:
Eci = 31 799 MPa
Long-term load factor Live load Superimposed dead load Prestress at release:
l = 0.6
Final prestress:
Pf = 954 kN
Creep factor:
cc = 2.1
wll = 10 kPa wsdl = 5 kPa Pi = 1136.51 kN
Determine the amount of deflection at erection and at final position for the above flooring member.
Soffit position at erection Prestress hog
ps =
Pi ecc Ls2 8 Eci Ixx
=
1136.15 x 103 x 84 x 95002 8 x 31 799 x 1254 x 106
= 27.01 mm
Positive direction is upwards
Self-weight deflection
sw =
-5 wsw Ls4 384 Eci Ixx
=
-5 x 4.11 x 9500
4
384 x 31 799 x 1254 x 106
= - 10.94 mm
Soffit position at release i = ps + sw = 27.01 - 10.94 = 16.07 mm Deflection at erection assuming, say, half the long-term losses
cc Eci
P sw + f x ps Ec Pi 2.1 31 799 954 = 16.07 + x - 10.94 + x 27.01 = 26.82 mm 2 36 455 1136.51
erection = i +
2
x
cont…
Final soffit position
Imposed dead-load deflection
sdl =
-5 B wsdl Ls4 384 E c Ixx
=
-5 x 0.6 x 5 x 9500
4
384 x 36 455 x 1254 x 106
= - 6.96 mm
Full live-load deflection
ll =
- 5 B wll Ls4
=
384 Ec Ixx
-5 x 0.6 x 10 x 9500 4 6
= - 13.92 mm
384 x 36 455 x 1254 x 10
LT prestress and self weight with all the LT losses
it = i + cc x
Eci Ec
sw +
= 16.07 + 2.1 x
Pf Pi
x ps
31 799 36 455
- 10.94 +
954 1136.51
x 27.01 = 37.56 mm
Long-term imposed loading
lt = (1 + cc)(sdl + l+ ll) = (1 + 2.1)(- 6.96 - 0.6 x 13.92) = - 47.47 mm Deflection in final position
final = it + lt = 37.56 - 47.47 = - 9.91 mm Determining deflection by multipliers
See Clause 6.7.3, this Handbook
Soffit position at erection erection = Cps ps + Csw sw
Table 6.6 for 'C' values
= 1.8 x 27.01 - 1.85 x 10.94 = 28.38 mm Final soffit position
final = Cps ps + Csw sw+ Csdl sdl + Cll l ll
Table 6.6 for 'C' values
= 2.45 x 27.01 - 2.7 x 10.94 - 3.0 x 6.96 - 3.0 x 0.6 x 13.92 = - 9.3 mm end
Example 6A.5a Design of gymnasium floor for vibrations
A 2400 x 600 double-tee beam with 50-mm topping and with a 15-m span on block walls is to be used as a gymnasium floor, aerobics being the critical rhythmic activity. There are no sensitive occupancies that might be affected by the vibrations.
Check the above beam, used for a gymnasium floor, for vibrations due to aerobic activities.
Acceleration limit
Table 6.10, this Handbook
Adopt 7%g (0.070) The weight of participants is assumed to be 0.15 kPa spread out over the total span (based on 0.2 kPa over an occupied area of 3.5 m 2 per person for a typical class), while the floor plus contents weigh 4.2 kPa. Table 6.9, this Handbook
Forcing frequencies, fi, for aerobics ffirst = 2.75 Hz
f
= second
5.5 Hz
f
third
= 8.25 Hz Clause 6.8.4, this Handbook
fn fn ! fi The natural frequency for the double-tee is determined from the simple beam deflection formula. Section properties Ag = 260 000 mm2; I = 11 600 x 10 6 mm4; Zt = 68 100 x 10 3 mm3; Zb = 24 100 x 10 3 mm3 The modulus of elasticity of concrete is increased by 20 % to allow for the expected short-term dynamic stiffness and taken as E cd = 35.5 MPa. cd
fn A natural frequency of 4.5 Hz is unacceptable because second harmonic resonance can occur at f = 4.5 Hz with accelerations of the order of 30%g by the application of the equation for a/g. To obtain acceptable performance (fn = 7.0 Hz), the double-tee spanning 15 m must be increased to a 2400 x 1000 with 50-mm topping or the span must be shortened.
Example 6A.5b Design of stadium seating for vibrations
The precast stadium seating shown below. Weight of the seating, including non-structural components, is 5 kPa on a horizontal projection.
Determine the maximum acceptable span for vibration acceptability using an acceleration limit of
7%g for concerts and sports events.
For such events, adopt a UD L of 1.5 kPa for the weight of participants and two harmonic loads with forcing frequencies up to 3 and 5 Hz respectively.
Table 6.9, this Handbook
Because the seating vibrates in a direction normal to the plane of seating, these forces are multiplied by cos 22. Clause 6.8.4, this Handbook
fn fn ! fi
The natural frequency for the precast seating is determined from the simple beam formula using the principle moment of inertia in the Timoshenko and Gere6.16
most flexible direction, which is found by standard formulae 10-3
m4.
to be 3.35 x Thus, the simply-supported deflection is: cd
fn Also check strength limit state and other serviceability requirements at this span.
Example 6A.7 Load distribution for precast hollowcore floor
An untopped hollowcore floor, supporting a loadbearing wall and concentrated loads as shown below.
Determine the design loads for the plank supporting the wall and concentrated loads.
Each step corresponds to step number in summary table below, calculated at given distances from support. 1 Calculate the shears and moments for the non-distributable (uniform) loads.
wf = 1.25(2.7 + 0.5) + 1.5 x 2.0 = 7.0 kN/m 2 2 Calculate the shears and moments for the distributable (concentrated and line) loads.
wf = 1.2 x 9.5 + 1.5 x 15.2 = 34.2 kN/m P1f = 1.2 x 2.2 + 1.5 x 4.4 = 9.24 kN P2f = 1.2 x 4.4 + 1.5 x 13.3 = 25.23 kN 3 Calculate effective width along the span.
At the support:
See Figure 6.10, this Handbook
width = 1200 mm
At 0.25L(1905 mm): 1
width = 0.5L
1=
3810 mm
Between x = 0 and x = 1905 mm: width = 1200 + (x/1905)(3810 - 1200) = 1200 + 1.37x 4 Divide distributable shears and moments from Step 2 by the effective widths from Step 3. 5 Add the distributed shears and moments to the non-distributable shears and moments from Step 1.
Once the moments and shears are determined, the planks are designed as described in Clause 6.10. Summary of design loads
NOTE: This method is suitable for computer solution. For manual calculations, the procedure can be simplified by investigating only critical sections. For example, shear may be determined by dividing all distributable end loads by 1.2 m, and flexure at midspan can be checked by dividing the distributable loads by 0.5L1.
Example 6A.8 Design for handling
A window unit, detailed below, with sandblasted finish and cast face down.
Locate the pick-up points to minimise tension stress in the concrete during stripping.
Table 6.11, this Handbook, for multipliers
Dead load of element assuming 1.6 multiplier
NOTE: Multiplier could be 1.3 (proper stripping tapers) but is taken as 1.6 due to possibility of jamming on fixed window-opening forms
wd1 = 1.6 x 7.2 = 11.5 kN/m
wd2 = 1.6 x 26.3 = 42.1 kN/m
Wd = 5.0 x 11.5 + 2.19 x 42.1 = 150 kN Lifting loops or inserts should be placed symmetrically about the centre of gravity of the element. Assume critical cracking stress will occur in the narrow mullion sections of the unit where they join the spandrel. For equal stresses on each face, f t = fb M - yt M + yb = I I y M+ 369 M+ M- = b = = 1.6 M + yt 231 cont…
Fx =
Fy tan
=
150 2 x tan 60
= 43.3 kN
yc = yt + 70 = 301 mm
M = yc Fx =
301 x 43.3 103
= 13.0 kN.m
M+ = Fy(3.1 - ) -
2
x 2.5 - w d2 x 0.6 x 2.8 + M 2 11.5 = 75(3.1 - ) x 2.5 2 - 42.1 x 0.6 x 2.8 + 13 2
wd1
= 139 - 75
M- = wd2 x 0.6 ( - 0.3) +
wd1 2
x ( - 0.6)
= 42.1 x 0.6 ( - 0.3) - 11.5 ( - 0.6) 2
2
2
= 5.82 + 18.4 - 55
Now, M- = 1.6 M+ 5.82 + 18.4 - 55 = 1.6 (139 - 75 ) 5.82 + 138.4 - 227.9 = 0 Solving for
= 1.54 m
Adopt = 1.5 m
M+ = 139 - 75 = 139 - 75 x 1.5 = 26.5 kN.m M- = 5.82 + 18.4 - 55 = 5.8 x 1.52 + 18.4 x 1.5 - 55 = 35.2 kN.m
ft =
fb =
MZt
=
M+ Zb
=
35.2 x 106 2 x 18 600 x 10 3 26.5 x 106 2 x 11 600 x 10 3
= 0.9 MPa
= 1.1 MPa
This stress would allow stripping at f'ci as low as 15 MPa without theoretically cracking the section. cont…
Handling reinforcement
For example, for the mullions: -*
M =
+*
M
=
2
x 1.2 = 21.12 kN/mullion
26.52
x 1.2 = 15.9 kN/mullion
2 This reinforcement is often sufficient for in-service conditions. For example, for a wind load suction of 2.75 kPa (ultimate), the in-service moment at mid-height of the mullion is: *
35.2
Mw =
2.75 x 1.94 2
x
(7.188 - 0.4)2 8
= 15.4 kN/mullion
Compared with 15.9 kN/mullion for stripping Transporting and site handling
A unit of these dimensions would probably be transported on edge on A-frames. It would be lifted on site on edge and rotated in mid-air until hanging vertically by its top lifters so that at no time is the panel spanning in its most slender direction.
Example 6A.9 Lateral stability of a beam during handling and transport
A 30-m-long Austroads I-girder, supported 1 m from each end and having the following properties.
Concrete:
IXX = 105 333 x 10 9 mm4
Eci = 29 910 MPa
IYY = 8250 x 10 6 mm4
Ec = 35 750 MPa
f’ci = 35 MPa f’c = 50 MPa
Section properties:
Stresses at hold-down point (x = 13 000 mm): ft(ps) = - 6.50 MPa (at transfer) = - 6.00 MPa (at 40 days - transport) ft(sw) = + 7.00 MPa (self weight) Msw = 169.5 x 12 - (11.30 x 13 2/2) = 1079 kN.m
Evaluate the lateral stability of this beam during handling and transporting.
Handling (a hanging beam)
cont…
1 Initial eccentricity
Offset factor = (L 1/L)2 – 1/3 = (28/30)2 - 1/3 = 0.538 Lateral bow (say) = 20 mm Lift loop placement offset = 10 mm ei = Offset factor x Bow + Lifting offset = 0.538 x 20 + 10 = 21 mm
2 Height of roll axis above CG of beam
Initial camber = 50 mm Yr = 775 - 50 x 0.538 = 748 mm
4 Theoretical lateral deflection
Z0 = =
w[0.1L5 – a2 L13 + 3a4 L1 + (6/5)a5]
Page 6-31, this Handbook
12Eci Iy L 11.3 x 10-3[0.1 x 28 0005 – 10002 x 2 8 0003 + 3 x 1000 4 x 28 000 + (6/5)10005]103 12 x 29 910 x 8250 x 10
6
x 30 000
= 216 mm
Page 6-32, this Handbook
Page 6-31, this Handbook
> 1.0
OK
> 1.5
OK
cont…
Transporting (beam supported f rom below)
1 Radius of stability
Assumed rotational stiffness of vehicle, K 0 = 4200 kN.m r = K 0/W = 4200 x 106/339 x 103 = 12 390 mm 2 Initial eccentricity Offset factor = (L 1/L)2 – 1/3 = (28/30)2 - 1/3 = 0.538
Lateral bow (say) = 20 mm Placement offset on truck, say = 25 mm ei = Offset factor x Bow + Placement offset = 0.538 x 20 + 25 = 36 mm 3 Height of CG of beam above roll axis
Camber = 50 mm Height of beam CG above road = 1800 + 100 + 625 = 2525 mm Height of roll axis above road, h r = 600 mm Y = 2525 + 50 x 0.538 – 600 = 1952 mm 4 Theoretical lateral deflection
Z0 = Z0(initial)
Eci Ec
= 216 x
29 910 35 750
= 181 mm
5 Tilt angle at cracking
fr = 0.6f'c = 0.650 = 4.242 MPa Mlat = (ft(ps) + ft(sw) +fr)(Iy)/(b t/2) = [(- 6.00 + 7.00 + 4.24)(8250 x 10 6)/(500/2)10 -6 = 173 kN.m
max = Mlat/Msw = 173/1079 = 0.1603 rad
cont…
6 Tilt angle at maximum FS’
Roadway superelevation of vehicle, say maximum 8%,
= 0.0800 rad
Distance center of truck to center of tyre, z max = 900 mm
'max = (zmax – hr )/r + = (900 – 600 x 0.08)/12 390 + 0.0800 = 0.1488 rad
7 Theoretical lateral deflection at tilt angle 'max Z'0 = Z0 (1 + 2.5 'max) = 181(1+ 2.5 x 0.1488) = 248 mm 8 Factor of safety against cracking
FS = r( max - )/(Z0 max + ei + Y max) = 12 390(0.1603 – 0.08)/(181 x 0.1603 + 36 + 1952 x 0.1603)
= 2.63 > 1.0
OK
9 Factor of safety against rollover
FS' = r( 'max - )/(Z'0 'max + ei + Y 'max) = 12 390(0.1488 – 0.08)/(248 x 0.1488 + 36 + 1952 x 0.1488) = 2.34 > 1.5
OK end
What you will find in this Chapter •
A comprehehensive explanation of numerous factors which affect connection and fixing design.
•
Design principles for cladding panels and methods for the attachment of cladding panels to structures.
•
Design methods for the connection of loadbearing units.
•
Details and design of bearings and bearing areas.
•
Design criteria and details of a wide variety of connection systems used successfully in Australia.
•
Design examples of typical connections.
7.1 Definitions and Notation 7.1.1 Definitions 7.1.2 Notation 7.2 Introduction 7.3 General Design Criteria 7.3.1 General 7.3.2 Resistance 7.3.3 Ductility 7.3.4 Volume change considerations 7.3.5 Durability 7.3.6 Fire resistance 7.3.7 Production issues 7.3.8 Construction issues 7.4 Loads, Load Factors and Capacity Factors 7.5 Cladding-Panel Connections 7.5.1 General design principles for cladding panels 7.5.2 Cladding panel connection categories 7.5.3 Bearing connections 7.5.4 Restraint connections 7.5.5 Industrial wall panel connections 7.6 Loadbearing Connections 7.6.1 Column units 7.6.2 Wall units 7.7 Bearing Pads 7.7.1 General 7.7.2 Design details 7.7.3 Material requirements 7.7.4 Friction under bearings 7.8 Shear Friction 7.9 Bearing Areas of Reinforced Concrete Members
7.10 Strut-and-Tie Model 7.10.1 General 7.10.2 Truss geometry 7.10.3 Design basis 7.10.4 Design examples 7.11 Cast-in Anchors 7.11.1 Introduction 7.11.2 Failure modes 7.11.3 The CCD method 7.11.4 Failure in tension 7.11.5 Failure in shear 7.11.6 Combined tension and shear 7.12 Connection Angles 7.13 Column Base Plates 7.14 Dowel Connections 7.15 Cast-in-Place Connections 7.16 Drilled Inserts 7.17 Welding of Reinforcing Bars 7.18 Permanent Formwork 7.18.1 Introduction 7.18.2 Connection detail principles 7.19 References 7.20 Appendix 7A – Design Examples 7A.1 Steel Corbel and Top-Restraint Fixings for a Cladding Panel 7A.2 Reinforced Bearing for a Rectangular Beam 7A.3 Corbel to a Column Supporting a Beam 7A.4 Dapped-End Connection for a Beam 7A.5
Reinforced Beam Ledge for a Double-T Leg
eI ev
= the eccentricity of the vertical load
Fn
= the design friction force
f'c 7.1.1
Definitions
For the purpose of this chapter the following definitions are used: Connection The system or assembly used to fix a precast member to the supporting structure or to an adjacent member to form the structure. Joint An intentional gap between adjoining elements (typically cladding) or between an element and some other portion of the structure. Ductility The ability to accommodate large deformations without failure. (Note this is a different definition to that used in ear thquake design, see Chapter 5 Analysis and Design of Buildings. ) Fixing
fsu
= the specified tensile strength of stud anchor
f
= the specified yield strength of reinforcement, anchor, stud or base plate
sy
fsy.f = the specified yield strength of A sh reinforcement g
= the gauge of angle
h
= the overall thickness of the concrete member
he
= the effective embedment depth of a ferrule
hmin = the minimum concrete thickness of the element to develop the full capacity of the studs Lsy.t = the development length of bars in tension le
= the embedment length of stud anchor or bolt
m
= the minimum cover from the anchor head to any free edge
N
= the unfactored permanent compressive load perpendicular to the shear plane
Notation
The following notation is used in this chapter:
Ndf = the factored dead load force normal to the friction face
A
= the projected area of a stud; or the net area under a bolt head or hook
Ab
= the area of a stud anchor; or the total area of anchor s and studs
Nt = the tension acting across the shear plane
Acr
= the area of the crack interface
Pf
= the factored tension force
Acv
= the area of concrete section resisting shear transfer
Pr
Ae
= the effective stress area
= the tensile resistance of the concrete surrounding a headed stud; or the factored resistance of a headed stud
Ag
= the gross area of the shear plane
t
Ash
= the area of vertical reinforcement across potential horizontal cracks
= the minimum thickness of non-gusseted angles; or the base plate thickness
Vf
At
= the additional amount of reinforcement resisting tension across shear plane
= the shear force along the crack face; or the factored shear force; or the ver tical force on the angle
Avf
= the area of shear-friction reinforcement; or the cross-sectional area of studs
Avf(min) = the minimum area of shear-friction reinforcement a Br
= a dimension = the factored bearing resistance of a stud
b
= the average width of the element; or the width of angle; or a dimension
c
= the cohesion stress
c1
= the characteristic compressive strength of concrete at 28 days
fcm = the mean value of concrete strength at the relevant age
The hardware component of a connection. 7.1.2
= the distance from the centre of the bolt to the horizontal reaction
= the edge distance of a ferrule
d
= the diameter of a ferrule
db
= the diameter of a stud anchor or bolt
Nf = the factored horizontal or axial force Nuo = the tensile capacity of a ferrule
Vr = the factored shear resistance Vuo = the shear capacity of a ferrule vr
f
fa fc fs m ms l rv se
= the factored shear stress = the capacity reduction factor; or a strength reduction factor = 0.9 = 0.65 = 0.85 = the coefficient of friction = the static coefficient of friction = a coefficient = the ratio of shear friction reinforcement = the effective normal stress
The design of connections is one of the most important phases in the design of precast structures. Generally, structural redundancy is eliminated to minimise forces at connections. Therefore, it
7.3.1
General
is critically load paths for forces through theimportant str ucture,that from elements through connections down to the footings and foundation, are carefully reviewed. Where possible it is prudent to design a statically determinate system, which will accommodate long-term, incremental volume-change movement. Consideration of connection behaviour over the whole life of the structure, including erection, is important.
being considered. Some of the items discussed in this chapter are self-evident. Other requirements may not be so obvious and may require particular consideration or specification by the owner or occupier of the building.
This chapter presents concepts of analysis and equations for design of connections and fixings for precast concrete members. Design equations have been developed from field experience, laboratory tests, and structural analysis and the source of each is stated. The design of practical and economical connections should consider production of the elements and the construction matters pertinent to structures incorporating precast concrete, as well as the performance of the connections for both serviceability and ultimate limit states. The recommendations made take into consideration design actions as given in AS 1170 7.1, design procedures and precast construction practice and are intended as reasonable guidelines for the analysis and design of connections. Other types of connections and fixings are in use and some have been extensively tested. Continuing research will lead to new and improved details and methods of analysis. Designers should not necessarily restrict themselves to the design methods and examples covered, but should feel free to explore other viable approaches. The information provided is intended for use by those with an understanding of structural design, and in no case should the information replace good structural engineering judgment for a particular project.
Connections and fixings must meet a variety of design and performance criteria, the appropriate set of criteria varying with the type of connection
7.3.2
Resistance
A connection must resist the forces to which it will be subjected during its lifetime. Some of these forces are apparent, for example those caused by permanent and imposed actions, wind, earthquake, and soil or water pressure. Others are not so obvious and are frequently overlooked. These are the forces caused by restraint of volume changes in the elements (see below) and forces required to maintain stability. Instability can be caused by eccentric loading (intentional or unintentional), as well as lateral actions
from wind and earthquake. Very often, measures taken to resist instability will aggravate the forces caused by volume changes, and vice versa. The connection resistance can be categorised by the type of forces to which it is subjected, viz: •
Compression
•
Tension
•
Flexure
•
Shear
•
Torsion.
Many connections will have a high degree of resistance to one type of force, but little or no resistance to another, eg a connection may have a high shear capacity and little or no moment capacity. For a given type of connection it may be unnecessary, or even undesirable, to provide a high capability to resist certain types of forces. In any structure, the number of connections designed to transfer axial force, shear and moment, should be minimised consistent with stability requirements. The remainder should be designed to allow movement and generate minimum force build-up, eg floor units continuous over a number of bays may have a simple support every third bay.
7.3.3
Ductility
For the purpose of design of connections, 'ductility' is defined as the ability to accommodate large deformations without failure. In structural materials, ductility is measured by the amount of deformation that occurs between first yield and ultimate failure. This definition is different from that used in Chapter 5 Analysis and Design of Buildings when discussing earthquake design. Ductility in building frames is usually associated with moment resistance (rotational ductility) and in the case of precast structures has a major impact on connection design. Flexural or direct tension are normally resisted by steel components, either reinforcing bars or structural steel sections. Connections are proportioned so that first yield occurs in this steel component, and final failure may be from rupture of the steel, crushing of the concrete, or a failure of the connection of the steel to the concrete. 7.3.4
Volume change considerations
The supports and connections of a precast member must take into account shrinkage, creep and temperature effects within the member and from the surrounding structure. Resistance to these strains results in the build-up of large forces and cracking, usually the support area of elements the member where it is least in desirable. Prestressed rarely exhibit cracking at locations further from the ends than the transfer length of the strand. Concrete can accommodate a limited amount of restraint by plastically deforming (creeping) concurrently with the strain; however, short-term effects such as temperature changes result in immediate load build up. If it is necessary to resist restraint forces some judgement is necessary in quantifying the restraint force. About half of the shrinkage will have taken place by the time a unit is fixed and a small movement at a support or flexing of the member can dissipate a large proportion of temperature strain.
7.3.5
Durability
Connections must be protected from degradation by their environment for the expected life of the structure. Failure to do so will result in corrosion of exposed steel components and cracking or spalling of concrete in the vicinity of cast-in metal fitments. Reinforced concrete connections should meet the provisions of AS 36007.2 Section 4. The use of corrosion-resistant materials (eg stainless steel) is usually required in exposure classifications B1, B2, C1, C2 and U (as defined in AS 3600) when directly exposed or the required concrete cover cannot be provided. Mild steel connections should be hot-dip galvanised but can be coated with other corrosion-resistant materials. Dissimilar metals, including different grades of stainless steel, should not be directly coupled in moist conditions. All exposed connections should be periodically inspected and maintained and this should be taken into account in the design of the structure. Table 7.1 provides guidance for typical material types and coatings for applications in various exposure environments. Comments on the various types of coatings are given in Chapter 3 Materials and Material Properties, along with a fuller discussion on the topic.
7.3.6 Fire resistance Many precast concrete connections are not vulnerable to the effects of fire and require no special treatment. For example, the bearing between slabs or tee-units and beams do not generally require special fire protection. If the slabs or tee-beams rest on elastomeric pads or other combustible materials, protection of the pads is not generally needed because deterioration of the pads will not cause collapse.
Other connections should be protected from the effects of fire to the same degree as that required for the members connected. The requirements in the BCA7.3 will need to be satisfied in this regard. For example, an exposed steel bracket that supports The preferred course is to allow enough elasticity or a beam has to be protected because it may be movement in the connection to keep induced restraint softened enough to cause failure. Steel connections forces within the nominal tensile strength of the can be protected by encasing in concrete or spraying concrete.The movement required is usually quite small. with fire-protection material. Other methods A variety of means are available, viz: are enclosing with plasterboard or coating with intumescent paint. • Neoprene bearing pads acting in shear •
Flexible metal connection
•
Oversize or slotted holes
•
Compressible material at dowels
•
Offset supports to allow flexing of the member.
Even when provision has been made for dissipation of these strains, supports and the supporting member should be designed for a minimum restraint force of 20% of the vertical reaction.
There is evidence that exposed steel hardware used in connections is less susceptible to firerelated strength reduction than other exposed steel elements. This is because the concrete elements provide a heat sink, which draws off the heat and reduces the temperature of the steel.
Table 7.1 Materials and Coating for Cast-in Items in Various Environments
3.11
7.3.7
Production issues
Maximum economy of precast concrete construction is achieved when connection details are kept as simple as possible, consistent with adequate performance and ease of erection. Furthermore, complex connections are more difficult to control and will often result in poor fit in the field. This can contribute to slow erection and less satisfactory performance.
The following checklist is suggested to improve production procedures: •
At connections, the need for extra reinforcement, embedded plates, inserts, blockouts, etc often causes congestion. Frequently, the number of items concentrated into an area may make it difficult to properly compact the concrete and maintain cover. In some cases, it may be economical to increase the element dimensions to avoid
congestion. Dapped or recessed ends require special reinforcement in a constricted space and careful detailing is needed. These areas should be drawn to a scale of at least 1:5 to check fit of the connection hardware and reinforcement. •
•
•
Reinforcing bars and prestressing strands or ducts, which appear as lines on drawings, have real cross-sectional dimensions. Reinforcing bars are larger than the nominal diameter because of the deformations. The effect on cover and clearance must be considered in the design phase. Large bars require anchorage lengths and hook sizes that may be impractical. It may be better to use welded bearing plates or other types of mechanical anchorages than to rely on bond or cogs for critical anchorages. Bends in reinforcing bars require minimum radii, which can cause fit problems or lead to loss of cover. Generally, and especially if congestion is suspected, details of the area in question should be drawn to a scale of at least 1:5 to ensure everything can be fitted together and concrete placed and compacted. Anchorage of stirrups and their bend radii must be taken into account in locating main bars and strands. Remember that elements are cast in moulds with concrete deposited from the top and sufficient space for vibrators should be provided.
•
Repetition is desirable for economy and quality assurance. Similar details should be identical even if it results in some over-design. Fewer mould changes will improve production scheduling. Wherever possible, cast-in hardware such as inserts and steel sections should be standard items that are readily available. Special items for a project should be standardised in size and shape as much as possible. For example, if half of the cast-in inserts are required to receive 20-mm-diameter bolts, and the other half 24-mm-diameter, the use of the larger size for all will avoid the chance of error.
•
Fixings that project through the mould and require cutting of the mould, are difficult and costly to place. Where possible, these fixings should be placed only in the top of the element as cast.
•
Items that are embedded in the element, such as inserts, plates, reglets, etc require time and care to locate precisely and attach securely.
•
Tighter dimensional tolerances than industry standards are difficult to achieve. Tolerances, which have proved over a long period of time to be suitable for normal building construction, are given in Chapter 4 Tolerances. Connections that require close-fitting parts without provision for adjustment should be avoided.
•
Inserts used for lifting should not be easily confused with other erection or fixing inserts that may be of a lesser capacity.
•
Reinforcement can often be suspended from the mould so that bar-chairs are not required on the finished face.
•
Precast concrete manufacturers should be allowed to use alternative details, methods or materials that meet the design requirements. Allowing alternative solutions will often result in the most economical and best-performing connection.
7.3.8
Construction issues
Much of the advantage of precast concrete construction is due to the opportunity for rapid erection of the structure. To fully realise this benefit, and achieve maximum economy, field connections should be kept simple. In order to fulfil the design requirements, it is sometimes necessary to compromise fabrication and erection simplicity. The following is a list of items that should be considered during the selection, design and detailing of connections to facilitate speedy and safe erection: •
Hoisting and connecting the precast elements is an expensive and time-critical process. Connections should be designed so that the element can be lifted, set, and unhooked in the shortest possible time. Before the crane can be unhooked, the precast element must be stable, secure and close to its final position. Elements such as slabs and double-Ts and hollowcore floor slabs are inherently stable and require no additional connections before releasing the crane. Other elements, such as columns, deep beams, wall panels and single-Ts require some supplementary shoring, guying, or suppor t. Bearing pads, shims, or other devices upon which the element is to be set should be placed ahead of hoisting, while loose hardware to complete the connection should be ready for quick attachment.
•
In some cases, it may be necessary to provide temporar y fasteners or levelling devices, with the permanent connection made after the crane is released. These temporary devices must be given careful attention to ensure that they will hold the element in its proper position during the placement of all elements that are erected before the final connection is made.
•
A certain amount of field adjustment at the connections is always necessary. Normal fabrication and building tolerances preclude the possibility of a satisfactory fit in the field without adjustment. Adjustment can be allowed for by slotted or oversize holes for bolts and dowels, by field welding, shims and grout for other items.
•
Connections should be planned so that they are accessible either from the completed structure or a stable deck or platform. The type of equipment necessary to perform such operations as welding, post-tensioning, or pressure grouting should be considered. Operations that require welding in an overhead position should be avoided. Room to place wrenches on nuts and swing them in a wide arc should be provided. Dry-packing column or wall panel bases in confined spaces is difficult.
•
Materials such as grout, dry-pack, cast-in-place concrete and epoxies need special provisions if they are to be placed in abnormal weather. Welding may require special procedures such as pre-heating when the ambient temperature is low. Connections that require these types of processes to be completed before erection can continue are costly.
•
Reinforcing bars, steel plates, dowels and bolts that project from the precast element can be damaged if care is not taken during handling. It is better to use a ferrule than a projecting bolt. Anchor bolts that project from cast-in-place footings should be at least 24 mm in diameter so that there is less chance of them being bent. Threads on projecting bolts should be protected from damage and rust.
•
High-strength cast-in bolts should not be used as these are prone to brittle subject to special requirements forfracture welding.and It isare much safer to use a larger diameter bolt if required.
In summary: •
Standardise products, details and hardware
•
Avoid reinforcement and hardware congestion
•
Avoid penetration of forms
•
Reduce post-stripping work
•
Be aware of material sizes and limitations
•
Consider clearances and tolerances and avoid non-standard production and erection tolerances
•
Plan for the shortest possible crane hook-up time
•
Provide for field adjustment
•
Use connections that are not susceptible to damage in handling
•
Ensure the panel has stability when the crane is unhooked, by using temporary connections if required, and allow for later adjustment for correct alignment
•
Try to locate the connections of a member so that they are all accessible from the same floor level.
Design methods are based on limit-state principles, unless noted otherwise, and incorporate the load factors (specified in AS 1170) and capacity reduction factors (f) specified in the relevant Australian Standards. In the design of any connection, all actions should be considered, including dead and live load, wind and seismic forces, forces from restraint of volume-change strains, forces induced by restrained differential movements between the element and the structure, and the forces required for stability and equilibrium. Determination of these forces should be in accordance with the relevant Australian Standards.
Haunches, corbels, and similar seats for flexural elements should be designed so that the flexural element’s ultimate resistance will be fully developed prior to that of the support. This will ensure that adequate warning of an impending flexural failure is provided. The increased capacity of the supporting element is a matter of judgement for the particular circumstances but would not normally exceed 30%. Flexural elements seated on bearing pads should be designed for the vertical load at that joint and also for the horizontal load imparted to the element as a result of the longitudinal restraint provided by the bearing pad. This restraint force is generally small and it is usually sufficient to design for a horizontal load equal to 20% of the vertical reaction load. Where bearing pads are not used, Table 7.2 (Clause 7.7.4) can be used to estimate lateral restraint loads.
•
7.5.1
General design principles for cladding panels
In addition to the above general principles the following should be followed in the design of connections for precast cladding units: • Connections should be designed to transmit the calculated forces, taking account of temperature, creep and shrinkage of the member and movements in the supports. If there is doubt about the action of an unusual connection or its load capacity, examples should be load-tested. An experienced precast manufacturer will have encountered most fixing types and will be able to give advice on the capacity of typical connections. •
Panel connections must resist the self-weight of the panel in combination with the external forces imposed on them. The main external forces are from wind and earthquake. Induced forces may also arise from movement of the panels and building frame due to shrinkage and creep. Concrete building frames shrink about the same amount as the precast units attached to them. However, differential deflection of supporting members needs to be considered. The restraint forces can be calculated with reasonable accuracy and resisted or dispersed by simple detailing. Temperature variation will cause panels to bow and move axially, giving rise to restraint forces at the fixings. For panels, such as spandrels, that are supported near each end by cantilever haunches or brackets, the bowing nearly compensates for the axial movement and the fixing is generally able to absorb the small differential dimensional change.
•
loads. Clearance holes and packing at the restraint fixings absorb building tolerances and isolate the panel from differential movement of the structure. Other support methods substitute steel fabricated sections for the haunch and clips for the restraint angle. •
Units should be provided with fixings functioning as illustrated in Figure 7.1. The arrows show the freedom to movement that can be provided at each of the fixings in the plane of the panel. Each of the fixings must provide resistance to stability, wind and earthquake forces perpendicular to the plane of the panel. The above points are summarised in Figure 7.2.
•
Frequently, the centroid of load of the precast unit and the line of the support do not coincide. This leads to a rotation effect on the unit with eccentricity forces being generated on the fixings. The design of the component fixing must provide stability resistance to these forces, Figure 7.3. Where these fixings are to steel beams, possible rotations and torsional twisting of the beam may need to be taken into account.
Figure 7.1 Typical Panel Fixings
The panel should be attached to the building frame so as to reduce the effects of any supportinduced forces. This means that the panel should be supported in a statically-determinate manner. Thus there should be no more than two supports and two restraints. Supports and restraints should be as far apart, vertically, as the panel dimensions and structure permit; small lever arms make the panel susceptible to out-of-plane rotation. Spandrel panels should be supported at floor level and restrained at a column or other vertical member rather than at the soffit of the supporting member. This prevents creep rotation of the edge member affecting the alignment of the panel.
The entire weight of the unit should be carried at the one level. The restraint fixings should preferably be accessible from this level for ease of erection. The panel weight should be carried in direct bearing if possible. The preferred fixing system to a building frame consists of two concrete haunches (corbels) and two restraint angles. This gives a robust but flexible attachment of the panel to the structure. Dowels are commonly used in the haunches to resist lateral
Figure 7.2
Figure 7.3
Design Principles for Cladding-Panel Connections
Examples of Eccentricity Forces
1
3
2
4
5
6
7
8
9
10
•
•
The member and its supports may also be subjected to loadings during construction for which they may not have been designed. The forces on the member and its connections may be higher or different to those in service due to load eccentricities, impact during placement and temporary restraints. These should be assessed and taken into account at the design stage. Increasing the dead load by 30% is usually sufficient to account for incidental impact during handling and placement. For non-loadbearing units, the unit and its fixings should be detailed to ensure that unintended loading is not transferred from the unit above or from adjacent structure. Horizontal joints should be clear and unobstructed to prevent the unit being axially loaded as a result of building shortening or differential deflection. Unintended bearing between panels will result in spalling and
overload of fixings. Frequently, temporary erection shims are placed in joints between the units. They must be removed after erection to prevent permanent transfer of load between panels. •
The relative movements between precast unit and support structure should be allowed for in the design width of the joint. The design of the building frame will provide estimates of the sway and deflections to be expected in the supporting structure. Provision should be made to accommodate these without impar ting load to the unit, generally by clear ance or flexibility in the panel fixings.
•
•
The capacity of an insert depends largely on its depth of embedment, spacing and distance from free edges. Inserts should be located in both the panel and the structure with this in mind and local reinforcement provided to ensure ductility. All inserts that resist primary load should be fitted with anchor bars of adequate embedded length. They should be plugged to ensure they are kept free of debris prior to erection. Lifters located in panels at joints should be either hot-dipped galvanised steel or stainless steel.
Figure 7.4 Concrete or Steel Corbel Bearing Connections
It is recommended that the smallest bolt used be an M20 size and it is not uncommon for designers to allow one bolt out of two to carr y all the lateral loads.
7.5.2
Cladding panel connection categories
There are many possible combinations of anchors, plates, bolts and angles, etc to form various connection assemblies. They can be divided into three broad categories: bearing; restraint; and industrial wall panel connections.These three categories are discussed below and typical arrangements illustrated. The details shown are not to be considered 'standard' but are presented as ideas on which to build. 7.5.3
Bearing connections
Bearing connections transmit load by direct bearing of one unit on another or the structure. Particular care should be taken in the detailing to prevent cracking in the supported as well as the supporting member. The interface material must cater for the vertical, horizontal, and rotational forces. Some form of variable-thickness packing material is necessary to absorb tolerances, eg mortar, epoxy, pads or shims. High bearing stresses may be developed at edges of a bearing surface due to deflection and twisting of the supported unit, as well as mismatching of the bearing surfaces. This can cause cracking and spalling unless they are taken into account or avoided in the design of the connection. Chamfered or protected edges will alleviate this problem.
connection for cladding panels. These can be either concrete or steel. Typical concrete haunches, cast on a cladding unit, are shown inFigure 7.4(a) and (b). Haunches can also be fabricated from a rolled steel section such as an angle or channel, a plate on edge, or, for lightweight units (up to 3t), a plate on flat, Figure 7.4(c).
Haunches (corbels) are the preferred type of bearing
Concrete corbels should be designed in accordance with the principles given inSection 7.10. An example of the design of a concrete corbel, which can be applied to a cladding panel, is provided in Example 7A.3 (Appendix 7A). An example of the design of a steel corbel is provided inExample 7A.1 (Appendix 7A).
7.5.4
Restraint connections
These stabilise the panel against out-of-balance gravity loads and resist horizontal loads. For ease of erection they should preferably be accessible from the same level as the suppor t fixings. The simplest is an angle as shown in Figure 7.5(a).
Panel-to-panel restraint connections can also be used in the horizontal direction to hold adjacent panels together (alignment connection). See Example 7A.1 (Appendix 7A) for design of steel corbel and top-restraint fixings for a cladding panel.
The panel-to-panel restraints shown in Figures 7.5(b) and (c) are suitable only if the lower unit is laterally restrained, eg by a corbel.
Figure 7.5 Restraint Connections
7.5.5
Industrial wall panel connections
The connection details and requirements are also similar for solid precast panels. The primary loading is wind or, sometimes, retained soil. They can also carry significant vertical structural load.
Precast wall panels are commonly used to clad single-storey industrial buildings. In these applications, the panels span vertically from footing to the roof level or are stacked horizontally and restrained at the columns. Details of typical fixings for hollowcore panels in vertical and horizontal configurations are shown in Figure 7.6.
These buildings often have a high risk of fire and the panel connections must be designed to keep the external wall attached to the building during the fire in accordance with the BCA7.3.
Figure 7.6 Hollowcore Wall Panel Connections
The forces on the fixings will be determined by the relative deformation of the structure and the panel. The material properties at elevated temperature (or appropriately factored forces) are required to be used in checking the adequacy of the fixing design. As a guide, for a design temperature of 750°C the force factor for concrete and embedded steel is about 2.2 and for exposed steel, about 6.6. Concrete panels bow inwards during a fire, expand laterally and joints close. A concrete supporting structure will deform relatively little, while steel structures deform significantly. Steel frames consisting of columns and rafters will usually collapse inwards due to the rafter failing and draping in tension between columns. With the use of clips to restrain panels, designers should be aware that there are limited tolerances on erection. While this will not usually be a problem with industrial buildings and similar, for commercial buildings with high-quality finishes where lining and levelling of panels is important, then packing or other types of connections may be required.
7.6.1
Column units
Introduction The connections of a column element must be detailed to carry the required design loads in service and to allow quick of and easy erection. Therecolumns are a number of means splicing or connecting into a structure; the two most common are by grouted dowels and by steel base plates. Precast concrete units are accurately-made factory products. Advantage can be taken of this by connecting precast unit to precast unit. Column connection detail principles. •
The column length between splices should be as great as possible to minimise the number of joints and the number of units to be erected. Internal columns are usually single-storey because of bracing requirements and to avoid corbels or cutouts to support floors. External columns, however, have a typical length of two storeys in multi-storey construction. Three storeys is a normal maximum. The columns must be braced and not rely on the splice for frame stability.
•
The connection should be easily accessible during construction, located in a zone between floor levels and, say, 1.5 m above the floor. The latter will place it where bending is low. Any changes in column section should be located at floor level..
•
The type of connection is selected on convenience and cost. The most convenient is the bolted baseplate; the most economical is the grouted pocket. The grouted pocket is usually used only at footing level. A baseplate connection is the quickest to erect. Plumbing is by adjusting the holding-down or connection bolts, the column is immediately stable and the crane can be released. The baseplate is flush with the outside of the column for intermediate splices. In this case, the bolts are housed in recesses at the corners of the section.
