CE 223 NOTES 2010.2011.pdf

CONSTRUCTION TECHNOLOGY II, BEng II, 2010/2011

Department of Civil and Building Engineering

P. O. BOX 1, KYAMBOGO – KAMPALA, UGANDA

FACULTY OF ENGINEERING

BARCHELOR OF ENGINEERING IN CIVIL AND BUILDING ENGINEERING

CE223: CONSTRUCTION TECHNOLOGY II YEAR TWO-SEMESTER II

2010/2011

Tel: 0772-459779/ 0712-459779; E-mail [email protected]

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Course Outline

1. History of Construction, forms of Construction: Buildings, Heavy/Civil Construction and industrial construction works 2. The process of construction: Building team; Inception, Design, submission of plans for approval, tendering, construction, supervision and inspection. The Construction Site. Drawings and Documentation. Site Layout. Sequence of Work. Site Clearance. 3. Site Visits and industrial tours. 4. Setting out: Buildings, Drainage and Roads Using Tapes Only. 5. Foundations: Site Selection and investigation; Preliminaries: Hoarding, Services e.g water, electricity, toilet, kitchen, staff accommodation, storage, security etc. Types of foundations: Strip, Pad, Pile and Raft foundation 6. Exclusion and Removal of Water. Importance and methods of dewatering 7. Temporary Works: Formwork, scaffolding and timbering 8. Floors. Walls and Piers: Ground and suspended timber and concrete floors, types of wall: Load bearing walls, framed walls, retaining walls etc 9. Multi-storey Structures: Foundations, Steel frames, Concrete Frames, Floors, Claddings 10. Roof Structures and Roof Coverings. Pitched and Flat roofs: Structural materials: reinforced concrete, steel, and timber. Roof coverings: Tiles, Concrete, sheet metals, asphalt 11. Doors, Windows and Other Openings: Materials used: Metals, Timber, Plastics. Frames, Linings and Shutters 12. Services: Electricity, water, sewage, telephone and gas transmission 13. Internal Finishes and External Finishes. 14. Stairs, Ramps and Ladders. Choice of the method of ascending and descending: Types of stairs, materials used for the construction; Reinforced concrete, Timber and Steel (Details of Sketches to demonstrate understanding shall be emphasised in this Course.)

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Course Outline

1. History of Construction, forms of Construction: Buildings, Heavy/Civil Construction and industrial construction works 2. The process of construction: Building team; Inception, Design, submission of plans for approval, tendering, construction, supervision and inspection. The Construction Site. Drawings and Documentation. Site Layout. Sequence of Work. Site Clearance. 3. Site Visits and industrial tours. 4. Setting out: Buildings, Drainage and Roads Using Tapes Only. 5. Foundations: Site Selection and investigation; Preliminaries: Hoarding, Services e.g water, electricity, toilet, kitchen, staff accommodation, storage, security etc. Types of foundations: Strip, Pad, Pile and Raft foundation 6. Exclusion and Removal of Water. Importance and methods of dewatering 7. Temporary Works: Formwork, scaffolding and timbering 8. Floors. Walls and Piers: Ground and suspended timber and concrete floors, types of wall: Load bearing walls, framed walls, retaining walls etc 9. Multi-storey Structures: Foundations, Steel frames, Concrete Frames, Floors, Claddings 10. Roof Structures and Roof Coverings. Pitched and Flat roofs: Structural materials: reinforced concrete, steel, and timber. Roof coverings: Tiles, Concrete, sheet metals, asphalt 11. Doors, Windows and Other Openings: Materials used: Metals, Timber, Plastics. Frames, Linings and Shutters 12. Services: Electricity, water, sewage, telephone and gas transmission 13. Internal Finishes and External Finishes. 14. Stairs, Ramps and Ladders. Choice of the method of ascending and descending: Types of stairs, materials used for the construction; Reinforced concrete, Timber and Steel (Details of Sketches to demonstrate understanding shall be emphasised in this Course.)

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CONTENTS

Course Outline .................................................... ........................................................................................................... .......................................................................... ................... ii 1.0 Introduction ....................................................................................................................... 2 1.1 History of Civil Engineering ........................................................ ............................................................................................. ..................................... 2 1.2 HISTORY OF CONSTRUCTION ........................................................ .................................................................................... ............................ 3 1.3 The Building/Construction Building/Construction Profession .............................................................................. 4 1.4 Construction Construction processes ............................................................................................ .......... 8 2.0 FOUNDATIONS....................................................... ............................................................................................................. ...................................................... 14 2.2 Functions Functions of Foundations: .................................................................... .......................... 14 2.3 Essential Requirements Requirements of a Good Foundation....................................................... ............................................................... ........ 15 2.4 Site investigation: investigation: .............................................................. ............................................. 18 2.5 Foundation Foundation failure ..................................................... ........................................................................................................... ...................................................... 47 3.0 SOLID CONCRETE GROUND FLOORS .................................................... ..................................................................... ................. 49 3.2 Elements of solid concrete ground floors ....................................................................... 52 4.0 WALLS ........................................................................................................................... 55 4.1 Introduction Introduction ...................................................... ............................................................................................................. ............................................................... ........ 55 4.2 Types of walls................................................... ....................................................... ............................................................... ........ 55 4.3 Functional Functional Requirements Requirements .................................................... ................................................................................................. ............................................. 56 4.4 Methods for constructing constructing walls for buildings................................................. ................. 57 5.0 ROOFS ........................................................................................................................... 65 5.2 The functional functional requirements of a roof are: ........................................... .......................... 65 5.3 Flat Roofs ........................................................ ............................................................................................................... ............................................................... ........ 67 5.4 Flat roof coverings: .................................................. ...................................................... 74 5.5 Pitched Roofs .................................................. ....................................................... ............................................................... ........ 80 5.6 Trussed rafters ................................................. ....................................................... ............................................................... ........ 81 5.7 Purlin or double roof: ............................................... ...................................................... 82 5.8 Hipped roofs ................................................................................................................... 85 5.9 Connections .................................................................................................................... 91 6 Windows and Doors .................................................. ...................................................... 94 6.2 Windows .................................................. ....................................................... ........................................................................ ................. 94 6.3 Functional Functional requirements.......................................................................................... ........ 94 6.4 Materials used for windows ............................................... ............................................. 97 6.5 Window types .................................................. ....................................................... ............................................................... ........ 99 6.6 Glass and Glazing ..................................................... ......................................................................................................... .................................................... 105 6.7 Doors....................................................... .............................................................................................................. ...................................................................... ............... 105 6.8 Functional Functional requirements of doors ...................................... ........................................... 105 7.0 Temporary Works ...................................................... .......................................................................................................... .................................................... 114 7.2 Scaffolds ....................................................................................................................... 114 7.1 Formworks ....................................................... .............................................................................................................. ............................................................. ...... 119 7.2 Factors which influences influences the pressure of concrete on the formwork: .......................... 121 7.3 Timbering Timbering to excavations: ........................................................... ................................. 124 8.0 References..................................................................................................................... 126