•
Dowelled connections are economical but require the column to be separately stabilised until grouted. Two or three braces are required for stability. These are secured to the main structure and are adjustable for plumbing of the column.
•
There are a number of techniques for forming the dowelled splice. Usually, the column bars project from the unit below into core holes formed in the unit above. This can allow an insitu floor slab to be carried directly on the column with the bars projecting through. Proprietary splice sleeves are available to form the core hole, these minimise
the bond length required. The column bars may also project from each unit and be connected by welding to splice angles or by fusion. However, this requires very accurate construction. •
The number of bars to be spliced at the joint should be a minimum to avoid congestion and simplify erection. Eight bars is a practical maximum. Load can be transferred through the connection by bearing, with most of the column bars being discontinuous. Extra ties may be required to carry
•
Core holes may be grouted by pouring directly into access holes in the side of the column or by pumping into holes drilled or cast into the duct near the base. This ensures that all air is displaced, see Figure 7.7.
•
The duct size must be large enough to provide sufficient erection tolerance and clearance and to permit free flow of grout around the bars. Generally, a duct size two-and-a-half to three times the bar diameter is satisfactor y. The horizontal joint
local stresses. design References 7.4For and 7.5. of these connections see •
The mixing of the grout must be properly controlled and tested to ensure that the design strength is achieved. Premixed and proprietary grouts are the best means of doing this. The designer should examine the products available and specify a particular product type rather than employ generic names such as non-shrink grout.
between should betowide enough provide adequate units tolerance and permit free to flow of grout throughout the bearing area. A width of 20 to 25 mm is generally adequate. •
The pressure of the grout at the joint can be considerable. A 1.5-m head will lead to a pressure of 36 kPa. Thus the joint needs to be very securely sealed.
Figure 7.7 Typical Loadbearing Column Connections
7.6.2
Wall units
•
Hard packers used for levelling during erection must be removed. These create a stress concentration that leads to vertical splitting and spalling of the unit. Plastic packers or similar, which can deform under long-term load, should be used when they must be left in place. The packers should be located at points where a stress concentration would be least critical.
•
Progressive collapse must be considered in loadbearing wall panel construction. Providing
Introduction The design of connections for loadbearing wall elements follows principles similar to those given above for column units. Loads are transmitted either by direct bearing or by dowelled connections. Close attention to detail, planning, manufacture and site activities is required. Wall unit connection detail principles • Generally, the principles given for column units above apply. Reference should be made to these. •
•
Load transfer is through grout or dry-packed mortar. Figure 7.8 shows typical examples: (a) with the horizontal joint at slab level; (b) similar but with the joint clear of the floor where it is more accessible and visible; (c) a thickened wall panel where a double row of long dowels provides moment resistance as well as bearing support. Lateral joints are left open or are connected by insitu grout or concrete infill sections.
alternative load paths in the structure by continuity of reinforcement across joints helps achieve this. •
Realistic erection tolerance should be provided for.
•
Loading from floor and roof structure usually applies eccentric loads on wall units. Connections and the members must be designed for realistic eccentricities.
•
Details for shear connection between panels to form shear walls are shown in Figure 7.9.
•
Welded-type connections should be avoided where possible as they are difficult to make in confined spaces and are expensive.
Figure 7.8 Typical loadbearing wall connections
Figure 7.9 Shear wall details
7.7.1
General
Bearing pads are used when it is necessary to distribute or alleviate the build-up of forces at supports by allowing displacements and rotations to occur at those supports. Generally, these stresses or movements are small in normal building construction and a combination of good detailing (with concreteto-concrete bearing or cast-in steel bearings) is sufficient to prevent local damage. Where bearings are required for slab-type units, and movements are small, the simplest are slip-joint bearings made of strips of metal such as stainless steel, bitumen-coated aluminium or a hard plastic. These materials do not compensate for uneven interfaces; this requires a deformable material such as an elastomer. Natural or synthetic rubbers are commonly used for this purpose and as an example, elastomeric strips for use under hollowcore and solid slabs are 50 to 100 mm wide and 3 to 6 mm thick. For slab type bridge construction, elastomer strips are 150 to 300 mm wide and 20 to 30 mm thick. Where concentrated loads are involved in building and road or rail bridge structures, plain and laminated elastomeric bearings are suitable up to 35-m spans. These bearings are designed for the specific application and are 6 to 150 mm thick. 7.7.2
Design details
Selection of a bearing pad or strip will depend on the following considerations: •
Loads normal to the bearing surface
•
Misalignment resulting from construction tolerances
•
The effect of hog due to prestress
•
Rotation due to bending under applied loads
•
The effects of creep and shrinkage
•
Movement due to differential temperatures.
Specialist bearing manufacturers can advise on the correct bearing pad for the particular application. An elastomeric bearing accommodates translation and rotation by elastic deformation. Its shape and dimensions influence the deflection under compression, shear and rotation. Various grades of elastomeric bearings are available with different properties and behaviour. The shape factor varies depending on whether the pad is plain or laminated. The shape factor of a layer of elastomer is the area under compression divided by the area free to bulge.
The thickness of the pad is proportioned for the displacement and rotations expected and it deforms and recovers in response to the movement of the suppor ted member. Bearings requiring larger rotations or deformation are made up of a number of layers of rubber bonded between steel plates. Large longitudinal displacements (up to 50 mm) are catered for by combining an elastomeric pad and polished stainless-steel plate with a PTFE (Teflon) interface and lateral side restraints. The maximum compressive stress on plain and laminated bearings should generally not exceed 5 and 15 MPa respectively. The shear displacement should be in the range of 30–40% of the bearing thickness. Laminated bearings under high compression and having thick layers of elastomer should be checked to ensure the plates are not overstressed in tension. A large range of standard elastomeric bearings is specified in AS 5100.47.10 and the Bridge Design Set, AS 51007.11. Further details should be sought from a specialist bearing manufacturer.
The bearing should always be horizontal under permanent dead load. To achieve this, the bearing surface may need to be cast at an angle (ie recessed) to the axis of the member to compensate for the longitudinal gradient and rotation of the member at time of placement. When bearings are recessed in this way, care should be taken in detailing to ensure that the reinforcement and prestressing steel are located such that concrete covers are maintained. Alternative solutions to recessing are a tapered compensator plate bolted or dowelled to the underside of the member or to the top surface of the bearing. The bearing should be set back from the edge of a bearing surface a minimum distance of 25 mm to allow for spreading of the elastomer under load. Where bearings are subjected to shear displacements and/or rotations in two directions, circular bearings rather than rectangular are a better choice. 7.7.3 Material requirements Elastomeric pads are usually manufactured from natural rubber of IHRD 53 hardness and having properties that comply with the requirements of AS 5100.47.10. Natural or synthetic rubber having other hardness and properties may be used provided the in-ser vice performance is equivalent. Laminated bearings consisting of steel plates bonded into the elastomer during vulcanising should have an edge cover to the plates of not less than 6 mm to protect them from corrosion. Wherever possible, bearings should be selected and tested from the standard sizes in AS 5100.4.
7.7.4
The static coefficients of friction shown in Table 7.2 are conservative values for use in determining the upper limit of volume change forces for elements without 'hard' connections. Thus, the maximum force restraining axial movements, Fn, can be determined by:
Friction under bearings
Fn = ms Ndf where: F
n
ms
= design friction force = static coefficient of friction as given in Table 7.2
Ndf = factored dead load force normal to the friction face If temporary loads are to be resisted by friction, the coefficients in Table 7.2 should be divided by a factor of 5. Note, for vertical elements such as walls, AS 3850 7.16 requires a minimum of two positive fixings.
Table 7.2
The shear-friction design method is not covered by AS 36007.2. However, it can be used under the alternative approaches permitted by the BCA 7.3. The method is applicable to situations where it is inappropriate to assume that shear stresses are uniformly distributed over the depth of an element. A crack is assumed to occur in the shear area along a plane located in the most undesirable manner. Shear transfer across this plane is achieved by placing reinforcement across the assumed crack to create a force normal to the shear plane. This normal force in combination with friction at the crack interface provides the shear resistance. Where an area of shear-friction reinforcement, A vf, is placed perpendicular to the shear plane, the factored resisting shear stress, vr, is computed by: vr = fc (c +
Static Coefficients of Friction of Dry Materials
mse) fc f'c
ms
but shall not exceed 0.25 nor 7.0 fc MPa.
where: c = cohesion stress
se = effective normal stress m = coefficient of friction fc = 0.65 Values of c and µ, for certain crack interface conditions, are shown in Table 7.3.
Table 7.3 Values for c and
m for Given Interface Conditions
m
The effective normal stress, se, is calculated by: se = rvfsy + N/Ag where: A
g
= gross area of the shear plane
fsy = specified yield strength of reinforcement N = unfactored permanent compressive load perpendicular to the shear plane
rv = ratio of shear friction reinforcement The ratio of shear friction reinforcement, rv, is: rv = Avf /Acv where: A
cv =
area of concrete section resisting shear transfer
Avf = area of shear friction reinforcement
Except for continuously-grouted horizontal joints between wall elements, and for uniform bearing of hollowcore and flat slabs, it is recommended that reinforcement be provided in the bearing area of concrete This as reinforcement can be 7.8 designed by elements. shear-friction discussed in Clause above. Referring to Figure 7.10, the reinforcement Avf + At across the assumed vertical crack plane is determined to resist N f directly, and Vf by shearfriction. The area of vertical reinforcement across potential horizontal cracks can be calculated by: Ash =
The area of shear friction reinforcement, Avf, should not be less than:
1.0
where: c
Avf(min) = 0.9 Acr /fsy where: A cr = area of the crack interface
All reinforcement, on both sides of the assumed crack plane, should be properly anchored by development length or welding to angles or plates to provide mechanical anchorage.
fsy.f m
+ At)
f
- c Acr
= the cohesion stress (see Table 7.3)
b = average width of element Acr = Lsy.t b ld = development length of A vf bars in accordance with AS 3600, Clause 13.1.2.1
= additional amount of reinforcement resisting tension across shear plane
fs = 0.85
sy(Avf
fsy.f = minimum specified yield strength of Ash reinforcement, eg stirrups
computed by: At = N t /fsfsy t
f
fsy = minimum specified yield strength of flexural reinforcement
Tension, Nt, acting across the shear plane should be resisted by an additional amount of reinforcement
where: A
Fitments or mesh used as shear reinforcement can be considered to act as Ash reinforcement. See Example 7A.2 (Appendix 7A) for design of Reinforced Bearing for a Rectangular Beam.
See Example 7A.2 (Appendix 7A) for use of shear friction. Figure 7.10 Reinforced Concrete Bearing
q
7.10.2
7.10.1
General
The plane-sections assumption of flexure theory does not apply to the portion of a member for a length approximately equal to its height from a force
discontinuity or a geometric discontinuity. Figure 7.11 illustrates the regions as ‘B-Regions’ for flexure areas within a beam (or other member) and ‘D-Regions’ at areas of discontinuity. For design purposes, D-Regions can be idealised as a truss composed of a series of axially-loaded compression struts and tension ties connected at nodes and transferring loads to the supports or to adjacent plane flexure regions. The truss model described in this section is based on Appendix A of ACI 318M–08 7.6.
up the model all have finite dimensions that must be taken into account in selecting the dimensions of the truss, Figure 7.13. Once the geometry of the truss is known, the forces in the struts and ties are determined by statics in equilibrium with the applied loads and the reactions. For equilibrium at least three forces should act on a node. Nodes are classified according to the signs of these forces as C-C-C (all compression), C-C-T (compression-compression-tension) and so on. Ties are permitted to cross struts. Struts can cross or overlap only at nodes. The angle between the axes of any strut and any tie entering a single node should not be less than 25°.
Figure 7.11 Examples of Design Regions Within a Member
Truss geometry
The geometry of the notional truss is determined by following the flow of forces from the support reaction into the body of the supported element, Figure 7.12. The intersection of compressive struts with tension ties or support reactions delineates the nodal zones. The axes of the struts and ties are chosen to approximately coincide with the axes of the compression and tension fields in the real beam7.14,7.15 . The struts, ties, and nodal zones making
Figure 7.13
Extended Nodal Zone showing Effect of Distribution of Reinforcement in Tie
Figure 7.12
Description of Strut-and-tie Model
Struts Struts are normally idealised as prismatic or uniformly-tapered members. They can also be thicker at mid-length where the compressed concrete can spread laterally into the adjacent concrete to form a bottle-shaped strut. The cross-sectional area of a strut is taken as the smaller of the cross-sectional areas at its two ends. Ties A tie consists of reinforcement or tendons plus a portion of the surrounding concrete that is concentric with the axis of the tie. The surrounding concrete is included to define the zone in which the forces in the struts and ties are to be anchored. The concrete in a tie is not used to resist the axial tension in it. The effective thickness in elevation of a tie for design can vary with the distribution of reinforcement in it. If the bars are in one layer, the effective thickness can be taken as the diameter of the bars in the tie plus twice the cover to the surface of the bars. Multiple-layers of reinforcement should be distributed approximately uniformly over the thickness and width of the tie. The reinforcement in ties can be anchored by hooks, mechanical anchorages, post-tensioning anchors, or straight bar development so that the following objectives are achieved, Figure 7.13: •
The nodal zone develops the difference between the tie force on one side of the node and the tie force on the other side;
•
At nodal zones anchoring one tie, the tie force is developed at the point where the centroid of the reinforcement leaves the extension of the bearing area or the assumed prismatic outlines of the struts anchored by the nodal zone and enters the span, whichever is the larger;
•
At nodal zones anchoring two or more ties, the tie force in each direction is to be developed at the point where the centroid of the reinforcement in the tie leaves the extended nodal zone.
Nodes A nodal is termed to hydrostatic when its loaded faces arezone perpendicular the axis of the struts and ties acting on it and the loaded faces have equal stresses. In a C-C-C nodal zone, the ratios of the lengths of the sides of the node are in the same proportions as the three forces acting on it. A C-C-T nodal zone can be represented as a hydrostatic node if the tie is assumed to extend through it to be anchored by a notional plate on the far side of the node. The size of the notional plate has to result in bearing stresses that are equal to the stresses in the incoming struts.
7.10.3
Design basis
The design of struts, ties and nodal zones is based on the requirement:
fFu ! F* where: F* = design action effect in a strut or tie or acting on a nodal zone due to the factored loads. It is the largest force in the element for all load cases. Fu = ultimate strength of the strut, tie or nodal zone
f
= strength reduction factor with a value of 0.7 [AS 3600, Table 2.3(h)].
Strength of struts The ultimate compressive strength of a strut without longitudinal reinforcement, Fns, can be taken as the smaller value of the effective compressive strength of the concrete in the strut and the effective compressive strength of the concrete in the nodal zone at the same end of the strut.
Fns = f cu Ac where: A
c
= cross sectional area at one end of the strut.
The effective compressive strength of the concrete in a strut can be taken as: fcu = 0.85 bs f’c where: bs = 1.0 for struts in which the area of the mid-strut cross section is the same as that at the nodes, such as the compression zone of a beam = 0.6 for struts located such that the thickness of the mid section of the strut is larger than the thickness at the nodes (bottle-shaped struts) = 0.4 for struts in tension members, or the tension flanges of members = 0.6 for all other cases. The thickness of a strut used to compute A c is the smaller dimension perpendicular to the axis, at the ends of the strut. The width of struts in twodimensional structures may be taken as the width of the member. The value of bs may be increased for bottle-shaped struts if confining reinforcement is provided to resist the transverse tension developed across the strut7.6. The amount of transverse confining reinforcement is computed using a secondary strut and tie model with the spreading struts at a slope of 1:2 to the axis of the strut.
Longitudinal reinforcement within and parallel to the axis of the strut can be included in computing the ultimate strength of the strut. It must be enclosed in ties or spirals and adequately anchored. The strength of a longitudinally-reinforced strut can be taken as: Fns = f cu Ac + Asc fsc
where: A
sc =
area of the longitudinal reinforcement, mm2.
fsc = yield stress in A sc, MPa. Strength of ties The ultimate strength of a tie, F nt, can be taken as: Fnt = Ast fsy + Aps(fse + where: A
st
Dfp)
= area of reinforcement, mm2.
Aps = area of prestressing tendons, mm2.
fse = effective prestress in the tendons after all losses, MPa.
Dfp = increase in
stress in the tendons due to the factored loads, MPa.
(fse +
Dfp) ! f py, the yield stress of the tendon.
The axis of the reinforcement and tendons must coincide with the axis of the tie. Strength of nodal zones The ultimate compressive strength of a nodal zone, Fnn, can be taken as: Fnn = f cu An where: A
n
= area of the face of the nodal zone that F* acts on, taken perpendicular to the line of action of F*; or the area of a section through the nodal zone, mm2.
The calculated compressive stress on a face of a nodal zone due to strut-and-tie forces shall not exceed: fcu = 0.85 bn f’c where:
bn
= 1.0 in nodal zones bounded by struts or bearing areas or both (C-C-C)
= 0.8 in nodal zones anchoring one tie (C-C-T) = 0.6 in nodal zones anchoring two or more ties (C-T-T and T-T-T).
7.10.4
Design examples
Corbels The strut-and-tie model as discussed above applies to the design of corbels and is illustrated by Example 7A.3 (Appendix 7A) Corbel to a Column Supporting a Beam. Although this example is for a corbel to a column supporting a beam, it can be equally applied to a corbel on a cladding panel bearing on a structure. Half-joint connections Half-joint connections are similar to concrete corbels and may be designed using the strut-and-tie model as illustrated by Example 7A.4 (Appendix 7A) DappedEnd Connection for a Beam. Beam ledges The strut-and-tie model can be used in the design of continuous beam ledges supporting concentrated or uniformly-distributed loads. The truss model is very similar to the model used in dapped-end connection design and is illustrated by Example 7A.5 (Appendix 7A) Reinforced Beam Ledge for a Double-T Leg.
7.11.2
Failure modes
The following failure modes of an anchor are possible as shown in Figure 7.15: •
7.11.1
Introduction
Cast-in headed anchors provide a means of making bolted or welded connections to hardened concrete. The usual forms of these anchors are illustrated in Figure 7.14. They are: • Ferrules, which are internally-threaded steel sleeves to take a bolt. They are anchored by transverse reinforcement attached to the base by welding, or, through a transverse hole in the base of the sleeve, or, with a J-bolt screwed or buttwelded to the base. •
Studs with a forged head to form a bearing area at the base and welded to a surface plate for attachment of a fixing.
•
Bolts or embedded nuts which can have the bearing area augmented by a washer or plate.
•
Threaded bars anchored in the concrete by a hook (J-bolts), typically used as foundation bolts.
•
Threaded bars anchored in the concrete by a right-angled extension (L-bolts), also used as
foundation bolts. A common application for a ferrule is the restraint fixing for a wall panel. Ferrules for structural connections are typically 75–100 mm long and the usual bolt diameters are 20 and 24 mm. Plates with stud anchors provide a means to weld a fixing bracket in its correct position after erection of the member. Such brackets should reach their yield strength prior to the load reaching the calculated capacity of the cast-in stud assembly. Figure 7.14 Types of Anchors
Steel failure of the anchor shaft or bolt in tension
•
Steel failure of the anchor shaft or bolt in shear
•
Breakout of a prism of concrete surrounding the anchor in tension
•
Breakout of a prism of concrete towards an edge in shear
•
Crushing of the concrete over the bearing area at the base of the anchor (pullout)
•
Rotational pryout of an anchor body subjected to shear
•
Side-face blowout when the anchor is located close to an edge in conjunction with deep embedment (greater than 200 mm)
•
Splitting of the concrete in the vicinity of the anchor.
The capacity of the bolt or stud is checked in accordance with AS 4100, Steel structures7.12. The strength of any welding or means of attachment of the bearing at the base of the anchor must exceed the breakout capacity and the pullout capacity in bearing. The effective bearing length of transverse anchor reinforcement may be taken as twice the diameter of the bar and that of a plate or washer as twice its thickness measured from the side of the anchor shaft. The anchor capacities calculated in accordance with the relationships below are based on the following assumptions: •
The minimum characteristic strength, f ’c, of the concrete is 25 MPa and the maximum is 65 MPa
•
The maximum anchor diameter,d o, is 40 mm
•
The maximum effective depth, h ef, is 200 mm
•
The minimum centre-to-centre spacing of anchors welded to an attachment plate is 65 mm.
The effective depth, h ef, is the distance between the concrete surface and the embedded bearing area of the anchor. The limitation on effective depth is intended to ensure that side-face blowout does not govern for the above edge distances.
7.11.3
The CCD method
At failure in tension, the concrete surrounding an isolated anchor fractures along a surface in a flat conical shape, Figure 7.15(c). The failure load is proportional to the tensile strength of the concrete and the area of the fracture. A cone provides the closest shape but leads to overly complex expressions for the surface area, particularly if there are edges or intersecting failure cones.
7.6, 7.9, The Concrete Capacity Design (CCD) method 7.13 simplifies the geometry by assuming a flat-sided square pyramid so that intersections at the surface of the concrete are rectilinear as shown in Figure 7.16(a). The angle between the failure surface and the surface of the concrete and at edges is assumed to be about 35°. This means that the base length of the pyramid is 3 times the height, the effective embedment depth of the anchorage, hef. An edge distance of 1.5 hef is thus required to develop a complete failure cone.
Figure 7.15 Failure Modes for Anchors
Using the CCD model, the basic failure modes can be conveniently expressed as functions of the embedded depth and the square root of the concrete strength with the effect of influences, such as edges and overlapping zones, introduced as modifying factors. The equations below have been derived in this way and calibrated against existing test data as reported in the references. The equations and their constants are based on those in ACI 318M027.6 Appendix D and give characteristic (5% fractile) values. This code and the references should be consulted for detail on the application and limitations of the CCD method. The accuracy of the equations should be confirmed by test for an actual anchor shape and concrete strength. It should also be borne in mind that the capacity is a function of the tensile strength of the
Similarly in shear, a conical segment breaks out and is represented by a half pyramid as in Figure 7.15(h) with the apex at a distance of c 1 from the centreline of the anchor to the edge in the direction of the applied shear force, Figure 7.17. For a group of two or more anchors acting together, Figure 7.18, the effective edge distance is the distance to the furthest row selected as critical with the entire shear carried by this row alone. For two anchors acting together inline in a member whose thickness, h, is equal to or less than 1.5 c1 some judgment is needed in assessing c1. The edge distance c 1 may be to the nearest anchor with half the load on each, or it may be the distance to the furthest anchor with the total load taken by it with no contribution from the anchor nearer the edge. Both cases would require checking.
concrete and thus has considerable scatter. Prototype test data can be used in lieu of this section and is particularly appropriate for critical or highly-loaded edge or corner configurations.
7.11.4
Failure in tension
•
Breakout ca pacity The basic case, illustrated in Figure 7.16(a), is a single anchor at an edge distance equal to or greater than 1.5 hef in an area of concrete assumed to be cracked in tension at full working load, ie the concrete tensile stress sct ! f ’cf where f ’cf = 0.6Ãf ’c. The ultimate breakout capacity for the basic case is:
A factor, y3, accounts for the presence or absence of cracking in the breakout zone of the anchor or group.
The design tensile capacity is therefore:
Nb = 10 hef1.5Ãf ’c newtons where: h
•
A factor, y2, takes into account the presence of an edge within 1.5 hef of the centreline of the anchor in conjunction with the ratio AN / ANo. The factor y2 is a function of the edge distance c min, the minimum of c1 or c 2 in Figure 7.16(b).
N cb
= effective embedment depth,mm f ’c = characteristic compressive strength (" 65 MPa). ef
•
A ratio, A N / ANo, accounts for the shape of the failure surface of multiple anchors and their spacing. ANo is the breakout area of an isolated anchor, 3hef x 3h ef = 9h ef2, see Figure 7.16(a). AN is the breakout area of the actual anchor arrangement, see Figure 7.16(b) for illustrations of common situations.
)y No
= 0.6
y y 1
2
N 3
newtons
b
= ANo if anchor ! hef from an edge ANo =
y1
9 hef2,
Figure 7.16(a)
= 1.0 for a single anchor = 1/[1+ (2 e’N / 3 hef)] "1 and e’N " s 1/2 for multiple anchors
A factor, y1, accounts for the eccentricity, e’ N, of loading on a group of anchors. e’N is the distance of the tension force from the centroid of a group of anchors in tension in which s 1 is the distance between outer are anchors Only anchors inthe tension used in in tension. calculating thethe centroid of resistance and e’N is not to exceed s1/2 with loads determined elastically. See Figure 7.19(a) and (b).
N
f
AN = projected concrete failure area of the anchor or group of anchors at the surface as illustrated in Figure 7.16(b)
This equation is modified to take into account situations likely to be found in practice as follows: •
= f(A / A
where:
y2
= 1.0 if edge distance c min ! 1.5hef
= 0.7 + 0.3(c min / 1.5hef) if cmin <1.5 hef
y3
= 1.25 if analysis shows that sct < f ’cf in region of anchor at service load, otherwise 1.0.
When an anchor is located near three or four edges with the largest edge distance c max " 1.5hef the embedment depth hef used in any of the above equations is limited to cmax /1.5.
Figure 7.16 Projected Concrete Failure Area of Single Anchors and Groups of Anchors in Tension and Calculation of AN and ANo
Crushing capacity (pullout) of bearing at base of anchor The ultimate pullout capacity of an anchor secured in the concrete by a bearing area, Abrg, can be estimated as: Np = 8 Abrg f ’c newtons brg = stiff bearing area of anchor, mm2
where: A
The design pullout capacity accounting for the presence or absence of cracking is:
Npn = where:
f y4 Np newtons f = 0.6 y4 = 1.25 if analysis shows that sct < f ’cf in region of anchor
7.11.5
Failure in shear
Shear force toward an edge The basic case, illustrated in Figure 7.17, is a single anchor at an edge distance of c 1 in the direction of the load located in an area of concrete assumed to be cracked in tension, ie the concrete tensile stress sct " f ’cf at full working load. As the edge distance increases, the pryout capacity or shear capacity of the anchor material may govern and should be checked as well. The ultimate shear capacity for the basic case is: Vb = 0.6(lv /do)0.2 c 11.5 where: l
v
at service load, otherwise 1.0. Pullout capacity of a J-bolt The pullout capacity of a J- or L-bolt is based on bearing on the inside of the hook without any contribution from concrete bond to the shaft of the bolt. The ultimate pullout capacity can be estimated as: Np = 0.9 f ’c eh do newtons h = distance from the inner surface of the shaft to the outer tip of the J- or L-bolt
where: e
Npn = where:
fy4 Np newtons f = 0.6 y4 = 1.25 if analysis shows that sct < f ’cf in region of anchor
Figure 7.17 Shear Toward an Edge on a Single Anchor
•
•
The minimum edge distance for an un-torqued anchor is 50-mm cover The minimum edge distance for a torqued anchor is 6do
•
Minimum centre-to-centre spacing of anchors that will not be torqued is 4do
•
Minimum centre-to-centre spacing of torqued anchors is 6do.
Torquing does not include normal tightening of a fastening by the spanner size usually used for that bolt diameter, ie snug-tight.
at service load, otherwise 1.0. Splitting failure The following spacings and edge distance requirements should be adhered to unless supplementary reinforcement is provided to control splitting of the concrete in the breakout zone.
newtons
= hef for anchors with constant stiffness over full length (studs and ferrules)
do = diameter of bolt; and 3 do ! e h ! 4.5 do The design pullout capacity accounting for the presence or absence of cracking is:
Ödo Öf ’c
= loadbearing length of anchor, not exceeding 8 do
The equation for ultimate shear capacity for the basic case is modified to take into account situations likely to be found in practice as follows: •
A ratio, A V/ AVo, accounts for the shape of the failure surface of multiple anchors and their spacing. AVo is the shear breakout area of an isolated anchor at an edge. AVo = 1.5c1 x 3c 1 = 4.5c12 where c1 is the distance to the nearest edge in the direction of the shear force, see Figure 7.17. AV is the breakout area of the actual anchor
Figure 7.18 Projected Concrete Failure Area of Single Anchors and Groups of Anchors in Shear and Calculation of AV
•
•
A factor, y5, accounts for the eccentricity, e’ V, of loading on a group of anchors. e’V is the distance between the point of shear force application and the centroid of the group of anchors resisting the shear in the direction of the applied shear. In the equation for y5 below, s1 is the distance between the outside anchors in the furthest or critical row from the edge. The eccentricity, e’v, shall not exceed s1/2. The procedure is not applicable above this value, see Figure 7.19. If the shear load is above the plane of the concrete it should be resolved as a shear in the plane of the surface and a normal force that may cause tension in the anchors. A factor, y6, takes into account an anchor located near a corner. The limiting breakout strength is the minimum value in either direction. See Figure 7.18. A factor, y7, accounts for the presence or absence of cracking in the breakout zone of the anchor or group.
arrangement. See Figure 7.18 for illustrations of common situations. •
Figure 7.19 Definition of Dimensions e’N and e’V for Groups of Anchors
Reinforcement in the failure zone of the anchor increases the capacity when cracking exists. The y7 factor has three possible values depending on the confinement reinforcement provided. It is good practice to locate laterally-loaded anchors behind a longitudinal reinforcing bar equal to or greater than 12-mm diameter with stirrups at 100-mm centres each side. Minimum practical edge distance is about 100 mm for this detail. The bolt capacity will not be developed at this distance; anchorage of the top of
The design pryout capacity is given by: Vcp =
f kcp Ncb f = 0.6
where:
kcp = 1.0 for hef < 65 mm ef " 65 mm = 2.0 for h Ncb = ultimate breakout capacity in tension.
The design shear capacity is therefore: Vcb = f(AV /AVo) where: A
V
y5 y6 y7 V b
= projected concrete failure area on side face, Figure 7.18 = AVo if a single anchor
AVo = projected area of an isolated anchor in a deep member = 4.5 c12, Figure 7.17 c1 = edge distance in direction of shear force, mm
y5
= 1.0 if single anchor = 1/[1+ (2e’N / 3c1)] ! 1 and e’N ! s 1/2
y6
= 1.0 if edge distance c 2 " 1.5 c1 = 0.7 + 0.3(c 2 /1.5 c1) if c2 < 1.5 c1 c2 = edge distance perpendicular to c1, mm
y7
= 1.4 if no cracking ( sct < f ’cf) in region of anchor at service load.
= 1.0 if cracking at service load with no supplementary reinforcement or an edge bar is less than 12-mm diameter = 1.2 if cracking at service load and with edge bar " 12 mm placed between the anchor and the edge. = 1.4 if cracking at service load and with an edge bar " 12 mm placed between the anchor and the edge and stirrups spaced at 100-mm centres restraining the edge bar either side of anchor. Shear force parallel to a free edge For shear force parallel to an edge, V cb, can be taken to be twice the value determined for the perpendicular case above, with y6 taken equal to 1.0.
Concrete pryout Anchors remote from an edge may fail under shear loading by bodily rotating out of the concrete. Short anchors with hef less than 65 mm are more likely to fail in this manner.
the anchor by a U-shaped reinforcing bar is necessar y if this is required.
7.11.6
Combined tension and shear
The capacity of anchors under combined tension and shear loading can be estimated from the following relationships7.14. If V* ! 0.2 fVn full strength in tension is permitted If N* ! 0.2 fNn full strength in shear is permitted If V* > 0.2 fVn and N* > 0.2 fNn then: N*
fNn
+
V*
fVn
< 1.2
where: V* = factored shear load
fVn = design shear strength N* = factored tensile load
fNn = design
tensile strength.
Figure 7.21 Vertical Loads on Connection Angles
Angles used to support light precast elements can be designed by statics as shown in Figure 7.20. In addition to the applied vertical and horizontal loads, the design should include all loads induced by restraint of relative movement between the precast element and the supporting member or structure. The minimum thickness, t, of non-gusseted angles loaded in shear as shown in Figure 7.21 can be determined by:
t = [(4V f e v)/(fa fsy b)] 0.5 where:
fa = 0.9
b = width of angle ev = actual eccentricity, ev + 20-mm allowance for slotted holes. The tension on the bolt can be calculated by: Pf = (V f ev) / el For angles loaded axially, see Figure 7.22, either in tension or compression, the minimum thickness of non-gusseted angles can be calculated by: g) / (f f b)] 0.5
t = [(4N f
where:
a sy
fa = 0.9
Figure 7.22 Horizontal Loads on Connection Angles
g = gauge of angle (see
Figure 7.22)
b = width of angle
Figure 7.20 Design Relationships for Connection Angles
If in the analysis for erection loads or temporary construction loads before grout is placed under the plate, all anchor bolts are in compression, the base plate thickness required to satisfy bending is determined from:
Column bases must be designed for both erection loads and loads that occur in service, the former often being critical. Two commonly used base plate details are shown in Figure 7.23, although other details are also frequently used. Bars should be welded to the plates with full-penetration butt welds.
t=
SF 4 xc
Ö fa fsy b
where:
fa fsy
= yield strength of the base plate
SF
= greatest sum of anchor-bolt factored forces on one side of the column
If the analysis indicates that the anchor bolts on one or both of the column faces are in tension, the base plate thickness is determined from:
Figure 7.23 Column Base Connections
SF 4 xt
t=
Ö fa fsy b
where: x
= as shown in Figure 7.23.
t
Also, the base plate thickness, t, may be controlled by bearing on the concrete or grout. In this case, the base plate thickness is determined from: t = xo
where:
2fc g f ’c
Ö fa fsy
x
o
g
Vr = fa 0.66fsy b t
where:
f a = 0.9
The anchor bolt size should be determined using appropriate Australian Standards. When the bolts are near a free edge, as in a pier or wall, the buckling of the bolts before grouting may be a consideration. Confinement reinforcement, as shown in Figure 7.23, should be provided in such cases. A minimum of four 10-mm ties at approximately 75-mm centres is recommended for confinement.
= as shown in Figure 7.23 = [0.85 - 0.007(f’ c - 28)] but 0.65 ! g " 0.85
The factored shear resistance of a column base plate can be determined from:
= 0.9
xc, b = as shown in Figure 7.23.
The strength of the concrete when a bolt is in tension may be critical and can be determined by assuming a shear cone pullout failure as described for cast-in anchors. The length of the anchor bolts should be such that the concrete will develop the desired resistance of the bolt in bond and bearing on the hook projection or bolt head. The bearing area of bolt heads can be increased by welding on a washer or steel plate. The bottom of the bolt should be a minimum of 100 mm above the bottom of a footing, and above the footing reinforcement.
The ultimate tensile resistance of smooth anchor bolts, limited by the bond strength to the concrete, can be determined from: Pr = 1.7p fc db le where:
fc = 0.6 db = diameter of anchor bolt le = embedment length (minimum 150 mm)
The tensile resistance limited by the bearing strength of concrete under a bolt head or hook is: Pr = fc f ’c A where:
fc = 0.6
A = net area under bolt head or hook Compression on anchor bolts during erection can be substantially reduced by the use of steel shims. The bearing resistance of the concrete determines the required area of the shims.
Grouted dowels are often used where continuity of main reinforcement between a precast unit and its support is required, as in columns and wall panels. Core holes for moment connections should be formed with a proprietary splice sleeve or corrugated steel tube, eg post-tensioning duct as shown in Figure 7.24. If the connection is always in compression, then the core hole may be formed directly in the concrete. Cement grout is normally used but epoxy can enable short embedment lengths to be used. Non-shrink grout is required in moment connections. Recommended details for a cement-grout connection are as follows: •
The minimum concrete cover to a metallic conduit should be 75 mm to allow for ties or anchorage reinforcement.
•
The conduit should have a minimum clear, internal diameter of three bar diameters. (A review of fabrication and construction tolerances may dictate an even larger diameter.)
•
The grout material strength depends on the development length of the dowel.
•
Confinement reinforcement consisting of a spiral or ties may be required to prevent splitting or bond failure between the conduit and the surrounding concrete, particularly at the ends of the beams in beam/column dowel connections where significant axial loads may be present.
Smooth-formed dowel holes, approximately three times the dowel diameter, exhibit ver y good bond to the smooth concrete of the hole when non-shrink grout is used. At failure conditions, the dowel bar will usually de-bond before the grout plug de-bonds. A benefit of a cored hole is that the metal conduit thickness is eliminated, permitting a more compact and durable connection. Dowels are also commonly used to transmit shear between a haunch and its support, a detail commonly used in wall panels. Failure can occur by bending in the dowel or by breaking out a wedge of the concrete behind the dowel. The tension stress across an assumed failure plane for the wedge should be checked for the latter, similar to stud design. Reinforcement should be provided across the potential failure plane behind the dowel in both the support and the haunch.
The dowel should be sized for the bending and shear at the support interface and extend into each member about 7.5 times its diameter. For dowels in shear, there will be a point of contra-flexure in the dowel between haunch and support. Thus it acts as a cantilever with a lever arm from half the depth of the grout bed to its effective point of suppor t in the haunch. The effective point of support will depend on the strength of the grout and the depth of compression zone required to resist the shear force.
Three basic types of insitu concrete connections are used in precast construction: •
Figure 7.24
•
A thin topping layer to form a composite member, typically used with floor units such as hollowcore and double-Ts. It also acts as a levelling screed and there may be no mechanical connection between it and the unit. Longitudinal shear due to bending is transferred by bond and is also a function of the roughness of the interface. The design rules for longitudinal shear are set out in AS 3600 Section 8.
Grouted-Tube Connection
Composite construction in which the insitu concrete is a major component of the structural member. A typical example is a beam-shell or inverted T-beam where the precast unit forms the soffit and sides of the beam and contains the longitudinal reinforcement or prestressing and the shear steel. This type of construction allows continuous members to be easily formed by placing negative reinforcement in the insitu concrete over supports. Simple spans are usually propped until the insitu concrete attains sufficient strength to carry the dead weight on the composite section.
•
As a splice to connect a precast unit into a structure. This is an effective detail in structures where beam or column continuity is required as in earthquake-resistant construction. The bond length of the bars being lapped dictates the length of the splice. It may be necessary to connect large main bars by welding. Carbon content limits in AS/NZS 46717.8 mean that field welding of reinforcement is a safe and simple process.
Shear transfer through friction requires a clamping force normal to the interface. This force can be an external compressive force, post-tensioning, or arising from the transverse reinforcement. The latter uses the shear friction principles discussed in Clause 7.8 to calculate the magnitude of shear transfer. Essentially, relative displacement of the interfaces requires lateral movement, which is a function of roughness. This causes extension of the reinforcement crossing the interface and thus the generation of a clamping force.
Proprietary drilled inserts may be required where cast-in ferrules have been omitted or cannot be used. These inserts have mechanical or chemical anchorage. Mechanical anchors rely on the grip of an expansion
Welding of reinforcement is a practical method to transfer force in many connections. The type of welding rod and preheating requirements are determined by the carbon content of the steel.
sleeve on anby oversize hole. Chemical are bonded epoxy drilled into a drilled hole. Theanchors manufacturer should be consulted for advice on the capacity of these fixings since anchorage details vary.
AS/NZS limits the carbon equivalent of steel used4671 for reinforcement. At that limit, hydrogencontrolled consumables and processes are required and preheating is necessary for ambient temperatures below 1°C.
7.8
For temporary bracing inserts, chemical anchors are not permitted by AS 3850 7.16, unless individually load tested. Mechanical anchors, which are thick-walled, loadcontrolled expansion anchors, are required for bracing inserts. For more details, see Chapter 11 of this Handbook.
Welding of reinforcing bars is covered by AS 1554.37.7. It sets out the requirements for the welding of bars: •
to each other;
•
through splice members;
•
to structural steel members used as anchorages.
Common considerations in the design and detailing of welded-bar connections are that: •
Welding should not be done on or within three bar diameters of any bent portion of a bar.
•
Straightening or bending a bar must be at least 75 mm from a weld location as required by AS 1554.3.
•
When welding bars to structural shapes that are embedded in concrete, allowance should be made for the thermal expansion of steel to avoid concrete spalling or cracking.
•
When the latter item is a concern, adequate confinement reinforcement should be provided in the immediate area or a compressible material placed around the steel plate to allow for expansion.
For further background information on the design and detailing of welded bar connections, reference should be made to AS 1554.3. Common welds used with reinforcing bars are shown in Figure 7.25.
Figure 7.25 Typical Reinforcing Bar Welds
Figure 7.26 Examples of Precast Permanent Formwork to Columns and Spandrels
7.18.1
Introduction
Precast concrete units serving as the formwork for the cast-in-place concrete are a cost-effective means of using precast concrete. They provide three major
elements of the design: • architectural finish •
formwork
•
structure.
The units may be non-structural, in that they are not required to carry load as part of the final loadcarrying structure of the building. When used in this way, great care must be taken to isolate them from the structure, particularly loads arising from deflection, creep or shrinkage of the structure. Alternatively, these units may act compositely with the insitu concrete to form par t of the final loadcarrying structure of the building. 7.18.2
Connection detail principles
The connection of the formwork unit can consist of projecting reinforcement, or inserts with threaded rods. If interface shear is to be transmitted, the area of the projecting reinforcement required can be calculated using the provisions of AS 3600 Section 8 or by shear-friction principles (see Clause 7.8). The reinforcement is cast into the unit and ties into the insitu concrete. The ties form the permanent connection between the precast concrete unit and the insitu concrete, Figure 7.26. They are generally loop bars so that secure anchorage can be obtained within a short distance. The projection needs to be minimised so that the bond ties do not foul the reinforcement in the insitu concrete. The anchorage of the ties should be behind the main reinforcement in the insitu and precast. Ferrules may be used to provide anchorage bars, which perform a similar function to the ties. Alternatively, they may provide the fixing for the formwork tie as shown in Figure 7.26. These cast-in inserts should be anchored beyond the precast unit reinforcement. When the shell is to form part of the load-carrying section, the internal surface of the precast concrete unit should be roughened. For formed faces, retarder can be used and the concrete water-jetted after stripping. For unformed faces, the unit can be roughened by water washing.
Precast Loadbearing ‘T’ Unit Columns using Dowel Connections. Because Precast Concrete Units are Accurately-Made Factory Products, they can be Connected Unit-to-Unit for Quick and Easy Erection.
7.1 AS/NZS 1170 Set Structural design actions, Standards Australia, 2007.
7.2 AS 3600 Concrete structures, Standards Australia, 2009. 7.3 Building Code of Australia, Australian Building Codes Board and CCH Australia, 2007. 7.4 Design Manual Precast and Prestressed Concrete 4th edition, Canadian Prestressed Concrete Institute, Ottawa, Ontario, 2007. 7.5 Somerville G The Behaviour of Mortar Joints in Compression Technical Report, Cement and Concrete Association (UK), November 1972. 7.6 ACI 318M–08 Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, 2008. 7.7 AS/NZS 1554.3 Structural steel welding, Part 3: Welding of reinforcing steel, Standards Australia, 2008. 7.8 AS/NZS 4671 Steel reinforcing materials, Standards Australia, 2001. 7.9 Cheok G S and Long T Phan Post-Installed Anchors - A Literature Review, National Institute of Standards and Technology, USA January 1998. 7.10 AS 5100.4 Bridge Design, Part 4: Bearings and deck joints, Standards Australia, 2004. 7.11 AS 5100 Set Bridge design, Standards Australia, 2007. 7.12 AS 4100 Steel structures, Standards Australia, 1998. 7.13 Fuchs W, Eligehausen R and Breen J E ‘Concrete Capacity Design (CCD) Approach for Fastening to Concrete’ ACI Structural Journal , V. 92, No. 1, January-Februar y 1995. 7.14 Reineck, K H Rational Methods for Detailing and Design: Strut-and-Tie Modelling, Chapter 5 Large Concrete Buildings, edited by Rangan, B V and Warner, R F, Longman 1996. 7.15 Schlaich J, Schafer K, Jennewein M ‘Toward a Consistent Design of Structural Concrete’ PCI Journal, May–June 1987. 7.16 AS 3850 Tilt-up concrete construction, Standards Australia, 2003
Connections in Exposure Conditions C and U (marine tidal and splash zones) are Usually Required to be Non-Corroding. Use of Grades 316 and 316L Stainless Steel for these Connections would give a Service life of Well Over 100 Years
The following pages cover a variety of worked design examples, as set out below. •
7A.1 Steel Corbel and Top-Restraint Fixings for a Cladding Panel
•
7A.2 Reinforced Bearing for a Rectangular Beam
• •
7A.3 Corbel to a Column Supporting a Beam 7A.4 Dapped-End Connection for a Beam
•
7A.5 Reinforced Beam Ledge for a Double-T Leg.
The Preferred Fixing System for Cladding Panels (ABOVE) to a Building Frame consists of Two Concrete Haunches (Corbels) and Two Restraint Angles Industrial Wall Panels ( BELOW) are Typically Fixed by Steel Clips when used as Cladding to a Steel Frame, otherwise they are Loadbearing.
Example 7A.1 Steel corbel and top-restraint fixings for a cladding panel
Cladding panel of following dimensions and properties.
FRL = 240/–/–
Required thickness = 175 mm
(AS 3600 Table 5.6.1)
Concrete strength = 40 MPa Exposure class Outside face = B1 Inside face = A1
Required cover = 30 mm Required cover = 20 mm
Design steel bottom corbel and top angle-restraint for above configuration.
General Concrete cover Adopt 35 mm for outside face Adopt 25 mm for inside face Loads Panel self weight
3 Volume = 0.175(4.5 x 3.55 - 2.1 x 1.8) = 2.13 m
Mass = 2.13 x 25(kN/m 3) = 53.4 kN Reaction per fixing Vertical R* =
53.4 x 1.2
Horizontal H*sw = 2955 2
2
= 32.0 kN
32.0 170
(
+ 25 + 120) = 2.52 kN
cont…
Wind load (corner panel)
Design pressure = + 2.2 (kPa) x 4.5 = 9.9 kN/m height Design suction = - 3.5 (kPa) x 4.5 = 15.75 kN/m height
Wind load reactions due to pressure, per fixing 9.9 0.5(3.55 - 0.445) Top fixing H*wpt = x 3.55 = 7.9 kN 2 2.955
Bottom fixing H*wpb = 9.9 x 3.55 - 7.9 = 9.7 kN 2
Wind load reactions due to suction, per fixing Top fixing H*wst = 7.9 x
3.5 2.2
Bottom fixing H*wsb = 9.7 x
= 12.6 kN 3.5 2.2
= 15.4 kN
Design forces at connections Erection
Self weight + Wind presure
Self weight + Wind suction
- 2.5
- 2.4
+ 7.9
- 12.6
+ 5.6
- 15.0
+ 2.5
+ 2.4
+ 9.7 32.0 x 1.2 (impact)
- 15.4
+ 12.3
- 12.8
32.0
32.0
= 38.4 kN Connection design Bottom support corbel
C* = 38.4 x
265 105
= 96.9 kN
T* = 96.9 - 38.4 = 58.5 kN
cont…
Corbel plate size For compression zone depth = 30 mm Compression zone width =
3718 30
= 124 mm
Adopt plate width, b = 150 mm
BM in plate, M* = 38 100 x 160 = 6.14 x 10 6N mm 6.14
Required thickness, t =
4S
Ö b
=
4 x 19 494
Ö
150
Platefsy = 350 MPa 19 494 mm3
= 22.7 mm
Adopt corbel plate size = 150 x 25 mm
Tension bar size Bar fsy = 500 MPa 61 400 Required bar area = = 154 mm 0.8 x 500
2
Adopt N16 bar (A s = 200 mm 2) with full-penetration butt weld to plate Tension bar length k1= 1.0
= k2
1.7
a = 35 mm
db = 16 mm
k1 k2 fsy Ab 1 x 1.7 x 500 x 200 Required development length, Lsy.t = = = 312 mm (2a + db)Ö f'c (2 x 35 + 16) Ö40 Adopt bar length = 450 mm including cog bent around 5 db pin Connection design Top restraint bracket
Wind load plus dead load on bracket
L = Ö 2502 + 200 2 = 320 mm
5.4 x 200 65
Compression in gussets = cos 51.3 Tension in gussets = cos 51.3
Tension on ferrule =
15.1
= 16.6 kN
5.4
= 8.6 kN
= 24.2 kN
Two gussets, C* = 8.6/2 = 4.3 kN T* = 24.2/2 = 12.1 kN
cont…
Gusset (to AS 4100) Le = 2502 + 200 2 = 320 mm
2 off 40 x 6 mm r=
d
12
=
6
12
fsy = 250 MPa
Form factor, kf = 1
= 1.73 mm
AS 4100 Clause 6.2.2
Nominal section capacity of one gusset Ns = kf.Ag.fsy = 1 x 40 x 6 x 250/1000 = 60 kN
AS 4100 Clause 6.2.1
AS 4100 Clause 6.3
Slenderness
Strut design capacity AS 4100 Table 6.3.3 (1)
AS 4100 Table 6.3.3 (3)
> C* = 4.3 kN
> T* = 12.1 kN Restraint ferrule in insitu Windpressure
Wind suction
AdoptM20ferrule
Edge distance ratio =
c1 hef
=
120 100
= 1.2
Check M20 ferrule capacity Single anchor located in negative moment region of edge beam (ie concrete in compression) Anchor depth, hef = 100 mm Edge distance, c1 = 120 mm Tension Capacity Ultimate breakout capacity of ferrule Nb = 1 0 hef1.5f'c = (10 x 100/1000)1.540 = 63.2 kN
Clause 7.11.4, this Handbook
Clause 7.11.4, this Handbook cont…
AN = (c1 + 1.5 hef)(2 x 1.5 hef) for c 1 < 1 .5 hef
= (120 + 1.5 x 100)(2 x 1.5 x 100) = 81 000 mm
2
ANo = 9 hef2 = 90 000 mm 2
Shear Capacity Ultimate shear capacity of ferrule
do f'c x 1201.5 x 32 x 40
Vb = 0.6(lv /do)0.2 c11.5 0.6 100
= 1000 32 = 35.4 kN
0.2
Clause 7.11.5, this Handbook
lv = hef for ferrule = 100 < 8 do = 8 x 32 = 256
AV = AVo for edge beam depth > 1.5 c1
Combined tension and shear N* V* 25.2 15.1 Ncb + Vcb = 40.1 + 29.8 = 0.63 + 0.51 = 1.14
Clause 7.11.6, this Handbook
Bolt Adopt Grade 4.6, M20 25.2
15.1
= 0.23
AS 4100 Clause 9.3.2.3
Example 7A.2 Reinforced bearing for a rectangular beam
Rectangular beam 400 x 700 x 8 000 long carrying a factored UDL of 120 kN/m
Table 7.3,
this Handbook
Assume a horizontal load of 20%
of vertical can develop at bearing
Determine reinforcement requirements at the end of the element.
Factored end reaction =
120 x 8
= 480 kN
2 V=f 480 x 1.3 = 624 kN
N
f
= 624 x 0.2 = 125 kN
Considering the Vf only, it is balanced by V' and N' that are respectively parallel and perpendicular to the cracked surface. V'= 624 cos 20 = 586 kN
N' = 624 sin 20 = 213 kN
Therefore the shear stress to be resisted is: 586 x 10 3 v'f = = 4.01 MPa 146 190
cont…
Therefore the steel required is: Avf = 0.00439 x 146 190 = 642 mm2
The minimum reinforcement required for shear friction design can be calculated by: Avf(min) =
0.9 x 146 190 500
= 263 mm
2
The total amount of reinforcement required is: Ast = Avf + An = 1058 mm2 Use 4 number N20 bars (A
Where:
s
= 1256 mm2)
Acr = Lsy.t b = 500 x 400 = 200 000 mm2
The minimum reinforcement required for shear friction design can be calculated by: Avf(min) =
0.9 x 200 x 103 500
= 360 mm
2
< 974 mm2 Use 3 number N16 stirrups (A
s
= 1200 mm2)
Example 7A.3 Corbel to a column supporting a beam
A 350 x 350 mm column supports a beam carrying a factored vertical load of 250 kN Assume a horizontal force of 20% of vertical load due to shrinkage and creep f'c= 32 MPa
f
sy
= 500 MPa
Determine the corbel size and reinforcement.
General Given that V f = 250 kN, then Nf = 0.2 V f = 0.2 x 250 = 50 kN
Corbel dimensions
Choose an overall corbel depth of 500 mm. Choose a bearing pad 300 x 110 x 10 mm. The depth at the outside face must be not less than half the depth at the face of the support; choose 250 mm.
To allow for load eccentricities and construction
tolerances, assume the vertical load to be forward
of the pad centreline by 25 mm.
Strut and tie model The assumed compression strut, tension tie and nodal zone model is shown below-left. The truss model is shown below-right.
Nodes A and B are located at the intersection of the centrelines of the tension ties. Node C is located at the intersection of the centreline of the upper tension tie and the line of action of the applied vertical loads. Node D is located on centreline of the lower tension tie for the tension to be fully developed to equilibrate the compressive struts at the node.
cont…
The forces in the truss members and the reactions are determined from statics and are as follows Reaction
A
Force (kN) Member
Force (kN)
x
50 CB 149
Ay
D
294
544
BD 329
CD 269
BA 294
DA 50
The minimum area required for the primary reinforcement is: f'ct = 0.6 Öf'c = 3.39 MPa f ct 3.39 As.min = 0.22(D/d) 2 bd = 0.22(500/450) 2 350 x 450 = 290 mm2 fsy 500 Therefore use 4–N12 bars (440 mm 2) Although tie BA has a larger tie force, the vertical reinforcement in the column would have been designed for this force. Continue the 4–N12 bars down the column 300 mm to anchor them.
Use an additional 2-N12 column ties at location DA (220 mm 2) Additional ties with an area equal to one half the primary reinforcement area must be placed within two-thirds the effective depth adjacent to As Tie area = 425/2 = 213 mm 2 Use 2–N12 ties within 2 x 450/3 = 300 mm
ie no transverse reinforcement in bottle-shaped struts cont…
Maximum design stress in strut,
fcu = (0.85 s f'c) = 0.7(0.85 x 0.6 x 32) = 11.4 MPa FBD
Required thickness of Strut BD, tBD =
fcub FCD
Required thickness of Strut CD, tCD =
fcub
= =
329 000 11.4 x 350 269 000 11.4 x 350
= 83 mm = 68 mm
Struts fall within truss and concrete boundary outlines
Check nodal zones Node B: Node type is CTT,
n = 0.6
Maximum design stress in Nodes A and D, fcu = (0.85 n f'c) = 0.7(0.85 x 0.6 x 32) = 11.4 MPa FBD 329 000 Effective depth of Strut BD, dBD = = = 83 mm fcub 11.4 x 350 Similarly, dBC = 38 mm
d BA = 74 mm
A bar can engage an area of concrete of approximately six times its diameter, therefore,
these depths are acceptable and fit within the truss and concrete boundaries Node C: Node type is CCT,
n = 0.8
Maximum design stress in Node C, Effective depth of Strut CD, dCD =
fcu = (0.85 n f'c) = 0.7(0.85 x 0.8 x 32) = 15.2 MPa FCD
fcub
Node depth required by reaction at D = 15.2 x 350
=
269 000 15.2 x 350
544 000
= 51 mm
This depth acceptable
= 102 mm
Distance between Node C and column boundary is 350 – 50 - 228 = 72 mm, therefore, node width can be accommodated Final arrangement The final details of the corbel are shown below
end
Example 7A.4 Dapped-end connection for a beam
A dapped-end beam, shown below, carrying a factored distributed load of 127 kN/m including SW = 40 f'c MPa
f
= 500 MPa
sy
Determine the required reinforcement for the dapped-end connection.
Use 400-mm-long galvanised MS angle, 100 x 100 x 10-mm thick Strut and tie model
The tension tie, compression strut and nodal zone model is shown in the Figures below. The corresponding line drawing of the truss is shown in the Figure on next page.
cont…
Truss idealisation for dapped end.
531 000 Tie BC, A0.7sx=500
= 1517 mm2
663 000 Tie CF, A s = 0.7 x 500
= 1894 mm2
204 000 Tie EF, As = 0.7 x 500
= 583 mm2
4–N16 closed stirrups @ 60 crs (1600 mm 2)
5–N28 bars (3100 mm
2)
but check anchorage
4–N12 closed stirrups @ 150 crs (880 mm
2)
cont…
Check compression struts (AB, BD, CD, DE and BE)
Similarly t
BD =
44 mm, t CD = 149 mm, t DE = 101 mm, t BE = 24 mm
Available width for strut AB at Node A is MS bearing angle, w = 100 Ö2 = 140
> 89 mm
At Node B it is 174 mm as sketched below. Other struts OK by inspection
For equilibrium under a “hydrostatic” stress condition, the length of the faces of the nodal zone are proportional to the loads and the faces are perpendicular to the loads.
Hence, width of strut AB at nodal zone B = 531 + 42
508 x 196
= 174 mm
Remainder of struts fall within truss and concrete boundary outlines by inspection Check nodal zones and anchorages
Nodal zone available is height of leg of MS bearing angle, w = 100 mm
> 52 mm
Extend the 4–N20 tie bars 675 mm (bond length) past Node D, the assumed anchorage point
cont…
Node B: Node type is CCT,
n = 0.8
Maximum design stress in Node B, fcu = (0.85 n f'c) = 0.7(0.85 x 0.8 x 40) = 19.0 MPa
Effective depth of concrete to be engaged by tie BC, dBC =
FBC
fcub
=
531 000 19.0 x 400
= 70 mm
< 196 mm Add two additional N16 top longitudinal bars at Node B to improve support of strut AB Node C: Node type is CTT,
n = 0.6
Maximum design stress in Node B, fcu = (0.85 n f'c) = 0.7(0.85 x 0.6 x 40) = 14.3 MPa
Strut CD requires node width of 149 mm, which can be accommodated The reinforcement for Tie CF is 5–N28. Development length for an N28 bar is 1000 mm. The available development length in Node C is 180 + 16 = 196 mm The force developed in the N28 bars as they emerge from node zone is: F N28
196 -3 = 0.7( x 5 x 620 x 500) x 10 = 213 kN 1000
Provide additional U-bars in node zone, As =
= 663 kN CF
663 000 - 213 000 0.7 x 500
= 1286 mm
2
4–N16 U-bars (1600 mm2), required length = 196 + 400 (bond length) = 600 mm To improve crack control and ductility, provide a minimum area of horizontal reinforcement parallel to the primary tensile tie reinforcement in the region above the support. If the dapped end is treated as a bracket, the required area of such additional reinforcement would be: 0.5As = 0.5 x 4 x 300 = 600 mm 2 Use 2–N16 horizontal U-bars distributed over 2/3 of the effective depth. Extend these bars for bond length beyond the face of the dap. Summary of reinforcement requirements
cont…
Tie AD/Node A:
4–N20 bars extended 675 past Node D
Tie BC/Node B:
4–N16 closed stirrups @ 60 crs plus 2–N16 top bars at Node B
Tie CF/Node C:
5–N28 bars plus 4–N16 U-bars 600 long
Tie EF:
4–N12 closed stirrups @ 150 crs
Final arrangement of reinforcement
end
Example 7A.5 Reinforced beam ledge for a double-T leg
A 125-mm-wide double-tee leg is resting on an L-beam as shown below. f'c = 32 MPa
fsy = 500 MPa
Vf = 100 kN ultimate
Nf = 20 kN ultimate
Determine the required reinforcement for the beam ledge.
General
Strut and tie model The flow of compressive stresses and the strut and the tie model is shown below.
Forces in struts and ties: Tie F
DB
Vertical, F
100(135 + 510)
=
510 AB.v
Horizontal, F
= 126 kN tension
= 100 kN
AB.h
=
135 x 100 240
= 56 kN
Strut F AB = Ö1002 + 562 = 115 kN compression cont…
Design of tension ties
Reinforcement area required for tension tie DB: As =
126 x 103
0.7 x 500
= 360 mm2
Provide 4–N12 closed stirrups in addition to those 2
required for shear and torsion (4 x 110 = 440 mm
)
Check Struts
Strut BC: OK by inspection Check compressive stresses at nodal zones
< 200 mm
OK
Final reinforcement details are shown below
What you will find in this Chapter •
A description of the types of joints most commonly used with precast concrete.
•
Advantages and disadvantages of the various joint types that will assist in appropriate joint selection.
•
Design principles and formulae for predicting joint widths for correct specification.
•
A guide to the types of sealants available and their use, including fire-resistant sealants.
8.1
Definitions and Notation
8.1.1
Definitions
8.1.2
Notation
8.2
Introduction
8.3
Requirement for Joints
8.4
Functional Requirements
8.5
Design Considerations
8.5.1
8.6
General
8.5.2
Number of joints
8.5.3
Location of joints
Types of Joints
8.6.1
8.7
General
8.6.2
Open-drained joints
8.6.3
Face-sealed joints
8.6.4
Compression-seal joints
8.6.5 Hollowcore wall joints Width of Joints
8.7.1
General
8.7.2
Joint width
Example 8.1 8.8
Joint Sealants
8.8.1 8.8.2 8.8.3 8.9
General Sealant types Joint design and sealant application
Fire Rating of Joints
8.10 References
Joint width calculation
8.1.1
Definitions
Well detailed and constructed joints play a vital part in maintaining the integrity of the external envelope of the building, ensuring it is weatherproof and meeting any other requirements such as fire-
For the purpose of this chapter the following definition is used: Joint An intentional gap between adjoining elements (typically cladding) or between an element and some other portion of the structure. Joints may be horizontal, vertical or inclined. 8.1.2
Notation
For the purpose of this chapter the following notation is used: at
= allowance for manufacturing and erection tolerances
bj
= joint width
bj.min = larger value of minimum joint width determined for opening or closing movements taking account of the strain capacity of the sealant. L s
resistance and acoustic performance. Therefore, the importance of joint design and detailing cannot be overemphasised. This chapter provides guidance on planning joint locations, gives requirements for joint types, widths, the choice of sealant and discusses the fire rating of joints. The word ‘joint’ often has a broader meaning than the restricted definition adopted in this chapter. It can, for example, encompass the sense of a structural joint, as in the joint between beams and columns, and a connection between elements. For information on these uses of the term see Chapter 5 Analysis and Design of Buildings, Chapter 7 Connections and Fixings and Design of Joints in Concrete Buildings8.5.
= joint spacing or length of panel as appropriate = strain in sealant
DT
th
= temperature change = the larger of (maximum temperature – temperature at sealing) and (temperature at sealing – minimum temperature) = hypothetical thickness of member = 2 x gross cross-sectional area of the member / exposed perimeter of the cross section plus half the perimeter of any closed voids contained therein.
ac.temp
= coefficient of thermal expansion for concrete
DLcc
= joint movement due to concrete creep
DLcs
= joint movement due to concrete
DLt
shrinkage = joint movement due to thermal changes
ecc
= design creep strain of concrete (see Section 3 of AS 3600)
ecs
= design shrinkage strain of concrete (see Section 3 of AS 3600, see Table 8.2 for typical values)
Joints are necessar y because:
•
elements have to be of manageable size so that they can be manufactured, transported on public roads, and erected;
The function of a joint between precast elements is to provide physical separation between the units and, in conjunction with joint sealants, prevent the ingress of water and air into the building; and, if required, fire
•
allowance for relative movement is required between adjacent elements and between panels and the supporting structure;
resistance. Two aspects of joint selection need to be emphasised:
•
tolerances in the precast unit, the structure and placement during erection require clearance around the unit.
•
The positioning of joints in relation to openings (eg windows) and to the structure can affect the serviceability, construction and maintenance of the building envelope. Poor joint location will lead to problems which cannot be overcome by joint detailing (see Figure 8.1)
•
Careful control of construction tolerances is vital to ensure the integrity of the cladding system.
Joints of sufficient width will ensure that unintentional and indeterminate forces are not developed in the precast element and its fixings. These may be caused by shortening of the supporting structure, instantaneous and creep deflections of horizontal beam support structures, movement due to temperature variations, and lateral deflections of the supporting structure under applied loads, eg wind and earthquake.
8.5.2
8.5.1
General
It is recommended that joints be treated as a strong visual feature of architectural wall design. Recessing of joints and/or sealants will help diminish the visual
Number of joints
For maximum economy in manufacture and erection, panels should be as large as practical (see Chapter 5 Analysis and Design of Buildings ). This gives the minimum joint length in the facade, which in turn reduces the cost of the jointing and improves serviceability. If architectural requirements dictate more-closely-spaced joints, false joints can be used to achieve a similar visual effect.
impact of possible variations between adjacent surfaces.
8.5.3
The following general aspects need to be addressed:
The weathering of the building facade can be controlled to a large extent by careful joint location.
•
•
Buildability and minimum size Details that are simple to fabricate and install on site should be selected. Proven details should be used wherever possible. Concrete is a brittle material and the details must be robust. The aspect ratio, height : width, of upstands should not exceed 1.25 :1, with a maximum height of 75 mm. Such upstands should always be as robust as possible. The joint must be wide enough to accommodate manufacturing and building tolerances (see Chapter 4 Tolerances) and the anticipated strain in the sealant material. A practical range is 15 to 25 mm. Maintenance and repair Although modern sealants have a long service life they eventually will need replacement or repair. Access for repair and replacement must be taken into consideration in the design of the building. The positioning of services or other features in front of joints will make future access difficult. Consideration must be given to the fact that inspection and repair will usually have to be made from the exterior of the building.
Location of joints
Recessing the sealant in the joint, or use of an opendrained system, will minimise concentrated rainwater runoff and keep the facade free from unsightly water-stain patterns. Horizontal and vertical joints should be aligned throughout their length and not staggered, see Figure 8.1. The joint layout should lead water directly to the base of the building. Staggered joints can result in water that has managed to penetrate the facade being directed into the building interior.
Figure 8.1 Location of Joints [From Egan8.1]
Non-aligned joints can also lead to cracking of panels if adjacent units are not free to slide relative to each other. Problem-areas relating to joint location that have been identified are shown in Figure 8.2. These include: •
•
•
Window openings formed in adjacent panels so that the window frame crosses the joint. Any failure of the joint will result in water being directed into the window head. It also makes an open-drained joint more difficult to implement. Preferably, windows should be located entirely within a single panel.
Figure 8.2 Common Problem Areas
Where the face of the precast unit and the associated joint slopes outwards, an open-drained joint cannot be used unless a vertical seal can be located within the joint as shown in Figure 8.2. Alternatively, a face seal must be used. Misalignment of grooves in adjacent panels preventing installation of the baffle; or non-square faces preventing completion of the air seal.
Water that enters the joint in front of the baffle is drained downwards. At every intersection between the ver tical and horizontal joints, a short length of flashing (300 mm) is used to ensure water is discharged to the outside.
8.6.1
General
The most common types of joint between precast concrete cladding and/or wall panels are: • • •
The horizontal joints are usually of shiplap form with a 50- to 75-mm-high upstand/downstand. A horizontal air-seal is provided at the interior face, linked to the vertical seals.
open-drained;
The minimum design width of both vertical and
face-sealed; and compression-seal.
horizontal joints should be 20 mm. Sidesway and frame shortening due to shrinkage and creep in tall buildings may govern this dimension.
Each is discussed below and their advantages and disadvantages are summarised in Table 8.1.
The expansion chamber, which holds the baffle strip, is formed by 20-mm-deep and 20-mm-wide grooves in the panel sides. The aspect ratio (height:width) 8.6.2 Open-drained joints of the upstand should be the same as for a shiplap joint, ie 1:1 generally but may be 1.25:1 if there is no The open-drained joint is recommended for high-rise construction. It consists of a rain barrier in the form of alternative. A panel with a nominal 50-mm upstand an expansion chamber with a loose-fitting baffle and an would need to be 120 mm thick, allowing a 20-mm clearance. The preferred minimum panel thickness air-seal at the interior face of the panel. for this type of joint is 150 mm, see Figure 8.3. A The baffle prevents direct entry of the wind-driven chamfer (at least 12 mm) is required on the panel rainwater.The pressure in the chamber between the baffle and the internal air seal is at external air pressure. edges to minimise handling damage. There is, therefore, no pressure differential to drive rain The rear sealant for an open-drained joint should be chosen to accommodate differential movement as for past the baffle. The air-seal is the demarcationbarrier the face-sealed type. Being at the back face it is not between outside and internal air pressures. exposed to destructive UV light. If required it can be fire-resistant. Table 8.1
Advantages and Disadvantages of Joint Types
Figure 8.3 Design and Construction of Open-Drained Joints
For the joint to function correctly, this rear seal needs to be approximately only 95% air tight. Internal lining of the wall usually helps to ensure this requirement. Care is required in the detailing and installation of the flashing. The vertical baffle is installed so that the lower edge overlaps the horizontal flashing below. This flashing is illustrated in Figure 8.3. It should be flexible enough to tolerate any non-alignment between adjacent panels without causing installation difficulties. It is usually made of light gauge stainless steel. The rear of the joint must be easily accessible after the panel is erected in order to place the sealant.
The vertical joint should not be placed in front of columns and the horizontal joint should be above or below any edge beam or slab, Figure 8.4. Support corbels on the panels may also interfere with access to the rear face and should be considered at the design stage.
Figure 8.5 Solutions for Overcoming Difficulty in Forming Back Seal
Figure 8.4
Locations to be Avoided for Open-Drained Joints
8.6.3
Face-sealed joints
These joints are simple, economical and are most suited to low-rise construction. They are sealed by a single run of gun-applied sealant close to the exterior surface of the joint. The sealant is placed against a backing-rod. The external face seal should, where practicable, be supplemented by a seal near the inside face of the panel; this is particularly recommended when the risk of water ingress is to be minimised.
Figure 8.6 Face-Sealed Joints
The horizontal joint is preferably of the shiplap type. The upstand/downstand is typically 50 mm. Minimum design width is 15 mm but 20 mm is preferable. The actual width of the joint should be determined from the anticipated movement between panels. Joint widths greater than 30 mm are too expensive to be practical. The sealant must bond firmly to the concrete and accommodate panel movements without splitting, tearing or loss of adhesion. Sealant manufacturers will supply allowable strain capacities for their materials. The sealant must be resistant to ultra-violet (UV) light and other weathering effects.
The recommended profile is shown in Figure 8.6. Setting the sealant back from the face of the panel gives some protection from UV light to minimise deterioration. The sealant is extruded against a cylindrical foam backing-rod with a non-adherring surface. This profiles the rear surface to an efficient cross section for resisting tensile forces. To ensure proper bonding of the sealant, the concrete surface should be sound and free of porous concrete, ie panel edges must be of well-compacted concrete with a Class 2 finish (as defined in AS 3610 Formwork for concrete8.2) in respect of allowable air holes and other surface irregularities. The surfaces must be free from contaminants and laitance and be dry. A primer may be required on the joint surface for some sealants to assist with adhesion.
Figure 8.7 Compression-seal Joints
Fire-resisting sealants can provide fire ratings if required. For high ratings, fire-retardant joint filler materials may also be required 8.3.
8.6.4
This type of joint utilises a compressible impregnated polyethylene or polyurethanefoam strip.The strip is pre-compressed and inserted into the joint after the panels are erected or it is glued in position before placement of the second panel,Figure 8.7. It then 8.7.1 General expands to fill the joint. It may be necessary to pre-cool A nominal joint width of 20 mm will usually the strip to maintain compression during placement. be satisfactory for most conditions and is the Selection of an adequate thickness of strip is critical recommended design starting point. Typical fieldto the performance of the joint. A positive pressure moulded sealants of this width have a movement must be exerted against the joint faces at all times. range of ±5 mm. This is adequate for panels up to Panel edges must be Class 2 finish as defined in about 8 m in the direction of movement. 8.2 AS 3610 and the uncompressed width of the foam However, to satisfy all requirements on joint width, strip must be sufficient to maintain pressure on the the following factor s should be considered: concrete at low temperatures when the joint is at its maximum width
•
the dimensions of the adjoining panels;
The use of this type of joint seal is usually limited to low-rise buildings such as factories and warehouses where wind pressures are low. It can be used where spandrel beams, downturns or columns restrict the access required for placement of gun-applied sealants.
•
manufacturing tolerances of the panels and the support structure, see Chapter 4 Tolerances;
•
the anticipated movement of the joint due to elastic deformation, creep deformation, shrinkage of concrete, temperature expansion and contraction, horizontal displacement or rotation due to wind and earthquake;
•
both vertical and horizontal movement of the building frame due to deflection under permanent imposed load and self-weight, and lateral loading
8.6.5
Hollowcore wall joints
Hollowcore wall units are primarily used on lowrise commercial and industrial buildings. In these applications the walls are generally required to have a fire resistance level as specified in the Building Code of Australia8.4. The jointing system must therefore be both weatherproof and provide the relevant fire performance. Typically, hollowcore wall construction has been fire-tested by the manufacturer using normal building sealants. The face-sealed system can meet these requirements while giving the best appearance to the finished wall by masking erection tolerances in panel spacing. See Clause 8.6.3 for general face-sealed joint details. Walls with the panels vertical, and therefore the cores vertical, may be sealed at either or both faces. A single seal on the inside face will act as an opendrained system. The smallest practical joint width may be used since these panels are only 1.2 to 2.4 m wide. The joint width is usually governed by the edge profile used by the particular manufacturer. Horizontally-placed panels are typically 6 to 9 m long and stacked ver tically. The horizontal joint spacing is thus 1.2 to 2.4 m and the practical minimum joint width may be used. The edge profile and alignment packers between panels will govern this dimension. The panel length and factory-cutting tolerance dictate the width of the ver tical butt joint between adjacent stacks. A minimum nominal width of 20 mm should be used in this location. This type of wall must have a face seal to prevent water penetrating into the ends of horizontal cores.
Compression-seal joints
•
from wind and earthquake effects; in the case of reinforced concrete framing, allowance must be made for long-term column shortening and beam deflections due to shrinkage and creep of the concrete.
Material properties values are given in Chapter 3 Materials and Material Properties and for a discussion of volume-change deformations see Chapter 5 Analysis and Design of Buildings . Manufacturing tolerances for facade elements are set out in Chapter 4 Tolerances. The maximum tolerance on face dimensions is ± 6 mm and actual tolerances are generally much less due to the fact that these types of units are usually made in close-tolerance steel moulds. See discussion in Chapter 4 Tolerances. Insitu construction, building tolerances also need to be taken into account. These include tolerances on the completed structure and tolerances on dimension between parts of the structure. The joint width needs to be established taking these and manufacturing tolerances into account, see Chapter 4 Tolerances.
Dimensional changes in the panel due to concrete shrinkage will depend on the concrete mix, climate in area of construction, the panel thickness and percentage of reinforcement, the aggregate type and the ambient weather conditions. Approximately half the ultimate shrinkage will occur during the first few months after casting, expected shrinkage movements at the joint will thus depend upon the age of the panels when erected.
Temperature movement A change in temperature will result in the joint opening or closing. (A rise in temperature results in a closing movement and a fall in temperature, an opening movement of the joint.) DLt
=Lx
where:
Joint movement due to thermal expansion or
ac.temp
DLt ac.temp
contraction of the panel will be determined by the seasonal extremes of temperature of the panel concrete rather than the ambient temperature variations. The orientation of the facade in relation to the sun will have a major effect on the temperature variations experienced. Consideration must also be given to the expected temperature at the time of sealant application.
bj = b j.min + a t where: b
j
= joint width
bj.min = larger value of minimum joint width determined for opening or closing movements taking account of the strain capacity of the sealant. at
bj.min = 100 x maximum opening or closing joint movement / sealant strain capacity (%)
Shrinkage movement Shrinkage of panels will lead to an opening movement of the joint. Creep of panels usually needs to be considered only for prestressed elements, in which it may lead to an opening movement of the joint. DLcs
= L x ecs
where:
DLcs
L
= allowance for manufacturing and erection tolerances
Minimum joint width The minimum joint width, bj.min, should be calculated for both opening and closing movements, taking the width when the joint is sealed as the base width.
= coefficient of thermal
= joint spacing or length of panel as appropriate
Joint width
Estimated joint width An estimation of the joint width, bj, can be made from the following equation8.5
DT
expansion for concrete = temperature change. = the larger of (maximum temperature – temperature at sealing) and (temperature at sealing – minimum temperature)
DT
L 8.7.2
x
= joint movement due to thermal changes
= joint movement due to concrete shrinkage
= joint spacing or length of panel as appropriate ecs
= design shrinkage strain of concrete (see Section 3 of AS 3600, see Table 8.2 for typical values).
Creep and shrinkage movement of the supporting structure Creep and shrinkage in the supporting structure generally will lead to a closing movement of the joint. The full effect of creep shortening will be maximised in the lower storeys of the structure. DLcc = L x ecc where:
DLcc
L
= joint movement due to concrete creep
= joint spacing or length of panel as appropriate. ecc
= design creep strain of concrete (in the supporting structure, eg columns). See Section 3 of AS 3600.
Example 8.1 Joint-width calculation
Table 8.2 Typical Design Shrinkage Strains after 30 Years in Various Environments
The joint between two adjacent concrete spandrel panels each 5-m long. Design shrinkage strain, after panels erected,
joints as panels horizontal. Installation temperature of 25°C, and expected temperature range of 0°C to 38°C. Sealant movement capacity of 25%. Assume ac.temp = 12 x 10 -6 mm/mm/°C and at = 6.0 mm.
Determine the minimum vertical joint width.
Temperature movement Closing (temperature rise) DLtc
= 5 x 1000 x 12 x 10 -6 x (38 minus 25)
8.6 e
= 0.78 mm (closing) Opening (temperature fall) DLto
= 5 x 1000 x 12 x 10 -6 x (25 minus 0) = 1.5 mm
DL t
= 1.5 mm (opening)
Shrinkage movement (opening) DLcs
= 5 x 1000 x 400
x 10 -6
= 2.0 mm (opening) Minimum joint width bj.min = 100 x (1.5 + 2.0)/25 = 14.0 mm Joint width bj
= bj.min + at = 14.0 + 6.0 = 20 mm
e
of 400 x 10 -6; no design creep strain for vertical
Joint width must be related to sealant performance, see Clause 8.8. In general, sealants will allow movement of ± 25% so a 20-mm joint can close to 15 mm or open to 25 mm. Panel shrinkage will begin after casting and a significant proportion will probably have taken place before the panel is erected and the joint sealed. In terms of the shrinkage and creep movement the sealant has to accommodate, it is the differential movement between the panel and the suppor ting frame. This will be small for steel frames but could be significant for concrete frames. Opening and closing movements at movement (separation) joints between parts of the building can be substantial and need to be calculated carefully. Leaving the sealing of these joints as late as possible in the construction phase is recommended. This type of joint should be accessible for future resealing. Variations in unit dimensions during manufacturing and in joint width during building construction, see Chapter 4 Tolerances, will mean that the width of a joint may var y by up to 5 mm over its length. The variation usually is visually-acceptable in a nominal 20-mm joint but not in a nominal 10-mm joint. Thus, nominal 10-mm and 15-mm joints should be adopted only for small panels with good shape repetition.
8.8.2
Sealant types
Field-moulded sealants are available as either one-part or multi-part products 8.7.
8.8.1
General
Sealants are an integral part of most joints. The requirements for sealants reach beyond weather and waterproofing, they have to provide appropriate mechanical, structural, fire and acoustic properties and provide high adhesion to the concrete panels. Designers should consult with the various sealant suppliers to ensure they are specifying an appropriate sealant for the specific needs of the project. The ACI Guide to Sealing Joints in Concrete Structures8.7 provides sound advice regarding the various types of sealant, how they function, joint details, installation and performance, repair and maintenance. For successful operation a joint sealant should be able to deform readily in response to cyclical panel movement. Factors to be considered by the designer when choosing a suitable sealant material include: •
The sealant should be impermeable to water.
•
It should have a low elastic modulus to accommodate strain due to joint movement without significant stress, with the shape of the sealant influencing the stress in the sealant.
•
It should be able to recover its srcinal shape after cyclic deformation.
•
It must bond firmly to the joint face without failing in adhesion nor splitting or peeling under the anticipated joint movements.
•
It must not soften or flow at higher service temperatures and should not harden and become brittle at low temperatures.
•
It should not be adversely affected by ageing or weathering and should be stable when exposed to UV light.
•
For face-sealed joints the sealant should have a stable colour, be non-staining and resistant to pickup of dirt.
One-part sealants are easy to apply, economical, raise no concerns over mixing uniformity (as with multipart sealants), and offer a wide choice of chemical type (polyurethane, silicones, hybrids and acr ylics). Because most of these products cure by reaction with atmospheric moisture, they are dependent on climatic conditions, especially relative humidity, and therefore are prone to unpredictable cure rates. Some hybrid materials cure by reaction with oxygen in the air and as such are much less susceptible to uneven cure due to changes in the relative humidity. One-part sealants can be prone to splitting and adhesion failure due to being subjected to movement of the panels before the sealant has cured. For a onepart sealant to perform in the long term, it must first survive the movement during its cure period – up to three or four weeks in some cases.
Performance-based specifications, such as ISO 116008.8 (which are independent of chemical type) relate the performance of sealants to actual service conditions. This specification can be used in evaluating which sealant is best suited to a given application. Multi-part sealants are not as easy to use as onepart sealants due to the need to uniformly mix the components. Nevertheless, they can be more economical in initial cost. The most significant benefit with multi-part sealants is the cure rate. The mixing of a curing agent throughout the base polymer ensures that the sealant mass cures at an even rate throughout the sealant material rather than forming a skin and curing inwards, as is the case with one-part sealants. This generally means that the sealant is able to accommodate joint movement earlier, which is an important factor to consider with joints between precast panels. Polyurethane and polysulphide are the most common chemical types. Polyurethane sealants have high resistance to ultraviolet light and will remain flexible for periods of at least 8–15 years because of their stable chemical structure. They have a minimum safe strain capacity of ± 25% and as well as their excellent elastic properties are resistant to abrasion, tearing and indention. They are available in one-part and two-part formulations, can possess fire and acoustic certification, and have a short application time. They are therefore recommended for most applications.
Polysulphide sealants have been available since the early 70s. They remain flexible over a wide temperature range and are highly resistant to ultraviolet light, ozone and other chemicals. Polysulphide-based sealants bond well to concrete when a suitable primer is used, and can accommodate joint movements of ± 25%.