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1.0 Introduction 1.1 History of Civil Engineering Civil engineering is a professional engineering discipline that deals with the design, construction

and maintenance of the physical and naturally built environment, including works such as bridges, roads, canals, dams and buildings. Civil engineering is the oldest engineering discipline in the world after military engineering, and it was defined to distinguish non-military (civilian) engineering from military engineering. It is traditionally broken into several sub-disciplines including environmental engineering, geotechnical engineering, structural engineering, transportation engineering, municipal or urban engineering, water resources engineering, materials engineering, coastal engineering, surveying, and construction engineering. Civil engineers have saved more lives than all the doctors in history through development of clean and safe water and sanitation systems. Engineering has been an aspect of life since the beginnings of human existence. The earliest practices of Civil engineering may have commenced between 4000 and 2000 BC in Ancient Egypt and Mesopotamia (Ancient Iraq) when humans started to abandon a nomadic existence, thus causing a need for the construction of shelter. During this time, transportation became increasingly important leading to the development of the wheel and sailing. Until modern times there was no clear distinction between civil engineering and architecture, and the term engineer and architect were mainly geographical variations referring to the same person, often used interchangeably. The construction of Pyramids in Egypt (2700-2500 BC) might be considered the first instances of large structure constructions. In the 18th century, the term civil engineering was coined to incorporate all things civilian as opposed to from military engineering. The first self-proclaimed civil engineer was John Smeaton who constructed the Eddystone Lighthouse. In 1771 Smeaton and some of his colleagues formed the Smeatonian Society of Civil Engineers, a group of leaders of the profession who met informally over dinner. Though there was evidence of some technical meetings, it was little more than a social society. In 1818 the Institution of Civil Engineers was founded in London, and in 1820 the eminent engineer Thomas Telford became its first president. The institution received a Royal Charter in 1828, 2


formally recognizing civil engineering as a profession. Its charter defined civil engineering as the art of directing the great sources of power in nature for the use and convenience of man, as the means of production and of traffic in states, both for external and internal trade, as applied in the construction of roads, bridges, aqueducts, canals, river navigation and docks for internal intercourse and exchange, and in the construction of ports, harbours, breakwaters and lighthouses, and in the art of navigation by artificial power for the purposes of commerce, and in the construction and application of machinery, and in the drainage of cities and towns. The first private college to teach Civil Engineering in the United States was Norwich University founded in 1819 by Captain Alden Partridge. The first degree in Civil Engineering in the United States was awarded by Rensselaer Polytechnic Institute in 1835. The first such degree to be awarded to a woman was granted by Cornell University to Nora Stanton Blatch in 1905. In a nut shell, Civil engineering is the application of physical and scientific principles, and its history is intricately linked to advances in understanding of physics and mathematics throughout history. Because civil engineering is a wide ranging profession, including several separate specialized sub-disciplines, its history is linked to knowledge of structures, materials science, geography, geology, soils, hydrology, environment, mechanics and other fields. Throughout ancient and medieval history most architectural design and construction was carried out by artisans, such as stone masons and carpenters, rising to the role of master builder. Knowledge was retained in guilds and seldom supplanted by advances. Structures, roads and infrastructure that existed were repetitive, and increases in scale were incremental. One of the earliest examples of a scientific approach to physical and mathematical problems applicable to civil engineering is the work of Archimedes in the 3rd century BC, including Archimedes Principle, which underpins our understanding of buoyancy, and practical solutions such as Archimedes' screw. 1.2 HISTORY OF CONSTRUCTION

Ever since the dawn of civilization, man has indulged in some form of construction activity. Even in ancient times, man created architectural marvels which came to be regarded as the wonders of the world, for example the Pyramids of Egypt, the Great Wall of China, the Angkor temples of 3


Cambodia and the Tower of Babel. The pyramid of Giza in Egypt contains more than 2,000,000 blocks with an average weight of about 2.3tons each. About 100,000 persons worked on the pyramids for three to four months a year to build it in about 20 years. The Great Wall of China, built to provide protection against surprised enemy raids, is about 6400km long and its height and width at the top varies from 5 to 10metres. It has several high towers placed every few hundred meters. In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is

a feat of multitasking. Normally the job is managed by the project manager and supervised by the construction manager, design engineer, construction engineer or project architect. For the successful execution of a project, effective planning and good management is essential. Those involved with the design and execution of the infrastructure in question must consider the environmental impact of the job, the scheduling, budgeting, site safety, availability of materials, logistics, inconvenience to the public caused by construction delays, preparing tender documents, etc. 1.3 The Building/Construction Profession

Technical and specialized occupations require more training as a greater technical knowledge is required. These professions also hold more legal responsibility. A short list of the main careers with an outline of the educational requirements is as given below: •

Architect - Typically holds at least a 4-year degree in architecture. To use the title "architect" the individual must hold chartered status with the Royal Institute of British Architects and be on the Architects Registration Board; in Uganda called USA.

•

Civil Engineer - Typically holds a degree in a related subject. The Chartered Engineer qualification is controlled by the Institution of Civil Engineers. A new university graduate must hold a masters degree to become chartered, persons with bachelor’s degrees may become an Incorporated Engineer after four years of engineering practice in Uganda.

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Building Services Engineer - Often referred to as an "M&E Engineer" typically holds a degree in mechanical or electrical engineering. Chartered Engineer status is governed by the Chartered Institution of Building Services Engineers. 4


•

Project Manager - Typically holds a 2-year or greater higher education qualification, but are often also qualified in another field such as quantity surveying or civil engineering.

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Quantity Surveyor - Typically holds a degree in quantity surveying. Chartered status is gained from the Royal Institute of Chartered Surveyors.

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Structural Engineer - Typically holds a bachelors or masters degree in structural engineering, new university graduates must hold a masters degree to gain chartered status from the Institution of Structural Engineers

Types of construction projects

In general, there are three types of construction: 1. Building construction 2. Heavy/civil construction/infrastructure 3. Industrial construction Each type of construction project requires a unique team to plan, design, construct, and maintain the project. 1. Building construction

Building construction is the process of adding structure to real property. The vast majority of building construction projects is small works such as residential houses, renovations, addition of a room, or renovation of a bathroom. Often, the owner of the property acts as labourer, paymaster, and design team for the entire project. However, all building construction projects include some elements in common - design, financial, and legal considerations. Many projects of varying sizes reach undesirable end results, such as structural collapse, cost overruns, and/or litigation reason; those with experience in the field make detailed plans and maintain careful oversight during the project to ensure a positive outcome. Buildings come in a wide amount of shapes and functions, and have been adapted throughout history for a wide number of factors, from building materials available, to weather conditions, to land prices, ground conditions, specific uses and aesthetic reasons. Buildings serve several needs of society – primarily as shelter from weather and as general living space, to provide privacy, to store 5


belongings and to comfortably live and work. A building as a shelter represents a physical division of the human habitat (a place of comfort and safety) and the outside (a place that at times may be harsh and harmful). Ever since the first cave paintings, buildings have also become objects or canvases of artistic expression. In recent years, interest in sustainable planning and building practices has also become part of the design process of many new buildings Definition of building

Building is defined in many aspects as: • •

The act of constructing, erecting, creating, manufacturing or establishing. A structure that has a roof and walls and stands more or less permanently in one place;

The first shelter on Earth constructed by a relatively close ancestor to humans is believed to be built 500,000 years ago by an early ancestor of humans, Homo erectus. The Egyptian pyramids were built around 4000-2700BC. Types of buildings Residential: Residential buildings are called houses/homes, though buildings containing large

numbers of separate dwelling units are often called apartment buildings / blocks to differentiate them from the more 'individual' house. Building types may range from one-room wood-framed, masonry, or adobe dwellings to multimillion dollar high-rise buildings able to house thousands of people. Increasing settlement density in buildings (and closer distances between buildings) is usually a response to high ground prices resulting from many people wanting to live close to work or similar attractors. Multi-storey

A multi-storey building is a building that has multiple floors above ground in the building.

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Multi-storey buildings aim to increase the area of the building without increasing the area of the land the building is built on, hence saving land and, in most cases, money (depending on material used and land prices in the area). Creation

The practice of designing, constructing, and operating buildings is most usually a collective effort of different groups of professionals and trades. Depending on the size, complexity, and purpose of a particular building project, the project team may include: •

A real estate developer who secures funding for the project;

•

One or more financial institutions or other investors that provide the funding

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Local planning and code authorities

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Construction managers who coordinate the effort of different groups of project participants;

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Licensed architects and engineers who provide building design and prepare construction documents;

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Landscape architects;

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Interior designers;

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Other consultants;

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Contractors who provide construction services and install building systems such as climate control, electrical, plumbing, Decoration, fire protection, security and telecommunications;

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Marketing or leasing agents;

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Facility managers who are responsible for operating the building.