8.8.3 Joint design and sealant application To ensure the joint and sealant give the desired performance, appropriate joint design, preparation and application technique is required. The following guidelines should be adopted: •
Correct joint preparation The substrate should be clean and dry, with a surface temperature above 5°C. Early introduction of sealants onto ‘green’ concrete can result in severe bubbling, and adhesion loss.
•
Correct sealant-backing systems For sealants to perform to their optimum movement parameters they must adhere only to the joint sides and never to the base. Foam backing rods or bond-breaking tapes should be used to ensure adhesion only to the sides (Figure 8.8).
•
Correct joint geometry Correct sealant geometry is required to ensure longevity of the selected sealant. The depth of sealant should never be less than half the width, and never greater than the width. This may vary in narrow (10 mm) or wide (40 mm) joint applications. The minimum sealant depth should, within these requirements, be not less than the manufacturer’s recommendations. For example a specification clause could specify “The sealant depth shall be 0.5 times the specified joint width but not less than 10 mm, subject to the sealant manufacturer’s recommendations.”
•
Sufficient curing time The time the sealant takes to cure through its depth and width is very important, especially if the joint is to be exposed to traffic or be fully or partially immersed in service. Standard curing polyurethanes will cure at a rate of 0.6 mm of depth per day in temperatures over 5°C . Fast-curing versions and two-part systems will be faster depending on their curing mechanism. Acrylics and silicones will surface-skin relatively quickly but then have a slower cure rate through their depth, resulting in a longer period being required before they can be put into service.
However, they are available only in two-part formulation, do not possess fire certification, require a primer in all applications, have a high initial cost and
require a long application time. Acrylic sealants being water based are deemed more ‘user-friendly’ but have a low strain capacity of 5–10% and this, combined with curing shrinkage, limits their applications to internal, low-movement joints.
Butyl sealants are available in both gun and preformed extruded grades and are used mainly in bedding or compression seal applications.
Silicone sealants should be avoided where possible as they stain the concrete surface. Silicone sealants have the highest strain capacity of all modern sealant materials, being able to accommodate joint movements of ± 100% in many cases. They have high resistance to ultraviolet light and a wide range of chemicals and possess good colour stability. However, they are expensive to apply and cannot be painted. Silicones can be cured using acetic acid or neutral catalysts, although neutral-cured compounds are preferred for bonding to concrete surfaces to avoid possible reaction of the acid with the alkali in the concrete.
Figure 8.8 Joint Design Principles
External cladding may be required to have a specified Fire Resistance Period (FRP). Cladding panels will usually be designed or tested to meet these requirements in accordance with Section 5 of AS 3600. This Section also contains the requirement for joints between members and between adjoining parts to “be constructed so that the fire-resistance level of the whole assembly is not less than that required for the member’’. Thus there is a requirement that joints have the same FRP as that required for the wall. Most sealant manufacturers produce sealants that are designed to provide resistance to fire. Where the assembly is not tested, the joint details and sealants should be designed and applied in accordance with the manufacturer’s recommendations to give the required level of fire resistance. Fire-rated sealants can be acrylics, silicone or polyurethane. Acrylics are suitable only for locations where movement is low and the sealant is not subject to attack by UV light. Silicones possess good UV-light resistance . Polyurethane sealants are currently the preferred sealant type. When evaluating the various alternative fire-rated sealants it should be noted that primarily these sealants are required to seal the joint against the weather and provide long-term performance. The sealant must first meet the environmental and movement criteria and keep the building weatherproof. Hopefully, only a few joints on some buildings will have to function as a fire barrier. This is not to downgrade the requirements for fire resistance but to stress the importance of first correctly evaluating the environmental criteria required for fire-rated joint sealants. It may be necessary to install fire-retardant blankets in some joints to achieve the required fire-resistance 8.3.
8.1 Egan, D 'Joints and Sealants for Precast concrete Panels' Constructional Review Vol. 67 No. 1 February 1994, pp 54–59. 8.2 AS 3610 Formwork for concrete Standards Australia, 1995. 8.3 Gustaferro, A H and Abrams, M S ‘Fire Tests of Joints Between Precast Concrete Wall Panels: Effects of various joint treatments’ PCI Journal Vol. 20 No. 5, September-October 1975. 8.4 Building Code of Australia Australian Building Codes Board, 2008. 8.5 Design of Joints in Concrete Buildings (CPN 24) Concrete Institute of Australia, 2005.
8.6 AS 3600 Concrete Structures Standards Australia , 2009. 8.7 ACI Committee 504 ACI Report 504R-90 (Reapproved 1997) Guide to Sealing Joints in Concrete Structures, ACI Manual of Concrete Practice, Part 5 American Concrete Institute, Farmington Hills MI, 1999. 8.8 ISO 11600 Building construction - Jointing products - Classification and requirements for sealants European Standards NSAI, 2002.
Section 7 of CPN 24 8.5 Design of Joints in Concrete Buildings, has charts for calculating the extent of nonconbustible fibre blanket needed in a butt joint to provide the required fire-resistance periods.
It is Recommended that Joints be Treated as a Strong Visual Feature of Architectural Wall Design
Open-Drained Joints are the Recommended Type for most Medium- to High-Rise Construction.
Open-Drained Joints can Tolerate Relatively Large Movements and the Rear Sealant is Protected from UV Light
Open-Drained Joints can be Installed from Inside the Building
What you will find in this Chapter •
Thermal:
Basic concepts of heat transfer. Factors influencing heat transfer into and out of a building. Calculating the thermal resistance of precast construction from material data. Factors affecting moisture condensation and its control. The effects of thermal mass and shading to the comfort of building occupants. Acoustics:
Basic concepts of sound transmission in buildings. How transmission loss is measured. How different types of construction are compared. Factors affecting the efficiency of walls and floors in controlling noise. The control of noise transmission with precast concrete members.
9.1 Thermal Properties 9.1.1 Introduction 9.1.2 Heating and cooling energy 9.1.3 Economic considerations 9.1.4 Heating and cooling calculations 9.1.5 Basic heat transfer concepts 9.1.6 Rate of heat transfer through a building assembly 9.1.7 Surface-air-film Resistances 9.1.8 Typical Precast Walling and Floor Solutions Thermal Resistance of Precast Solid Wall Panel Example 9.1 Thermal Resistance of Sandwich Wall Panel Example 9.2 Thermal Resistance of Hollowcore Floor Assembly Example 9.3 Example 9.4 Thermal Resistance of 'Ultrafloor' Floor Assembly 9.1.9 Condensation 9.1.10 Vapour barriers 9.1.11 Control of condensation by insulation Condensation Analysis Example 9.5 9.1.12 Thermal mass, capacitance and inertia 9.1.13 The effects of thermal mass 9.1.14 Evaluation of mass effects 9.1.15 Mass and heating 9.1.16 Mass and cooling 9.1.17 Solar radiation 9.2 Acoustic Properties 9.2.1 Architectural acoustics 9.2.2 Airborne sound 9.2.3 Measurement of transmission loss 9.2.4 Types of wall construction Sound Reduction Index, Rw, of a Composite Panel Example 9.6 9.2.5 Structure-borne sound 9.2.6 Sound absorption of concrete walls 9.2.7 BCA sound insulation provisions 9.3 References 9.4 Bibliography
9.1.1
Introduction
The Building Code of Australia (BCA)9.1 has been progressively introducing energy-efficiency provisions for all building classifications since 2003 as part of a broader strategy being undertaken State, Territory and Federal governments to reducebygreenhouse gas emissions.
Thermal performance is a broad term that refers to the ability of a building to maintain comfortable indoor temperatures while minimising the use of heating and cooling energy. The BCA thermalperformance provisions are expressed as minimum thermal resistance (R value) requirements for walls and roof/ceilings. Under these provisions, different Total R-values are required depending on the building classification (Table 9.1) and the climate zone (Figure 9.1) in which the building is located. of the BCA covers all Class J in Volume 2Section to 9 buildings in all climate zones (Table 9.2) and Part J Building Fabric, addresses the energy-efficiency performance of various building elements. Each element within Part J is intended to work as part of a system to achieve overall building-energy efficiency. Insulation will be required to meet the required R-values for external precast walls in air-conditioned buildings and for precast flooring systems which have a non-air-conditioned space under them. Clause 9.1.8 provides guidence on economical BCA-compliant precast concrete solutions using common insulation products.
Table 9.1 Building Classifications [After BCA9.1]
Figure 9.1 Climate Zone Map of Australia [After BCA9.1]
Table 9.2 Minimum Total R-Values for each Climate Zone [After BCA9.1]
ß
ß
ßÝ
Ý
ß Ý
ÝÝÝÝ
ß
ßßßßßßßß
ß Ý
9.1.2
Heating and cooling energy
In most buildings, the main contributors to the cost of space conditioning (heating or cooling or both) are internal heat gains, heat loss by transmission, air leakage across the building envelope, and solar gains. The amount of thermal storage in the building elements can also be important, depending on the climate and the heating and cooling regime. A precast concrete enclosure has high thermal inertia and hence thermal storage, and thus may have an advantage over lightweight enclosures. Precast’s thermal proper ties can be varied to meet regulations and to control the environment within a building. In particular, the thickness, shape and density of a member can be varied within wide limits. The wall thickness and concrete density determine the resistance to heat flow and the capacity to store heat.The external shaping of the member can be used to control the amount of direct radiation entering the building.This reduces discomfort to occupants at the building perimeter when, as is often the case, air-conditioning levels are set for occupants in zones unaffected by radiation. In the clauses that follow, the emphasis is on controlling transmission heat transfer. However, it must be appreciated that heat transfer due to air leakage, or infiltration/exfiltration, may also be quite significant. Proper attention must therefore be directed to a number of design considerations such as the number, orientation and thermal resistance of windows, and the sealing of doors and joints. 9.1.3
Economic considerations
Energy costs are a significant part of life-cycle costs. More than half of the true total costs incurred during the economic life of a building may be attributable to operating and energy costs. Life-cycle costing is a sound means of assessing the cost of all elements involved in constructing and operating a building and allows rational decisions to be made on insulation levels for the building. 9.1.4
Heating and cooling calculations
Heating and cooling calculations are used to predict peak energy loads and annual energy usage. Peak load estimates are required to size equipment and to design distribution systems. Increasing a building’s thermal mass with precast concrete panels will sometimes lower and shift peak loads, which can reduce equipment size. Small equipment that runs continuously uses less energy than large equipment that has been sized to meet large peak loads but for most of the time runs intermittently. As design standards switch from prescriptive to performance requirements, annual energy usage calculations will
become imperative. Design calculations of both peak loads and annual energy usage in all but the simplest of buildings are relatively complex and often require special expertise and computer analysis. Some factors that influence peak loads and energy usage are: • Geographic location: latitude, longitude, building exposure (landscaping). • Occupancy demands: number of occupants, their requirements, activities, hours of occupancy. • Building characteristics: site, orientation, plan, configuration, insulation, insulation location, envelope mass, mass of the construction and contents, window glass, window frame, shading of glazing, shading of the building, surface colour and texture. • Climate: dry-bulb temperature, wet-bulb temperature, wind speed and direction, solar radiation, cloud cover. • Heat loss mechanisms: transmission, lowtemperature radiation, air leakage and ventilation. • Heat gains: transmission, solar and lowtemperature radiation, air leakage and ventilation. • Internal heat gains: occupants, lights, appliances, machines, power and equipment. •
Environmental (comfort): area, temperature, indoor relativewindow humidity andindoor indoor
air quality. occupancy, lighting, ventilation, equipment, changes in thermostat set points, changes in humidity. When computing energy consumption, equipment efficiency must also be considered.This is particularly important when making life-cycle cost studies involving factors such as mass, optimum insulation, glass area, environmental factors, energy type and energy costs. Most of these factors can be handled by software tools for calculating peak loads and annual energy usage. Designers of building envelope assemblies can provide overall energy efficiency by insulating different parts of the building in the most cost-effective manner. For example, the cost of adding additional insulation to roofs is usually less than for walls. •
Usage times:
9.1.5
Basic heat transfer concepts
In the SI system there are two temperature scales, Celsius and Kelvin. In the Celsius scale, 0°C is the temperature of melting ice; the Kelvin scale star ts at absolute zero (-273.15°C). A temperature difference of one degree is the same in both scales. The unit of heat is the joule (J). It is the amount of energy equivalent to the work done by a force of one newton when it moves a particle one metre in the direction of the force. Thus one joule is one newton.metre, or N.m. The unit of heat flow rate is the watt (W). A watt is the power developed when work is done or energy is expended at the rate of one joule per second, ie 1 W = 1 J/s. When a material is heated, the amount of energy stored in the body is increased. A larger mass of a particular material requires more energy to bring it to a cer tain temperature than a smaller mass. The specific heat (c p) is the amount of heat required to raise 1 kilogram of a material by one degree Kelvin. Its units are J/(kg.K). It is an intrinsic property of the material. For example, the specific heat of water at 15°C is 4185.8 joules per kilogram for a temperature rise of 1°K (or 1°C). The thermal capacitance (C) is the amount of heat required to raiseofthe temperature unit degree. area of aIt slab of material a given thicknessofbya one is calculated as the product of the material’s density, thickness and specific heat and its units are J/(m2.K). It is not an intrinsic property of a material. The thermal conductivity (k) is the rate of heat flow through unit thickness, across unit area for unit temperature difference. It is an intrinsic property of a material. Its units are W/(m.K). Unit conductivity means that a slab of material one metre thick will transmit heat at the rate of 1 watt per square metre for every degree of temperature difference between opposite faces. The thermal resistivity (r) is defined as the reciprocal of the conductivity, ie r = 1/k. Its units are thus (m.K)/ W. It is also an intrinsic property of a material. Table 9.3 gives the intrinsic properties of some common materials. The thermal resistance (R) of a material is the temperature difference required to establish a heat transfer rate of 1 W across a unit area of a slab of the material of a given thickness. Its units are m2.K/W. It is not an intrinsic property. The higher the R-value, the greater the resistance to heat transfer. The thermal resistance of a homogeneous material is calculated as: R = L / k, where: L = thickness of material in metres k = thermal conductivity.
Air gaps and surface-air-films also possess a thermal resistance and are discussed below. The thermal conductance is the reciprocal of the thermal resistance. Its units are W/(m2 K). It is not an intrinsic property. Table 9.4 gives the thermal resistance and conductance of selected building materials. The total thermal resistance of a building assembly can be calculated readily if the assembly is approximated as a series of layers of homogeneous materials and air gaps with parallel surfaces. With this simplification, the total resistance of the assembly is the sum of the resistances of each layer, including any air gaps. The simplification amounts to assuming that heat flow through the assembly is one-dimensional, eg there are no heat bridges, such as metal connectors, penetrating an insulation layer. The total thermal resistance can be calculated from surface to surface, or, more usefully, from air to air across the assembly. In the latter case, the resistances of the indoor and outdoor surface-air-films, R si and Rso, are included. These are discussed in detail below. For an assembly consisting of layers of materials, the total thermal resistance is given by: SR = R so + SRmaterials + SRair spaces + R si The overall thermal transmittance is the reciprocal of the total thermal resistance. It is the rate at which heat is transferred through a unit area of a building assembly for a unit temperature difference between indoor air and outdoor air. It is termed the U-value and has units of W/(m 2.K). U = 1 / SR
Table 9.3 Typical Properties of Common Building Materials
r
Table 9.4 Thermal Conductance and Resistance of a Material for a Particular Thickness
9.1.6
Rate of heat transfer through a building assembly
For a building assembly consisting of a series of layers of building materials and air gaps, the rate of heat transfer can be calculated as follows: Q = A DT / SR or Q = A DT U where: Q = rate of heat transfer (W) A = area of the assembly (m2) DT = difference in temperature across the assembly (air to air) (K) U = overall thermal transmittance (W/[m2.K]) The rate of heat transfer through the assembly is the same as through each layer. 9.1.7
Surface-air-film resistances
When calculating the rate of heat transfer through a building assembly, it is necessary to know the temperature difference across it, and to use the total thermal resistance appropriate to this temperature difference. The surface temperatures of an assembly are usually not known, but the air temperatures on both sides usually are. Thus, in order to use of thethe air temperature difference, the total resistance assembly must include the resistances to heat transfer from the surfaces to the surrounding air, commonly called the surface resistances or surface-air-film resistances, Rsi and Rso. Surface resistances have two components: a radiative component and a convective component. The radiative component deals with the radiant heat transfer between the surface of an assembly and all its surrounding surfaces. It is generally expressed as a radiative conductance or coefficient (hr). It depends on the temperatures, geometries, and emissivity of the various surfaces. The emissivity (e) of a surface is a measure of its ability to radiate and absorb energy, and can range from 0 to 1. Polished metal surfaces (such as in reflective foil insulation) have low emissivity (as low as 0.03), which means that they are poor radiators and poor absorbers (and thus good reflectors) of radiant energy. Most other materials have high emissivity (typically above 0.8), which means that they are good radiators and good absorbers (and thus poor reflectors) of radiant energy. Since, in general, the surrounding surfaces will have very different shapes and sizes, and can be at very different temperatures, the exact calculation of radiative heat transfer is very complicated, and it is necessary to simplify matters in order to enable routine calculations to be made.
The convective component deals with the heat transfer between a surface and the air via the thin boundary layer of air adjacent to the surface. It is generally expressed as a convective conductance or coefficient (hc). It depends on the temperatures of the surface and air, the degree of surface roughness, the speed of air moving across the surface, and the orientation of the surface (eg vertical, facing up, facing down, etc). Again, an exact calculation of the convective resistance can be complicated, and simplification is needed for routine calculations. From the above discussion it is clear that since the temperature difference for convective heat transfer is between the surface and the surrounding air, while the temperature difference for radiative heat transfer is between the surface and the surrounding surfaces, the radiative and convective temperature differences will generally not be the same. A key simplification that is commonly made is to assume that these temperature differences are the same, in which case the radiative and convective conductance can be added and the overall surface resistance, Rs, calculated as: Rs = 1/(hc + h r) For high-emissivity surfaces, the radiation coefficient, hr, has a value of 5.1 W/(m 2.K) at a mean surface temperature of 20° C, and a value of 4.2 W/(m2.K) at 0° C. For moving air, the convective coefficient may be calculated from the air speed (v) in m/s along the surface. While there is no definitive formula, a typical one is: hc = 5.8 + 4.1v For still air, the convective coefficient depends on the orientation of the surface and the temperature difference. Table 9.5 lists surface-air-film resistances for high- and low-emittance surfaces, ie e = 0.9 (nonreflective) and e = 0.05 (reflective). The values of the resistances decrease with both increasing roughness (although this is not shown in Table 9.5) and rate of air movement over the surface. Nonreflective (high-emittance) surfaces have a lower 9.5, the resistance than reflective surfaces. In Tableonly effect of emittance is taken into account for still air conditions, as in a wall cavity (see below), and internally, but it must be remembered that this is quite a gross simplification. Air speeds of 6 m/s and 3 m/s are usually adopted in building calculations for winter and summer conditions respectively for external surfaces. An air speed of 0.5 m/s may be used for internal surfaces subjected to forced ventilation (eg from air conditioning).
The thermal resistance of an air space depends on its orientation, the direction of heat flow (eg horizontal, up or down), the emissivity of the bounding surfaces, and the temperature of the space. Table 9.6 lists values of resistance for common situations. Use of the tables is shown in Examples 9.1 to 9.4 for wall, floor and roof assemblies in precast concrete.
Table 9.5 Thermal Resistance of Surface-Air-Film
Table 9.6 Thermal Resistance of Air-Space
*
*
*
*
9.1.8
Typical Precast Walling and Floor Solutions
Using Table 9.1, Figure 9.1 and Tables 9.2 to 9.6 designers should consider the following examples of composite precast concrete systems where insulation provides a level of thermal resistance to comply with the energy efficiency provisions of Section J of the BCA. Four steps are proposed: • Identify the Climate Zone for the project (Figure 9.2) • Identify the Building Classification for the project from Table 9.1 • Look up Table 9.2 which provides the minimum total R-Values for each climate zone. • Determine from Tables 9.3 to 9.6 the R-Value of the total system which includes insulation to meet the minimum R-value required. When using any insulation product it is recommended that actual R-values be verified with the insulation supplier. It is also recommended that the R-value of the total system be checked with an independent ABSA-accredited assessor.
Example 9.1 Thermal Resistance of Precast Solid Wall Panel
R-value of assembly: Outside surface-air-film 0.030
Table 9.5
0.123
Table 9.3
flow, one surface reflective) 0.580
Table 9.6
Glasswool insulation (50 mm thick, 11 kg/m 3)
1.200
Table 9.4
Plasterboard (13 mm)
0.069
Table 9.2
non-reflective surface)
0.120
Table 9.5
Total R-value of assembly
2.122 m2.K/W
(winter) Precast panel 0.175 x 0.700 Air-space (horizontal heat
Inside surface-air-film (still air, horizontal heat flow,
Steel furring channel adjustment: Use glasswool insulation
(50 mm thick, 14 kg/m3)
1.300
Less 30%
0.390
Table 9.4
Effective insulation value
0.910
Difference to above
0.290
Net Total R-value
1.832 m2.K/W
Thermal Resistance of Sandwich Wall Panel
Example 9.3 Thermal Resistance of a Hollowcore Floor Assembly
R-value of assembly: Outside surface-air-film
(winter)
0.030
Table 9.5
0.052
Table 9.3
Exterior concrete panel 0.75 x 0.700
Extruded polystyrene
–
(value from manufacturer) Interior concrete panel
1.850
0.15 x 0.700 Inside surface-air-film (still
0.105
Table 9.3
non-reflective surface)
0.120
Table 9.5
Total R-value of assembly
2.157 m2.K/W
air, horizontal heat flow,
Outside surface-air-film
0.030
Table 9.5
Plasterboard (10 mm)
0.058
Table 9.4
(14 kg/m 3)
1.300
Table 9.4
Hollowcore slab Concrete topping,
0.240
Table 9.4
(0.05 x 0.700)
0.035
Table 9.3
non-reflective surface)
0.160
Table 9.5
Total R-value of assembly
1.823 m2.K/W
Glasswool insulation,
Inside surface-air-film (still air, downward heat flow,
9.1.9
Example 9.4 Thermal Resistance of ‘Ultrafloor’ Floor Assembly
Outside surface-air-film 0.030 Expanded polystyrene (value from manufacturer) 1.240 Air-space (horiz, downward heat flow, non-reflective) 0.170
Table 9.5
–
Table 9.6
0.120 0.070
–
non-reflective surface)
0.110
Table 9.5
Total R-value of assembly
1.740 m2.K/W
Formboard (16 mm) Concrete slab (0.1 x 0.700)
Table 9.3
Inside surface-air-film (still air, upward heat flow,
Condensation
Under normal circumstances, air contains only a certain percentage of the maximum possible amount of water vapour. This percentage is called the relative humidity (RH) and is the ratio of the water vapour pressure present in air to the water vapour pressure present in saturated air (ie air containing the maximum possible amount of water vapour) at the same temperature and atmospheric pressure. As air is cooled, the maximum amount of water vapour it can contain decreases. Water vapour will condense when it comes into contact with a surface at or below a critical temperature called the dew point. For air containing a certain amount of water vapour, the dew point is the temperature at which the air becomes saturated, ie the temperature at which the water vapour content is the maximum that can be contained. Condensation can lead to the eventual breakdown of finishes when it occurs on the interior surface of walls and ceilings. It can also damage the structure or the insulation if it occurs in building cavities. Condensation on interior surfaces may be controlled or avoided by a combination of ventilation, vapour barriers and insulation. 9.1.10
Vapour barriers
The principal functions of a vapour barrier are to retard the passage of moisture as it diffuses through the assembly of materials in a building envelope, to control the location of the dew point in the assembly and to ensure a manageable flow of moisture across the assembly. It may be formed from such differing materials as a sound film of paint, a polyethylene film, or an impervious metallic layer such as aluminium foil. The vapour barrier should be installed on the warm side of any insulation, with the object of preventing the migration of moisture vapour from the warm, high-moisture-content side to the cooler side where it may condense in the wall or ceiling cavities or inside the materials. Note that the location of the warm side of the insulation depends on the circumstances, eg the climate. In cold climates, where indoors is warmer than outdoors, the warm side could be immediately behind the facing sheet, on the indoor side of the insulation. In hot climates with air-conditioned spaces, indoors may be cooler than outdoors, in which case the warm side is on the outdoor side of the insulation. In addition, there should be no other membrane on the cold side of the vapour barrier/insulation system with a lower resistance to water vapour transfer than the vapour barrier itself. While a vapour barrier need not be perfectly continuous, care should be taken to minimise the occurrence of imperfections such as unsealed laps, cuts and pinholes.
9.1.11
Control of condensation by insulation
The calculation of the temperature gradient profile through a roof or wall assembly can be used to determine whether there may be a problem with condensation or differential thermal movement. The temperature gradient alone is not sufficient to accurately locate the dew point within the assembly but it can be used as a guide where condensation might occur from exfiltrating or infiltrating air. The
Condensation analysis
assumption of steady-state conditions in such a calculation is seldom satisfied, owing to fluctuations in the temperatures to which a building envelope is exposed. Nevertheless, the calculation is useful to flag potential problems. Calculating the temperature-gradient profile through an assembly due to an indoor-outdoor temperature difference allows an estimation of the location of condensation planes, and thus an initial assessment of the suitability of wall and roof assemblies. Table 9.7 lists dew-point temperatures for a range of relative humidities and air temperatures. This table can be used once the thermal gradient is determined. If the calculated temperature at a plane within an assembly is less than the anticipated dew point temperature, it can be expected that condensation will form at that plane. The steady-state temperature at any plane in the assembly can be estimated as follows: ts = t i - ( DT/SR) Rpartial where: t s = temperature of the internal plane of interest (°C) ti = indoor air temperature (°C) DT = ti - t o (°C) to = outdoor air temperature (°C) Rpartial = total thermal resistance from indoor air to the plane of interest (m2.K/W) The thermal resistance of the wall or ceiling must be sufficient to keep the surface temperature above the dew point. Table 9.7 can be used as a guide in establishing the lowest anticipated dew point temperature. It lists the dew points for a range of inside air temperatures and relative humidities. An illustration of the use of Table 9.7 is given in Example 9.5. This shows that condensation will not occur on either side of the plasterboard. From Table 9.7 it can be seen that the indoor RH would have to be about 80% (a dewpoint of 16.5°C) for there to be a condensation risk at the outside surface of the plasterboard. It also shows that condensation from exfiltrating air will occur on the inside surface of the precast panel, since its temperature of 4.8°C is well below the dew point of 13.3°C.
Temperature drop across the wall: T = 20 - 2 = 18°C Temperature at any plane: Rpartial Tn = Tinside x T R
Plane
Temperature (°C)
Plasterboard
20 -
Inside face Outside face Precast panel Inside face Outside face
20 20 -
0.12 0.912 0.189 0.912 0.769 0.912
x 18 = 17.6 x 18 = 16.3 x 18 = 4.8
20 - 0.882 x 18 = 2.6 0.912
At 20°C / 65% RH, dew point
(12.1 + 14.5)/2 = 13.3°C
Table 9.7
therefore condensation will not occur at either face of the plasterboard (both > 13.3°C). RH of 80% required for condensation to occur.
Table 9.7 Dew Points for Ambient Air Temperature and Relative Humidity
9.1.12 Thermal mass, capacitance and inertia
These terms are often used interchangeably, and refer to the ability of a material to store heat. While the specific heat does not var y greatly for many inorganic materials, the density does (see Table 9.2). Thus, dense materials, or high-mass materials, such as concrete have a high thermal capacitance and can store much more heat in a given volume than lowdensity materials such as bulk insulation. Whereas thermal resistance is a steady-state concept, thermal mass comes into play only when outdoor and/or indoor temperatures or heat flows vary in time. The greater the variation in temperatures (eg between daily maximum and minimum outdoor temperatures), the greater the potential benefit of thermal mass. The BCA currently imposes requirements on the R-values only of building assemblies. However, to achieve better control over the environment within a building, both the thermal resistance and the thermal mass (or thermal capacitance) should be used. Figure 9.2 shows how the mass of a concrete roof affects the heat flowing through it. Solid or high-mass walling systems act in the same way. This ability to store heat causes the peak indoor temperature to be offset by approximately six hours in this example. Figure 9.2 Heat Flow Through a 200-mm-thick Concrete Roof [From Addleston9.5]
The effects of thermal mass
The mass of heavy materials such as precast concrete wall, floor and roof elements can reduce the annual cooling and heating requirements of a building. The effect of massive materials on peak loads and annual energy requirements is primarily governed by: • location and storage characteristics of the mass; • location of any insulation with respect to the mass; • effectiveness of the thermal coupling between the • • • •
mass and indoor air or heat sources; ratio of internal heat gain to heat loss; time of day when internal gains occur; solar radiation through glass; ventilation rate.
9.1.14
Evaluation of mass effects
Because of the complex interactions between climate, mass, insulation, and heating and cooling regimes, computer simulations are essential to fully evaluate the effect of mass on heating and cooling loads. A simulation study (CSIRO9.3) evaluated a typical residential and a small commercial building for a full year of weather conditions using meteorological data for Melbourne, Sydney and Brisbane, thus covering most climatic areas that contain significant populations. The NatHERS software was used for residential buildings and BUNYIP for commercial buildings. Various types of walling systems, including high-mass and low-mass, uninsulated and insulated, were evaluated. Some results are briefly described below.
9.1.13
9.1.15
Mass and heating
During the heating season, mass located on the indoor side of any insulation will help to stabilise indoor temperatures in unheated spaces, particularly if the mass is subjected to conditions that permit it to absorb solar radiation and heat from lights, equipment and occupants. At night, the absorbed heat is then released to nearby cooler surfaces or the air in the space, leading to more comfor table conditions. This process of absorption and release is largely dependent upon the location of the mass in the structure relative to the space being heated, the sources of heat and location of any insulation. Regardless of climate, the CSIRO study found that uninsulated solid cavity wall construction generally performs better than the other uninsulated wall types tested. Solid single-leaf concrete with plasterboard on battens gave similar performance to uninsulated solid cavity wall construction. Concrete walls with foil-backed board on battens provided similar Uvalues and heating energy performance, as did the
other insulated wall systems that would meet BCA requirements under certain circumstances (eg in Victoria, in houses with concrete slab floors). In cooler climates, where heating is the predominant requirement, the study found that once walls are insulated, the heating energy differences between the wall types were relatively small and the location of the insulation (inside face, outside face, central or both faces) had little impact. 9.1.16
Mass and cooling
Mass on the outside of building envelopes absorbs solar radiation, some of which is released back into the atmosphere during the cooler night-time. Mass on the indoor side of any insulation will help to stabilise the indoor temperature, reduce the maximum indoor temperature and delay its occurrence. Thermal coupling between the interior mass of the building and cool night-time outside air is important, as is forced ventilation, which will flush out excess heat stored within the structure during the day.The mass of precast concrete building construction can substantially reduce the need for mechanical cooling. In warmer climates, where cooling is the predominant requirement, the CSIRO study found that insulated masswalls wallswere performed better than uninsulated wallsthe if the insulated on the outdoor side and solid partition walls were not insulated, so that their thermal mass was well coupled to the indoor air. Solid internal partition walls gave better performance than lightweight par titions. 9.1.17
Solar radiation
Windows are a dominant source of heat gain regardless of the type of glass used. Shading devices can reduce solar radiation in the summer while still allowing solar radiation to enter the building for winter heating. Overhangs provide shading for windows when the summer sun is at a higher altitude and allow direct sun on the windows in winter when the sun altitude is much lower. Precast concrete is ideal for the construction of sunshades, often as an
Figure 9.3 Subjective Temperature of Skin [From Harkness9.6]
shows the subjective skin temperature of occupants behind various facade systems for a west-facing window in Sydney in December. The benefit of shading can be seen in curve (c). At an air-conditioned air temperature of 23°C, the skin reaches 33°C behind unshaded clear glass, 31°C behind green heat-absorbing glass and 26°C behind shaded clear glass. This last increase is mostly due to diffuse solar radiation. Figure 9.3
integral part of wall panels. Environmental temperature, which can be considered to be the effective temperature perceived by an individual, is affected by the air temperature, the rate of air movement and radiation from surrounding surfaces or from direct solar radiation. The radiation component has an important influence on the sense of thermal comfort. Occupants of offices sitting directly in the solar beam behind any type of glass experience higher environmental temperatures than in areas remote from the windows.
illustrates the air temperatures required to achieve comfor t for those parts exposed to solar radiation. At times of peak solar radiation the air temperature may need to be reduced to 16°C or lower, but this has the effect of chilling the shaded side of the body. Thermal comfort cannot be achieved by simply supplying cooler air in zones of direct radiation; the occupants have to be shaded from the direct solar beam. Figure 9.7 illustrates a facade clad in precast concrete that forms shallow Figure 9.4
sunscreens and shades glass for of the day. It also shades fromthe a portion of portion the diffuse radiation. This configuration has a horizontal voidto-solid ratio of 1.033 and 41% of glass. The effect of various shading configurations on air conditioning loads can be calculated for any particular weather data using a computer simulation program such as CAMEL . Cooling loads on a typical day derived from actual data for this precast layout compared to a flush glass facade is shown in Figure 9.5. An alternative solution is illustrated in Figure 9.8. This layout uses overhanging spandrels to shade the glass below. Simple shading elements are effective in reducing air conditioning plant size, recurrent cooling loads and in shielding occupants near windows from the direct component of solar radiation. Figure 9.6 shows preferred cross sections for economical use of precast concrete as shading elements. The depth of the overhang from the window plane, the height of the window opening and the size of the louvres may be designed to control sunlight penetration for the various facades. In temperate areas in the southern hemisphere: • simple horizontal shading is effective on the northern facade; • vertical louvres and mullions are effective on the southern facade and for a range of orientations in the quadrant southeast to southwest, provided the tops of the louvres are covered. These sunscreens may be designed by the method described in Sunshine and Shade in Australasia9.4. Sunscreens facing due east and west may also be designed using this method and will produce
Figure 9.4 Air Temperatures to Achieve Thermal Comfort [From Harkness9.6]
Figure 9.5 Cooling Load Comparisons for Shallow Sunscreen Facade [From Harkness9.6]
Figure 9.6 Preferred Sunscreen Configurations [Based on Harkness9.6]
designs which give a view out to the southeast and southwest.
Figure 9.7 Example of a Shallow Sunscreen Facade [From Harkness9.6]
Figure 9.8 Example of an Overhanging Strip Sunscreen [From Harkness9.6]
Transmission loss
9.2.1
Architectural acoustics
Architectural acoustics deals with the control of sound propagation within buildings. The objective is to provide environments where occupants hear what
When a sound wave strikes a partition it will be deformed and vibrate. This causes it to generate pressure variations in the adjoining space as a portion of the srcinal sound is transmitted through it. When the por tion is low, the partition is said to have a high transmission loss (TL). The loss increases with increasing frequency as illustrated in Figure 9.9, the rate of increase being a function of mass and stiffness. The interaction of sound with a non-homogenous
they want to hear and are control not seriously disturbed unwanted sounds. Sound in buildings maybybe broadly divided into two categories: • control within an occupancy – wanted sounds are heard properly by the recipients, without being blurred by reverberation or echoes. This is primarily done with sound absorbing materials; • insulation between occupancies – sound srcinating in an occupancy does not intrude into adjacent occupancies. Sound may be airborne or created by impact, travelling through the structure. Sound insulators are not effective sound absorbers and sound absorbers provide little insulation. They are treated separately in sound-control design. This chapter deals primarily with insulation. While walls are generally referred to, insulation principles apply equally to floors.
partition over the audio frequency range is most complex. It is not easily predicted by mathematical modelling which has largely been developed for lightweight materials. This is compounded by the fact that acoustic testing is laborious and must be conducted at full-scale for meaningful results so that calibration of theory with reality is a developing technique. Nevertheless, theoretical predictions of the performance of compound par titions are usually within the range of variance between laboratories9.17 and are useful for design purposes. Heavy materials like concrete are the most effective to use as single par titions for attenuating airborne sound. Also, a concrete panel in combination with lightweight materials can achieve very high insulation values (with less mass than an equivalent solid wall) by exploiting the different responses of the materials
9.2.2
in the everyday frequency range. The transmission loss in the audible range can be divided into three regions9.16; stiffness-controlled, mass-controlled and wave-coincidence, Figure 9.9.
Airborne sound
Airborne sound travels as waves of rapid air pressure variation. The frequency of sound is the rate at which successive crests of a sound wave pass a given point and is measured as cycles per second or hertz (Hz). Wavelength is the distance between two successive crests. Wavelength (l) and frequency (f) are related by the expression l = c/f where c is the speed of sound in air, about 344 m/s. Sound pressure level, the perceived loudness of a sound, is measured by the decibel (dB). The decibel is a logarithmic function of the ratio of the sound pressure to a reference pressure, taken to be 20 mpascals, the lower limit of hearing. The human ear can detect sounds from 20 Hz to 20 000 Hz and is most sensitive in the 1000 to 5000 Hz range. Loudness depends on both intensity and frequency. Changes in sound level of 3 dB or less are difficult to notice. A doubling of loudness for the average listener is an increase of about 10 dB. Sound attenuation of common building materials is most effective at shorter wavelengths, ie high frequencies. Long wavelength sounds, below say 100 Hz, can travel long distances unimpeded and are an increasing source of annoyance due to the growing use of powerful sound equipment in the home.
Figure 9.9 Characteristic Sound-Transmission-Loss Curve
Stiffness-controlled region
In the very low frequency region, below about 100 Hz for most building materials, the stiffness of the partition in bending controls the amount of sound reduction until resonance occurs. Mass and damping are of little consequence here. A partition of a particular construction has a series of vibration modes that are excited at certain frequencies. The first occurrence is the fundamental frequency with the greatest effect followed by a series of integer multiples with progressively less effect. Resonant frequencies can be calculated as: Fn = 0.45 n t [(n/L)2 + (n/H)2] where: Fn = resonant frequency at nth harmonic (Hz) t = panel thickness (m) L = panel length (m) H = panel height (m) n = harmonic, 1 = fundamental n = longitudinal velocity of sound in panel (m/s) in which: n = ÖE/r(1 - m2) where: E = material elastic modulus (MPa) r = panel density (kg/m 3) m = Poisson’s ratio, = 0.2 for concrete Mass-controlled region
Commencing at approximately twice the lowest resonant frequency, the greatest influence on the response of a partition to sound is its mass. The heavier the partition the greater the sound insulation it can provide due to the increase in energy required to set it in motion. The mass law is a semi-empirical expression that predicts transmission loss until wave-coincidence occurs. It has greatest accuracy for lightweight materials such as plasterboard in the mid-audio range. The mass law predicts that the transmission loss will increase by 6 dB for each doubling of the surface mass (mass per unit area) or doubling of the frequency (one octave). An increase in the transmission loss by the minimum discernible change, 3 dB, at a particular frequency requires an increase in the mass by a factor of 1.4 due to the logarithmic relationship.
The mass law is expressed as: TL = 20 log 10 (m f) - B where: TL = transmission loss (dB) m = surface mass (kg/m 2) f = frequency (Hz) B = 48, but can vary between 45 and 53 depending on angle of sound incidence, field conditions, etc While mass is concrete’s greatest asset, when used as a sound insulator, its transmission loss in the audible range is not accurately predicted by this relationship since wave-coincidence effects commence at a low frequency. Wave-coincidence region
Shear waves due to bending are generated in the surface of a partition during flexing from sound pressure variation. At a critical frequency above the masscontrolled region, the velocity of incident sound waves will equal that of these ripple waves, increasing the efficiency of energy transfer and reducing the effective insulation. This effect starts at a particular frequency that varies with the surface mass and modulus of elasticity of the partition.The stiffer or thicker the material, the lower the critical frequency. It is low for concrete walls, about 125 Hz, and high for lightweight partitions, in the range 1000 to 4000 Hz.
The transmission loss in this region has the relationship9.12: TL = 20 log 10 (m f) + 10 log10 (hf/fc) – 44 (dB) where: h = a loss factor dependant on material properties = 0.006 for concrete panels in which 9.9: fc = 0.555 (c2 / t) !(w / E) where: f c = critical frequency (Hz) t = thickness of the material (m) w = material density (kg/m 3) E = material elastic modulus (N/m2) c = speed of sound in air (344 m/s) Table 9.8 gives
typical values of the critical frequency for a range of materials and thicknesses. Table 9.8 The Product of Critical Frequency (Hz) and Thickness (mm) for Various Materials
x
For solid, normal density concrete, the critical frequency is given by f c = 18 700/t where t is the thickness in mm. For example, a 150-mm-thick concrete slab with a material density of 2300 kg/m 3 and an elastic modulus of 28 000 MPa has a critical frequency of 125 Hz. The transmission losses of two single-leaf walls are illustrated in Figure 9.10. For the lightweight partition, agreement with the mass law is good below about 2000 Hz. The transmission loss for the 150-mm concrete slab is below that predicted by the mass overlow most of the mid-audio frequency range duelaw to the value of the critical frequency. Figure 9.10 Tranmission-Loss Curves for Single-Leaf Partitions – 16-mm Plasterboard and 150-mm Concrete [from CPCI9.10]
Flanking is often the result of poor construction practices such as unsealed gaps at partition edges, cracks in mortar and the like. Flanking can therefore be controlled by effective details and quality construction. To control flanking and provide the maximum possible attenuation in a building: • avoid physical contact between layers in a built-up wall; • caulk the perimeter of walls with non-hardening sealant; • use resilient fastening systems to support plasterboard walls and ceilings; • isolate structures at intervals; • resiliently-suspend and insulate the ceiling, Figure 9.12; • install floating floors with the working surface isolated from the structure, Figure 9.13.