Regardless of their size or intended use, all buildings must comply with local zoning ordinances, building codes and other regulations such as fire codes, life safety codes and related standards. Residential construction practices, technologies, and resources must conform to local building authority regulations and codes of practice. Materials readily available in the area generally dictate the construction materials used (e.g. brick versus stone, versus timber). Cost of construction on a per square metre (or per square foot) basis for houses can vary dramatically based on site conditions, local regulations, economies of scale (custom designed homes are always more

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expensive to build) and the availability of skilled trades people. As residential (as well as all other types of construction) can generate a lot of waste, careful planning again is needed here. Infrastructure /Civil engineering construction

These are capital intensive and heavy equipment oriented works which involves movement of large quantity of bulk materials like earth, steel and concrete.

These works include dams, canal,

highways, airports, railways, bridges, gas/oil pipe lines, transmission lines, water supply and sewage disposal networks, dock and harbours, nuclear and thermal power plants, and other specialist construction activities which build-up the infrastructure for the growth of the economy. These works are designed by specialist engineering firms and are mostly financed by government/public sector. The engineers and builders engaged in infrastructure construction are usually highly specialized since each segment of the market requires different types of skills. However, demands for different segments of infrastructure and heavy construction may shift with saturation in some segments. For example, as the available highway construction projects are declining, some heavy construction contractors quickly move their work force and equipment into the field of mining where jobs are available. Industrial construction

These works include construction of manufacturing, processing and industrial plants like steel mills, petroleum refineries and consumer goods factories. Industrial construction, though a relatively small part of the entire construction industry, is a very important component. Processes in these industries require highly specialized expertise in planning, design, and construction. As in building and heavy/highway construction, this type of construction requires a team of individuals to ensure a successful project. 1.4 Construction processes Selection of Professional Services

When an owner decides to seek professional services for the design and construction of a facility, he is confronted with a broad variety of choices. The type of services selected depends to a large degree on the type of construction and the experience of the owner in dealing with various 8


professionals in the previous projects undertaken by the firm. Generally, several common types of professional services may be engaged either separately or in some combination by the owners. Design team

In the modern industrialized world, construction usually involves the translation of paper or computer based designs into reality. A formal design team may be assembled to plan the physical proceedings, and to integrate those proceedings with the other parts. The design usually consists of drawings and specifications, usually prepared by a design team including the client architects, architects, interior designers, surveyors, civil engineers, cost engineers (or quantity surveyors), mechanical engineers, electrical engineers, structural engineers, and fire protection engineers. The design team is most commonly employed by (i.e. in contract with) the property owner. Under this system, once the design is completed by the design team, a number of construction companies or construction management companies may then be asked to make a bid for the work, either based directly on the design, or on the basis of drawings and a bill of quantities provided by a quantity surveyor. Following evaluation of bids, the owner will typically award a contract to the most cost efficient bidder. The modern trend in design is toward integration of previously separated specialties, especially among large firms. In the past, architects, interior designers, engineers, developers, construction managers, and general contractors were more likely to be entirely separate companies, even in the larger firms. Presently, a firm that is nominally ”architecture" or "construction management" firm may have experts from all related fields as employees, or to have an associated company that provides each necessary skill. Thus, each such firm may offer itself as "one-stop shopping" for a construction project, from beginning to end. This is designated as a "design Build" contract where the contractor is given a performance specification, and must undertake the project from design to construction, while adhering to the performance specifications. Several project structures can assist the owner in this integration, including design-build, partnering, and construction management. In general, each of these project structures allows the owner to integrate the services of architects, interior designers, engineers, and constructors throughout design and construction. In response, many companies are growing beyond traditional offerings of design or construction services alone, and are placing more emphasis on establishing relationships with other necessary participants through the design-build process. The increasing complexity of construction projects creates the 9


need for design professionals trained in all phases of the project's life-cycle and develop an appreciation of the building as an advanced technological system requiring close integration of many sub-systems and their individual components, including sustainability. Financial advisors

Many construction projects suffer from preventable financial problems. Underbids ask for too little money to complete the project. Cash flow problems exist when the present amount of funding cannot cover the current costs for labour and materials, and because they are a matter of having sufficient funds at a specific time, can arise even when the overall total is enough. Fraud is a problem in many fields, but is notoriously prevalent in the construction field. Financial planning for the project is intended to ensure that a solid plan, with adequate safeguards and contingency plans, is in place before the project is started, and is required to ensure that the plan is properly executed over the life of the project. Mortgage bankers, accountants, and cost engineers are likely participants in creating an overall plan for the financial management of the building construction project. The presence of the mortgage banker is highly likely even in relatively small projects, since the owner's equity in the property is the most obvious source of funding for a building project. Accountants act to study the expected monetary flow over the life of the project, and to monitor the payouts throughout the process. Cost engineers apply expertise to relate the work and materials involved to a proper valuation. Cost overruns with government projects have occurred when the contractor was able to identify change orders or changes in the project resulting in large increases in cost, which are not subject to competition by other firm as they have already been eliminated from consideration after the initial bid. Large projects can involve highly complex financial plans. As portions of a project are completed, they may be sold, supplanting one lender or owner for another, while the logistical requirements of having the right trades and materials available for each stage of the building construction project carries forward. In many English-speaking countries, but not the United States, projects typically use quantity surveyors. Legal Aspects

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A construction project must fit into the legal framework governing the property. These include governmental regulations on the use of property, and obligations that are created in the process of construction. The project must adhere to zoning and building code requirements. Constructing a project that fails to adhere to codes will not benefit the owner. Some legal requirements come from malum in se considerations, or the desire to prevent things that are indisputably bad - bridge

collapses or explosions. Other legal requirements come from malum prohibitum considerations, or things that are a matter of custom or expectation, such as isolating businesses to a business district and residences to a residential district. An attorney may seek changes or exemptions in the law governing the land where the building will be built, either by arguing that a rule is inapplicable (the bridge design won't collapse), or that the custom is no longer needed (acceptance of live-work spaces has grown in the community). A construction project is a complex net of contracts and other legal obligations, each of which must be carefully considered. A contract is the exchange of a set of obligations between two or more parties, but it is not so simple a matter as trying to get the other side to agree to as much as possible in exchange for as little as possible. The time element in construction means that a delay costs money, and in cases of bottlenecks, the delay can be extremely expensive. Thus, the contracts must be designed to ensure that each side is capable of performing the obligations set out. Contracts that set out clear expectations and clear paths to accomplishing those expectations are far more likely to result in the project flowing smoothly, whereas poorly drafted contracts lead to confusion and collapse.

Procurement Procurement describes the merging of activities undertaken by the client to obtain a building.

There are many different methods of construction procurement; however the three most common types of procurement are: 1. Traditional (Design-bid-build) 2. Design and Build 3. Management Contracting 11


There is also a growing number of new forms of procurement that involve relationship contracting where the emphasis is on a co-operative relationship between the principal and contractor and other stakeholders within a construction project. New forms include partnering such as Public-Private Partnering (PPPs) aka Private Finance Initiatives (PFIs) and alliances such as "pure" or "project" alliances and "impure" or "strategic" alliances. The focus on co-operation is to ameliorate the many problems that arise from the often highly competitive and adversarial practices within the construction industry. Traditional/Design-bid-build: This is the most common method of construction procurement and

is well established and recognized. In this arrangement, the architect or engineer acts as the project coordinator. His or her role is to design the works, prepare the specifications and produce construction drawings, administer the contract, tender the works, and manage the works from inception to completion. There are direct contractual links between the architect's client and the main contractor. Any subcontractor will have a direct contractual relationship with the main contractor. Design and build/turn-key: This approach has become more common in recent years and includes

an entire completed package, including fixtures, fittings and equipment where necessary, to produce a completed fully functional building. In some cases, the Design and Build (D & B) package can also include finding the site, arranging funding and applying for all necessary statutory consents. The owner produces a list of requirements for a project, giving an overall view of the project's goals. Several D&B contractors present different ideas about how to accomplish these goals. The owner selects the ideas he likes best and hires the appropriate contractor. Often, it is not just one contractor, but a consortium of several contractors working together. Once a contractor (or consortium/consortia) has been hired, they begin building the first phase of the project. As they build phase 1, they design phase 2. This is in contrast to a design-bid-build contract, where the project is completely designed by the owner, then bid on, then completed. Construction management

In this arrangement the client plays an active role in the procurement system by entering into separate contracts with the designer (architect or engineer), the construction manager, and 12


individual trade contractors. The client takes on the contractual role, while the construction or project manager provides the active role of managing the separate trade contracts, and ensuring that they all work smoothly and effectively together. Management procurement systems are often used to speed up the procurement processes, allow the client greater flexibility in design variation throughout the contract, the ability to appoint individual work contractors, separate contractual responsibility on each individual throughout the contract, and to provide greater client control.

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CHAPTER TWO 2

FOUNDATIONS

Every building consists of two basic components: the superstructure and the substructure/ foundation. The superstructure is usually that part of the building which is above the ground, and which serves the purpose of its intended use. The substructure or foundation is the lower portion of the building, usually located below ground level, which transmits the load of the super-structure to the sub-soil or the ground to which the loads are transmitted. A part of the super-structure, located between the ground level and the floor is known as the plinth. The foundation is therefore that part of the structure (walls, piers and columns) which is in direct contact with and transmitting loads to the ground. The foundations of structures bear on and transmit loads to the ground. The concrete base of walls, piers and columns is what is called the foundation. The soil which is located immediately below the base of the foundation is called the sub-soil or foundation soil, while the lowermost portion of the foundation which is in direct contact with the sub-soil is called the footing. The basic function of a foundation is to transmit the dead loads, super-imposed loads (live loads) and wind loads from a building to the soil on which the building rests, in such a way that: •

Settlements are within permissible (allowable)limits, without causing cracks in the superstructure

•

Soil does not fail in shear. When loads are transmitted to the sub-soil, it settles. If this settlement is slight and uniform throughout, no damage will be caused to the structure. If the settlement is excessive and unequal, serious damage may result in the form of cracked walls, distorted doors and window openings, cracked beams and lintels, walls thrown out of plumb etc and sometimes complete collapse of the structure.

The foundation is thus one of the most important parts of a building. The principal foundation types are: strip, pad, raft and pile foundations (R.Barry, 1984) as shown below in figure 1 and all can broadly be classified as shallow and deep foundations. The size and shape of a building has an effect on the type of foundation used; though this is also dependent on soil and site conditions (R. L. Fullerton, 1977) 2.2 Functions of Foundations:

Foundations serve the following purposes;

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•

Reduction of load intensity: Foundations distribute the loads of the super-structure to a larger area so that the intensity of the load at its base does not exceed the safe bearing capacity of the sub-soil. In the case of deep foundations, it transmits the superimposed loads to the sub-soil both the the skin friction as well as through the end bearing.

•

Even distribution of loads: Foundations distribute the non uniform load of the super-structure

evenly to the sub-soil. For example, two columns carrying unequal loads can have a combined footing which may transmit the load to the sub-soil evenly with uniform soil pressure. Due to this, unequal or differential settlements are minimised. •

Provision of level surface: Foundations provide levelled and hard surface over which the super-

structure can be built. •

Lateral stability. It anchors the super-structure to the ground, thus imparting lateral stability to

the super-structure. The stability of the building against sliding, overturning, due to horizontal forces (such as wind, earth quake etc) is increased due to foundations. •

Safety against undermining (deterioration). It provides the structural safety against undermining

or scouring due to burrowing animals and flood water •

Protection against soil movements: Special foundation measures prevent or minimises the

distress (or cracks) in the super-structure, due to expansion or contraction of the sub-soil because of moisture movement in some problematic soils. 2.3 Essential Requirements of a Good Foundation

Foundations should be constructed to satisfy the following requirements: a) The foundation shall be constructed to sustain the dead and imposed loads and transmit these to the subsoil in such a way that pressure on it will not cause settlement which would impair the stability of the building or adjoining structures b) Foundation base should be rigid so that differential settlement are minimised, especially for the case when super-imposed loads are not evenly distributed. c) Foundations should be taken sufficiently deep to guard the building against damage or distress caused by swelling or shrinkage of the sub-soil. d) Foundations should be so located that its performance may not be affected due to any unexpected future influence

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Despite the precautions taken, the foundation of large buildings imposes considerable loads on subsoils so that consolidation of the subsoil may be appreciable either during the erection or for some years after the completion of the building. Owing to variations in the subsoil or to different intensities of pressures in the subsoil at various points below buildings or to both, unequal settlement of the foundation could occur which may damage the structure. The intensity of pressure in the subsoil below the foundation of a large building may be considerable to some depth below the foundation so that a stratum of weak subsoil in this region may give way. If the intensity of pressure below a foundation is sufficiently great it may cause the sub-soil to collapse by shear failure, either forcing a column of subsoil down, or by the displacement of soil each side of the foundation. To anticipate the likely behaviour of subsoil under the foundation of a large building the engineer must know the nature of the sub soil for some depth below the surface and have knowledge of its behaviour under load. Soil is defined as sediments and deposits of solid particles produced by the disintegration of rocks and it is the size of the particles of a particular soil and the degree to which the particles bind together which is of interest to an engineer. Soils can be defined as; •

Non-cohesive or coarse grained soils (sand and gravel)

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Cohesive or fine grained soils (clay and silt)

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Organic soils (peat)

The characteristics of soils of interest to engineers include: 1. Compressibility 2. Cohesion of particles 3. Internal friction and 4. Permeability 1. Compressibility

Under load sand is only slightly compressed due to expulsion of water and some rearrangement of the particles. Because of its high permeability sand is rapidly compressed due to quick expulsion of water, and compression of sand subsoil keeps pace with the erection of buildings so that once the building is completed no further compression can take place. This condition makes sand to be safe against settlement. 16


Clay is very compressible, but due to its impermeability, compression takes place slowly because of the very gradual expulsion of water through the narrow capillary channels in the clay. The compression of clay subsoil under the foundations of a building may continue for some years after the building is completed, with consequential gradual settlement.

2. Cohesion of particles (plasticity):

There is negligible cohesion between the particles of sand and in consequence it is not plastic. There is marked cohesion between the particles of clay which is plastic and can be moulded, particularly when wet. 3. Internal friction:

There is considerable friction between the coarse particles of sand which strongly resists displacement of the particles. When the internal friction is overcome, the soil shears and suddenly gives way causing sudden collapse of the building. There is very little friction between the fine particles of clay due to its plastic nature and as a result, shear failure, under the loads of the building may take place along several strata simultaneously with consequent heaving of the soil.

Figure1.2 showing heaving of the soil 4. Permeability:

When water can pass rapidly through the pores of a soil the soil is said to be permeable. Coarse grained soils such as sand and gravel are permeable, and because water can drain rapidly through them, they can consolidate rapidly under load. 17


Fine grained soils such as clay have low permeability and because water passes very slowly through the pores, they consolidate slowly.