Figure 9.11
Direct (D) and Flanking (F) Sound Paths for Air-borne and Impact (Structure-borne) Sound in a Concrete Building
Flanking
Sound can bypass an element that is intended to be the only significant sound path between two occupancies by travelling for considerable distances along or around a flanking path. Even in a test laboratory, flanking will ultimately determine the limit to which it can accurately test. Figure 9.11 illustrates a building where all the components are rigidly connected and shows how sound energy can be transmitted by several paths through the ceilings, walls, and floors of the structure to reach nearby rooms and cause annoyance. Sound is also transmitted through ceiling spaces, ducts and piping.
Figure 9.12 A Resiliently-Suspended Ceiling in a Concrete Building Reduces Direct Sound but not Flanking Sound
9.2.3
Measurement of transmission loss
The sound transmission loss of a building element is measured in specially-constructed reverberation rooms in accordance with Australian Standard AS 11919.7. Typical transmission-loss test results are shown in Figure 9.14 for 150-mm and 200mm hollowcore panels. Transmission loss is not an intrinsic material property and due to the number of variables and their complex interaction, laboratories will report differing results for nominally identical partitions. The spread of results between laboratories for a partition of a particular description is likely to be of the order of 4 dB9.17, a variance of about 10%. Some of the causes of this variation are: • the actual size of the test room and the materials used in its construction; • the effectiveness of its calibration; • the presence of flanking paths in the test room or in the construction of the test wall; • variation in the physical dimensions of the test wall, eg actual thickness of panel; • variation in the physical properties of the test wall, such as density, moisture content, and aggregate type in concrete walls; • quality and calibration of the sound-generating and measuring equipment; • normal testing error.
When comparing results between countries there is also likely to be differences in test standards and the quality of testing, introducing uncertainty in comparing regulations of one country with those of another. Field measurements can be made in buildings on site in accordance with AS ISO 140.49.8 to determine the insulation actually achieved. These measurements take into account the type of building construction and flanking or deficiencies in workmanship, but are available too late in the construction process to be of use in design. Figure 9.14 Sound-Transmission-Loss Test Data for Hollowcore Panels [After PCI9.11]
Figure 9.13 A Floating Floor Reduces the Transmission of Impact Sound to the building Structure
Sound Reduction Index
In acoustic design and regulation, it is convenient to replace the detailed transmission loss data by a single-number rating known as the weighted sound reduction index, R . The R is determined by w transmission w comparing measured loss values of a test specimen in the 16 one-third octave bands from 100 to 3150 Hz with a reference contour covering this frequency. The basic reference contour is defined in AS/NZS ISO 717.19.14 as a curve and in tabular form. The contour method and rating numbers (STC) were srcinally derived subjectively by the American Society of Testing Materials 9.15 using everyday sounds. The spectrum of normal sounds has changed with time and the contours can be adjusted with the spectrum adaptation terms described below to accommodate this shift.
The Rw is calculated by adjusting the reference contour up or down in steps of 1 dB at each one-third-octave band relative to the measured data until the sum of unfavourable deviations is as large as possible but not more than 32 dB. An unfavourable deviation at a particular frequency occurs when the measured value is less than its reference. Only the unfavourable deviations are taken into account. The sound reduction index is then the value, in decibels, of the reference curve at 500 Hz after shifting by
Spectrum adaptation terms
Figure 9.15 this procedure. transmission-loss data for a particular 150-mmshows slab and the position of the reference contour after the fitting process is complete. The sound reduction index arrived at by the above testing is a pragmatic value intended to allow comparison between walls of different construction over a limited frequency range. Walls of the same rating may have vastly different performance at each end of the frequency spectrum. The R w values given in this chapter are for guidance and are largely arrived at by calculations using algorithms which have been calibrated to test data 9.17. Given the scatter in actual test results, the values are a reasonable assessment of the performance that can be expected from a particular wall type.
described in Table 9.9. For common forms of construction, C is approximately -1 to -2, Ctr has a much larger range, generally -1 to -15. Precast panels typically have a C of -1 and Ctr of -5, indicating that they perform well in the full range of living environments.
The spectrum adaptation terms C and C tr are added to R w to account for the characteristics of particular sound spectra. The terms have negative values. The sound reduction index with spectrum adaptation is expressed as Rw(C; Ctr). AS/NZS ISO 717.19.14 defines two spectra in tabular form for calculating C and Ctr from the measured transmission-loss values of a test specimen. Some noise sources covered by the two spectra represented by C and C tr are
Table 9.9 Spectrum Adaptation Term for Different Types of Common Noise Source [After AS/NZS ISO 717.1 9.14 Table A.1]
Figure 9.15
Example of Fitting the Rw Contour to Measured Data for a 150-mm-thick Concrete Slab [From CPCI9.10]
Sound Transmission Class
The Rw rating has replaced the sound transmission
class rating, STC, which has been traditionally used to classify the performance of partitions and define acoustic requirements. The STC is a single-number value arrived at as described for Rw except that no single deviation may exceed 8 dB.
9.2.4
Types of wall construction Single-leaf walls
The term single-leaf wall refers to all types of partitions where the faces are rigidly connected. Examples are concrete panels, plasterboard-stud walls, concrete block and rendered brick. Also included in this category are composite walls which use a dry second layer as a finish rather than for sound proofing. In fact, the direct fixing of the second layer will usually degrade the insulation by 1 or 2 dB duegap to permitting the rigid mechanical connection air sound to travel easily and fromnarrow one leaf to another. Typical walls of this type are concrete panels sheeted with plasterboard direct-fixed or fixed to solid furring battens. The transmission loss of a single-leaf partition depends mainly on its surface mass as described in Clause 9.2.2. The heavier the partition, the less it vibrates in response to sound waves and therefore the less sound it radiates from the side opposite the sound source.
Simple concrete partitions can provide Rw ratings from about 45 to 55 dB. This range is sufficient for attenuation of everyday noise in most situations. For Rw ratings much greater than 55 dB, the weight required is likely to be prohibitive unless the panel is used for loadbearing purposes. Hollowcore slabs have lower values than solid slabs of the same thickness due to a lower surface mass which is partially compensated by their greater stiffness. Tables 9.10 and 9.11 give representative Rw values for some
common single-leaf or floors. They are calculated valuesconcrete using thewalls INSUL computer program which predicts the sound insulation of building assemblies based on simple mass law and coincidence frequency models using work by Sharp 9.18, Cremer9.19 and others. The program can make reasonable estimates of the transmission loss (TL) and weighted-sound-reduction index (Rw) for use in noise transfer calculations. The Rw ratings in the Tables 9.10 and 9.11 are based on a mass of 2400 kg/m 3 for wet-cast panels and 2300 kg/m3 for hollowcore panels. Values in the field may be 5–10% less due to leakage and flanking.
Table 9.10 9.12 Calculated Rw Values (dB) for some Common Concrete Walls
Table 9.11 Calculated Rw Values (dB) for some Common Concrete Floors9.12
*
*
*
9.10
When a partition with an R w in excess of 55 dB is required, it is generally necessary to utilise multilayer construction. A multi-layer wall can have a substantially higher sound insulation than a single-leaf partition of the same total mass. However, it will not be as high as the sum of the individual R w ratings due to coupling across airspaces. A composite wall allows the individual selection of components to meet a particular R w rating. Rw values of up to
Prevention of flanking becomes particularly important at high-insulation values. Flanking paths bypass the cavity wall and reduce its effective transmission loss. Rigid mechanical connections across the wall must be avoided; for example, by constructing the two leaves to stand independently of each other. Where mechanical connections are required, they should be sufficiently resilient to dampen sound transfer. Leaves of different thickness assist in mismatching resonant and critical frequencies across the wall.
65 dB can bebased economically obtained construction on precast panelswith such as shown in Figure 9.16. In selecting the components of the wall, the aim should be to reduce the frequency at which resonance commences and to raise the critical frequency, thereby increasing the region over which the mass law applies. For a concrete panel, from the equations given in Clause 9.2.2, it can be seen that: • reducing the stiffness (E value and moment of inertia, I) of the assembly lowers its resonant frequency and raises its critical frequency; • increasing panel mass lowers the frequency at which resonance commences and raises the critical frequency; • decreasing panel thickness (reducing the I value) raises the critical frequency but reduces the mass.
The addition of sound-absorptive material such as mineral wool to a cavity, at least 75-mm wide, will improve the sound insulation by 5 to 8 dB. The type and density of the fibrous material does not influence the Rw significantly but the width of the cavity does. Closed-cell foams such as polystyrene do not improve sound insulation or absorption. The position or arrangement of the sound absorptive material inside the cavity has no significant effect provided the whole area of the partition is covered, preferably with some cavity remaining. For example, in staggered-stud construction, it does not matter whether the material is against one face or zigzags between the studs. A common method of finishing a precast wall is to add a layer of plasterboard on one or both sides. In order to be effective the plasterboard must
Multi-layer walls
Figure 9.16 Typical Sections of High-Performance Multi-Layer Walls
be supported independently of the precast or by metal furring using resilient attachments. The latter arrangement will give an improvement of only 1 dB for 10-mm sheeting on one side and 3 dB for sheeting on both sides of the panel. In some cases, the addition of the lining will also seal extraneous leakage and yield higher apparent improvement. The air in the cavity of a multi-layer wall or floor system acts as a spring between the leaves, transferring vibrations from leaf to leaf. The apparent stiffness of the spring depends on the width of the air gap. This interaction between leaves can cause a resonance called the mass-air-mass resonance and results in a dip in the transmission-loss curve. The transmission loss can be reduced to less than that for a single leaf of the same total weight. Commonlyused partitions can show this effect in the range of normal low-frequency sound.
The frequency of the mass-air-mass resonance can be calculated from: fmam = K Ö(m1 + m 2) / (d m1 m2) where: f mam = mass-air-mass resonance frequency (Hz) m1 = surface mass of the first layer (kg/m2) m2 = surface mass of the second layer (kg/m2) d = their separation (m) K = 60 for an empty cavity = 43 for a cavity filled with sound-absorbing material To maximize the improvement due to a cavity, the resonance should be as low as practical. From the above relationship, this is given by a large cavity, say greater than 75-mm, heavier materials and soundabsorbing material in the cavity. A cavity giving a resonance of 80 Hz will improve the transmission loss from 125 Hz upward, thereby ensuring an increase in the Rw. For a given total weight of wall, the resonant frequency is lowest when the mass of each leaf is equal. However, having both leaves of the same construction could lead to low transmission loss around the coincidence dip. The best results are obtained when the leaves have significantly different weights and stiffness.
Multi-component walls
Components having low values of sound insulation, such as windows and doors, drastically reduce the overall insulation of a wall. The assembly is unlikely to provide much better insulation than that of the component with the least insulation. The transmission loss of a partition made up of one or more such components depends on the area of each, their sound insulating properties and the area of any gaps or openings. The transmission loss values at each frequency over the considered required for thisband calculation. Therange composite soundare transmission loss at each frequency band is calculated from individual areas and transmission coefficients and the Rw derived for the resultant transmission-loss curve. The sound transmission coefficient is the ratio of the transmitted acoustic power through a building element to the incident power on the element. The transmission loss is related to the coefficient by: TLn = 10 log10 (1/ tn) where: TL n = transmission loss of an individual element, dB tn = transmission coefficient of the element and thus: TL -
tn = 10
n
10
Where a partition is composed of a number of components, the transmission coefficient of the composite partition is: tT = S(An tn) /AT where: tT = transmission coefficient of composite partition An = area of each component, m 2 tn = transmission coefficient of component with area An AT = Total area of composite partition, m2 The composite transmission coefficient (tT) is calculated at each octave frequency band to give the transmission loss at that frequency. The R w is then derived from the resulting curve.page) The method illustrated in Example 9.5 (next using theis R w values of the elements as an approximation.
Sound reduction index, R w , of a compos ite panel
A 150-mm-thick precast concrete partition with a solid-core, plywood-faced door as shown below
Calculate the Rw with door edges sealed and with a 3-mm air gap With door edges sealed Surface area of door A1 = 0.82 x 2.04 = 1.673 m2 Sound reduction index of door TL1 = 33 dB
From Reference9.16, No 7031
Transmission coefficient of door
= 10
-3.3
= 5.012 x 10
-4
Surface area of precast panel, excluding door A2 = 4.0 x 2.4 - A 1 = 9.6 - 1.673 = 7.927 m 2 Sound reduction index of precast panel TL2 = 54 dB
Table 9.10, this Handbook
Transmission coefficient of precast panel
= 10
-5.4
= 3.981 x 10
-6
cont…
Total surface area of partition AT = 4.0 x 2.4 = 9.6 m2 Transmission coefficient of composite partition
=
(5.012 x 10 -4 x 1.673) + (3.981 x 10 -6 x 7.927) 9.6
= 9.062 x 10
-5
Sound reduction index, R w, of composite partition with door edges sealed 1 = 10 log( 9.062 x 10
= dB ) 40.482
R
-5
w
= 40 dB
With 3-mm air gap around door Surface area of air gap
A1 = (2 x 0.82 + 2 x 2.04) 0.003 = 0.017 m 2 Sound reduction index of air gap TL1 = 0 dB Transmission coefficient of air gap 0
= 10 = 1 Surface area of composite panel, excluding air gap A2 = 4.0 x 2.4 - A 1 = 9.6 - 0.017 = 9.583 m 2 Sound reduction index of composite partition with door edges sealed
Calculated previously
TL2 = TLT = 40.482 dB Transmission coefficient of composite partition with door edges sealed
= 10
-4.0428
= 9.062 x 10
-5
Total surface area of partition AT = 4.0 x 2.4 = 9.6 m2 Transmission coefficient of composite partition
=
(1 x 0.017) + (9.062 x 10 -5 x 9.583) 9.6
= 1.878 x 10
-3
Sound reduction index, R w, of composite partition with 3-mm air gap around door 1 = 10 log( 1.878 x 10
= dB ) 27.263
-3
R
w
= 27 dB
9.2.5
Structure-borne sound
Structure-borne sound refers to sound that srcinates as impacts or vibrations directly on the structure and travelling through it. Noise from footsteps, service pipes, machinery and dropped objects is a common source of annoyance. It is more difficult to control than airborne sound because the source and path may be obscure. A construction that provides good insulation against airborne sound does not necessarily provide good insulation against impact sound and vice versa. Impact Sound Level
The impact sound transmission through floors can be expressed as a single-number rating such as the Weighted Normalised Impact Sound Pressure Level (Ln,w)20. Ln,w is a laboratory-measured value referenced in the Building Code of Australia and is replacing the commonly used IIC (Impact Isolation Class) parameter. Tests to measure Ln,w use a standardised tapping machine that strikes the floor at a prescribed rate. Sound pressure levels are measured in the frequency range 100 to 3150 Hz in the space below and the resulting data are fitted to a reference contour to obtain the Ln,w. Some examples of test results are shown in Figure 9.17 for both solid and hollowcore floors. It is important to note that the lower the L value, the better the performance. n,w of the more recognised IIC This is the converse parameter. The Ln,w is determined by those values
lying above the reference contour once the fitting process is complete. As with the R w, the total deficiency must not exceed 32 dB and no single deficiency may exceed 8 dB. Spectrum Adap tation Terms The spectrum adaptation term CI21 is
added to Ln,w in order to take account of the unweighted impact sound level. The term has been developed to be more representative of the characteristics of typical walking noise spectra. Concrete floors that have hard or ineffective floor coverings can have CI values between -15 dB and 0 dB. Concrete floors with carpet or other effective floor covering will have C I values in the order of 0 dB. Improving Ln,w ratings for concrete floors
To provide a minimum standard, the L n,w should be 62 dB or less. However, for a reasonable protection the L n,w needs to be about 55 dB or less. Bare concrete floors or those with hard finishes such as tile or hardwood offer poor impact sound insulation. The Ln,w is usually between 85 dB and 75 dB. This is shown in Figure 9.17 where the rating of a bare-concrete floor is controlled by the high frequencies. Adding carpet with an underlay makes a very large difference. Most of the high-frequency noise is reduced and the Ln,w is controlled by the lower frequencies.
Figure 9.17 Tapping Machine Data for a 150-mm Concrete Floor, Tested Bare, with a Carpet, and with a Carpet and Foam Underlay, [From CPCI9.10] Similar Data is shown for 150- and 200-mm Hollowcore Floors [After PCI9.11] NOTE: Ln,w and CI data has been derived from the srcinal graphs.
It is best to prevent the problem at the source. A soft, resilient floor covering, such as carpet, cushions impact forces at their source and reduces the energy transferred to the building structure, improving the Ln,w. The improvement in the Ln,w also depends on the characteristics of the floor structure. Lightweight floors vibrate more in response to impacts and thus generate more sound. Typically, heavy concrete floors generate about 10 dB less noise at low-impact frequencies than do timber
Where a penetration is essential, it must not form a rigid connection between the floating slab and the structural slab or walls. It should be noted that in service areas such as laundries and kitchens impacts on walls can also occur. When these are adjacent to sleeping areas, treatment such as shown in Figure 9.16 should be considered.
floors and provide a better living environment in domestic constructions. Typical soft carpets and underlay used in the home give L n,w ratings between 20 to 40 dB. Vinyl floor coverings and similar products improve the Ln,w of the bare slab by about 5 to 10 dB.
Normal-density concrete is not an efficient sound absorber. It is necessary to add an absorptive layer over the concrete such as sprayed or trowelled acoustic plasters. Some low-density concretes made with porous aggregates (expanded shale, slag or expanded mica) are reasonably-effective absorbers, providing a coating of slurry does not seal the individual aggregate particles at the surface and the surface is not painted. Layers of fibrous sound-absorbing materials (mineral-wool) may be attached and protected by a durable covering. Low-frequency absorption is improved by resonant cavity systems, porous layers behind a perforated screen or sound-absorbing material mounted on furring or a suspension system that leaves space behind it and the wall. Manufacturer’s trade literature provides soundabsorption characteristics for sound-absorbing materials on typical mounting systems.
Floating floors
When a hard floor surface such as tiles is specified, impact sound transmission to the building structure can be reduced by using a floating floor. A floating floor is a loadbearing slab as shown in principle in Figure 9.18. It is supported by a structural floor but isolated from it by resilient and sound-absorbent support material. Floating floors can improve the impact sound insulation performance of a concrete slab by about 30 toor40increasing dB. Decreasing theofstiffness the support layer the mass the slabof lowers the frequency at which the floating assembly becomes effective in attenuating impact sound. The L n,w values are not as good as those provided by a soft carpet and underlay. This construction is generally more expensive but necessary in wet areas such as laundries, kitchens or bathrooms which have living areas underneath, as may occur in multi-unit dwellings. Further improvements in the Ln,w can be obtained with a resiliently-supported ceiling and insulated ceiling space as in Figure 9.12. The improvement depends on the method of support, the cavity depth, the weight of the ceiling and the amount of sound-absorbing material in the cavity. The ceiling has to be a continuous sheet, not the individually suspended tile type. Sound insulation measures act in both directions, ie the upper space is also protected against sound srcinating in the space below. The floating slab is most effective if it is relatively heavy, at least 50 mm of concrete and 100 mm or thicker in areas such as plant rooms. Residential applications may also use bonded layers of fibrecement or structural particleboard. It is crucial that the floating slab does not contact the building structure. Figure 9.18 shows an edge detail using caulking to seal the air gap. Penetrations of the floating slab by pipes, ducts, etc should be avoided.
9.2.6
Sound absorption of concrete walls
Figure 9.18 Floating Floors and Plasterboard either Direct-Fixed or on Furring Channels Attenuate Direct and Flanking Transmission of Airborne and Impact sound
9.2.7
BCA Sound Insulation Provisions
Acoustic requirements
In May 2004, the sound insulation provisions of the Building Code of Australia (BC A) underwent a significant modification in response to increasing evidence that community expectations were not being met. The modifications included an increase in the required level of airborne sound insulation performance and the introduction of a minimum performance standard for impact sound insulation. The sound insulation provisions are described in the three par ts of the BCA applicable to residential buildings and are intended to provide a minimum acceptable standard in order to safeguard occupants from illness or loss of amenity: Part F5 Volume One for Class 2, 3 and 9c buildings; Part 2.3 Volume Two for Class 1 buildings; Part 3.8.6 Volume Two for Class 1 buildings. The following comments are intended as a guide to the general requirements of separating walls and floors. The BCA should be referenced for compliance details and for the respective requirements in each State and Territor y.
Broadly speaking, there are three ways that a building system can meet the performance requirements: • Achieve the Deemed-to-Satisfy levels by specifying building systems that have been laboratory tested, or are as described in the BCA (Specification F5.2). • Perform on-site tests confirming that each building element meets the requirements specified in the Verification Method. • Provide a documented Expert Judgement (opinion) that the system meets the required BCA performance values. Examples of some of the performance requirements for Class 2 and 3 buildings are provided in Figure 9.19. The BCA’s acceptable forms of construction for a 50 dB Rw+Ctr wall includes a 200-mm-thick concrete panel with 13-mm-thick plasterboard or render on each face, Figure 9.20. An impact sound insulation performance of 62 dB L n,w+CI is achieved with a floor construction consisting of a 200-mm-thick concrete slab with carpet on underlay, Figure 9.20.
Figure 9.19 Summary of BCA Acoustic Performance Requirements for Class 2 and 3 Buildings
When conducting on-site testing, the magnitude of the performance requirement for airborne sound is nominally relaxed by 5 dB to ! 45 dB D nT,w+Ctr in order to allow for on-site performance tolerances. DnT,w is the Weighted Standardised Level Difference and describes the on-site sound level difference between two rooms, also taking account of sound flanking paths. It is a field measurement that relates back to the Rw laboratory measurement. For impact sound, there is no relaxation of the on-site verification value and the requirement is " 62 LnT,w+CI.
Design advice Good design advice is important when designing a building to comply with the BCA as it will ensure cost-effective solutions in addition to minimising the risk of noise transfer via flanking paths. Particular attention should be paid to wall and floor junctions where concrete and lightweight constructions meet. Figure 9.21 shows an example of bad design where flanking sound travelling via the wall cavity will compromise the performance of the sound-rated party wall. As indicated, the party wall should extend, and be effectively sealed, to the external wall panel. Care should also be taken with services penetrations as these can compromise the final performance values. Advice should be sought from appropriately-qualified and experienced acoustics professionals who will employ both prediction tools (such as INSUL 9.12) and their previous experience to arrive at projectspecific solutions.
Figure 9.20
Figure 9.21
BCA Acceptable Forms of Construction to satisfy certain Acoustic Values
Flanking-Sound Control at External Walls showing Good and Bad Design Practice
BCA implementation issues
The improved airborne sound insulation performance provisions of the BCA have generally been welcomed. However, concern has been expressed by acoustics professionals over the use of the term C I in the requirement for the impact sound insulation to be Ln,w+CI • 62 dB. CI was actually developed for use in the impact noise rating of timber floors which have a tendency to generate acoustic and vibration resonances; the adaptation term is not suitable for application in concrete floor installations. It is considered by many in the acoustics industry that the BCA application of C I is in error and submissions are being made to the Australian Building Codes Board to have this adjusted. Note that in the development of the equivalent BCA document in the UK, the use of CI was rejected after review by the industry. Therefore, the recommendation from the acoustics profession is to use the CI term with caution. Other Guidance
The BCA sound insulation provisions provide a minimum performance standard. Experience shows that residents in luxury developments demand a higher level of acoustic amenity, typically in the range of 5 to 20 dB better than BCA values for airborne and impact Guidance on appropriate designsound valuesinsulation. can be found in publications such as Acoustical Star Ratings for Apartments and Townhouses9.22 (Table 9.12).
Table 9.12 Requirements of Star Rating System for Activities between Tenancies of Apartments and Townhouses9.22
9.14 AS/NZS ISO 717.1 Acoustics - Rating of sound insulation in buildings and of building elements Part 1 Airborne sound insulation , Standards
Australia, 2004. 9.1 Building Code of Australia Australian Building Codes Board and CCH Australia, Sydney, 2007. 9.2 AS 2627.1 Thermal insulation of dwellings Part 1: Thermal insulation of roof/ceilings and walls in dwellings, Standards Australia, 1993.
9.3 Thermal Benefits of ‘Solid’ Construction, Cement and Concrete Association of Australia, June 1999. 9.4 Phillips, R O, Sunshine and Shade in Australasia Commonwealth Experimental Building Station, Bulletin no. 8 (reprinted as CSIRO Building Construction and Engineering Technical Report TR92/2, 1992).
9.5 Addleston, L, Materials for Building Vol 4 , Newnes-Butterworth, 1976. 9.6 Harkness, E L, Precast Concrete Energy-CostEffective Building Facades, Precast Concrete Manufacturers Association of NSW, August 1987. 9.7 AS 1191 Acoustics – Method for laboratory measurement of airborne sound transmission loss of building partitions, Standards Australia, 2002.
9.8 AS ISO 140.4 Acoustics - Measurement of sound insulation in buildings and of building elements Part 4: Field measurements of airborne sound insulation between rooms, Standards Australia, 2004.
9.15 Tentative Classification for Determination of Sound Transmission Class, ASTM Designation E413-70T. 9.16 Weston, E H, Burgess, M A, Whitlock, J A, Airborne Sound Transmission Through Elements of Buildings, Experimental Building Station Technical
Study 48, Commonwealth of Australia, 1973. 9.17 Farina, A, Fausti, P, Pompoli, R, Scamoni, F, Intercomparison of Laboratory Measurements of Airborne Sound Insulation of Partitions for the Determination of Repeatability and Reproducibility Values. Proceedings INTERNOISE, Liverpool, UK,
1996. 9.18 Sharp, B H, ‘Prediction Methods for the Sound Transmission of Building Elements’ Noise Control Engineering Vol. 11, 1978.
9.19 Cremer L, Heckel M, Ungar E E,Structureborne Sound, Springer Verlag, 1988. 9.20 AS ISO 140.6 Acoustics - Measurement of sound insulation in buildings and of building elements Part 6 Laboratory measurements of impact sound insulation of floors, Standards Australia, 2006.
9.21 AS ISO 717.2 Acoustics - Rating of sound insulation in buildings and of building elements Part 2 Impact sound insulation, Standards
Australia, 2004. 9.22 Acoustical Star Ratings for Apartments and Townhouses, Association of Australian Acoustical Consultants, Version 9.7, 2007.
9.9 The AIRAH Application Manual DA2, Noise Control, Commonwealth of Australia, 1995. 9.10 Design Manual, Canadian Prestressed Concrete Institute, third edition, Ottawa, 1996. 9.11 PCI Design Handbook – Precast and Prestressed Concrete, The Precast/Prestressed Concrete Institute of America, Chicago, Edition 4, 1996.
9.12 INSUL computer program, version 5.0, Marshall Day Acoustics, Melbourne, 2002. 9.13 Uno P,Acoustic and Thermal Advantages of Concrete, Proceedings: Australian Building Industry Conference, 1992, pp 67–69.
The AIRAH Design Manual, The Australian Institute of
Refrigeration, Air-Conditioning and Heating. Air Conditioning Load Estimation, AIRAH Application Manual DA9, third edition, Commonwealth of
Australia 1994. Concrete in Energy-Efficient Design and Noise Control,
(TN62) Cement and Concrete Association of Australia, 1994. Acoustic Benefits of ‘Solid’ Construction, Cement and
Concrete Association of Australia, September 1999. Condensation – Design Strategies, Cement and
Concrete Association of Australia, July 2000. Fahy, F,Sound and Structural Vibration, Academic Press, 1985. Sound Insulation in Buildings, Guideline Document,
Australian Building Codes Board, 2004.
The use of sunscreens and moulded facades, which are most economical in precast concrete, are a popular way of controlling solar radiation. The thermal mass of precast concrete can also be an important factor in creating comfortable living and working conditions. Precast concrete components, along with some insulation products, can easily achieve the BCA-required thermal resistance values for efficient thermal design.
Architectural acoustics deals with the control of sound propagation within buildings. Sound control in buildings may be broadly divided into two categories –
The BCA stipulates minimum levels of airborne sound insulation performance and a minimum performance standard for impact sound
control within an occupancy and insulation between occupancies. Precast concrete components can be used effectively for the control of sound between occupancies, which is a major concern of the BCA.
insulation. These requirements, which are particularly important for Class 2 and 3 buildings (apartments, units, hotels and similar accommodation), can be met with precast concrete wall and flooring systems.
What you will find in this Chapter •
Information to demystify the subject of architectural precast concrete.
•
Procedures for selecting and approving finishes and a review of typical finishes.
•
An explanation of relevant production issues such as mould design.
•
How to specify and administer colour control and other important architectural criteria.
•
Information on rectification, protective coatings and maintenance.
10.1 Definitions 10.2 Introduction 10.2.1 Scope 10.2.2 The decision to use architectural precast concrete 10.3 Samples and Prototypes 10.3.1 Samples 10.3.2 Prototypes 10.4 Shape, Form and Size 10.4.1 General 10.4.2 Moulds 10.4.3 Separation of finishes 10.4.4 Profiled surfaces 10.4.5 Two-layer casting 10.4.6 Dimensions and overall panel sizes 10.4.7 Design for weathering 10.5 Colours and Off-form Surfaces 10.5.1 Colours 10.5.2 Colour control 10.5.3 Quality of off-form surfaces 10.6 Surface Finishes 10.6.1 General 10.6.2 Smooth off-form 10.6.3 Water-washed 10.6.4 Retarded 10.6.5 Honed or polished 10.6.6 Sandblasted 10.6.7 Acid etched 10.6.8 Bush-hammered 10.6.9 Hammered-nib or fractured-fin 10.6.10 Form liners 10.6.11 Applied finishes 10.6.12 Brick- or tile-faced 10.6.13 Stone-faced 10.6.14 Multiple finishes within a single panel 10.7 Hollowcore Architectural 10.8 Other Matters 10.8.1 Treatment of unformed (face-up) surfaces 10.8.2 Acid cleaning 10.8.3 Remedial work after stripping 10.8.4 On-site rectification work 10.8.5 Protective coatings 10.8.6 Matching to insitu concrete or existing precast concrete 10.8.7 Maintenance of precast concrete 10.9 References
For the purpose of this chapter the following definitions are used: Architect A person with suitable qualifications and experience registered with the Architects Registration Board of State or Territor y. Customer Usually the owner of a building, who engages the Architect to design and control the project. Designer Usually an Architect or Engineer, or both, who is involved in the conceptual and detailed design or the structural design of the structure . Engineer A person qualified for admission to Corporate Membership of the Institution of Engineers, Australia, or equivalent, and competent to practice in the appropriate field. Precaster or Precast Manufacturer or Manufacturer The manufacturer of the precast concrete members. Wetcast Conventional concrete process, as opposed to those used in hollowcore, concrete block, some pipe manufacturing techniques and the like, which are drycast processes.
10.2.1
Scope
This chapter covers the use of architectural precast concrete. It describes the development of factorymade precast concrete units from the conceptual design and sample stage through to mould design. It then describes the variety of shapes, colours and textures that can be achieved. Architectural precast concrete is a man-made product manufactured from natural materials and ingredients. Some variation in uniformity within and between units can be expected. As with natural stone such as granite and sandstone, precast concrete has its own character, aesthetic appeal and uniqueness. Unlike natural stone, architectural precast concrete may be made in complex shapes for products such as walling and structural elements.
Prestressed hollowcore units can be used as architectural cladding and their application is covered under Clause 10.7. 10.2.2
The decision to use architectural precast concrete
Precast concrete will be selected for its architectural aesthetic qualities, for durability, for buildability reasons and for economy. Other factors which will influence a decision to use architectural precast concrete include its fire resistance, acoustic and thermal proper ties, its loadbearing capacity and speed of construction. Architectural precast concrete is a high-technology transformation of the masonr y construction used since ancient times. Only experienced persons can differentiate polished reconstructed stone (ie precast concrete) from polished natural stone on a building, while sandblasted and other finishes impart the same ambience to a well-designed building as does natural stone. It is the favoured cladding material for monumental buildings, other prestige buildings and hotels. It is also used extensively for office and public buildings of all sizes as well as for industrial buildings. Its use in low-, medium- and high-rise residential buildings is increasing, while it is the ideal material for street furniture and a host of other uses. Issues such as the structural design of architectural precast concrete and tolerances are dealt with elsewhere in this Handbook.
The Formwork Code, AS 361010.1, which deals with grey off-form finishes, recognises that surface defects will occur, even on Class 1 finishes, and provides for the preparation of samples to agree the acceptable extent of these at the outset. Similar procedures should be followed for all architectural finishes.
10.3.1
Samples
The process of selecting a surface finish needs to be handled properly to avoid misunderstanding or, later in the project, dispute. At the conceptual stages of a project, the precaster typically supplies to the architect samples within the colour range and type of finish proposed for the project. These samples are usually no larger than 300 x 300 mm. The sample range will indicate the texture and colour that can be achieved throughout the project. Finishes to an off-form surface (eg acid etching) will have visual characteristics such as blowholes, and the extent to which they are to be expected in the final product may not be evident from the sample and may need to be established from a unit with shape characteristics similar to those to be used in the project. The fine aggregate, cement and any coloured oxides provide the colour in off-form finishes. When the aggregate is exposed by polishing, sandblasting or other technique, however, the coarse aggregate also affects the colour. If the coarse aggregate colour is significantly different to the matrix, it is advisable to tint the matrix to blend in with the aggregate by use of appropriate sand, cement and, if necessary, oxides. This masks patchiness caused by unavoidable variations in the mix proportions at the surface. When aggregate from a particular quarry is known to be inconsistent in colour, it is advisable to stockpile all the aggregate for a project.
In setting the criteria for acceptable colour variation, variation between horizontally-cast and verticallycast surfaces, and acceptability of rectification, it is helpful to reference an existing building with similar characteristics. This will highlight the reality of fullscale production which cannot be represented by small flat panels. The architect should inspect the first production unit of each distinct type to ensure that the selections made at the sample and prototype stage, have provided an acceptable result. In making this comparison, it is important to understand that concrete changes colour with time. In particular, finishes using off-white cement have an initial green tinge that lightens to a light-fawn hue. The aggregates proposed should be readily available at acceptable cost. They should preferably be of known performance by prior use. If this is not the case then AS 114110.2 and AS 2758.110.3 specify a range of tests to confirm properties required for the application. The compressive strength of the proposed mix will usually need to be confirmed at this stage and other proper ties such as shrinkage and absorption may need to be checked. Appropriate tests are covered in AS 1012 10.4.
It is sometimes difficult to match an existing building exactly when later extensions or alterations are required. Buildings weather with age; however, with cleaning, a similar finish to the srcinal can be This early stage of the selection process often takes achieved. A concrete mix identical to the srcinal some time but it is an essential part of the design mix should be used, if possible, rather than one process and must be allowed for in the project that gives the best initial match. As the old and schedule.When the required colour and finish have new components weather, they will draw closer been selected, larger samples, say 600 x 600 mm may in appearance. Many buildings, such as the Sydney be made. Opera House, have successfully had panels added or replaced without the changes being obvious. Figure 10.1
Sample Panels Illustrating Different Architectural Finishes
10.3.2 Prototypes
Limited prototype A full or part unit may be constructed to aid in the final assessment of the appearance of the panel. Such a prototype may be specially made, or may be one of the first panels in the main production run.
Full Prototype A full prototype is constructed to evaluate performance characteristics that may lead to the redesign of the components. They are a faithful reproduction of the intended design. Such prototypes must be made and evaluated before the project shop drawings are started. It is too late in the building process to attempt to use the first production panels for this purpose.
Typical reasons for a limited prototype include:
Typical uses for a full prototype include:
Prototypes may be specified or requested to cater for a variety of needs. There are two basic types.
•
Assessment of depth of aggregate exposure or degree of etch or polish in relation to the scale of the panel.
•
Selection of colours and finishes for companion elements in the facade (eg window frames).
•
Selection of the size of false joints. These can often be varied in size to achieve the desired effect.
•
Assessment of the effect of attitude of casting on adjacent surfaces in different planes. (Aggregate density will differ, while the presence of bleed water and air bubbles at the mould interface of a vertical surface will affect the finish.)
Adjustments to dimensions of a prototype, made from production moulds, may result in additional cost and time in revising drawings and changing or remaking the mould. Figure 10.2
Sirowet Test being Set Up with a Full Prototype Facade Section
•
Full-scale review of the shape, colour and finish of the panel
•
Light and shade effects produced by modelling of the surface
•
Selection and testing of the waterproofing system
•
Selection and testing of window details for wind and water penetration
•
Practicality of fixing details
•
Weathering
•
Handling trials
•
Transpor t and erection.
The full prototype has a substantial cost and long lead-time and is sometimes dealt with in a separate contract. It is mostly used to evaluate waterproofing details with the Sirowet test10.5 and should be used only where there is real doubt as to the feasibility of the design – typically where complex or innovative panels are involved. It is part of the design process and should be built and tested before tenders are called for the main precast work. It is now rare that an experienced precaster will not have constructed a similar project and solved any problems. It is also rare for any facade configuration to not have been previously weather tested. A typical time framework for a full prototype for Sirowet test would be: Shop • drawings Mould • manufacture •
Production and installation
4 weeks 8 weeks 4 weeks.
Any time needed for evaluation of the results and redesign must be added to these times.
10.4.2 Moulds Moulds may be constructed from any suitable material including steel, concrete, timber, fibreglass or a combination of these. Steel and concrete are the most common mould materials in Australia.
10.4.1
General
Factory-produced precast concrete allows designers to enjoy freedom of form with few restrictions. Width, length and weight tend to be governed by the practical limits imposed by delivery and site craneage. The shape of a panel tends to be governed by the practicalities of pouring and stripping the unit in the factory. Panels may contain window openings, have return legs, be heavily-modelled or, like lift cores, be box shaped. It is always preferable for window openings to be contained wholly within one panel. This provides the best waterproofing detail between window frame and panel by isolating this joint from building tolerances and movement. It allows the efficient use of the open-drained joint system between panels. The window sub-frame or even the finished window can be installed at the precast factory, allowing improved supervision and inspection of the workmanship and therefore improved quality. When the window opening is formed by separate panels the size and squareness of the opening will be subject to panel and erection tolerances. The window framing must then be detailed to allow for these tolerances. Panel joints should not intersect a window opening – the positioning of a vertical joint partway along a window will allow rainwater to enter the window head should the sealing system break down.
Figure 10.3
Window Contained within a Cladding Panel
Computer programmes and CAD/CAM profiling machines can produce a wide variety of shapes and give designers unprecedented architectural freedom. Such profiles usually require flexible formliners or fibreglass or concrete moulds (see Figure 10.5). A mould that requires minimal dismantling is always preferable since it can be made watertight more effectively. Loss of water and cement paste at joints in the mould will discolour the panel locally. Hence, manufacturers have developed techniques to ensure watertight seals between removeable mould components and minimise loss of water at joints. To enable the product to be stripped, surfaces produced by fixed forms cannot be parallel or have reverse taper to the direction of removal. Depending on the type of mould and the material of manufacture, the required taper varies from 2.5° to 10°. Edges and corners of precast panels should be chamfered. The purpose of a chamfer is to lessen the risk of damage during de-moulding, transportation and installation. Visual considerations usually restrict their size to under 20 mm. Chamfers also mask minor misalignment of adjacent panels in the structure.
Figure 10.4
Steel Mould and Finished Product
10.4.3 Separation of finishes A groove or a recess should separate different finishes or textures where they are required on the same face of a panel. The size of the groove is dependent on the types of adjoining finishes and will usually be 20 to 75 mm wide by 15 to 20 mm deep. An alternative approach is for the finishes to be on separate planes. This is often used when combining polished surfaces with, say, sand-blasting. In this case, the surface to be polished would be slightly higher to allow clearance for the polishing head. Designers must allow for adequate cover to the reinforcement at such grooves and recesses. 10.4.4
Profiled surfaces
The panel face may be broken up in various ways by shaping the mould. The complexity of the surface can vary from simple banding to profiling of the mould. Shape does not restrict the production of finishes such as acid etching or sandblasting after casting. Machine polishing, however, can generally be used only on flat surfaces or those with a gentle convex curve. Other surfaces, eg window reveals, can be polished only with hand equipment and are therefore more expensive.