Bearing Pressure

The intensity of pressure on subsoil is not uniform across the width or length of a foundation and decreases with depth below the foundation. To determine the probable behaviour of a soil under foundations, the engineer has to know the intensity of pressure on the subsoil at various depths. If points of equal stresses are joined the result is a bulb of unit pressure extending down wards. Thus the bulb of the pressure gives an indication of stress in sub-soils at various points below a foundation. If there are separate foundations close together as for example where there is a group of columns then the bulbs of pressure can be combined to form one large pressure bulb diagram as shown in figure 1.3. Where bulbs of pressure of adjacent foundations intersect an increased intensity of pressure occurs. For an engineer to be able to select the type and determine the depth of foundation to use an in-depth study of the soil below the ground has to be carried out and this is referred to as site investigation.

Figure 1.3 2.4 Site investigation:

Before foundation design can begin there are a number of preliminary stages. These, separate stages, are generally referred to as site investigation. Site Investigation normally involves three basic stages: 18


1. The desk study:

This is the first stage in the site investigation. Essentially, it comprises the collection and analysis of existing information about the site. The desk study has two main objectives: • To determine the nature, past use, and condition of the site. • To determine whether this has any implications for the proposed building and its foundations. A sensible starting point is to consult large scale maps of the proposed site and check site boundaries, building lines, existing buildings and other man-made, or natural, features which will affect the future buildings. A comparison with older maps may give some clues to determine former use and, therefore, potential hazards. Geological maps, other written records, and local knowledge will help identify the likely nature of the subsoil and determine the extent of difficult ground conditions. Most subsoil, including firm and stiff clays, compact sands, gravels and rocks will easily support the relatively low loads of two and three storey buildings using simple strip foundations. However, soft cohesive soils, peaty soils, and of course, fill, pose problems. A site that has been mined also needs treating with caution - foundation solutions can be costly. Large scale historical maps, often held at city and county libraries, show the extent of former mining. Thousands of old shafts and tunnels still exist in many countries. Other items which should come to light during the desk study include the likelihood of: • Filled or contaminated ground • Quarrying or mining • Rights of way • Ponds, watercourses, ground water levels and the risk of flooding • Utility services (drains, electricity, gas, telephone, and optical cables etc - see left-hand plan) • Previous vegetation (ie large felled trees) • Landslip • Naturally occurring aggressive chemicals (eg sulphates), harmful gases (radon) and landfill gases e.g (Methane and CO2). 2. A walk-over survey:

It is the second stage in the site investigation. It's a detailed site inspection which: • Enables much of the material discovered in the desk study to be confirmed or further investigated • Identifies other potential hazards 19


• Enables the surveyor to collect photographic records • Gives the surveyor/engineer the opportunity to make detailed drawings of all those items (trees, existing buildings, watercourses, etc) which will have implications for the building design 3. Sub-soil survey:

A direct ground investigation is the third stage in the site investigation. As far as low rise housing is concerned its main objective is to determine whether strip foundations will be suitable and, assuming they are, whether they can be designed in accordance with the simple 'rule of thumb' approach contained in the Building Regulations. The ground investigation will provide detailed information on: • Nature and thickness of made up ground/top soil above the subsoil • Nature, thickness and stratum depth of subsoil • An assessment of allowable bearing pressure • Groundwater levels, chemicals in the ground etc. • Existing structures or hazards in the ground The natural vegetation at a site gives guide to the nature of the soil, and the conformation of the natural surface will be a guide to the nature of the subsoil. Any adjacent earth work such as quarries and railway or road cuttings will give some indication of subsoil. Geological maps of the area and information from the Local Authority Surveyors will supply further information. This preliminary inspection will be a guide to the preferred siting of buildings on open land and will provide background information in built up areas. Once the preliminary designs of the buildings are completed and the position of the buildings on the site established the Engineer will require a precise knowledge of the subsoil under the proposed buildings for some depth below the surface. Depth of Exploration

The depth to which exploration of the subsoil should be carried depends on the nature of the subsoil strata, the size of the structure and the type of foundation. Exploration in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible settlements or shear failure of foundations. Such a depth, known as significant depth, depends upon the type of structure, its weight, size, shape and disposition of the loaded areas, and the soil profile and its properties. The significant depth may be assumed to be equal to one-and-a-half or two times the width (smaller of the lateral dimension) of the loaded area. 20


The depth of exploration at the start of the work may be decided according to the following guide rules, which may need modification as exploration proceeds: 1.

Isolated spread footing or raft: One and a half times the width.

2.

Adjacent footings with clear spacing less than twice the width: One and a half times the length.

3.

Pile foundation: 10 to 30 metres, or more, or at least one a half times the width of the structure.

4.

Base of the retaining wall: One and a half times the base width or one and a half times the exposed height of face of wall, whichever is greater.

5.

Floating basement: Depth of construction.

6.

Weathering considerations: 1.5 m in general and 3.5 m in black cotton soils.

Building Codes suggests that normally the depth of exploration should be one and half times the estimated width (lower dimension) of the footing, single or combined, from the base level of he foundation; but in weak soils, the exploration should be continued to a depth at which the loads can be carried by the stratum in question without undesirable settlement or shear failure. In any case, the depth to which weathering processes affect the soil should be regarded as a minimum depth for the exploration of sites and this should be taken as 1.5 metres. But where industrial processes affect the soil characteristics, this depth may be more. Sub-soil exploration is done for the following purposes;

(a)

For New Structures

1. The selection of type and depth of foundation. 2. The determination of bearing capacity of the selected foundation. 3. The predication of settlement of the selected foundation. 4. The determination of the ground water level. 5. The evaluation of the earth pressure against walls, basements, abutments etc. 6. The provision against constructional difficulties. 7. The suitability of soil and degree of compaction of soil. (b)

For Existing Structures

1. The investigation of the safety of the structure. 2. The prediction of settlement. 21


3.

The determination of remedial measures if the structure is unsafe or will suffer detrimental settlement.

Methods of Site Exploration

The various methods of site exploration may be grouped as follow: 1. Open excavations 2. Borings 3. Sub-surface soundings 4. Geo-physical methods.

1.

Open Excavation (Open Trial Pits)

Trial pits are the cheapest method of exploration in shallow deposits, since these can be used in all types of soils. In this method, pits are excavated at the site, exposing the sub-soil surface thoroughly. Soil samples are collected at various levels. The biggest advantage of this method is that soil strata can be inspected in their natural condition and samples (distributed or undistributed) can be conveniently taken. A typical trial pit is shown below.

Sketch of a typical trial pit

The method is generally considered suitable for shallow depths, say up to 3 m. The cost of open excavation increases rapidly with depth. For greater depths and for excavation below ground water table, especially in pervious soils, measures for lateral support and ground water lowering becomes necessary. 2.

Boring Methods

The following are the various boring methods commonly used; (i)

Auger boring 22


(i)

(ii)

Auger and shell boring

(iii)

Wash boring

(iv)

Percussion boring

(v)

Rotary boring

Auger boring

Augers are used in cohesive and other soft soils above water table. They may either be operated manually or mechanically. Hand augers are used up to a depth of up to 6 m. Mechanically operated augers are used for greater depths and they can also be used in gravelly soils. Augers are of two types; (a) spiral auger and (b) post-hole auger

Sketch of a typical spiral auger and post –hole auger

Samples recovered from the soil brought up by the augers are badly distributed and are useful for identification purposes only. Auger boring is fairly satisfactory for explorations at shallow depths and for exploratory borrow pits. (ii)

Auger and shell boring

23


Cylindrical augers and shells with cutting edge or teeth at lower end that can be used for making deep borings. Hand operated rigs are sued for depths up to 25 m and mechanized rigs up to 50 m. Augers are suitable for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for sandy soils. Small boulders, thin soft strata or rock or cemented gravel can be broken by chisel bits attached to drill rods. The hole usually requires a casing.

Sketch of typical sand pump (iii)

Wash boring

Wash boring is a fast and simple method for advancing holes in all types of soils. Boulders and rock cannot be penetrated by this method. The method consists of first driving a casing through which a hollow drilled rod with a sharp chisel or chopping bit at the lower end is inserted. Water is forced under pressure through the drill rod which is alternatively raised and dropped, and also rotated. The resulting chopping and jetting action of the bit and water disintegrates the soil. The cuttings are forced up to the ground surface in the form of soil-water slurry through the annular space between the drill rod and the casing. The change in soil stratification could be guessed from the rate of progress and colour of wash water. The samples recovered from the wash water are almost valueless for interpreting the correct geotechnical properties of soil.

24


Sketch of wash boring (iv)

Percussion boring

In this method, soil and rock formations are broken by repeated blows of heavy chisel or bit suspended by a cable or drill rod. Water is added to the hole during boring, if not already present and the slurry of pulverised material is bailed out at intervals. The method is suitable for advancing a hole in all types of soils, boulders and rock. The formations, however, get disturbed by the impact. (v)

Rotary boring

Rotary boring or rotary drilling is a very fast method of advancing hole in both rocks and soils. A drill bit, fixed to the lower end of the drill rods, is rotated by a suitable chuck, and is always kept in firm contact with the bottom of the hole. A drilling mud, usually a water solution of bentonite, with or without other admixtures is continuously forced down to the hollow drill rods. The mud returning upwards brings the cuttings to the surface. The method is also known as mud rotary drilling and the hole usually requires no casing. Rotary core barrels, provided with commercial diamond-studied bits or a steel bit with shots, are also used for rotary drilling and simultaneously obtaining the rock cores or samples. The method is then known as core boring or core drilling. Water circulated down the drill rods during boring. Record of borings

In all exploration work it is very important to maintain an accurate and explicit record of borings. Soil/rock samples are collected at various depths, during boring. These samples are tested in the laboratory for identification and classification. The samples are suitably preserved and arranged serially according to the depth at which they are found. A boring chat is prepared for each bore hold. A site plan should be prepared; showing the disposition of various bore holes on it.

25


Boring Record

Number and disposition of trial pits and borings

The number and disposition of the test pits and borings should be such as to reveal any major changes in the thickness, depth or properties of the strata affected by the works, and the immediate surroundings. The following are recommended number of pit: (a) For a compact building site covering an area of about 0.4 hectares, one bore hole or trial pit in each corner and one in he centre should be adequate. (b) For small and less important buildings, even one bore hole or trial pit in the centre will suffice. (c) For very large areas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Dynamic or static cone penetration tests may be performed at every 100 metres by dividing the area into grid patterns and number of bore holes or trial pits decided by examining the variation in the penetration curves. 3. Sub-Surface Soundings

The sounding methods consist of measuring the resistance of the soil with depth by means of penetrometer under static or dynamic loading. The penetrometer may consist of a sampling spoon, a cone or other shaped tool. The resistance to penetration is empirically correlated with some of the engineering properties of soil, such as density index, consistency bearing capacity etc. The values of these tests lie in the amount of experience behind them. These tests are useful for general exploration of erratic soil profiles, for dinging depth to bed rock or stratum, and to have an 26


approximate induction of the strength and other properties of soils, particularly for cohesionless soils, from which it is difficult to obtain undisturbed samples. The two commonly used tests are standard penetration test and the cone penetration test. 4.

Geo-Physical Methods

Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. Geo-physical investigations involve the detection of significant differences in the physical properties of geological formations. These methods were developed in connection with prospecting of useful minerals and oils. The major method of geophysical investigations are; gravitational methods, magnetic methods, seismic refraction method and electrical resistivity methods are the most commonly used for Civil Engineering purposes. Seismic refraction method

In this method, shock waves are created into the soil at their ground level or a certain depth below it by exploding small charge in the soil or by striking a plate on the soil with a hammer. The radiating shock waves are picked up by the vibration detector (also called geophone or seismometer) where the time of travel of the shock waves gets recorded. A number of geophones are arranged along a line. Some of the waves, known as direct or primary waves travel directly from the shock point along the ground surface and are picked first by the geophone. The other waves which travel through the soil get refracted at the interface of two soil strata. The refracted rays are also picked up by the geophone. If the underlying layer is denser, the refracted waves travel much faster. As the distance between the shock point and the geophone increases, the refracted waves are able to reach the geophone earlier then the direct waves. By knowing the time of travel primary and refracted waves at various geophones, the depth of various strata can be evaluated by preparing distance-time graphs and using analytical methods. Seismic refraction method is fast and reliable in establishing profiles and different strata provided the deeper layer have increasingly greater density and thus higher velocities and also increasingly greater thickness.

Different kinds of materials such as gravel, clay, hardpan, or rock have

characteristics seismic velocities and hence they may be identified by the distance-time graphs. The exact type of material cannot, however be recognized and the exploration should be supplemented by boring or soundings and sampling.

27


Sketch Electrical Resistivity Method

The electrical resistivity method is based on the measurement and recording of changes in the main resistivity of various soils. Each soil has its own resistivity depending upon its water content, compaction and composition; for example, it is low for saturated silt and high for loose dry gravel or solid rock. The test is conducted by driving four meal spikes to serve as electrodes into the ground along a straight line at equal distances. A direct voltage is imposed between the two outer electrodes, and the potential drop is measured between the inner electrodes. The mean resistivity

 (ohm-cm)

is

computed from the expression  =

2*3.14*D*E/I, D= distance between the electrodes (cm)

Sketches

The depth of exploration is roughly proportional to the electrode spacing. For studying vertical changes in the strata, the electrode system is expanded, about a fixed central point, by increasing the 28


spacing gradually from an initial small value to a distance roughly equal to the depth of exploration required. The method is known as resistivity sounding. To correctly interpret he resistivity date for knowing the nature and distribution of soil formation, it is necessary to make preliminary trial or calibration tests on known formations. Choice of Exploration Method

The choice of a particular exploration method depends on the following factors (a) nature of ground (b) topography (c) cost

1. Nature of ground

In clayey soils, borings are suitable for deep exploration and pits for shallow exploration. In sandy soils, boring is easy but special equipments should be used for taking representative samples below the water table. Such samples can however, be readily taken in trial pits provided that, where necessary, some form of ground water lowering is used. Borings are suitable in hard rocks while pits are preferred in soft rocks. Core borings are suitable for the identification of types of rock but they cannot supply data on joints and fissures which can only be examined in pits and large diameter borings. When the depth of exploration is much, and where the area of construction site is large, geophysical methods (specially the electrical resistivity method) can be used with advantage. However, borings at one or two locations should be carried out, for calibration purposes.

In soft soil, sounding

method may also be used to cover large area in relatively shorter duration. 2.

Topography

In hilly country sides, the choice between vertical openings (for example, boring sand trial pits) and horizontal openings (for example, headings) may depend on the geological structure, since steeply inclined by strata are most effectively explored by headings and horizontal strata best explored by borings which may have to be put down from a floating craft. 3.