Figure 10.5
Computer-Controlled Patterned Concrete Mould used for Facade Panels on National Museum, Canberra
Recesses can be formed by steel, timber or plastic battens attached to the mould face. An effective means of shaping a panel surface is to cast it against a concrete mould taken from a positive fashioned from another material. Plaster castings or readily worked materials such as polystyrene are commonly used as master moulds. Mould shapes can be cut with computer-controlled equipment enabling the most complex and irregular shapes to be transferred straight from the designer’s CAD system to the master mould. Flexible mould liners are commercially available in a range of patterns and are manufactured by some precasters. When using form liners it is important to make sample panels to ensure the desired result is achievable. 10.4.5
Two-layer casting
The finished surface of a panel can be cast as a separate layer shortly before or after the remainder of the panel, depending on whether the unit is cast face-down or face-up. This minimises the amount of expensive ‘architectural’ concrete and improves compaction and control of stone density on the exposed face. The first layer is usually 20 to 50 mm thick. The second layer is placed and compacted before the first layer achieved final set,surface. or placed and compacted ontohas a suitably-prepared The layer of ‘architectural’ concrete should be as thin as is allowed by practical considerations such as the maximum aggregate size and the need to avoid colour bleeding from the first layer to the second. Research work carried out by Mahaffey Associates10.6 illustrates that two-layer casting does not affect durability. Figure 10.6
Patterned Columns to John Curtin Medical School, Canberra
10.4.6 Dimensions and overall panel sizes
Panel size is generally governed by the weight that can be handled on the building site and the size that can be transpor ted on public roads, as well as by vehicle and crane access to and around the site. Restrictions within the precast factory are seldom the limiting factor. The designer needs to be aware of the likely craneage that will be used for erection. Usually, mobile cranes can be used on buildings up to about six storeys in height, after that fixed tower cranes are
Figure 10.7
Commonwealth Bank Computer Centre, Sydney, Showing use of Two-Layer-Cast Facade Panels
needed. Mobile cranes provide great flexibility in lifting capacity and positioning on site. It is necessary to use high-capacity machines only for the lifts that require it. However, access and firm standing adjacent to the structure is essential. Crane use may not be permitted in inner city streets or may be limited to work out of hours. Tower cranes are fixed in position but give easy site coverage. However, their lifting capacity is restricted (usually less than 10 tonnes) and is least at remote corners of the building where the heaviest panels often occur.Tower-crane capacity is too often determined by general workload rather than by the size of the precast panels. Road regulations vary from State to State, but generally the size limits for unrestricted travel are 2.5-m width x 4.3-m height x 17.5-m length. Loads, which exceed any of these limits, will have restrictions on the hours of travel and may require escort vehicles. This can impact on job schedules. For information and guidance on transporting larger panels, refer to Chapter 11, Handling, Transport and Erection. Figure 10.8
Wall Panels for Gold Coast Highway, Qld, being Transported to Site
10.4.7 Design for weathering Atmospheric pollution and uneven rainwater runoff will stain the surface of precast wall panels (and any other walling materials) if appropriate steps to control run-off are not taken. Rainwater should be able to uniformly wash the facade. Sills, copings, drip grooves and recesses should be used to channel rainwater to vertical joints, false joints or beyond the plane of the facade. Horizontal ledges and returns above windows should always have a drip groove in the underside to prevent water running back onto the facade and causing staining due to an alkali reaction with the window glass. Parapet and roof edges should be designed to direct rainwater away from the facade. Horizontal surfaces collect dust and dirt which will wash unevenly down the facade. Efflorescence is a deposit of soluble and insoluble salts that sometimes forms on the surface of concrete. It is a result of evaporation of moisture containing dissolved salts from the cement and aggregates. These react with carbon dioxide in the atmosphere to form a white film. Efflorescence appears on panels soon after they have been manufactured. The amount decreases naturally with time. It is one of the main reasons architectural panels require an acid clean prior to delivery.
Figure 10.9
Use of Drip Grooves on Facade to Control Weathering
While efflorescence is not a factor in the appearance of any but black or very dark colours, it may be reduced by: •
using blended cement containing a pozzolan, eg fly ash;
•
using materials with a low content of soluble alkaline salts, eg washed sands.
Efflorescence can be removed by washing the surface with a solution of weak hydrochloric acid (2 to 5% acid in water) and thoroughly rinsing off with water.
their effectiveness on being adequately dispersed throughout the mixed concrete. They do not dissolve and stain the concrete as a dye colourant does.
10.5.1
Colours
The colour of concrete is determined by its age as well as its components, viz: •
grey, off-white or white cement;
•
coloured sands and aggregates;
•
oxides;
•
moisture content.
Cements The decision to use grey, off-white or white cement will be determined by the architectural requirements. Generally, grey will be used with dark aggregates, off-white with mid-range aggregate colours and white with white quartz and granite aggregates.
The dosage rates for precast concrete are typically 1 to 3% by weight of cement. The oxide dosage rate should be governed by architectural requirements rather than by achieving saturation levels. Some offform and other precast concrete finishes may require as much as 6%. The rates best suited to any project will be advised by the precaster and confirmed by architectural samples. Such samples should be precast concrete samples rather than cement and oxide samples. 6% is generally regarded as the dose at which saturation is achieved, for most pigments, after which more oxide gives only marginal increases in colour intensity (see Figure 10.12). The primary reasons for apparent colour changes of concrete are efflorescence and atmospheric pollution leading to etching and staining and any accumulated dirt and grime which obscure the oxide particles.
Off-white cement is produced in Australia but white cement is imported, and is far more expensive. Coloured sands and aggregates Mechanically or chemically treating the panel surface exposes the colour of the sand and stone aggregates to impar t an overall colour to the concrete. The coarse aggregates haveuse theofmost influence theofffinal colour. With the coloured sandson and white cement the cement matrix can be the lighter shades of brown, red or yellow. Darker grey tones can be achieved using dark sands, where available, and grey cement. Pigments (oxides) Where the desired colour cannot be achieved by the cement and aggregates alone a wider range of colours can be achieved by the use of pigments. In concrete for exterior use they must be colourfast and not reduce the durability of the concrete. Colouring pigments used in precast concrete are predominantly metal oxides. They are chemically inert and alkali resistant, insoluble and inorganic to prevent fading by photochemical degradation. Mineral-oxide pigments retain their colour and do not exhibit colour changes due to age or weathering. Mineral (metal) pigments such as oxides of iron (reds, blacks and yellows), chromium (greens) titanium (white), etc meet these requirements. Pigments are available in a wide range of colours, from deep to pale pastel colours and hues. The major non-blended standard pigments are green, black, red, brown, yellow, blue and white. These can be obtained in commercially-blended form to produce many intermediate colours (see Figure 10.11). Fine, solid, oxide pigment particles provide colour by reflecting at the surface. They therefore rely for
Figure 10.10
General Purpose Building North, University of Quensland, Brisbane, with Facade Panels Incorporating a Range of Oxides
These effects can be controlled by producing welldetailed precast units of high-performance, quality concrete. Just like all material surfaces left in the open, precast concrete must be occasionally cleaned to remove pollutants and restore colour. The same rules should apply to pigmented precast concrete sample evaluation as apply to the assessment of other architectural precast, see Clause 10.3.1. The first panels of a production run should always be inspected by the architect to ensure that the design requirements are being achieved. Figure 10.11
Sample of Colour Range in Commercial Pigments
10.5.2 Colour control Some colour difference between nominally identical precast units is inevitable, but colour variation, both between and within panels, should be kept within an agreed range. It is therefore important, at the sample stage, to reconcile the expectations of the customer and architect with the practical limits of colour uniformity. Some designers prefer to see colour variation akin to timber and natural stone, while others desire the consistency and uniformity of paint. Colour control is thus about ensuring that panels or other elements for a project have acceptable tonal range. Supplement 1 (Appendix B) to AS 3610 10.1 provides a tonal scale of some ten colour tones ranging from very-light grey to near charcoal and represents the range of shades which may be expected with the use of grey cements. This assessment method is more relevant to off-form finishes that are to receive no further treatment than to those that are (eg polished, sand-blasted and water washed). Having selected a shade, the range of variation is controlled by the surface finish class nominated, ie:
Recognising the difficulty of achieving complete colour control, the Code has provided the opportunity for assessors to exercise discretion. An example, explained in AS 3610 Supplement 2 – Commentar y, Clause C3.5.4 states: “There may be situations where the subject work will be accepted, even though elements have tonal variations outside those recorded. Examples could include: a) some elements slightly darker (or lighter) overall than the specified tone, but with the overall effect acceptable, and
Figure 10.12
Typical Saturation Point of Standard Pigments
b) some elements with local dark (or light) patches of colour which do not detract from the overall
appearance”. These comments highlight the need for the use of discretion in assessing colour-range and for the use of samples to set acceptable and achievable results. It is important to consider the age of the units inspected as very early-age panels will mellow to a more uniform colour as the concrete matures. In assessing colour, panels should not be inspected at close range – the facade should be inspected as a whole, from a distance.
In work where the aggregates are exposed (waterwashing, polishing and sandblasting), the major concerns for colour uniformity are the uniformity of aggregate size and distribution and uniformity of the cement matrix together with depth of exposure. Assessment of colour uniformity of the panels prior to such treatments may offer little information but the sample process referred to in Clause10.3 of this Handbook will allow management of the colour range. Factors within the precaster's control include: • Keeping the basic mix design, manufacturing and curing processes constant. •
•
Ensuring that there is no change in aggregate colour or cement supplier during the project. Compliance with AS/NZS ISO 900110.7 Quality Assurance standards.
Factors outside the precaster's control include: •
Changes in cement colour.This is more likely to be associated with grey cements rather than with offwhite or white, as the latter are manufactured to very close colour tolerances.
•
Variations in curing as a result of changes in ambient temperature and humidity.
•
Variations in pouring and finishing procedures due to normal human factors. Even where the highest standards of training and supervision apply, a change of operator or even the varying performance of one operator may, for example, affect the degree of vibration or the depth of aggregate exposure in certain finishing techniques. Appearance may be affected – but not structural adequacy.
•
Variation between horizontally- and verticallycast or complex shaped units. This can often be resolved by such units not being adjacent to each other in the completed structure or by a variety of casting techniques.
10.5.3
Quality of off-form surfaces
Clause 10.3 described the process of setting standards for finish by reference to samples, prototypes and existing buildings. One basis commonly used for surface finish documentation, however, is AS 3610 supported by AS 3610 Supplement 1 (Blowhole and colour evaluation charts) and AS 3610 Supplement 2 (Commentary ). AS 3610 defines five classes of surface finish. Class1 is exceptionally demanding and is unlikely to be achieved over large areas or between elements. Consequently, the Standard defines it as being suitable for use in very special features, generally in small areas, in buildings of a monumental nature. A Class 1 finish may, nevertheless, incorporate a certain number of blowholes and other minor blemishes.
Class 2 is intended for external and internal facades that can be viewed in detail. Many specifiers appear to believe that Class 2 is readily achievable for no other reason than it is one class removed from first-class. Nonetheless, a Class 2 finish is a highquality finish requiring very substantial input by the designer and the precaster and is the finish appropriate for prestige architectural projects. A Class 3 finish is required to give good visual quality when viewed as a whole. The essential differences between Classes 2 and 3 are the type, number and dimensions of permitted surface defects including: •
face deflection
•
blowholes
•
face steps, undulations and fins
•
flatness
•
shape – squareness
•
dimensional tolerance.
Additionally, there are Classes 4 and 5. Class 4 is intended for surfaces that are to receive thick applied coatings such as cement render, while a Class 5 finish is intended for surfaces that are totally concealed such as the inside unseen face of a panel. The Standard is a quality-guidance document but no document could possibly eliminate the subjective component of surface finishes and colour control. Interpretation of the Standard is variable and often unrealistic.
InFigure summary: 10.13
Former Australian Taxation Office Building, Adelaide, Showing a Class 2 Surface Finish
In summary: •
Appreciate that AS 3610 is written for off-form grey finishes and is therefore not applicable to architectural precast concrete involving colour and/or secondary finishes.
•
If using AS 3610 in documentation, then read it in conjunction with the Commentary which sets out the intent of the Code committee.
•
Like most documents, it requires pragmatic interpretation.
•
Don't use the document in an attempt to obtain unrealistic quality. Good design and detailing are still the prime requirements for quality appearance.
•
Avoid the temptation to specify the impossible; be realistic in terms of the status of the project and, importantly, the budget.
•
Appreciate that the most realistic measure of what is achievable is that which has been achieved on previous projects of a similar nature.
•
Talk to your local precaster during the design stage for input into practicality of design.
Figure 10.14
Water Feature, Melbourne City Square, Constructed in Precast Panels to Class 1 Finish
For further reading on the subject, refer to NPCAA Data Sheet No. 310.8.
10.6.2
Smooth off-form
For this finish, trueness of the surface plane is a critical requirement, especially for flat surfaces. The mould face must be carefully checked for compliance with tolerance criteria. 10.6.1
General
Decorative finishes for precast concrete products combine the traditional skills of the concrete artisan, the design skills of the professional engineer and the practical experience of the precaster. Knowing that these skills are available, architects can direct their efforts towards combining shapes, textures and colour s to achieve the desired effect on the structure and its environment. While the finishes described in this chapter are most commonly used on wall panels, the information provided is, in most cases, relevant to any precast elements requiring a special finish. The finishes listed hereunder are the ones commonly used in the industr y today.The list does not cover all finishes nor is it intended to suggest limitations in developing new finishes. With the exception of water-washed finish, the usual casting orientation is face-down. Building facades may be oriented such that sunlight just grazes the surface at a par ticular time of day. This causes otherwise imperceptible ripples, projections and misalignments on the surface to cast long shadows and be grossly exaggerated in appearance. Precast concrete, like any building surface, is subject to manufacturing and alignment tolerance so that this effect cannot be avoided. It is usually most noticeable in the early morning or late after noon. The shadows last briefly, about ten minutes or so. The actual time at which they appear varies with the season for a particular wall. Honed or polished panel surfaces will minimise shadows from surface irregularities but minor dimensional deviations such as twist and installation tolerances will always be present.
Since the mould face is generally concrete or flat steel sheet and there is no additional treatment to the panel surface after stripping from the mould, this may at first be thought to be a low-cost finish. This, however, is not necessarily so. The care required to produce a face of appropriate quality and level of colour uniformity, whilst preventing blotchiness of the face, may outweigh the costs of some moresophisticated finishes. Smooth off-form units may be expected to have some minor surface imperfections. Joints in steel plates, minor variations in the surface texture of a steel or concrete casting table and voids caused by entrapped air may be visible in this finish. This finish is the most difficult of all precast finishes to repair and chamfers to the edges of panels are essential to minimise chipping. The cement will dominate the face colour, while the fine aggregate (sand) will have some minor influence and oxides have a considerable influence. Note that the sand colour becomes more noticeable when a surface is exposed as in sandblasting or acid-etching. Coarse aggregate colour will have no impact unless heavy vibration of the concrete induces aggregate transparency, in which the presence of the coarse aggregate is visible as a hazy, shaded outline. The use of a plasticiser will generally control this effect. Moulds must be constructed to prevent leakage. Wherever possible, tapers should be built in so that the mould does not need to be dismantled after every cast and can therefore be permanently sealed. Smooth off-form units produced with high-cementcontent mixes may exhibit some surface crazing after curing. This is usually visible only when viewed at very close quarters or with some magnification. Such crazing has no effect on durability or strength. It is recommended that wherever possible, offform surfaces be modulated by grooving, shaping or profiling. Such techniques provide an architectural design opportunity by developing light and shade, and minimise the visual effect of any minor discolouration.
Advantages •
No finishing required after stripping
•
Sharp lines at intersecting planes can be achieved
•
Can be painted
Limitations •
•
•
Minor mould imperfections will show. Moreexpensive moulds required Colour variation will generally be more pronounced as the finish is more sensitive to a range of variables Difficult to make visually-satisfactory repairs
•
Air voids are more obvious
•
Surface crazing may develop
•
Aggregate transparency may occur.
Figure 10.15
Off-Form, Unpainted, Ribbed Finish using Off-White Cement in an Industrial Building
10.6.3
Water-washed
This finish is both visually pleasing and economical. It is often used in conjunction with other finishes such as smooth off-form and sand-blasted. Additionally, it weathers well in service due to:
•
the surface exhibiting a high proportion of dense stone with very low absorption characteristics;
•
rainwater tending to be distributed over a large area of the face because of its texture, thus providing more-uniform weathering.
Aggregates should be selected for their colour and shape. Generally, a gap-graded mix is preferred since the washing process will remove much of the smaller sized material from a fully-graded mix, reducing the density of the aggregate at the surface and hence the consistency of the finished appearance. Aggregate size will depend upon a number of considerations, the most critical being: •
•
The amount of exposure required. As a general rule, it is normal to ensure that about two-thirds of any aggregate particle remains bedded in the matrix of the mix. Aggregate size should be compatible with the dimensions of the area to be exposed. The larger the aggregate, the more difficult it will become to accommodate returns, reveals, etc.
It is good practice to blend the matrix, using cement, sand or oxide to approximate the colour to that of the exposed stone. Such treatment will offset any minor variation in aggregate distribution. Water washing can be applied to a wide range of flat and shaped products. Aggregate density will be less on a face cast at an angle to the horizontal. Such faces may be water washed or retarded. The former is achieved by removing the mould face and washing after initial set has occurred. Before delivery, it is normal to wash down the exposed face using a weak solution of hydrochloric acid. This treatment removes traces of laitance, which adhere to the face of the stone particles, thereby dulling their appearance. Subsequently, the surface is well rinsed with water to remove any residual acid solution.
Advantages •
Consistency of finish as the finisher can see and monitor the product as it is produced
•
The backs of the panels are off-form
•
Easy to repair
• •
Economical Weathers well
Limitations Figure 10.16
Water-Washed Retarded Exposed Quartz Aggregate and White Cement, Commonwealth and Family Law Courts, Perth
•
Trowel and screed marks can be seen in oblique light. This usually occurs for only a few minutes each day on an affected wall panel.
10.6.4
Retarded
Surface retarders have not been extensively used in Australia but may be used to produce an exposedaggregate finish to formed panel faces similar to that achieved by water-washing. Retarders are painted onto the mould face to chemically delay the surface set of the concrete so that the aggregate may be exposed after stripping. This is achieved by washing and brushing the retarded faces. Retarders are available in a range of formulations to give different depths of exposure. Advantages •
Flatter finish than water-washing
•
Finish can be achieved on profiled shapes
•
Easy to repair
Limitations •
Less reliability in achieving uniform texture and aggregate distribution, therefore seldom used in Australia
•
The backs of the panels must be finished by hand.
Figure 10.17
Retarded, Exposed Aggregate Finish to Ribbed Panels, Majura Aeropark Office
10.6.5
Honed or polished
Aggregates used for both honed and polished finishes are chosen for colour, durability and ability to hold a polish. The surface of the concrete is ground with diamond and carborundum abrasives to reveal and polish the stone. It is the aggregate rather than the matrix which holds the polish. Hard igneous rocks retain the polish best, eg granites. While these are among the most expensive precast finishes they usually cost far less than dimension stone and curtain-wall alternatives. The term honed refers to a level of grinding which exposes the aggregate and produces a smooth but matt finish, usually with a 120 or 220 grit abrasive. Further grinding with finer abrasives produces a polished finish. Manufacturers will allow on their shop drawings for the effect of grinding on unit thickness where necessary. The polishing machine is essentially a beam travelling along tracks with a grinding head traversing it. The movement of the head can be controlled longitudinally, vertically and laterally. In automated equipment, this three-dimensional movement is computer controlled. Edge polishers are similar and polish vertical return faces which cannot be accessed by the larger heads. The cutting process uses a range of abrasive sizes; coarser abrasives or diamonds for the removal of material to expose the aggregate, finer abrasives to polish the aggregate. Small and awkward areas can be polished with hand-held equipment. The rules of economy are straightforward: maximise machine work and reduce hand work. This requires that, for maximum economy, polished surfaces should be large, flat and readily orientated beneath the polishing machine. Returns should not terminate in re-entrant corners. Reveals, false joints and awkward profiles should ideally be left off-form, etched or sandblasted. Panel shapes should be kept simple and generic to permit high mould re-use. Where expensive aggregates are specified, often in association with more-expensive white cements or oxides, a two-layer technique (Clause 10.4.5) is normally used. Since aggregates for polished finishes are often expensive, the coarse aggregate proportions should be as close to the as-crushed grading as the finished appearance and good concrete technology will allow. The colour of the matrix should be matched with the coarse aggregate and this is achieved through cement colour, fine-aggregate colour and oxides. This will offset any incidental variation of the aggregate density at the surface.
Where a high stone density is required, some experimentation with aggregate grading and cutting depth will be necessary. As with most precast work, a high-strength concrete is required for polished work. High early strength provides good aggregate bond during polishing. Polished surfaces provide for easy shedding of water and dirt and minimal surface absorption. Thus a polished finish can be expected to remain clean and fresh in appearance and have excellent durability characteristics. Polished surfaces will also reflect more heat than other finishes. Washing of the surfaces with dilute acid in the factory removes any alkali bloom which develops after polishing. Special considerations that need to be noted include: •
Areas requiring hand work should be minimised, eg window reveals.
•
On panels incorporating more than one surface finish, the surface to be polished should be proud of other surfaces or separated by a wide groove.
•
Convex surfaces are suitable for polishing if the radius is 3 m or more.
•
Polishing up to internal corners is impossible. An off-form recess should be provided for clearance.
•
Recesses should sandblasted be left unpolished and can be painted, etched, or left off-form.
•
Square edges and small chamfers are susceptible to fretting during polishing, handling and fixing.
•
Chamfers may be honed or polished.
•
Circular columns should be sized to enable rotation under the polishing machine.
Polished finishes using selected aggregates, cements and possibly colouring oxides compare visually with natural stone but at a much lower cost. Further, the ability to mould the panel into a range of profiles offers the architect great freedom. However, designers should be aware that surfaces cast vertically will have a different stone orientation to surfaces cast on the flat and so will appear slightly different when polished. Manufacturers are often able to alleviate the effect of this with appropriate casting techniques.
Advantages •
Superior finish – similar to natural stone
•
Just as robust as other finishes
•
Surface sheds dirt
•
Most natural granites can be simulated
Figure 10.19
Polished Loadbearing Facade to Westin Hotel, No. 1 Martin Place, Sydney
Limitations •
Internal reveals must be polished by hand
•
Polishing curves, especially concave curves, can be difficult.
Figure 10.18
Honed and Acid-Etched Forecourt Panels, Sydney Opera House
s in r e i S c i r E y b s h p a r g o t o h P
10.6.6
Sandblasted
Advantages
A good off-form surface is required for this finish. When the concrete is about a week old, the surface is sandblasted to remove the matrix and expose the aggregate. The depth of the blast is determined by the desired texture and the target colour. Sandblasting is carried out on units poured face down and hence may be used on flat panels or on panels with ribs, grooves or other features, thus giving tremendous scope for crisp architectural detail.
A sophisticated sandstone-type finish can be achieved at reasonable cost
•
The depth of sandblast can be varied to give a fine to coarse texture
Limitations •
The type and hardness of the abrasive is selected to give the required result. Silica sand should not be used for health reasons. Experienced precasters will select suitable sandblasting techniques and media – the specification should concentrate on the required appearance. The abrasive is carried by air or by an air and water mixture. It is a medium-cost way of achieving an excellent architectural finish. The technique requires considerable skill in preparation of shop details, in mix design, in pouring technique and in the blasting operation itself.
Visible air voids may occur on vertical and sloped as-poured surfaces.
Figure 10.20
Office Building Clad with Sandblasted, Coloured-Aggregate Panels
While sandblasting is used on returns and other parts of panels not cast horizontally, the finish on such surfaces will often not be a perfect match for the horizontal surfaces. This is usually not a problem but should be dealt with during the sample-approval process. Sandblasting is always followed by a light acid wash to provide an even, clean finish. It may be combined with other finishes such as painting or polishing in the one panel. Varying degrees of sandblasting are possible and are normally typified as follows:
Light exposure – the cement surface skin is removed to expose the fine and some coar se aggregate Medium exposure – exposes approximately equal amounts of fine and coarse aggregate Deep exposure – the coarse aggregate becomes the dominant surface feature.
s in r ie S ic r E y b h p a r g o t o h P
•
10.6.7
Acid-etched
A very good off-form surface with a minimum of voids is required for this finish. It is achieved by casting concrete against a smooth, hard surface. About a week after removal from the form, the product is washed with an acid solution and scrubbed to remove the cement skin. The result is a flat, sand-textured surface. The panel surface is first wetted and the acid is brushed over the surface until the desired depth is achieved. The panel is then thoroughly rinsed. The acid used is generally undiluted commercial grade 33% hydrochloric acid. It is possible to achieve a light etch with phosphoric acid but the reaction is slow and leaves a white surface laitance and irregular aggregate exposure. The high cement content and good compaction used in precast panels means that chloride penetration to the reinforcement is not a risk. Testing by the Volhardt Method10.11 has shown that etching poses no threat to the durability of good-quality concrete and has been accepted by the industry as a standard.
Figure 10.21
Polished Facade Featuring Heavy Acid-Etched Bands in Office Building, 120 Collins Street, Melbourne
The specification may call for units to be tested by the Volhardt Method to ensure that the levels of chlorides present do not exceed acceptable levels as shown in AS 1379. Those values are total chlorides available from all sources including aggregates, water and acid cleaning. Having the work done by an experienced precast concrete manufacturer provides assurance of quality. See also Clause 10.8.2 Acid cleaning .
Advantages •
Produces a fine surface finish resembling sandstone and limestone
•
Crisp shape details can be achieved
Limitations •
Difficult to patch
•
Imperfections in moulds show readily
•
Panels cast vertically may have air voids.
10.6.8
Bush-hammered
This finish requires considerable skill on the part of the person undertaking the work to achieve a consistent appearance.
The hardened concrete surface is abraded by a pneumatic bush hammer (a hammer having a serrated face comprising rows of pyramidal points) which removes the surface fines and breaks or abrades the surface of the coarse aggregate. Alternatively, a pneumatic needle gun may be used which results in a finish similar to exfoliated natural stone. Advantages •
Bush hammering achieves a unique architectural effect
10.6.9
Hammered-nib or fractured-fin
This finish is achieved by casting concrete against specially-textured or patterned formwork to create ribs. After removal from the mould, the hardened surface is treated mechanically by breaking the nibs with a hammer to create the desired effect. Whilst providing a pleasing effect, it is critical to require a full-scale trial, to establish a mix design which will provide a suitable aggregate grading and colour.
Advantages •
An excellent, robust finish can be easily achieved
Limitations Limitations •
•
Detailing at returns and reveals can be difficult.
Relatively expensive as it is labour intensive.
Figure 10.22
Facade with Bush-Hammered Finish
Figure 10.23
Hammered-Nib Finish to Car Park Spandrels
10.6.10 Form liners Form liners are used to produce patterned, profiled finishes. In Australia, they have been extensively used on products such as highway noise barriers. Polystyrene can be cut to shapes relatively cheaply for single pours. Flexible form liners made from silicone or polyurethane can be used to produce intricate shapes including slight undercuts if required. While imported form liners are available, in Australia they are generally produced by the manufacturer from a master pattern. These moulds are expensive but useful for very difficult shapes and economical when there is good repetition. The life of liners depends on the quality of the liner material and the complexity of the formed shapes – 200 pours from a liner are quite possible.
Figure 10.24
Formliner Finish used in Place on Brougham, North Adelaide
Advantages •
Limitless range of patterns – may be produced by CAD/CAM technology
•
Intricate details can be produced
Limitations •
Not suitable for all shapes.
10.6.11 Applied finishes
Painted
Applied finishes including Paint, Stain and Textured Coatings are commonly used on precast concrete surfaces.
Paint, in a variety of textures, often used as a siteapplied finish to precast concrete elements.
Some precast manufacturers have the capacity to permit the application of an applied finish in their factory. However, it should be noted that:
•
Application of the finish will require additional space and panel handling in the factory.
•
Applied finishes can only be applied to cured concrete surfaces. This may impact on the delivery program.
•
Joints and erection damage to the surface finish will require attention on site following completion of associated on-site activities.
•
The application could be intermittent depending on element availability and accordingly, a greater opportunity for colour variation between elements exists.
Surface preparation of precast elements is important to ensure adhesion of the base coat to the concrete. Advantages •
Availability of a large range of colours and textures
•
Uniform colour and colour matching can be achieved
•
Colour schemes can be changed or updated
Limitations • Applied finishes are not as durable as other finishes described throughout this Chapter.
Figure 10.25
Applied Surface Finish to Showroom/Warehouse, NSW
Gloss finishes should be avoided due to the high cost of surface preparation necessary to achieve satisfactory results in appearance. Advantages •
Consistent colour of finished precast elements is achievable
•
Colour control of the precast concrete surfaces is not required
•
Increased durability over unpainted concrete surfaces
Limitations • Natural look of concrete is lost • Paint systems are usually 3- or 4-coat to achieve satisfactory results • Repainting will be required at regular intervals.
Figure 10.26
Perth Apartment Block Featuring a Fully-Precast Structure with a Painted Surface Finish
Stained
Texture-coated
Staining is the application of coloured emulsions and stains that penetrate the surface of the concrete so that the concrete substrate is preserved. This is in contrast to painting where a film is applied over the surface.
Textured coatings provide a thicker membrane to a surface than paint coatings and generally require a base coat prior to the application of the finish coat.
There are different types of stains with varying chemical properties. These properties require consideration with respect to environmental impact and colour consistency. When combined with form-liners, staining can replicate the appearance of traditional brickwork, blockwork, stonework, sandstone, timber and numerous other materials. Matching of existing masonry and concrete colours along with numerous surface effects are achievable. Skilled applicators can offer multi-layered and multicoloured effects if required.
These coatings can be applied using various methods including spray, trowel, roller and brush to achieve the required thickness and texture. Advantages •
• •
•
Can mask defects in less-than-perfect concrete surface finishes
Limitations • Colours can be limited •
Advantages • Colours can be translucent, semi-transparent as well as opaque •
Can be painted over to maintain or update the colour Various finishes can be achieved
Expansion joints are required (high-build textured finishes should not be applied across panel joints).
Metallic colours are available Environmentally sensitive applications are a possibility
Limitations • Staining is permanent and cannot be reversed.
Figure 10.27
Example of a Stain Finish used in Conjunction with a Formliner
10.6.12 Brick- or tile-faced
10.6.13 Stone-faced
Traditional clay brickwork or tiled finish can be simulated in precast wall panels by using brick wafers or tiles.
In much the same way as a brick- or tiled-look is achieved by the method described in Clause 10.6.12, a stonework-look can be provided by placing natural stone pieces into the mould prior to casting the panel.
To achieve such finishes, a veneer of brick wafers or tiles is placed into the mould and the concrete panel cast behind it. The wafers or tiles are held in place by the bond between them and the concrete. There is a basic conflict between clay, which expands, and concrete which shrinks. The designer and manufacturer must take account of this to prevent separation. Advantages •
It is usual for the natural stone to be mechanically fixed with stainless steel pins. A bond breaker is placed between the concrete and the stone where necessary to prevent adhesion which would result in cracking of the stone facing due to temperature differential and concrete shrinkage. Advantages
The brick or tile effect can be achieved without on-site labour, scaffolding, etc
•
Fixing natural stone to precast in a factory is a safer and better way to use natural stone
•
Fixing of the natural stone is done off-site ahead of site requirements
Limitations •
Loss of flexibility in detailing. Limitations •
Figure 10.28
Tiles Placed in Moulds Prior to Casting to Achieve a TileFinish Effect in Orange Civic Centre
Some complex shapes do not lend themselves to this finish.
Figure 10.29
Stairwell Panels made of Natural Stone Fitted into Mould Prior to Casting, Grosvenor Place, Sydney
Photograph by Eric Sierins
10.6.14 Multiple finishes within a single panel It is not uncommon to have panels with more than one finish. Typical is any combination of face-down finishes such as polished, honed, sandblasted, off-form and painted, or combinations of face-up finishes such as water-washed, steel-trowel, sponge-float and painted. There are practical problems to be resolved with many combinations. For instance: surfaces in the same plane as, and immediately adjacent to, polished or honed finishes will need to be set down so that they will not be abraded by the polishing process; finishes not sandblasted will need to be protected during blasting operations, and so on. Where different concrete mixes are to be used within panels they obviously will need to be kept apart to avoid contaminating one another. It is sensible to get feasibility and cost advice from an experienced precaster before designing combination finishes.
Advantages •
Greater architectural scope
Limitations • More expensive than a single finish due to extra labour.
Figure 10.30
Multiple Finishes to Noise Barrier Panels
Figure 10.31
Adelaide Office Building, Featuring Combination of Polished, Sandblasted and Acid-Etched Finishes
For this form of architectural walling, these other points should be noted:
Prestressed hollowcore units are often used as architectural cladding for commercial and industrial buildings. Being prestressed and made from lowslump concrete, they are very durable.They are generally used either as a cladding panel fixed to a steel portal frame in either a vertical or horizontal configuration or as loadbearing panels. Hollowcore units are usually either painted with a high-build paint or prefinished in exposed aggregate. Panels are nominally 1200 mm wide and are usually supplied in thicknesses ranging from 150 to 250 mm. Manufacturers have standard details for window, door and service openings.
Figure 10.32
Hollowcore Panels used as Architectural Cladding in a Horizontal Configuration for an Industrial Building
Samples and Prototypes Hollowcore units are specified with regard to samples but, if the specifier is unfamiliar with its use, a finished building constructed by the intended manufacturer should be inspected to appreciate the characteristics of the product. As hollowcore cladding is usually restricted to low-rise industrial buildings, a prototype is seldom warranted. Colour Control Hollowcore units cannot be specified with regard to tonal scales in AS 3610. Designers should obtain details, from the precaster, appropriate to the method of manufacture. Quality of Off-form Surfaces Standards for hollowcore manufacture are set by agreement with each manufacturer to reflect the characteristics and capabilities of the manufacturing equipment. Attempts should not be made to apply wetcast standards to hollowcore units. Surface Finishes Hollowcore finishes are generally restricted to mechanically-trowelled, water-washed (Clause 10.6.3) and painted (Clause 10.6.11). For water-washed finish, options for size and type of aggregate and colour of cement should be discussed with the manufacturer.
10.8.2
10.8.1
Treatment of unformed (face-up) surfaces
The treatment of unformed upper surfaces needs to be specified. There is no Australian Standard for guidance, so where the finish is important, a sampleapproval system should be used. Typical finishes used for these surfaces and applications are: •
Broomed or raked – where units are to be topped with insitu concrete.
•
Broomed finishes – for slip-resistance.
•
Wood floating – will leave some ridges and is typically used only where the face will not be seen.
•
Sponge floating – gives a sandy finish suitable for many applications where the face will be seen.
•
Steel floating – yields a flat surface which may have some trowel marks and be dark in colour.
•
For some multi-faced units, such as columns, manufacturers will use techniques which allow sandblasting, polishing or other treatment of
Acid cleaning
Architectural precast concrete, by its nature, requires a clean surface finish. Precasters universally use a dilute solution of hydrochloric acid to clean panels after all finishing processes are complete. This acid cleaning10.9 is distinct from acid etching, the acid being much more dilute. There has been some concern that such use of acid will adversely affect the durability of the precast units. There is no evidence to support this. On the contrary, all studies available to the NPCAA indicate that it is a safe practice. For instance, see Chloride Movement Through Precast Concrete Panels10.10. Acid concentrations, vary from approximately 1 part commercial grade (33%) hydrochloric acid to 15 parts water for washed and retarded finishes, to 1 to 40 for polished panels. Acid is applied evenly and thoroughly rinsed off with water.
face-up surfaces to match the off-form faces. Figure 10.33
The Finish to 'Face-up' Surfaces Needs to be Specified
10.8.3
Remedial work after stripping
Architectural precast panels are rarely taken straight from the mould ready to transport to the building site. An example of work required after de-moulding is the removal of arrises (with a hand stone) on grey off-form panels.
Some minor remedial work will also often be required, eg bagging of the surface to fill in blow holes or repairing chips to panel edges. Remedial work and cleaning is generally undertaken in the precaster’s yard prior to delivery to the site. An exception is hollowcore panels where the nature of the production and materials-handling process usually dictates that all such work is carried out onsite. 10.8.4
On-site rectification work
While every precaution should be taken to prevent the need for rectification work or patching, some will always be required on architectural projects. This is brought about by the inevitability of transport or erection damage or staining after installation. It is important that this need be recognised and, where it causes concern, the architect sees trial patches at the sample stage. Good materials are available to ensure a sound and durable repair, but it is the matter of
10.8.5
Protective coatings
Anti-graffiti coatings Anti-graffiti coatings generally put a matt or gloss film over the surface and may be acrylic, polyurethane or epoxy-based. They all will affect the surface of precast units, usually by darkening them. They are either sacrificial, where a layer of the coating is removed with the graffiti or permanent, where the graffiti will not stick to or can be removed from the coating. These coatings are being improved with time, and those seeking to use them should be satisfied with regard to: •
• •
Film breakdown mechanism – will it peel or dust away Recoatability Discolouration with age.
Durability enhancers – silanes and siloxanes Most high-strength precast concrete will be exceptionally durable. Nevertheless, there are situations where aggressive environments or doubts about concrete quality will lead specifiers to look for greater durability.
matching colour and texture which is subjective and which should be resolved early. Such rectification must be carried out by a skilled repairer. Figure 10.34
Some Remedial Work after Str ipping will Usually be Required on Architectural Panels
Surface treatments for concrete are designed to make the pores in the substrate repel water. The most-commonly used materials for pore blocking and to repel water are products containing silanes, siloxanes or a combination of both. These chemicallyactive materials react with the cementitious materials, lining the pores and hairline cracks in concrete to make it hydrophobic. Silanes and siloxanes are chemically similar, the only difference being that siloxanes are typically made up of about four silane molecules chemically combined to form a larger and less volatile molecule. A major disadvantage of silanes is their high volatility. In Australian summer conditions, during silane application in direct sunlight with surface temperatures over 24°C , more than 80% of the applied silane could be expected to evaporate into the atmosphere, compared to about 6% for siloxanes. The primary advantage of silanes and siloxanes is that they do not change the appearance of the concrete, thereby making them ideally suited to architectural finishes. The invisibility of silanes is also a disadvantage since it is very difficult to detect where they have been applied.
Silane/siloxane treatments should be used only on atmospherically-exposed concrete and are not suitable in water-ponded or semi-immersed applications as even very low water pressures will eventually allow water to pass. Silane/siloxane-treated surfaces remain breathable, freely allowing the passage of water vapour and gases through the surface treatment. The advantage of this is that they will not become separated from the surface by trapped moisture.Silane/siloxane treatments are most effective when applied to capillary-open concrete. The presence of any curing compounds or release agents on the surface will prevent these materials from penetrating and reacting. Silane/siloxanes find their ideal application on exposed concrete in coastal areas where wind-blown salt spray is a problem and no changes to the surface appearance is desirable. They are not effective on wide cracks or on new cracks that form after the silane treatment has been applied.
Figure 10.35
Anti-graffiti Coating to Panels of Transport Interchange, Parramatta, NSW
10.8.6
Matching to insitu concrete or existing precast concrete
Matching the appearance of precast and insitu concrete is very difficult. It should be attempted only where there is no other option and after samples have been approved. Similarly, the matching of new to old precast can be difficult unless the exact mix can be duplicated. It will be necessary to clean the existing precast concrete if a good match is to be achieved. 10.8.7
Maintenance of precast concrete
Precast concrete is a durable and long-lasting building product. By following a programme of inspection and maintenance, precast concrete will maintain its appearance for the service life of the building. To ensure the continued performance of external wall systems, visual inspections should be carried out at intervals of about five years. Attention should be paid to caulked joints and the surface appearance. Repairs should be made as necessary. Atmospheric pollution should not affect the performance of precast panels. However, they should be cleaned at appropriate intervals to preserve their appearance. Maintenance will be minimised by ensuring windowcleaning run-off is not allowed to cause staining. Where acid or sandblasting is used to clean surfaces, a small inconspicuous area should be treated first to ensure units will not be damaged by the treatment. Precautions should be taken to avoid damaging or staining precast units by ensuring access equipment does not scratch or chip precast surfaces. Figure 10.37
Example of a Well-maintained Facade
Figure 10.36
Good Matching of New and Old Finishes in Sydney Opera House Extension
10.1
AS 3610 Formwork for concrete, Standards Australia, 1995.
10.2
AS 1141 Methods for sampling and testing aggregates, Standards Australia, 1974.
10.3
AS 2758.1 Aggregates and rock for engineering purposes Part 1: Concrete aggregates, Standards Australia, 1998.
10.4
AS 1012 Methods of testing concrete, Standards Australia.
10.5
AS/NZS 4284 Testing of building facades (The SIROWET Method), Standards Australia, 1995.
10.6
Mahaffey Associates, The Suitability of Veneering in Precast Concrete Applications, Sydney, 2001.
10.7
AS/NZS ISO 9001 Quality management systems – Requirements, Standards Australia, 2008.
10.8
Surface Finishes – Specification of Surface Finishes
under AS 3610, Data Sheet No. 3, National Precast Concrete Association Australia, September 2000. 10.9
Surface Finishes – Acid Cleaning of Architectural Precast Concrete, Data Sheet No. 1, National Precast Concrete Association Australia, September 1999.
10.10 Symons, M and O’Sullivan, P ‘Moisture and Chloride Movement through Precast Concrete Panels’ Concrete in Australia, Vol. 20 No. 4, December 1994, pp 13–16. 10.11 AS 1012.20 Methods of testing concrete – Determination of chloride and sulfate in hardened concrete and concrete aggregates, Standards Australia, 1992. 10.12 AS 3600 Concrete Structures Standards Australia, 2009.
Details of other examples of surface finishes on following pages A Red polished ground-floor band, off-white sandblasted upper levels – The R G Casey Building, York Park, Canberra. B Alternate honed and sandblasted panels giving banded effect – Park Hyatt Hotel, Campbell Cove, Sydney. C Polished reconstructed-granite facade using pink Tarana granite and off-white cement – CML Building, Brisbane. D Example of muti-finish detailing at a dummy joint. The recessed ‘joint’ is left off-form giving a suitable contrast to the polished portion. This technique is also useful around window openings in panels with either polished or sandblasted finish. E Quartz aggregate, red sand and off-white cement were used here as an economical alternative to reconstructed granite. The panels were polished and the recessed joints and dummy joints were lightly acid washed – Casselden Place, Melbourne.
Other Examples of Excellent Excellent High-Quality High-Quality Surface Surface Finishes Finishes
s in r ie S ic r E y b h p a r g o t o h P
A
s in r ie S ic r E y b h p a r g o t o h P
B
C
D
E
What you will find in this Chapter •
Guidelines for the handling and storage requirements within the precast factory.
•
An appreciation of the transpor tation factors in the preplanning stage of projects.
•
Mass, size and access limitations for product transportation.
•
Key criteria for preplanning the erection of precast elements.
•
Design recommendations for efficient erection – an understanding of the problems and solutions.
•
General information on cranage, rigging and temporary bracing equipment.
11.1 Introduction 11.2 Handling and Storage in the Factory 11.3 Transportation to Site 11.3.1 General 11.3.2 Preplanning 11.3.3 Delivery arrangements 11.3.4 Mass and size limitations 11.3.5 Loading of vehicles 11.3.6 Prestressed elements 11.3.7 Non-standard elements 11.3.8 Off-loading on site 11.4 Site Erection 11.4.1 Erection design engineer 11.4.2 Preplanning and work methods 11.4.3 Cranes 11.4.4 Rigging 11.4.5 11.4.6 11.4.7 11.4.8 11.4.9 11.4.10
Erection of wall panels Erection of hollowcore floor planks Erection of Ultrafloor system Erection of prestressed bridge elements Bracing – General Bracing – Wall panels
11.5 Bibliography
The effects of loads and stresses on precast elements during the handling stages between fabrication and erection are covered separately in Chapter 6 Design of Elements. This chapter covers recommended
The handling and storage of precast elements in the factory is the manufacturer’s responsibility. •
practices handling, storage, transport and andprocedures erection of for precast elements used in both building and civil construction. The most economical precast element that can be used is the largest that can be manufactured and handled. The size limitation of individual elements is determined more by transport restrictions or site lifting capacity rather than by the lifting capacity of the precast manufacturer.
Figure 11.1
Methods of Temporary Strengthening of Panels with Significant Openings
Precast components are generally demoulded and lifted from their casting position at between 12
and 24 hours after placement of the concrete. The expected concrete strength would be in the range of 15 to 25 MPa at the time of lifting. This may be achieved through the use of high-early-strength concrete or by means of accelerated curing (eg steam).
Flat panels are usually rotated to the vertical position using specially-designed edge lifters. Where a large number of slender panels are to be made, the manufacturer may use tilt tables.
•
Panels with large openings sometimes require strongbacks, braces or ties to keep stresses within safe limits (see Figure 11.1).
•
Wherever possible, an element should be stored on only two points of support. These should be arranged so that overall stresses are at a minimum. Generally, supports located at L/5 from the ends of a unit will produce positive moments at the centre equal to the negative moment at the supports. Wall cladding panels are usually stored vertically and braced in position by A-frames or a racking system (see Figure 11.2).
If more than two supports are provided, precautions must be taken so that the element does not bridge over one of the supports due to differential support settlement and, therefore, be overstressed.
Floor and beam units should be handled and supported in the as-erected attitude near their ends unless specifically indicated otherwise by the designer.
•
The primary cause of warping is incorrect storage of the units. They should be stacked at points so that there is no twisting moment even if one of the supports crushes slightly.
The following general guidelines are provided for supporting precast units so as not to cause damage or staining: •
Use softwood packers which deform slightly and reduce edge chipping and do not stain.
•
When hardwood packers are required for strength, units must be protected from wood stains which are very difficult to remove from the concrete.
•
Where packers are required on smooth faces visible in the finished job, dimpled plastic packers should be used so as to reduce curing hydration marks.
•
Rust stains from unprotected steel can be difficult to remove from concrete.
•
Storage should be planned to minimise handling before delivery. There is always a risk of damage every time a unit is moved.
General guidelines on stacking and handling of precast elements are provided in Figures 11.3 and 11.4.
Figure 11.3
Horizontal Stacking of Slab Units
Figure 11.2
Methods for Vertical Storage of Precast Panels. TOP: A-Frame BOTTOM: Racking System
Figure 11.4
Handling of Beams and Slabs
Government road authorities issue regulations governing weight, size and hours of travel which may vary between States but the size of precast units is generally limited as follows.
11.3.1
General
The design, tendering and planning stages of all precast concrete projects should consider the following transport limitations: •
•
Mass and size Transport is usually by road on semi-trailers, Figure 11.5. Where units are too large or heavy for a standard semi-trailer, special vehicles or travelling conditions will be necessary and may affect delivery rates and times. Slenderness When units are too slender to resist normal transport stresses, special strong backs or frames are required and their cost and availability may limit delivery rates.
11.3.2
Preplanning
The following information should be available at tender stage: •
Date of first delivery for each type of element and rate of deliveries required.
•
Site limitations on space and time. There may be restrictions on the times when trucks can be parked in construction zones. Where space is available, it is desirable to unhitch trailers within crane reach to make delivery times less critical.
11.3.3
Delivery arrangements
The builder/erector should advise the manufacturer the number and type of units required, the erection sequence and the delivery times required. For local deliveries the notice required is usually 48 to 72 hours although for some units a minimum of 24 hours may be possible. The notice required for long-range deliveries needs to be agreed early in the project.
Mass Mass is limited for standard loads by permissible axle loadings which are approximately 24 tonnes for a semi-trailer and 18 tonnes for a step-down trailer or low-loader.
For indivisible heavy loads, special vehicles are required, with consequent restraints on travelling speeds, times and routes. These heavy loads are expensive and may not be possible over some routes. Size Height For units that would be over 4.3 m high when loaded on a semi-trailer, a step-down or lowloader must be used. For loads over 4.6 m high, the truck must travel a prescribed route which has been checked for obstructions, Figure 11.6. For loads over 5 m high provision has to be made to lift power lines over the load as it proceeds.
Width There are no restrictions for loads up to 2.5 m wide and some restrictions in travelling times for loads over 2.5 m up to 3.5 m. Loads between 3.5 m and 4.0 m wide require one escort vehicle, loads between 4.0 m and 4.6 m require two company escorts and loads over 4.6 m require two company escorts and one police escort.
As loads get wider, travelling times become more restricted. Length There are no restrictions for articulated vehicles up to 19.0 m long; this allows a product length of about 12 m. For vehicle lengths of 19 to 25 m a permit is required and there will be some restrictions on travelling times. Over this length, escorts and special vehicles are required. Figure 11.5
Typical A-Frame Load on Standard Semi-Trailer
Prior to any delivery, the carrier should visit the site to consult with the builder on all site-specific requirements, including access practices adopted for the site. and safe working The manufacturer should advise the erector how individual units are to be loaded onto trucks and when each truck is dispatched to site. 11.3.4
Mass and size limitations
While precast units may be delivered by rail, sea or road, they usually finally reach the site by road on tray trucks, semi-trailers, drop-down trailers, lowloaders, timber jinkers or special rear-steer trailers for very long loads.
11.3.5
•
Load restraints may be chains or webbing straps. Restraints should be checked and tightened as necessary during transit; they tend to loosen due to settling of the load and stretch in the restraints, particularly if webbing straps are used. Load safety in transit is the carrier’s responsibility. Some States have very prescriptive regulations for restraint but general guidelines are shown in Figures 11.8 and 11.9.
•
Special restraints may be required for long
The manufacturer should advise the carrier of any special requirements for support and restraint of units. The carrier is responsible for the selection of the appropriate truck, adequacy of the restraints, safety of the load in transit and obtaining any travel permits required by law. •
Where possible, units should be loaded in as-erected orientation.
•
Where possible, trucks should be loaded so that units can be removed in the sequence required for erection. This requirement may conflict with the need to achieve optimum payload for the truck.
Loading of vehicles
•
prestressed bridge girders, and similar, to control whip and torsional forces, especially when hauling long distances, see Figure 11.7.
Packers between units and support frames should be softwood or non-marking rubber.
Figure 11.6
Special Step-Down Trailer for High Loads
Figure 11.7
Special Rear-Steer Trailer for Long Loads (Note Cable Bracing to Control Whip)
Figure 11.8
Restraining Panels on an A-Frame when using a Flat-Top Trailer
Figure 11.9
Restraining Horizontal Panels when using a Flat-Top Trailer
11.3.6 Prestressed elements Bridge Girders Prestressed concrete girders are manufactured, handled and stored in their upright position. The method of handling and storage should be such as to avoid fracture from impact, undue bending, twisting and whipping.
Prestressed units are to be lifted only by the lifting devices provided unless approval has been received to lift by alternative means.
Figure 11.10
Recommended Procedure for Stacking Large Prestressed Girders
During transport, torsional forces generated must be limited to allowable values. During storage, units need to be supported on level bearers placed near the ends of the units. They should not rest on any support at locations between the approved support points. Supports should be level at all times to ensure that the units do not develop twist. Where units are stacked in more than one layer, the supports for each layer should be placed directly above the lower supports. Figure 11.10 shows the recommended procedure for stacking large prestressed girders.
Piles The method of handling piles and the location and the method of storage on site should be such as to avoid damage by impact or by overstressing. Piles are not designed to be handled by dragging across the ground. Bending stresses induced in the piles during handling and transportation are limited in the specifications.
11.3.7
Long piles transported by jinker may need to be laterally restrained. They may also require load-sharing rigging when lifting or pitching on site in order to
Limitations will tend to be determined by the width and height of a load for transport, and by crane capacities. These limitations may vary greatly from metropolitan to country areas.
reduce bending stresses. Piles should be stored on two level bearers of adequate dimension to prevent settlement. Generally, they should be located at the fifth points of the piles. Where piles are stacked in more than one layer, the supports for each layer should be directly above the supports in the lower layers and need to be of sufficient dimension to carry the imposed loads. Piles stacked in layers should be stored in accordance with Figure 11.11. Piles stacked stored as in Figure 11.11 (a) and (b) can be unstable and because of the small width of the pile they are prone to roll if the units settle or are bumped. Multiple-layer storage stacks need to be considered as structures and it is important that foundations for these stacks are capable of carrying the loads without distor tion.
Figure 11.11
Correct and Incorrect Pile Storage Methods
Non-standard elements
The handling, storage and transportation of nonstandard products will be dependent on the shape, mass and dimensions of the product and the availability of suitable transport equipment to carry the load within the limitations specified by the various road authorities.
In general, consideration needs to be given to methods of handling and storage that will limit torsional forces on the units to limit tor sional cracking. 11.3.8
Off-loading on site
The site off-loading area should be regarded as an exclusion zone where only persons inducted in the safe-work method and directly involved with the lifting, should be allowed access. The transpor t driver should leave this zone until the unloading operation is complete. Restraints to vertical panels should not be released until the crane slings are engaged. Site off-loading of units will be subject to various OH&S requirements which vary from state to state.
account the above items and include:
11.4.1
Design erection engineer
•
a general description of the erection process,
•
a statement identifying who is responsible for each activity,
•
a risk analysis or similar;
•
erection design computations and documentation covering temporary bracing, brace footing details, temporary fixings necessary for erection stability, lifting insert details and rigging systems where
The design of precast concrete will often involve two separate engineers, a project design engineer and a design engineer for erection. The design of the precast concrete is carried out by the project design engineer as part of the overall design of the structure. This covers the in-service performance of the precast concrete as part of the complete structure. The design erection engineer, if one is involved, is responsible for the design-for-construction, including the handling, transportation, erection, bracing and propping of the individual precast concrete elements during the manufacturing, transporting and erecting processes. In some cases, the precast erector will fill this role if the panels are simple or standard units. In some States (eg Western Australia), a design erection engineer is legislated through a Code of Practice for flat panel precast units. In WA, the Worksafe Western Australian Commissioner must be notified of the intention to manufacture precast wall panels and certain documentation is required on site. In Victoria, a design erection engineer is required where precast wall units in buildings or portions of buildings are:
required, required; and any propping (eg for floors) if •
a component casting schedule;
•
an erection schedule.
The erection schedule and erection design are the key to being able to safely erect precast concrete. Close liaison is required between the precaster, the erector and the builder/contractor so that the full details of each process are fully understood by all parties. For simple projects, this may be part of the project design and be shown on the working drawings or shop drawings. For a complex projects it will almost certainly be a separate set of computations, sketches and erection drawings that cover the stability of the complete structure during erection as well as bracing of individual components. • • •
Braces
•
Brace fixings
•
weigh more than 8 t; or
•
Brace footings
•
are not nominally flat or rectangular; or
•
Propping.
•
are not directly fixed and supported by a freestanding structural frame.
11.4.3
Preplanning and work method
method of erection. The erection method should take account of: •
site ground conditions, in particular, hardstanding areas required for cranes and trucks;
•
overhead obstructions, eg power lines and trees;
•
access and egress for cranes and trucks;
•
crane capacity;
•
unit size and mass; and
•
delivery sequence and rate.
A work-method statement should be prepared for the erection of the precast units. This should take into
Lifting inserts Rigging system
greater than 8 m in height; or
Except for very small units, precast units are erected by crane. The designer should take into account site access and planned erection equipment capacities to ensure the design is buildable. At shop drawing stage, the units should be designed to suit the adopted
The key elements in the erection design are:
•
11.4.2
Cranes
Site craneage must comply with AS 2550 Cranes, hoists and winches - Safe use [set] and have sufficient capacity to lift the units into their final locations in the structure. Erection equipment will frequently influence the size of precast elements. The designer must consider access to the site to be certain that there is sufficient space in proximity to the structure to allow erection to proceed as contemplated. This requires coordination with other trades to ensure that there will be no interference with crane access, eg by wall footings. It should be noted that cranes are rated by the safe capacity they will lift with the shor test boom and at the steepest boom-up angle. Maximum lifts will reduce rapidly as boom length and angle change. On multi-level or very tight sites, the use of a tower crane may be necessary and will have a significant effect on the planning of the structural frame and the sequencing of construction.
Cranes used for erection of precast elements can be classified into three broad groups: •
•
•
Tower cranes These are characterised by having a jib at the top of the tower with the jib clearing the whole of the structure. The capacity reduces with radius and is sometimes expressed in metre tonnes, eg a crane that lifts 1.5 tonnes at 20 metres would have a rating of 30 metre tonnes. The use of tower cranes is common on buildings of five or more storeys, especially where access is
restricted. Mobile cranes These cranes can be driven from job to job. The crane capacity is usually based on the crane being supported on outriggers, though some cranes can move with a small load. Depending on the height/reach required, space may be needed at the site to rig the crane. The most common types of mobile cranes are allterrain, rough-terrain, articulated, and crawler cranes. These range in lifting capacities from 3 t to 800 t Safe Working Load. Floor cranes These are designed for travelling on suspended floor slabs. A mobile crane lifts the floor crane to the required floor, where the floor crane lifts the precast units into position. These cranes are used to lift and place small precast elements into position in tight places, ie multi-storey building cladding units, and can be a useful addition to materials-handling resources. It is important that the building designer confirms that the floor can carry the loads involved.
11.4.4
Rigging
The mass of an element should be marked on each unit and lifting points should be shown on the shop drawings. Where possible, proprietary lifting inserts with visible capacity markings should be used. Where more than the minimum of two lifting points for a wall unit or three for a slab unit are to be used, balancing sheaves or multiple-spreader beams are required. These systems are complex and should be avoided where possible.
11.4.5 Erection of wall panels The adoption of the following recommendations will improve erection efficiency of wall panels.
•
Make vertical joints between adjoining panels butt joints. Grooved, lapped and similar joints may limit the choice of erection sequence.
•
Design panels so that they can be landed on the floor, eg haunched, and the crane hook can be freed once the two restraint connections are made.
•
Provide a leverage point so that final panel adjustment, shimming and movement (in, out or sideways) can be effected by hand, using nothing more than a crowbar.
•
Give careful consideration to corners. Corner units made up of two panels, butt jointed or mitred, are the easiest to install but the column behind causes problems with the installation of joint sealants. Single L-shaped corner units are more difficult to handle but alleviate the sealant problem, Figure
•
11.12. Where possible, off-set joints from columns to facilitate installation of sealants.
•
Locate the four fixing points per unit so that they can be reached by erection crew working on the same floor. To have a team split between floors makes communication difficult.
•
Locate the fixing points such that the fixer does not have to lean out of the building to reach them and provide room physically to make the connection, eg turn spanner to tighten bolts.
•
Allow reasonable tolerance in the design, especially in the position of builder’s cast-in fittings and in the loose hardware connections (precast to structure).
•
Unless carried out off the critical path, avoid fixings that require welding, which in turn alleviates time delays for the erector, especially with multi-storey projects.
Figure 11.12
Erection and Detailing Options at Corners
With good, simple fixing designs, the following erection rates are achievable in an eight-hour shift: Hollowcore floor panels
40 to 50 units
Ideally, precast is loaded on to the transport “as erected” so as to permit its removal and erection directly into its final position.
Industrial wall panels: Fixed to steel structure Braced with raking braces
25 units 20 units
When panels are too tall to travel “as erected”, they should be delivered laid on their long edges supported against an ‘A’ frame and erected as follows:
Architectural wall panels
10 to 15 units
The key to achieving high erection rates is to check the accuracy of the structure in which units are to be erected and to set out the design position of units before erection commences.
Figure 11.13
Procedure for Lifting Long Panels from a Transport and Turning ready for Fixing
•
•
Turning in the air with a two-point lifting system; this is usually possible with one mobile crane with two winches, but tower cranes will require an extra crane to ‘tail’ the panel during turning. Using a specially designed turning frame.
The procedure for lifting a long panel from the transpor t and turning it ready for fixing is illustrated in Fig 11.13. The centre of gravity of the unit in its vertical position governs the position of the lifting inserts to ensure a ver tical lift. Should this position not be acceptable (eg the lifting inserts may need to be in the exposed sloping face of the panel), then a special hook may need to be used.
The situation becomes even more complicated if the centre of gravity of the unit in its final position falls inside the perimeter of the structure. If this distance is small then it may be possible to pull the unit in before it is lowered onto its bearing points, but some control over the positioning of the unit during its last stage of travel is lost. If the number of such units warrants it, a counter-balanced lifting jig as shown in Figure 11.14 can be used to facilitate the placing of units in their final position.
Figure 11.14
Counter-Balanced Lifting Jig
11.4.6 Erection of hollowcore floor planks
Before erecting any planks, the bearing surfaces should be checked to ensure that they are smooth and level, bearing strips should be set where required and temporary shoring and bracing provided as necessary to maintain the stability of the structure. Hollowcore planks are generally lifted with long chains or a spreader bar and a chain choker or webbing slings placed close to each end or gravity clamps and safety chains,Figure 11.15. Alignment of planks is generally carried out using come-a-longs to pull units into position. Prior to grouting the keyways, planks should be levelled while keeping the units tight and at right angles to the bearing wall or support beam. In grouting the keyways, a 3:1 sand-cement grout or an approved topping concrete should be used so that the keyways are properly filled. To prevent grout from flowing into the voids, suitable dams should be provided in the voids at plank ends as required.
Figure 11.15
Typical Example of Erection of Hollowcore Floor Planks
11.4.7
Erection of Ultrafloor system
The Ultrafloor® system typically comprises prestressed shell-beams and I-beams with formboard or metaldeck as infill material. Temporary propping/ bracing is normally carried out by the installer. Prior to installing the Ultrafloor system, the builder should ensure that perimeter protection is in place and props/frame supports are level and installed to the Figure 11.15 correct RLs. Example of Load-controlled (Torque-controlled) The Ultrafloor beams are positioned on the Heavy-duty Safety permanent support Anchor structure at centres designed to satisfy a variety of load/span cases. Due to the sharp/ abrasive nature of the beam edges, chains are used for all lifts involving shell and I-beams, Figure 11.16. Multiple beams may be lifted in a single lift subject to crane capacity and safety considerations. The beams are placed into their approximate positions while on the hook and manoeuvred into final position by the installers while the crane is picking up the next load. The infill material is placed as soon as practicable to progressively create a safe deck for the installers and other trades that will follow. Once the deck is handed over and accepted by the builder, other trades such as steel-fixers and service trades (pipes, conduits, and penetrations) can access the deck. Figure voids 11.16 Typical Anchor Load v Displacement Curves Figure 11.16 Erection of Ultrafloor® Beams by Crane with Metal-deck Infill Progressively Installed to Form Safe Working Deck
11.4.8
Erection of prestressed bridge elements
Erection procedures for prestressed bridge elements, such as girders, piles and planks are quite specialised and beyond the scope of this Handbook. Reference should be made to appropriate State transport authorities, who provide guideline specifications for the erection of such elements.
11.4.9
Bracing – General
The greatest risk during erection of precast concrete elements, particularly flat panels, occurs while the elements are in the temporarily-erected position prior to being connected to the structure. Braces are often required to provide temporary stability to prevent a precast concrete element overturning. Both ends of braces for vertical units are fitted with a hinged foot to allow for variable fixing angles. Adjustable braces should have stops on the threads to prevent over extension. Formwork props are not suitable as braces; proprietary braces should be used. Braces are designed to resist wind actions, temporary imposed actions and impact actions calculated in accordance with AS 3850 and AS/NZS 1170 Set, but using Working Limit Loads (WLL), not limit state design. The permissible gust wind speed may be obtained by dividing the wind speed given in AS/NZS 1170.2 by the square root of 1.5.
Acceptable post-fixed anchors include: •
•
Mechanical anchors – Heavy-duty undercut anchors and drilled-through fixings. (Note that undercut anchors generally have load capacities less than expansion anchors and are therefore not used) Expansion anchors – Only load-controlled (torque-controlled) types (Figure 11.17) that have load/displacement curves similar to those shown in Figure 11.18 should be used.
Deformation-controlled expansion anchors, including self-drilling anchors, chemical anchors and drop-in and spring-coil anchors must not be used.
Figure 11.17
Example of load-controlled (Torque-controlled) Heavyduty Safety Anchor
Type of brace and location dimensions are to be shown on the design erection drawings or shop drawings. Unless otherwise specifically designed, braces should not be used to carry significant lateral loads or actions due to backfilling and the like. Where possible, the precast unit should be secured and
form part of the final structure before such loads or actions are applied.
Braces should have the maximum Working Load Limit permanently marked on them. On adjustable braces, the Working Load Limit for both zero extension and maximum extension should be shown. Whenever possible, the bracing should be fixed to the panel before lifting. A minimum of two braces should be used for each panel.
Figure 11.18
Typical Anchor Load v Displacement Curves
During the lifting process, the braces should not hang below the base level of the panel. This may be achieved by the use of adjustable brace lengths or by the use of tailropes. If bracing inserts are on the opposite face of the panel to the lifting inserts, the panel should be tilted just past vertical in order to install the bracing.
Whenhas it isbeen necessary to attach braces after panel positioned, the the panels should bethe held firmly, safely and just past vertical by the crane while the braces are installed. Where a post-fixed concrete anchor is to be used in place of a cast-in insert, the load case should be established taking into account bracing loads, bracing geometry and other contributing factors in accordance with AS 3850 and AS/NZS 1170; the appropriate anchor is then selected using the manufacturer’s published performance data. Post-fixed anchors are usually used as bracing inserts in the floor.
The tensile and shear performance of anchors should be established in accordance with the test methods described in Appendix A of AS 3850 and the results published by the manufacturer in a certificate, along with shear and tensile capacity. AS 3850 suggest four design cases need to be considered when designing anchors. The certificate should also list the installation details required to achieve the published capacities, including: •
Drilled hole diameter and depth
• •
Setting criteria (eg installation torque) Minimum concrete edge distance
•
Minimum anchor spacing
•
Minimum concrete compressive strength
•
Maximum fixture thickness.
Only anchors with certificates detailing their performance as stated in this clause, should be used for the purpose of fixing braces to concrete footings and elements. At present, only a limited number of expansion anchors in Australia comply with Appendix A of AS 3850.
•
The brace fixings are typically load-controlled expansion anchors. They can also be a cast-in ferrule or cast-in bolt but these are not usual.
•
Most post-fixed brace inserts require a minimum slab thickness of 125 mm. For thin slabs such as topping slabs to hollowcore panels and Ultrafloor panels, specialist advice from the fixing supplier will be required.
•
The concrete anchor should be at least 3 days old and have attained a strength of at least 20 MPa (or the nominated concrete strength by the manufacturer or supplier) before drilled brace fixings can be made.
•
There should be a minimum distance of 300 mm between anchors and edges of concrete as required by AS 3850.
•
The WLL for load-controlled anchors is limited to 0.65 of the first slip load in accordance with AS 3850.
•
Brace anchors have to be set to a nominated torque by the manufacturer or supplier with a torque wrench.
•
Brace anchors should be retightened 24 hours after initial fixing.
•
The base of precast panels must be restrained to prevent kick-out under wind load or construction loads. Friction cannot be relied upon. At least two restraints should be provided, one at each end of the base of the panel. These can be dowel bars between the footing and a grout tube in the precast, or correctly-bolted brackets or similar. Grout tubes or dowel bars in precast panels should have horizontally-restraining reinforcement either side of the grout tube or bar to avoid break-out under lateral loads. The restraints should be in place before the crane is unhooked.
11.4.10 Bracing – Wall Panels
The following are the basic principles involved in bracing of flat precast wall panels:
•
The same wind speeds as for the project design, factored down for working loads should be used.
•
Bracing design should be carried out by an experienced design erection engineer familiar with precast concrete.
•
Braces generally are at 45–60° to the horizontal and nominally perpendicular to the face of the panel in plan, but the angles can be different if properly considered and designed by the design erection engineer.
•
Braces generally connect at about 2/3 height of panel. Note that it is possible to have the braces lower but it needs detailed design by the design erection engineer.
•
A minimum of 2 braces per panel are required. For columns it is usual to have 2 braces at right angles to two adjacent faces.
•
Only proprietary braces with known working load limits should be used. With more than 2 braces it is difficult to apportion the load although long panels may need 3 or more braces.
•
The top of the braces should connect to an M20 ferrule with an anchor bar or enlarged base cast into the back of the panel with an M20 bolt.
•
Braces connect to a concrete anchor at their base. This is typically a footing or slab. The anchor must be designed for the brace loads (not guessed). They should not bear on fill unless designed to do so by the erection design engineer.
Figure 11.19
Foot of Brace with a Load-controlled Fixing Anchor
Figure 11.20
Typical Arrangement of Wall Panel Braces
Design Manual: Precast and Prestressed Concrete, 4th Edition, Canadian Prestressed Concrete Institute (CPCI), Ottawa, 2007.
Erection SafetyPrecast/Prestressed Manual for Precast and Prestressed Concrete Concrete Institute (PCI), Chicago, PCI MNL-132-95.
PCI Design Handbook, 6th Edition, Precast/Prestressed Concrete Institute (PCI), Chicago, 2004.
AS 3850 Tilt-up concrete construction Standards Australia, 2003. AS/NZS 1170 Structural design actions [set] Standards Australia, 2002.
AS 2550 Cranes, hoists and winches – Safe Use [set] Standards Australia, 2002. Figure 11.21
Typical Bracing of Precast Industrial Wall Panels
A Guide to Restraining Concrete Panels, VicRoads Publication No. 00091, 1999. Tilt-up and Precast Concrete Construction, Code of Practice: WorkSafe, Western Australia Department of Consumer and Employment Protection, 2004. Industry Standard for Precast and Tilt-up Concrete for Buildings, Worksafe, Victoria, 2001.
Tilt-up and Precast Construction Code of Practice, Queensland Government, Department of Industrial Relations, 2003.
What you will find in this Chapter* •
An approach to fair risk allocation in contracts involving precast concrete.
•
Recommended resolution of typical issues in precast concrete contracts.
•
Explanation of the normal choices for the allocation of design responsibility.
•
Explanation of the way many specification issues are dealt with in contracts.
* DISCLAIMER : The information contained in this Chapter is for guidance only and is not a substitute for specific legal advice.
12.1 Introduction 12.1.1 General 12.1.2 Precast concrete classification 12.1.3 Manufacturer’s credentials 12.1.4 Contracts 12.2 Contractual Framework and Contract Administration 12.2.1 General 12.2.2 Risk allocation 12.2.3 Tendering 12.2.4 Entering into a contract 12.2.5 The contract agreement 12.2.6 Payment for off-site work 12.2.7 Retention 12.2.8 Liquidated damages 12.2.9 Deeds of release 12.3 Responsibility for Engineering Design 12.3.1 General 12.3.2 Design practices 12.3.3 Recommendations 12.4 Samples and Prototypes 12.4.1 Samples 12.4.2 Prototypes 12.5 Schedules 12.6 Manufacture 12.6.1 Documentation and information 12.6.2 Shop drawings 12.6.3 Testing and inspection 12.6.4 Finishes 12.6.5 Reinforcing steel 12.6.6 Prestressing strand 12.6.7 Fittings 12.6.8 Concrete 12.6.9 Curing 12.6.10 Secondary processes 12.6.11 Storage 12.6.12 Marking of units 12.7 Delivery and Erection 12.7.1 Manner of delivery 12.7.2 Site access 12.7.3 Sequencing of erection 12.7.4 Tolerances 12.7.5 Continuity of work on site 12.7.6 Fittings cast into the structure 12.7.7 Site set-out 12.7.8 Temporary bracing 12.7.9 Site services 12.7.10 Correction of errors 12.7.11 Repairs 12.7.12 Site security 12.7.13 Acceptance 12.7.14 Occupational health and safety provisions 12.7.15 Industrial relations 12.8 References
12.1.1
•
Architectural Precast Concrete usually refers to elements such as building cladding and other components whose appearance is important. Architectural precast may also be structural as in the case of loadbearing walls and facades or beams and columns used in architectural or visually-sensitive locations.
•
Hollowcore is a distinct product type (produced with automated equipment) which is typically used for floors or walls. Hollowcore can be classed as
General
Many specifications and contracts covering precast concrete in Australia are adaptations or direct copies of documents written for insitu construction and are often inappropriate. As a consequence, project contract documentation is often either extensively altered or left in an inappropriate form. In the former case, the result is seldom perfect as resistance to change or lack of understanding brings compromises which lead to hybrid documents. In the latter, the specification may be impossible to comply with and hence the parties head into a contractual wilderness where proper guidance and issue resolution cannot be delivered by the contract documents. The goal of this Chapter is to promote better understanding and better relationships between parties by highlighting some of the issues that are important for the proper delivery of precast concrete into projects. The purpose is to explain standard practice in the Australian precast industr y so that precast
architectural or structural or both but differs in important respects from conventional structural and architectural precast concrete by virtue of its design, concrete technology and manufacturing processes. •
Prestressed Concrete Both structural and architectural precast concrete may be reinforced or prestressed. Hollowcore, Ultrafloor and bridge components are generally prestressed.
12.1.3
12.1.2
12.1.4
It is common for well-equipped manufacturers to have invested many millions of dollars in precasting facilities to ensure that the necessary quality and performance can be delivered. Contracts
The following definitions categorise precast concrete into its main groups. Inevitably there is overlap
Most precast supply is carried out under the terms of supply contracts or subcontracts. The Australian
between the categories and the specifier must interpret accordingly.
construction contracting environment is often adversarial. Contracts are often offered which seek to impose an inappropriate risk allocation regime. Any party to a contract should be asked to accept only those risks which it can control and which it can price. Even-handed forms of contract such as those published by Standards Australia are recommended.
Structural Precast Concrete usually refers to bridge and other civil engineering components, building frames, flooring and other products such as piling and grandstand seating.
Manufacturer’s credentials
The history isofreplete the purchasing of precast concrete in Australia with examples of contracts being let solely on price with often unsatisfactory consequences. The first but vital step to ensure quality is to use only manufacturers who have a proven track record, who have the experience and personnel, and who have invested in the installation and maintenance of suitable facilities and a quality system appropriate for the work involved.
•
Design, manufacture, transportation and erection of precast concrete should be carried out by specialist companies. It is desirable for such a company to have a third-party-accredited quality assurance system complying with AS/NZS ISO 9000 12.1.
concrete can be used confidently and efficiently. Recommendations made in this Chapter cover those matters which should be considered but not the form in which they should be expressed in contract documents. Wherever the words owner, designer, contractor or builder are used they mean the owner of the project or the people to whom the owner has let the design and building roles. The word contract is used to include direct contracts with the owner as well as subcontracts and purchase orders. The precaster, irrespective of whether a contract is for supply only or supply-and-install is referred to as the supplier or subcontractor. Precast concrete classification
Contract documents are dealt with more fully in Clause 12.2.5.
12.2.3
12.2.1
General
Numerous contractual arrangements are in use in Australia, ranging from formal governmentsponsored agreements to verbal offers sealed with
a handshake. Businesses and individuals are free to use any contractual arrangement (within the law) which suits them. The construction industry, however, despite many successful and ethical relationships, has a history of adversarial behaviour, coercion and business failures. The purpose of this section is to identify the essential and desirable features of contracts for the supply or supply-and-installation of precast concrete. Contracts should include all terms contemplated by the par ties, be even-handed, be easy to read, contain all required information and have fair risk allocation. Very few proposed contracts in use comply with these criteria and substantial changes are therefore required on a job-by-job basis. Many standard products such as street furniture and drainage products are sold as stock items and do not
conforming offer which points out the conflicts and of proposing alternatives. Very often, the architectural and engineering information provided to tenderers is incomplete and inappropriate. In other cases the contractor may be preparing a design-and-construct proposal and seeks advice from the manufacturer. Proposed conditions of contract relating to terms of payment, liquidated damages and other commercially-based clauses are likely to be unacceptable to the manufacturer. The manufacturer should condition his tender so that it is clear just what risk he is accepting or rejecting. A period of validity for the offer should be stated and whether the offer provides for the acceptance of penalties and liquidated damages. 12.2.4
come within the scope of this section. 12.2.2
Risk allocation
The manufacture and erection of precast concrete usually involves considerable risk. Unfair terms of contract will add another risk layer. For a contractual arrangement to work, it must provide fair risk allocation, ie the parties to a contract must be required to accept only risk which is within their control and which has been priced. Unfair contracts, where all the risk is forced onto the manufacturer, lead to losses, poor-quality structures, business failures and dissatisfied customers and suppliers. For example, time-of-the-essence and fit-for-purpose clauses are not practical, are seldom understood by the parties and, because of the inherent risk, are seldom able to be priced and usually represent very unfair risk allocation. These types of clauses should not therefore be used. If you are asking the manufacturer to carry risk, ask yourself the question – would you accept such risk if it was applied to your business or profession? If the answer is ‘no’, then do not specify it for others. Risk is often able to be priced but seldom is. Competitive pressures and other factors encourage many firms to shut their eyes to the possible consequences. This contributes to the high rate of business failure in the construction industry.
Tendering
Manufacturers are typically asked to price projects or respond to advertisements calling for quotations or tenders. Specifications and proposed contractual arrangements are seldom totally appropriate and it is very seldom that a conforming tender is able to be submitted. For example, a specification written for architectural precast concrete will be proffered for a hollowcore project or be applied to civil construction. Manufacturers then have the task of making a non-
Entering into a contract
Bid shopping occurs frequently in Australia as contractors seeks to improve their commercial position through a process of re-tendering. Manufacturers must ensure that they do not take on too onerous a risk and customers should understand that as margin is eroded the first sacrifice is usually quality. Manufacturers should appreciate that any counter offer made by a potential contractor gives them an oppor tunity to withdraw or improve their price or conditions as well as the opportunity to degrade their price and conditions. Following submission of an offer the contractor will seek to place an order and the parties must negotiate on any outstanding points of difference. Should the manufacturer decide to start work during this period he must appreciate the considerable risk involved. A good practice is to starting work until the contract is finalised or, defer at very least, until an unambiguous letter of intent is received which authorises expenditure to a defined limit. Should negotiations not be finalised before the first progress payment is due, the manufacturer can come under pressure to yield in the negotiations in order to obtain payment. (Thus offers should be conditioned to ensure payment of progress claims pending finalisation of contract negotiations.)
12.2.5 The contract agreement
12.2.6 Payment for off-site work
Irrespective of what form of contract is used it should cover all of the following:
Manufacturers often incur a very high percentage of their contract costs before making a deliver y to site. Costs can include samples, design, shop drawings, mould manufacture, stockpiling of special aggregates, purchase of fittings and completed precast units. This can typically mean that a large percentage of the contract value is expended before delivery is made. Manufacturers should not be expected to fund this work for more than a normal commercial 30 days.
•
A statement of who the contractor and manufacturer are, including the ABN for both parties. (Businesses often ask for an offer in one name and then issue a contract in another.)
•
A full description of the project, its location, the names of the owner, developer, architect or designer, consulting engineer and other basic information.
•
An appropriate specification, drawings and a scope of work as part of the contract documents setting out the manufacturer’s design, manufacture and erection responsibilities along with ancillary matters such as the supply of cast-in fixings, site fitments and cast-in services. Inclusions and exclusions should be sufficiently detailed to avoid all possibility of misunderstanding.
•
•
The price should be clearly stated, either including or excluding GST, as well as provisions for cost adjustment for inflation and any agreed basis for variations.
It is normal in Australia, for progress claims to be submitted for all off-site work and completed components, and for contracts to provide accordingly. A matter, however, which often concerns customers is obtaining security for goods which they have not taken possession of. The most common concerns expressed are fear of damage to the product or the commercial failure of the manufacturer with possession of the goods then being taken by a receiver or liquidator so that the contractor may not have custody and control of the goods.
•
The manufacturer should carry a contractor’s all-risk insurance policy which covers all aspects of the work both in the factory and during delivery including loss of or damage to the moulds or finished products. The policy should note the contractor as Principal.
•
The contractor should check the financial viability of the manufacturer.
•
The manufacturer should provide, in an agreed form, a statement that the goods will, upon payment being made, be marked as the property of the contractor and stored on his behalf.
The offer, amended if appropriate, from the manufacturer should be incorporated into and attached to the contract. Alternatively, all the
The terms of payment should be set out, including dates for progress claims, dates for payment, penalties for late payment and the process for making claims for work done off-site. Issues such as retention, taxes and other such matters should be quite clear. It should be noted that paid-if-paid and paid-when-paid clauses are illegal in most States.
•
The insurances required and responsibility for their provision.
•
The schedule for the work, provisions for extension of time, responsibility for costs of delay and any conditions precedent for claiming time and costs.
The steps which can be taken to manage this risk are:
terms and conditions of the offer should be incorporated into the contract. •
Occasionally, customers request bank guarantees for the full value of payment for work off site. The provision of such guarantees is, in the aggregate, very onerous for manufacturers and would diminish their financial resources and ability to fund capital expenditure and working capital needs. They are inequitable and are not usually provided.
• •
The procedure for treatment of variations. The procedure for settlement of disputes.
•
Rights of both parties in the event of default.
It is wise to remember that contracts and contract documents are legal documents which are subject to detailed scrutiny by the legal fraternity in the event of a significant problem. They must be clear and unambiguous.
12.2.7 Retention Retentions are intended to provide the paying party to a contract some recourse in the event that all work or rectification is not carried out on projects that include installation on site. They are used for that purpose but also, too frequently, as an unauthorised source of funding for the party holding the retention. Retentions held as cash are very much at risk in the event of the holding party going into receivership or liquidation and good practice dictates that such
security be held in the form of unconditional bank guarantees. Because of the continual difficulties experienced with bank guarantees not being returned they should be issued with an expiry date. Retention arose from the need to ensure on-site performance and is not appropriate for supply-only contracts such as reinforcing steel, hardware and precast concrete.
12.2.8
Liquidated damages
Many contracts seek to impose a liability for preascertained liquidated damages in the event of delays. Customers usually argue that they will incur substantial costs in the event of delays to the project. Liquidated damages are, however, related only to the schedule agreed between the contractor and the manufacturer and will be payable even if the contractor does not incur cost. Debate about liquidated damages range from the real cost of delays to a project to consideration of the situation, for example, where a small firm supplying the last fire door holds up completion. It is usually agreed that such a firm cannot be expected to pay, say, $25,000 per day. It remains only, therefore, to determine what is a fair rate of liquidated damages for any contracts intermediate between a head contract and that of the smallest subcontractor. Parties carrying out such negotiations should have regard to: how the contractor dealt with time during the letting of the contract and how achievable the proposed schedule is; whether the manufacturer knew all the details of lead time, float, the nature of acceptable events for extensions of time, level of liquidated damages sought and the other relevant proposed conditions of contract when the offer was made; and the general reasonableness of the proposals. A very common statement from customers is that liquidated damages are never applied but that they are useful as a tool for extracting better performance out of the manufacturer. Such paternalistic and condescending attitudes should have no part in the supply of sophisticated products such as precast concrete in the legalistic and adversarial world in which we live.