Cost

For deep exploration, borings are usual, as deep shafts are costly. However, if the area is vast, geophysical methods or sounding methods may be used in conjunction with borings. For shallow exploration in soil, the choice between pi and borings will depend on the nature of the ground and the information required for shallow exploration in rock; the cost of boring a core drill to the site will only be justified if several holes are required; otherwise trial pits will be more economical. 29


Soil Samples and Samplers

Soil samples can be of two types; (i)

Distributed samples

(ii)

Undistributed samples

A distributed sample is that in which the natural structure of soil gets partly or full modified and destroyed although with suitable precautions the natural water content may be preserved. Such a soil sample should, however, be representative of the natural soil by maintaining the original proportion of the various particles intact. An undistributed sample is that in which the natural structure and properties remain preserved. The sample disturbance depends upon the design of the samplers and the method of sampling. To take undisturbed samples from bore holes properly designed sampling tools are required. The sampling tube when forced into the ground should cause as little remoulding and disturbance as possible. The design features of the sampler that govern the degree of disturbance are (i) cutting edge (ii) inside wall friction and (iii) no-return value. The area ratio should be as low as possible. It should not be greater than 25 percent; for soft sensitive soil, it should preferably not exceed 10 percent. The inside clearance should lie between 1 to 3 percent and the outside clearance should not be much greater than the inside clearance. The walls of the sampler should be smooth and should be kept properly oiled so that wall friction is minimised. Lower value of inside clearance allows the elastic expansion of soil provided in samplers, should permit easy and quick escape of water and air when driving the sampler.

30


Sketch of cutting edge

31


Types of foundation 1.

Shallow foundations

2.

Deep foundations

Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip

footings and rafts. Deep foundations include piles, pile walls, diaphragm walls (Diaphragm walling refers to the in-situ construction of vertical walls by means of deep trench excavations. Stability of the excavation is maintained by the use of a drilling fluid, usually a bentonite suspension) and caissons. 1. Shallow foundations •

Pad foundations

•

Strip foundations

•

Raft foundations

Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (D f ) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorlycompacted fill, peat, and alluvial deposits, etc.

32


Pad foundations

Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or rectangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used. Strip foundations

Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would

be

inappropriate.

The

absolute

minimum

thickness of this strip is 150mm.

Raft foundations

Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact. A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.

33


Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area. 2. Deep foundations •

Piles

Deep foundations are those founding too deeply below the finished ground surface for their base

bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to deeper, more competent strata at depth if unsuitable soils are present near the surface. Piles are relatively long, slender members that transmit foundation loads through soil strata of low

bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces. Piers are foundations for carrying a heavy structural load which is constructed in-situ in a deep

excavation. Caissons are a form of deep foundation which are constructed above ground level, then sunk to the

required level by excavating or dredging material from within the caisson. Compensated/floating foundations are deep foundations in which the relief of stress due to

excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement. Piles •

Types of pile

•

Types of construction 34


•

Factors influencing choice

•

Pile groups

Piled foundations can be classified according to the type of pile (different structures to be supported, and different ground conditions, require different types of resistance) and the type of construction (different materials, structures and processes can be used). Types of pile •

End bearing piles

•

Friction piles

•

Settlement reducing piles

•

Tension piles

•

Laterally loaded piles

•

Piles in fill

Piles are often used because adequate bearing capacity can not be found at shallow enough depths to support the structural loads. It is important to understand that piles get support from both end bearing and skin friction. The proportion of carrying capacity generated by either end bearing or skin friction depends on the soil conditions. Piles can be used to support various different types of structural loads. End bearing piles

35


End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their carrying capacity from the resistance of the stratum at the toe of the pile. Friction piles

Friction piles obtain a greater part of their carrying capacity by skin friction or adhesion. This tends to occur when piles do not reach an impenetrable stratum but are driven for some distance into a penetrable soil. Their carrying capacity is derived partly from end bearing and partly from skin friction between the embedded surface of the soil and the surrounding soil. Settlement reducing piles

36


Settlement reducing piles are usually incorporated beneath the central part of a raft foundation in order to reduce differential settlement to an acceptable level. Such piles act to reinforce the soil beneath the raft and help to prevent dishing of the raft in the centre. Tension piles

Structures such as tall chimneys, transmission towers and jetties can be subject to large overturning moments and so piles are often used to resist the resulting uplift forces at the foundations. In such cases the resulting forces are transmitted to the soil along the embedded length of the pile. The resisting force can be increased in the case of bored piles by under-reaming. In the design of tension piles the effect of radial contraction of the pile must be taken into account as this can cause about a 10% - 20% reduction in shaft resistance. Laterally loaded piles

Almost all piled foundations are subjected to at least some degree of horizontal loading. The magnitude of the loads in relation to the applied vertical axial loading will generally be small and no additional design calculations will normally be necessary. However, in the case of wharves and jetties carrying the impact forces of berthing ships, piled foundations to bridge piers, trestles to overhead cranes, tall chimneys and retaining walls, the horizontal component is relatively large and may prove critical in design. Traditionally piles have been installed at an angle to the vertical in such cases, providing sufficient horizontal resistance by virtue of the component of axial capacity of the pile which acts horizontally. However the capacity of a vertical pile to resist loads applied normally to the axis, although significantly smaller than the axial capacity of that pile, may be sufficient to avoid the need for such 'raking' or 'battered' piles which are more expensive to install. When designing piles to take lateral forces it is therefore important to take this into account.

37


Piles in fill

Piles that pass through layers of moderately- to poorly-compacted fill will be affected by negative skin friction, which produces a downward drag along the pile shaft and therefore an additional load

on the pile. This occurs as the fill consolidates under its own weight. Types of pile construction •

Displacement piles

•

Non-displacement piles

Displacement piles cause the soil to be displaced radially as well as vertically as the pile shaft is driven or jacked into the ground. With non-displacement piles (or replacement piles), soil is removed and the resulting hole filled with concrete or a precast concrete pile is dropped into the hole and grouted in. Displacement piles •

Totally preformed displacement piles

•

Driven and cast-in-place displacement piles

•

Helical (screw) cast-in-place displacement piles

Methods of installation

38


Sands and granular soils tend to be compacted by the displacement process, whereas clays will tend to heave. Displacement piles themselves can be classified into different types, depending on how they are constructed and how they are inserted. Totally preformed displacement piles

These can either be of precast concrete; full length reinforced (prestressed) · jointed (reinforced) · hollow (tubular) section or they can be of steel of various section. Driven and cast-in-place displacement piles

This type of pile can be of two forms. The first involves driving a temporary steel tube with a closed end into the ground to form a void in the soil which is then filled with concrete as the tube is withdrawn. The second type is the same except the steel tube is left in place to form a permanent casing. Helical (screw) cast-in-place displacement piles

This type of construction is performed using a special type of auger. The soil is however compacted, not removed as the auger is screwed into the ground. The auger is carried on a hollow stem which can be filled with concrete, so when the required depth has been reached concrete can be pumped down the stem and the auger slowly unscrewed leaving the pile cast in place.

Methods of installation •

Dropping weight

•

Diesel hammer

•

Vibratory methods of pile driving

•

Jacking methods of insertion

Displacements piles are either driven or jacked into the gound. A number of different methods can be used.

39


Dropping weight

The dropping weight or drop hammer is the most commonly used method of insertion of displacement piles. A weight approximately half that of the pile is raised a suitable distance in a guide and released to strike the pile head. When driving a hollow pile tube the weight usually acts on a plug at the bottom of the pile thus reducing any excess stresses along the length of the tube during insertion. Variants of the simple drop hammer are the single acting and double acting hammers. These are mechanically driven by steam, by compressed air or hydraulically. In the single acting hammer the weight is raised by compressed air (or other means) which is then released and the weight allowed to drop. This can happen up to 60 times a minute. The double acting hammer is the same except compressed air is also used on the down stroke of the hammer. This type of hammer is not always suitable for driving concrete piles however. Although the concrete can take the compressive stresses exerted by the hammer the shock wave set up by each blow of the hammer can set up high tensile stresses in the concrete when returning. This can cause the concrete to fail. This is why concrete piles are often pre-stressed. Diesel hammer

Rapid controlled explosions can be produced by the diesel hammer. The explosions raise a ram which is used to drive the pile into the ground. Although the ram is smaller than the weight used in the drop hammer, the increased frequency of the blows can make up for this inefficiency. This type of hammer is most suitable for driving piles through non-cohesive granular soils where the majority of the resistance is from end bearing. Vibratory methods of pile driving

Vibratory methods can prove to be very effective in driving piles through non cohesive granular soils. The vibration of the pile excites the soil grains adjacent to the pile making the soil almost free flowing thus significantly reducing friction along the pile shaft. The vibration can be produced by electrically (or hydraulically) powered contra-rotating eccentric masses attached to the pile head usually acting at a frequency of about 20-40 Hz. If this frequency is increased to around 100 Hz it can set up a longitudinal resonance in the pile and penetration rates can approach up to 20 m/min in 40


moderately dense granular soils. However the large energy resulting from the vibrations can damage equipment, noise and vibration propagation can also result in the settlement of nearby buildings. Jacking methods of insertion

Jacked piles are most commonly used in underpinning existing structures. By excavating underneath a structure short lengths of pile can be inserted and jacked into the ground using the underside of the existing structure as a reaction.