In a properly-priced contract with a normal schedule, liquidated damages of around 0.8% of the manufacturer’s contract value per week up to a maximum of 10% is a figure that has gained widespread acceptance. Where a crash schedule is required through no fault of the manufacturer, it is normal for the contractor to agree to the schedule on the basis that liquidated damages do not commence until the lateness exceeds a period equivalent to the compression of the schedule, or not at all. 12.2.9
Deeds of release
Deeds of release which ask for confirmation that wages and creditors are paid as they fall due, are appropriate. Such deeds will also usually set out the final contract sum and list any amounts still to be paid, as well as confirming the amounts of any retention or security and manufacturer’s entitlement to it. Deeds which seek to bar the manufacturer from further entitlement under the contract in respect of any matter whatsoever, irrespective of when that entitlement may arise, and irrespective of whether the manufacturer may be aware of the entitlement, should not be accepted as they may void the manufacturer’s relevant insurance policies. They are also inequitable.
12.3.1
General
responsibility sometimes being taken by a party who is not qualified and who does not understand the possible consequences. The same process also often does not provide a contractual relationship between those with design responsibilities. The result is a lack of clear delineation of responsibilities and the possibility of future problems.
The design and construction of precast concrete structures is an important, often complex, process which must be managed so as to produce a safe and
The authorities having jurisdiction over construction approval usually ask no more than that a chartered engineer (CPEng) sign off on the work. Chartered
high-quality result. The employment of professional design engineers must not be treated as just another cost element to be minimised without regard to the level of risk and the possible consequences. Critical design functions must not be let out to individuals or firms who are not competent or who do not have the contractual power to ensure implementation of their design.
engineers are authorised to practice only in the area of their expertise so this offers inadequate assurance of design quality if there has been little coordination between the various designers who are each responsible for part of the structure.
The term, head consultant is used to mean the architect and/or engineer, usually employed by the owner or his representative, who has responsibility for the design and certification of the whole structure or building. 12.3.2
Typical arrangements Precast concrete’s contribution to structures ranges from cladding for simple warehouse-type structures to sophisticated bridge and loadbearing high-rise building components. The following are examples of typical arrangements: •
A substantially-complete design, carried out by the head consultant, is presented to the precaster who has no input other than gaining approval for minor modifications for lifting or other purposes which are unrelated to the service performance of the final structure.
•
Designs, carried out by the head consultant and the design team, are prepared for all the in-ser vice loads with the manufacturer required to design the elements for handling and erection loads, propose modifications to the srcinal design and submit them to the head consultant for approval.
•
The body calling tenders provides a performance brief and the manufacturer is asked to provide a design. The manufacturer may then be asked to submit that design to the head consultant for checking and approval, or the manufacturer may be asked to certify the design.
•
Products used in a structure are standard elements such as hollowcore walling and flooring, where the manufacturer has developed standard sections and applications akin to standard beam
Design practices
Fragmentation of responsibility Traditional design methods have given way to a multiplicity of practices which, as far as the design is concerned, often blur the relationships between owner, consultants, contractors, subcontractors, suppliers and others such as project managers and construction managers. Listing here all the permutations and combinations of design responsibilities which occur would be too lengthy.The position can be appreciated, however, by considering that on any major project the design work may be shared between design consultants appointed by the owner and the owner’s representative, a design-and-construction firm which may have won the project with a modified design, the specialist subcontractors and suppliers who may be contracted to design all or part of their own work, while consulting engineers may be employed by any of the parties. Design concepts or proposals promoted by unsuccessful tenderers at contractor or supplier level may also find their way into the project. The fragmentation of the design process is a result of the increasing specialisation of subcontractors and suppliers. Head consultants no longer have understanding of every aspect of the design. This fragmentation is abetted, however, by the treatment of design as just a commodity to be tendered or auctioned with the sole aim of reducing cost. Once costs are driven down, design quality deteriorates as participants naturally look for ways to manage their risk and costs. This process results in the design
and column sections produced by the steel industry. •
The manufacturer offers an alternative which may involve the conversion of an insitu structure to precast or the substitution of one precast system for another. In these circumstances such offers are usually accepted only if the precaster provides the design. The precaster may carry out the design or employ a consultant.
More than one precast concrete manufacturer may be involved on any one project being carried out under any of the above arrangements.
The design role of precast concrete manufacturers The devolution of design responsibility has had a considerable impact on the attitude of manufacturers to their role in the design process. Most manufacturers employ professional engineers to supervise the preparation of shop drawings and to supervise production. Seldom, however, have the precasters set out to become consulting engineers and nor can they fully understand the complexity
of many projects in total. Manufacturers have responded in a variety of ways to pressure to have more involvement in the design of structures. Some manufacturers employ consulting engineers on a project-by-project basis, some have in-house design staff, some refuse to carry out any design other than that required to manufacture and handle their products, while still others are prepared to offer engineering input readily on either an informal or formal basis. Any engineering design responsibility which is contracted to the precaster should be clearly determined in the contract and such responsibilities be certified by the supplier's engineer as a supplement to the project design certification.
Recommendations to the owner Owners must understand that on any project there is always risk that they must share. While some owners intend to retain ownership of their projects, others are developing projects for subsequent and sometimes immediate sale . The most common arrangements for the owner to enter into regarding design are: Arrangement 1 The owner retains either an architect or consulting engineering firm to carry out the design, in conjunction with other professional consultants, and prepare contract documents sufficient for construction without further design by the builder, subcontractors or suppliers. Arrangement 2 The owner retains either an architect or consulting engineering firm to carry out the design, in conjunction with other professional consultants, and prepare contract documents with further design required by the builder, subcontractors or suppliers. Arrangement 3 The owner contracts with a builder to carr y out design and construction.
Risks obviously escalate when inexperienced and poorly-briefed manufacturers are prevailed upon to
While there may seem to be other alternatives they are generally only versions of the above. For instance the owner may dispense with a builder and hire a construction manager who employs the consultants
carry out, components and perhaps and evensystems. certify the design of structural
and lets the head contracts to the specialist contractors.
12.3.3
Recommendations
General These recommendations have only one purpose, viz to ensure that all aspects of the design process are carried out and certified by professional engineers who are competent, experienced and who have the authority to exercise design control. It should be little comfort to a building owner and the community at large to know that sufficient professional indemnity insurance policies are in place to pay for the consequences of mistakes. The intention in making these recommendations is to support the many owners and other participants in the Australian construction industry who have a very clear view of the risks and responsibilities in design and who act appropriately and with integrity. The unacceptable risks which are being taken arise from a lack of understanding of those risks and their possible consequences combined with the, often extreme, commercial pressures arising from the competitive nature of the industry. If those ethical firms who understand the risks, however, are at all guilty of contributing to the chances of design failure it is that they too often refuse to speak out. To recast Edmund Burke – The only thing necessary for a design failure to occur is for good engineers to do nothing.
The best control over structural integrity will be achieved when a head consultant is appointed and given responsibility and authority for the whole structural design, even if this involves the head consultant appointing a sub-consultant to carry out such design on his behalf. For precast concrete this includes responsibility for the design of the precast elements and their connections, for review and approval of the shop drawings and for inspection during manufacture and during construction on site. If Arrangement 2 is used, then the head consultant will provide all the design criteria including loadings and service conditions and must check and certify any design carried out by the manufacturer. If Arrangement 3 is used, then the owner is advised to employ a head consultant to ensure that the design intent is realised in all respects by the designers employed by the contractor. Nothing in these recommendations should be read as preventing the head consultant from recognising and utilising the contribution to design that an experienced precaster is able to provide, so long as the head consultant provides cer tification.
Recommendations to the head consultant The role of the head consultant varies according to which of the contractual arrangements listed above applies to the project. The head consultant often has to resolve the fact that it has little money to spend and a huge incentive to take shortcuts and devolve risk to other parties. Nevertheless, the overriding objective must be to ensure structural integrity. The head consultant must ensure that procedures are in place to prevent any aspect of design, design verification, manufacture or construction inspections being overlooked. Where, for commercial or contractual reasons, the head consultant is unable to perform that complete role he must ensure that the other parties involved are aware of the position in regard to inspections and certification. Recommendations to the builder When the owner carries out the design of a structure then the builder’s responsibility is to construct it in accordance with the contract documents.
Recommendations to the manufacturer Manufacturers must ensure that the design responsibilities for precast concrete that they supply are clearly delineated. Where manufacturers make products to drawings and designs supplied by others, then their liability will be limited to matters of good practice in manufacture and compliance with the contract documents. Where manufacturers do any design work, however, they must employ an experienced professional engineer to carry out such designs in accordance with good practice. Such an engineer may be a member of the manufacturer’s staff. The manufacturer should also understand who has responsibility for any necessary certification and ensure that he discharges any responsibilities that he has in that process. In instances where the manufacturer requests changes to fixing, reinforcing or other details, the manufacturer must ensure that approval of the certifier is gained for the changes.
When the builder has the design responsibility, however, the professional nature of this function must be recognised and the responsibility discharged in such a manner that integrity and/or quality is not compromised. The builder must ensure that there is at all times an engineer or architect or other appropriate person with the total responsibility for the design of the project who will act as the head consultant and will act in a professional and independent manner. In those cases where the manufacturer has partial or full design responsibility for the precast elements and their connections, the builder must ensure that the necessar y processes of transmittal of information, approval, verification and certification are followed. The builder must ensure that all necessary information for design is passed to the manufacturer in sufficient time, and that approvals are given where required by the contract.
12.4.2 Prototypes
12.4.1
Samples
Few matters cause as much trouble in precast contracts as misunderstanding of surface finish requirements.
Structural members Samples are seldom required for structural precast concrete. In some instances, samples of the off-form finish may be required and, while small samples can provide an indication of finish, other variables such as colour variation can be more realistically evaluated by reference to existing structures of similar configuration. Commonly-used structural members produced using form vibration should not be specified to have colour control. Architectural precast concrete Where an architectural finish is required, samples are usually necessar y. Ideally, samples should be evaluated and accepted before a contract for manufacture is let but for a variety of reasons this seldom happens. It is necessary, however, for tenderers to have a basis for their pricing of the project. Where a sample is held by the contractor he must ensure that the mix design is made available to all tenderers or that they are given the opportunity to examine the sample and to allow for matching it. In Australia, architectural precast is often transported over long distances. It is often not economical, however, to do things such as transporting a Queensland aggregate to an Adelaide precaster for delivery to a Sydney project. Thus it is always wise to ask tenderers to price using their local aggregates and cements that provide the best match for the sample. It should be understood that a small sample, or even a number of them, cannot properly represent the finish to be expected from full-sized panels, especially where complex shapes are involved. This is particularly true of factors such as colour variation, degree of segregation or depth of sandblast. These are best assessed by reference to existing buildings with similar finishes. Where there is a need to demonstrate the consequences of design features it is often useful to manufacture full-scale sections of panels from specially-made moulds or from existing moulds for similar panels.
A prototype is the planned design, manufacture, assembly and testing of a precast concrete unit or assembly of units. This must be finalised prior to commencement of shop drawings for the project and preferably before the precast concrete is tendered. Prototypes can be expensive and must be specified only where there is a demonstrated need and where there is enough time to evaluate the testing and incorporate any consequent modifications into the design. They are generally used only for larger projects. Prototypes may be used to test the structural adequacy of connections, the shear capacity of a non-standard section, the waterproofness of a facade, the architectural impact of a window and precast assembly, the fixing of reinforcement, or many other factors. It is wasteful to build prototypes to test in areas where there has been adequate previous testing. For instance, most facade configurations have been tested for waterproofness and it should not be necessary to repeat those tests.
Figure 12.1 illustrates the typical activities and potential delays which can be expected on complex projects.
No work should start before a contract or letter of intent authorising expenditure to defined limits has been signed or an order issued and its terms agreed. Within the contract or order should be a construction schedule which has been agreed by all parties. This schedule should be achievable and it should list clearly the responsibilities of the parties with regard to time. In particular, both parties should have an obligation to keep each other informed on matters of time and schedule. If the builder falls behind schedule he must be required to inform the manufacturer so that excessive stockpiling within the precasting factor y does not become a major problem.
It is recommended that a bar chart, customised for the complexity and features of the project be submitted with tenders so that the lead time required can be clearly demonstrated.
Schedules should be compiled so that the time for samples, shop drawings, approvals, mould manufacture, processing after casting and other activities are clearly understood by the contractor. Times must be allocated for all critical activities such as the approval of shop drawings. The critical path and the float should be identified on the schedule. Figure 12.1
Typical Schedule – Complex Architectural Project
stamp. Each party must retain one copy of approved drawings for future verification. This procedure must take precedence at all times, unless otherwise agreed in writing.
12.6.1
Documentation and information
The contract should clearly define the scope of work and deal with the technical and commercial issues set out in this Chapter as well as any additional issues specific to the particular project.
Prior to commencing any design and shop drawings, the manufacturer should have all architectural and engineering drawings and specifications along with any other information such as loadings, site measurements provided by the contractor and shop drawings of other trades, which are necessary for completion of the precast shop drawings. It is the responsibility of the contractor to provide the manufacturer with up-to-date construction drawings and other relevant information. 12.6.2
Shop drawings
The responsibilities for all aspects of producing and approving shop drawings should be set out in the contract. It is normal for the manufacturer to prepare shop drawings in contracts for supply of architectural and structural precast concrete in the building construction industry. It is not normal practice for shop drawings to be prepared in contracts that involve supply of precast concrete in the civil engineering construction industry. In this situation, it is normal practice for the precast elements to be manufactured directly from the 'approved for construction' contract drawings. It is also normal that the par ty responsible for preparation of the approved-for-construction contract drawings is also responsible for correctness of all information, including dimensions, shown thereon. Shop drawings assist in the design and construction of moulds, ordering of reinforcement, fittings and other materials, manufacture of the precast components and, where applicable, outline the erection procedure. The manufacturer should take responsibility for correctly transferring all information provided by the contractor to the shop drawings. The contractor should take responsibility for providing correct and relevant information, resolving ambiguities and for inspection and reviewing the drawings to ensure that they do not conflict with any aesthetic or physical attribute of the structure, particularly any of which the manufacturer is unaware. The structural design engineer and the project architect must inspect and review the shop drawings and indicate amendments or approval for manufacture to commence in the form of a signature and a ‘resubmit’ stamp or ‘approval for manufacture’
Adequate time must also be allowed for review and checking of shop drawings prior to manufacture. In the event that the customer supplies shop drawings, responsibility for their correctness must be stated in the contract. 12.6.3
Testing and inspection
Manufacturers generally carry out tests as required by the contract and their quality assurance system. The specification should clearly specify all testing which is required under the contract. In some cases, the contractor may wish to carry out regular or intermittent audits of manufacture. This is welcomed by most manufacturers and is normally done at the expense of the contractor. It is recommended that the contractor assess the risk inherent in using a manufacturer who does not have third-party quality-assurance cer tification and arrange for suitable inspection of manufacture to minimise risk. The contractor and the appropriate designer should inspect the first precast units manufactured for any project at the earliest possible time prior to and immediately after stripping. This is particularly important if full-scale samples or prototypes have not been produced by the manufacturer. In instances where there are units with different finishes or there are units with significantly-different shapes, then the first of each type should be inspected. 12.6.4
Finishes
It is very important to ensure that misunderstandings do not occur regarding the surface finish of precast units. To ensure this, the requirements of the contractor should be clear ly defined in the specification. The offer made by the manufacturer should state whether these requirements can be met, or if they cannot be met then an alternative should be proposed. A regime of samples and inspections should be set up to allow verification that what is being offered is acceptable. It is particularly important that customers understand the limitations of the precasting process. For instance, manufacturers are limited in the finishes possible for hollowcore units due to the characteristics of the hollowcore production process. It is also important that finishes are not overspecified. For instance, specification of a Class 1 finish under AS 361012.2 is inappropriate except for individual features of monumental buildings that are produced from a single concrete pour without further treatment. The appropriate class applicable to the highest quality facade is Class 2. Customers should not confuse this with a second-class finish in the colloquial sense.
12.6.5
Reinforcing steel
Reinforcement should comply with Australian Standards. It is usually left untreated, ie not galvanised or otherwise treated.
12.6.8
Concrete
For some projects, especially architectural facades, hot-dip galvanising may be specified. Galvanising is wasteful except in very special circumstances. It may give some additional years of life if poor quality concrete is used or where cover is difficult to achieve in thin elements. There is no evidence that it will
Some manufacturers have their own batch plants. Some purchase concrete from premix suppliers while others use both methods. Many concrete specifications apply to insitu concrete and may not be appropriate for concrete used in precasting. For example, the concrete used in some hollowcore manufacture has zero slump and concrete mix designs for architectural concrete are often nonstandard because of requirements to incorporate
significantly increase the life when used in goodquality concrete with adequate cover.
coloured in gradings designed to produce particular aggregates visual effects.
12.6.6
Prestressing strand
Prestressing strand should be specified to comply with AS/NZS 467212.3. Australian-made and most imported prestressing strand have chemical and mechanical properties which are equal to or better than that required by the Australian Standard. 12.6.7
Fittings
Within the tender price, the manufacturer usually allows for all fittings actually cast into the precast units and which are needed for handling and for fixing to the structure. Any other arrangement for the supply of loose or other fittings should be stated in the contract documents or specification. Where the manufacturer is also carrying out the erection, the manufacturer usually supplies the fixings to be cast into any insitu structure by the builder, and the loose fittings used by the erector to connect the units to the structure . Where the contract is for supply only, the supply of the loose and insitu fittings needs to be made clear in the offer and in the contract. The party responsible for the supply could be the manufacturer, the builder or the erector. Both parties to the contract must ensure that the responsibility for supply of other than fixing fittings is clearly understood. Such items could include electrical conduits, BMU (building maintenance unit) attachments, window fixing points and the like.
High early strength for transferring of the prestressing force in pretensioned elements almost invariably requires the use of superplasticised concretes which have mix designs developed to allow placement of concrete into heavily-congested, minimum-thickness cross sections. Manufacturers should qualify their offers where the tender specification is inappropriate. 12.6.9
Curing
A wide variety of effective curing regimes is used in Australia. Manufacturers will base their offers on their own procedures and in some instances the approval of the designer may be necessary if the proposed method differs from the specification. 12.6.10 Secondary processes The manufacture of many precast units is complete once they are stripped from the mould; they require no secondary treatment other than cleaning before delivery. There are many situations, however, where secondary processes are carried out to complete the manufacturing process. These include the assembly of two or more units into a single unit, sandblasting, polishing, acid etching/washing, painting and the like. Customers should be aware of, and understand the time requirements of, secondary processes.
Fittings should be made of appropriate materials. Galvanised mild steel is the generally accepted and specified material but stainless steel or other materials may be warranted in corrosive environments or where required by the Authorities.
12.6.11 Storage
It is normal to store and stockpile a predetermined number of precast concrete units at the factory for a limited period prior to delivery. The period of storage and size of the stockpile will depend on the nature of the project and the size of the elements. It is not uncommon for builders who have fallen behind schedule to insist that the manufacturer adhere to the srcinal manufacturing schedule in the hope of making up lost time. Holding precast units beyond
Delivery is made either to the contractor or to his erection subcontractor or, when the manufacturer is also responsible for erection, to the erector employed by the manufacturer. Erection is most
the scheduled deliver y date may, however, to congestion in the storage area and require lead additional resources to transport and double-handle units. This may cause considerable disruption to other projects and is therefore often costly.
efficiently carried outcircumstances directly from the delivery trucks unless special apply. For bridgeworks, the development of an on-site stockpile is encouraged to expedite erection and to avoid factory congestion
Manufacturers generally will seek to recover these costs and this is often done on the basis of a cost per tonne per day stored, in addition to any extra handling costs.
12.7.1
Excessive storage costs are best avoided by the contractor keeping the manufacturer well informed of his progress and needs, so that alternatives to long-term storage can be explored. In some cases, the manufacturer may have space available, in others the manufacturer may be able to adjust his rate of manufacture to suit site progress, while in others it may be appropriate for the contractor to take delivery at a storage area remote from the building site. 12.6.12 Marking of units The manufacturer should mark and record all units so that they can be clearly identified for type, date manufactured and weight. The type and date can also provide traceability with the position of each unit in the structure uniquely identified by reference to the shop drawings. This procedure may often be inappropriate in work such as the production of identical pavers.
Manner of delivery
The manufacturer should deliver the precast concrete to the site at the times and in the sequence agreed with the contractor. The time allowed by the manufacturer for unloading on site should be stated in the contract together with a rate per hour for waiting time. Prompt unloading is especially important when the trucks are required back at the factory to reload for the same or another project. The contractor should always give, preferably in writing, the notice for delivery specified in the contract. This will generally be between 48 and 72 hours. Timely unloading and release of delivery vehicles within an agreed period will ensure that demurrage costs are minimised. 12.7.2
Site access
One of the most common sources of delay and additional cost in the delivery and erection process is inadequate site access. The contractor must ensure that there is unobstructed, firm and adequately level access for the delivery trucks and mobile cranes and sufficient working space adjacent to the work area. This may require the paving of the access road and site surrounds, the removal or covering of overhead wires, the provision of traffic control, the removal of other construction materials or debris impeding progress, and whatever other measures are appropriate. Generally, all of these activities are to be performed by the contractor and this must be stated in the contract. Where the manufacturer is erecting, the contract will normally specify that the contractor remove any roofing structure such as purlins or other obstacle to erection. Removal of any structural members should be carried out only with the approval of the engineer. Responsibilities such as these should be stated in the contract.
12.7.3
Sequencing of erection
The sequence of erection will normally be specified in the contract or agreed between the manufacturer and contractor before manufacture commences. While changes in sequence are often possible, requested changes may not always be able to be complied with. This may be because units cannot be produced in time to meet the new sequence, or because units are stored in a prearranged sequence and cannot be accessed. In some cases, a new sequence can be accommodated by the provision of additional moulds or the working of overtime. This would normally be done at the cost of the party making the change necessary. 12.7.4
Tolerances
The manufacturer and the customer will normally work to the tolerances given in Chapter 4 of this Handbook unless there are special requirements. Any special requirements for tighter-than-normal tolerances should be covered in the contract. Customers should be aware that unnecessarily-tight tolerances will increase the cost and, where they are very difficult to achieve, may lead to conflict and dispute. Where the geometry of the structure dictates that normal tolerances cannot be applied, then resolution must be reached between the manufacturer and the contractor and this is normally shown on the shop drawings. Such resolution often involves changing precast concrete or construction details so that adequate tolerance is provided. Before deliveries commence, the contractor must ensure that his structure, including the location of any fixings, etc, is verified as being within tolerance and that any necessar y rectification has been carried out. 12.7.5
Continuity of work on site
12.7.6 Fittings cast into the structure The contractor will normally be responsible for the casting into the structure of ferrules, plates, anchor bolts, holes and other fittings required for the erection of the units. The contractor should ensure that the location of these is verified before the erector commences work. In some cases, the erector may agree to do this checking and any such responsibility should be stated in the contract. Insitu fittings can be supplied by the manufacturer. Where such arrangements apply, this should be stated in the contract. 12.7.7
Site set-out
The contractor should supply benchmarks and grid lines to every floor or section of the structure or project.
12.7.8
Temporary Bracing
When precast units are braced, or otherwise temporarily restrained, the braces and other devices are normally provided, installed and removed by the erector unless the contract provides otherwise. The provision of appropriate bearing for the braces is usually the responsibility of the contractor. Brace hire is expensive and the responsibility for time overruns which result in additional hire cost should be stated in the contract. Braces should be removed only after certification by the project design engineer or the erection design engineer. 12.7.9
Site services
Power, water, and other site amenities are normally supplied by the contractor. 12.7.10 Correction of errors
Where the manufacturer is erecting and supplying the craneage, then it is normal to state in the contract how many site crane setups have been allowed for and a rate for additional setups which may be required by the contractor. Productivity is greatly diminished if the erector is forced to erect in small
Corrections of minor errors are considered part of the erector’s work. Modifications to fixings, grinding and cutting of units and other such procedures will be carried out as permitted by the approved work method statement. Necessary modifications falling outside this will be carried out after permission is
uneconomical quantities.
gained in accordance with any requirements of the contract.
12.7.11 Repairs Repair of minor damage caused during transport, handling and erection will sometimes be necessary. Determining precise liability for every chip or mark is often difficult and it is sound practice, on building projects particularly, for all such repairs to be carried out by the manufacturer with the liability for costs stated in the contract. On high-rise buildings, the contractor will normally provide scaffolding or other means of access for repairs free of charge; on low
rise buildings the erector normally provides a boom lift or other access. For civil engineering construction, ie bridge works, rectification of damage, other than delivery damage, is effected by the contractor. There are a number of ways of allocating these responsibilities and the agreed arrangement should be stated in the contract. 12.7.12 Site Security Site security is normally the responsibility of the contractor. 12.7.13 Acceptance In supply-only contracts, acceptance normally takes place on delivery. In supply-and-erect contracts, the contractor should be prepared to accept the erected precast concrete in stages where appropriate and the procedure for achieving this should be stated in the contract. 12.7.14 Occupational health and safety provisions Manufacturers will have legal obligations to discharge in regard to OH&S as they apply to their manufacturing operations. It is not usual for details of factory procedures to be required in contracts. Where the manufacturer is responsible for installing its products, the manufacturer shall provide work method statements and proof of compliance with safety standards and any OH&S conditions of contract.
12.7.15 Industrial relations It is illegal in Australia for contract documents to make union membership a condition of contract. Contracts may call for any appropriate industrial award or enterprise agreement to be complied with.
12.1 AS/NZS ISO 9001Quality management systems – requirements, Standards Australia, 2000. 12.2 AS 3610 Formwork for concrete, Standards Australia, 1995. 12.3 AS/NZS 4672 [set] Steel prestressing materials, Standards Australia, 2007.
What you will find in this Appendix •
Technical data on materials used in association with precast concrete members.
•
Bending moment and shear diagrams for common loading arrangements.
•
Properties of reinforcing bar and prestressing strand along with application tables.
•
Section properties of common geometric shapes and metric conversion factors.
A.1 Design Information
A.1.1 Permanent actions of floors, ceilings, roofs and walls A.1.2 Imposed actions on floors and roofs A.1.3 Beam design equations and diagrams A.1.4 Camber and end-rotation coefficients for prestress force and load A.1.5 Moments in beams with fixed ends A.1.6 Moving load placement for maximum moment and shear A.2 Material Properties
A.2.1 Values of concrete stresses A.2.2 Concrete modulus of elasticity as a function of density and strength A.2.3 Coefficients of thermal expansion A.2.4 Properties of prestressing strand, and round and deformed prestressing bars A.2.5 Reinforcing bar and mesh data A.2.6 Development and lap-splice lengths for grade D500N bars in tension A.2.7 Development and lap-splice lengths for grade D500N bars in compression A.2.8 Minimum beam web widths and column sizes for 2db clear distance between bars A.3 Properties of Geometric Sections A.4 Metric Units and Conversion Factors
A.4.1 SI units A.4.2 Conversion factors
A.1.1
Permanent actions of floors, ceilings, roofs and walls
Wherever possible, manufacturer’s data sheets should be consulted for information on specific materials, otherwise AS/NZS 1170.1 may be used as a guide. The unit weights for materials, given in Table A.1, are for guidance only and can vary between suppliers.
Table A.1
Permanent actions of floors, ceilings, roofs and walls
A.1.2
Imposed actions on floors and roofs
For recommended minimum floor or roof imposed actions, refer to AS/NZS 1170.1. An extract of some common imposed floor actions is given in Table A.2 and roof actions in Table A.3. Where the use of an area of floor is not provided in AS/NZS 1170.1, the specified imposed action due to use and occupancy of an area can be determined from an analysis of the actions resulting from consideration of the weight of the probable:
•
assembly of persons;
•
accumulation of equipment and furnishings;
•
stored materials.
Any such analysis should be based on acceptable engineering principles.
Table A.3
Minimum imposed actions on roofs [Extract from AS/NZS 1170.0, Section 3]
Table A.2
Minimum imposed actions on floors [Extract from AS/NZS 1170.0, Section 3]
A.1.3
Beam design equations and diagrams
This collection of bending moment and shear diagrams, for common loadings, is for the following beam types: A Simply-supported beam B Beam overhanging one support C Cantilever beam D Beam fixed one end, supported at other E Beam fixed both ends (see also A.1.5) F Beam with variable end moments
This information can be used in combination for other arrangements of loads.
D
D
D
D
D Ö Ö
D D
D
D
D
Ö
Ö
D Ö Ö
D
D
D
D
D
D D
D Ö Ö D
D
D
D
D
D
D
D
D Ö
D
Ö
D
D
D
D
D Ö
D
D D
Ö
D
D
D
D
D
D
D D D
D
D Ö D
Ö
D
D
D
D Ö Ö D
D
D
D Ö D
D
D D
D D
D
D D D D
D
D
D
f
D
f
f
f
D f D f
D
D
D D D
f
f f f
D
D
f
f
D f
f
D D D D D D
D D D D
Ö D
D
D
A.1.4
Camber and end-rotation coefficients for prestress force and load
The end eccentricities of Cases 1, 2 and 3 may be added to the remaining cases, as appropriate. The sign notation adopted for camber and rotation is: Camber: - = downwards + = upwards End rotation: - = clockwise + = counterclockwise
The following camber and end-rotation values are for the effects of prestressing. However, if the directional notation is adjusted, they may also be used for the effects of loads.
Camber: End rotation:
- = downwards + = upwards - = clockwise + = counterclockwise
A.1.5
Moments in beams with fixed ends
See also A.1.3 for other design equations for beams with fixed ends.
A.1.6
Moving load placement for maximum moment and shear
Ö
Ö
A.2.1
Values of concrete stresses
Table A.4
Concrete Stresses (MPa) as Functions of f ’
Ö
Ö
Ö
Ö
Ö
A.2.2
Concrete modulus of elasticity as a function of density and strength
Figure A.1
Concrete Modulus of Elasticity, E (MPa) as affected by Density and Strength
A.2.3
Coefficients of thermal expansion
Table A.6
Average Coefficients of Linear Thermal Expansion of Rock (Aggregate) and Concrete
Table A.5
(10/°C)
Average Coefficients of Linear Thermal Expansion of Common Building Materials (10/°C)
(1)
(2)
(2)
(2)
A.2.4
Properties of prestressing strand, and round and deformed prestressing bars
Table A.7
Properties of Common Seven-Wire Stress-Relieved (Relax 1) Ordinary Strand to AS/NZS 4672.1
Table A.8
Properties of Common High-Strength Prestressing Bars [After AS/NZS 4672.1]
A.2.5
Reinforcing bar and mesh data
Table A.12
Design Areas (mm) for Specific Numbers of Grade D500N Bars Table A.9
Nominal Values for Hot-Rolled Deformed Bars of Grade D500N
Table A.13
Design Areas (mm/m) of Grade D500N Bars at Specific Spacings
Table A.10
Nominal Values for High-Strength Deformed Bars of Grade D500L
Table A.11
Nominal Values for Hot-Rolled Plain Round Bars of Grade R250N
Table A.14
Information on Mesh Sizes Commonly Available in Australia [Based on AS/NZS 4671]
*
Rectangular
Square, with edge side-lapping bars
Square, without edge side-lapping bars
Table A.15
Overall Dimensions (mm) of 180° Hooks and 90°Cogs
180° Hooks
90° Cogs
Table A.16
Minimum Length of Bar, La (mm) to Form a Standard Hook or Cog
A.2.6
Development and lap-splice lengths for grade D500N bars in tension
The following Tables are in accordance with the stress development rules given in Section 13 of AS 3600:2009 and differ from those in AS 3600:2001. Designers using AS 3600:2009 are given the option of determining the development length, both in tension and compression, as either a basic development length or as a refined development length. In most designs, the basic development length will be used. In tension and compression, the development lengths and lapped-splice lengths are different.
The development length in tension can be reduced if hooks or cogs are used, as shown in Figure A.2. Hooks or cogs do not reduce the development length for bars in compression. Hooks and cogs can cause congestion in members such as thin sections and because of the tensile stress generated in the concrete in the plane of the hook they should not be used in sections thinner than about 12 bar diameters. Figure A.2
Effect of Hooks or Cogs on Development Length
For bars in tension, the basic development length Lsy.tb shall be multiplied by: •
1.5 for epoxy-coated bars;
•
1.3 when lightweight concrete is used; and
•
1.3 for all structural elements built with slip forms.
AS 3600 does not state that for the refined development lengths in tension, they are multiplied by the above values, but one should assume that the same multipliers would apply. Designers should remember that in most designs for bars in tension, they are not lapped at the point of maximum tension and good design practice will minimise bars being developed or lapped in highstress areas. An example is top bars in a cantilever beam or slab which are usually spliced at about the quarter points in the back span depending on the length of the cantilever span and back span. AS 3600 allows pro-rata-reduced development lengths (and lap-splices) where the stress in the bar is less than the yield stress both in tension and compression. For tension there is a minimum development length of 12db or for slabs as permitted by Clause 9.1.3.1(a)(ii).
For development and lap-splice lengths, this Handbook, in the following pages, provides a range of values of lengths for: •
Basic development length in tension, Table A.18, where for horizontal bars there is less than or equal to 300 mm of concrete cast below the bar and where k1 = 1.0 and 0.7 ! k3 ! 1.0
•
Basic development length in tension, Table A.19, where for horizontal bars there is more than 300 mm of concrete cast below the bar and where k1 = 1.3 and 0.7 ! k3 ! 1.0
•
Splice length of bars in tension, Table A.20, where for horizontal bars there is less than or equal to 300 mm of concrete cast below the bar and where k1 = 1.0 and 0.7 ! k3 ! 1.0 and k7 = 1.25
•
Splice length of bars in tension, Table A.21, where for horizontal bars there is more than 300 mm of concrete cast below the bar and where k1 = 1.3 and 0.7 ! k3 ! 1.0 and k7 = 1.25
In wide elements or members (such as flanges, band beams, slabs, walls and blade columns) where the bars being lapped are in the same plane of the element or member, the tensile lap length, Lsy.t.lap, for either contact or non-contact splices is the basic development length, Lsy.t, multiplied by a factor, k7, which is generally taken as 1.25. The factor k 7 =1.25 has been used in the following Tables. Depending on whether the As provided is greater than twice the As required and not more than one-half of the
Table A.17
Factor, k, Accounting for Staggering of Bars in the Splice Region [After AS 3600, Clause 13.2.2]
reinforcement at the section is spliced, k7 may be taken as 1. Refer to Table A.17.
In narrow elements or members (such as beam webs and columns) the tensile lap length, L sy.t.lap, is dependent on whether the clear distance, s b, between bars of the lapped splice is less than or greater than 3d b (see Figure A.3). If sb does not exceed 3d b then Lsy.t.lap is equal to k7 Lsy.t as above, where k7 = 1.25 but may equal 1 if conditions in Clause 13.2.2 are met (see Table A.17). Otherwise, if s b is greater than 3d b then Lsy.t.lap is equal to the larger of k7 Lsy.t and Lsy.t + 1.5s b. The staggering of lapped splices not only affects the k7 value as shown in Table A.17 but also the value of c d which is used to determine the k 3 value in the formula for basic develop length, Lsy.tb, which in this Handbook is equal to the development length, L sy.t. For a fuller explanation of determining the cd value, see Figure A.4.
Figure A.3
The Lap-splice Length of Adjacent Bars in Tension in Webs of Beams and in Columns (Narrow Elements)
Figure A.4
Values of cd [After AS 3600 Figures 13.1.2.3(A) and 13.2.2]
The value of cd used to calculate the factor k3 and
Refined development length using the factors k 4 and
to produce the Tables A.18, A.19, A.20and A.21, is purely a dimension (in millimetres) derived from the clear spacing between adjacent parallel bars (horizontally), critical covers to the bar under consideration and the staggering or otherwise of lapped splices, see Figure A.4.
k5 in tension and k6 in compression are complicated factors to calculate for general design and tables for these have not been included in this Handbook and indeed are not likely to be used by designers.
Table A.18 Basic Development Lengths, Lsy.tb (mm) for Grade D500N Bars in Tension where there is Less than or Equal to 300 mm of Concrete Cast below the Bar k = 1.0
k2 = (132 - db )/100 0.7 • k3 • 1.0
k3 = 1.0 - 0.15(c d - db )/db
fsy = 500 MPa
Table A.19 Basic Development Lengths, Lsy.tb (mm) for Grade D500N Bars in Tension where there is More than 300 mm of Concrete Cast below the Bar k = 1.3
k2 = (132 - db )/100 0.7 • k3 • 1.0
k3 = 1.0 - 0.15(c d - db )/db
fsy = 500 MPa
Table A.20 Basic Lapped-Splice Lengths, Lsy.t.lap (mm) for Grade D500N Bars in Tension where there is Less than or Equal to 300 mm of Concrete Cast below the Bar k = 1.0
k2 = (132 - db )/100
k3 = 1.0 - 0.15(c d - db )/db
0.7 • k3 • 1.0
k7 = 1.25
fsy = 500 MPa
Table A.21 Basic Lapped-Splice Lengths, Lsy.t.lap (mm) for Grade D500N Bars in Tension where there is More than 300 mm of Concrete Cast below the Bar k = 1.3
k2 = (132 - db )/100
k3 = 1.0 - 0.15(c d - db )/db
0.7 • k3 • 1.0
k7 = 1.25
fsy = 500 MPa
A.2.7
Development and lap-splice lengths for grade D500N bars in compression
The following Tables are in accordance with the stress development rules given in Section 13 of AS 3600:2009 and differ from those in AS 3600:2001.
Designers using AS 3600:2009 are given the option of determining the development length, in both compression and tension, as either a basic development length or as a refined development length. In most designs, the basic development length will be used. In compression and tension, the development lengths and lapped-splice lengths are different. In compression, the basic development length (Table A.22) is largely independent of variables but all values are shown for different concrete grades. The minimum development length for deformed bars in compression is 200 mm. A refined development length equal to 0.75 of the basic development length can be used subject to complying with Clause 13.1.5.1 of AS 3600 but is not shown in the Table. Hooks or cogs do not reduce the development length for bars in compression.
The lap-splice length for deformed bars in compression (Table A.23) is a minimum of 300 mm and not less than 40d b in Clause 13.2.4(a) which is independent of the concrete strength. However, there are two other conditions in AS 3600:2009, Clause 13.2.4, which allows the lap splice length to be reduced to 0.8 of the 40d b value . This reduced value of 0.8 has also been included in Table A.23. AS 3600 does not give guidance on how close the bars are to be lapped and it is assumed that they are lapped or spaced less than 3d b apart as for bars in tension
Table A.22
Basic Development Lengths, L (mm) for Grade D500N Bars in Compression
Table A.23
Lap-Splice Lengths (mm) for Grade D500N Bars in Compression for bars in Contact or Spaced less than 3db apart
b(1)
(2)
A.2.8
Minimum beam web widths and colunm sizes for 2db clear distance between bars
AS1480 required a minimum clear spacing between parallel bars of the greater of 25 mm, 1d b or 1.5 times the maximum nominal aggregate size. AS 3600:2009 requires the reinforcement to be placed to allow the concrete to closely surround the reinforcement, Section 17.
Table A.18 is based on a clear distance between bars of twice the size of the main bar (2d b) ie the centre-to-centre spacing is 3db. These limitations are related to AS 3600:2009 Section 13 for tensile stress development. One of the factors is that the cover to the main bar (cmain= c fitment + d b.fitment) must not be less than one half of the clear distance between parallel bars, ie d b.main. It is assumed that the member has properly designed fitments to control longitudinal splitting. The minimum widths in Table A.24 are also practical minimum sizes which allow the concrete to be placed and properly compacted – remembering that sufficient room must be allowed for a vibrator to be properly used.
Table A.24
Beam Web Widths and Column Sizes (mm) for 2db Clear Distance Between Bars
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A.4.1
SI unit
Table A.25
Preferred SI Units
SI base units
SI supplementary units
Derived units with special names
W
W
Derived units with complex names
Table A.26
Table A.27
Preferred Multiples and Submultiples*
Multiples of SI Units in Common Use
m
Table A.28 Non-SI Units in Common Use
A.4.2
Conversion factors
Table A.30
Conversion Factors – Metric/Imperial
Plane angle
Length
Area
Volume, Capacity and Modulus of section
Conversion Factors – Metric/Imperial, continued
Second moment of area
Velocity, Speed
Acceleration
Volume rate of flow
Equivalent temperature value (°C = K - 273.15)
Temperature interval
Mass
Mass/unit length
Mass/unit area
Density (Mass/unit volume)
Mass/unit time
Moment of inertia
Conversion Factors – Metric/Imperial, continued
Force
Moment of force, Torque
Force per unit length
Pressure, Stress, Modulus of elasticity (1 Pa = 1 N/m2)
Work, Energy, Heat (1 J = 1 W . s)
Power, Heat flow rate
Intensity of heat flow (Heat loss from surface)
Thermal conductance (Heat transfer coefficient)
Thermal conductivity
Calorific value (Mass and volume basis)
Thermal capacity (Mass and volume basis)
Illumination
Luminance