Non-displacement piles •

Small diameter bored cast-in-place piles

•

Large diameter bored cast-in-place piles

•

Partially preformed piles

•

Grout or concrete intruded piles

With non-displacement piles soil is removed and the resulting hole filled with concrete or sometimes a precast concrete pile is dropped into the hole and grouted in. Clays are especially suitable for this type of pile formation as in clays the bore hole walls only require support close to the ground surface. When boring through more unstable ground, such as gravels, some form of casing or support, such as a bentonite slurry, may be required. Alternatively, grout or concrete can 41


be intruded from an auger rotated into a granular soil. There are then essentially four types of non displacement piles. This method of construction produces an irregular interface between the pile shaft and surrounding soil which affords good skin frictional resistance under subsequent loading. Small diameter bored cast-in-place piles

These tend to be 600mm or less in diameter and are usually constructed by using a tripod rig. The equipment consists of a tripod, a winch and a cable operating a variety of tools. In granular soils, the basic tool consists of a heavy cylindrical shell with a cutting edge and a flap valve at the bottom. Water is necessary to assist in this type of excavation. By working the shell up and down at the bottom of the bore hole liquefaction of the soil takes place (as low pressure is produced under the shell as the liquified soil is rapidly moved up) and it flows into the shell and can be winched to the surface and tipped out. There is a danger when boring through granular soil of over loosening the material at the sides of the bore. To prevent this, temporary casing should be advanced by driving it into the ground.

42


In cohesive soils, the borehole is advanced by repeatedly dropping a cruciform-section tool with a cylindrical cutting edge into the soil and then winching it to the surface with its burden of soil. Once at the surface the clay which adheres to the cruciform blades is paired away. Large diameter bored cast-in-place piles:

A spiral or bucket auger as shown in this diagram is attached to a shaft known as a Kelly bar (a square section telescopic member driven by a horizontal spinner). Depths of up to 70m are possible using this technique. The use of a bentonite slurry in conjunction with bucket auger drilling can eliminate some of the difficulties involved in drilling in soft silts and clays, and loose granular soils, without continuous support by casing tubes. One advantage of this technique is the potential for under reaming. By using an expanding drilling tool the diameter at the base of the pile can be enlarged, significantly increasing the end bearing capacity of the pile. However, under-reaming is a slow process requiring a stop in the augering for a change of tool and a slow process in the actual under-reaming operation. In clay, it is often preferable to use a deeper straight sided shaft.

Large boreholes from 750mm up to 3m diameter (with 7m under-reams) are possible by using rotary drilling machinery. The augering plant is usually crane or lorry mounted.

43


Partially pre-formed piles

This type of pile is particularly suitable in conditions where the ground is waterlogged, or where there is movement of water in an upper layer of the soil which could result in cement being leached from a cast-in-place concrete pile. A hole is bored in the normal way and annular sections are then lowered into the bore hole to produce a hollow column. Reinforcement can then be placed and grout forced down to the base of the pile, displacing water and filling both the gap outside and the core inside the column. Grout- or concrete-intruded piles

The use of continuous flight augers is becoming a much more popular method in pile construction. These piles offer considerable environmental advantages during construction. Their noise and vibration levels are low and there is no need for temporary borehole wall casing or bentonite slurry making it suitable for both clays and granular soils. The only problem is that they are limited in depth to the maximum length of the auger (about 25m). The piles are constructed by screwing the continuous flight auger into the ground to the required depth leaving the soil in the auger. Grout (or concrete) can then be forced down the hollow shaft of the auger and then continues building up from the bottom as the auger with its load of spoil is withdrawn. Reinforcement can then be lowered in before the grout sets. An alternative system used in granular soils is to leave the soil in place and mix it up with the pressured grout as the auger is withdrawn leaving a column of grout reinforced earth. Factors influencing choice of pile •

Location and type of structure

•

Ground conditions

•

Durability

•

Cost

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There are many factors that can affect the choice of a piled foundation. All factors need to be considered and their relative importance taken into account before reaching a final decision. Location and type of structure

For structures over water, such as wharves and jetties, driven piles or driven cast-in-place piles (in which the shell remains in place) are the most suitable. On land the choice is not so straight forward. Driven cast-in-place types are usually the cheapest for moderate loadings. However, it is often necessary for piles to be installed without causing any significant ground heave or vibrations because of their proximity to existing structures. In such cases, the bored cast-in-place pile is the most suitable. For heavy structures exerting large foundation loads, large-diameter bored piles are usually the most economical. Jacked piles are suitable for underpinning existing structures. Ground conditions

Driven piles cannot be used economically in ground containing boulders, or in clays when ground heave would be detrimental. Similarly, bored piles would not be suitable in loose water-bearing sand, and under-reamed bases cannot be used in cohesionless soils since they are susceptible to collapse before the concrete can be placed. Durability

This tends to affect the choice of material. For example, concrete piles are usually used in marine conditions since steel piles are susceptible to corrosion in such conditions and timber piles can be attacked by boring molluscs. However, on land, concrete piles are not always the best choice, especially where the soil contains sulphates or other harmful substances. Cost

In coming to the final decision over the choice of pile, cost has considerable importance. The overall cost of installing piles includes the actual cost of the material, the times required for piling in the construction plan, test loading, the cost of the engineer to oversee installation and loading and the cost of organisation and overheads incurred between the time of initial site clearance and the time when construction of the superstructure can proceed. 45


Pile groups

Piles are more usually installed in groups, rather than as single piles. A pile group must be considered as a composite block of piles and soil, and not a multiple set of single piles. The capacity of each pile may be affected by the driving of subsequent piles in close proximity. Compaction of the soil between adjacent piles is likely to lead to higher contact stresses and thus higher shaft capacities for those piles. The ultimate capacity of a pile group is not always dependent on the individual capacity of each pile. When analysing the capacity of a pile group 3 modes of failure must be considered. Single pile failure · Failure of rows of piles · Block failure The methods of insertion, ground conditions, the geometry of the pile group and how the group is capped all effect how any pile group will behave. If the group should fail as a block, full shaft friction will only be mobilised around the perimeter of the block and so any increase in shaft capacity of individual piles is irrelevant. The area of the whole base of the block must be used in calculating the end bearing capacity and not just the base areas of the individual piles in the group. Such block failure is likely to occur if piles are closely spaced or if a ground-contacting pile cap is used. Failure of rows of piles is likely to occur where pile spacing in one direction is much greater than in the perpendicular direction. Combined foundation: spread footing which supports two or more columns is termed as combined

footing. The foundation of adjacent columns are combined; i) when a column is so close to the boundary of the site that a separate foundation would be eccentrically loaded and ii) where foundations of adjacent columns are linked to resists uplift, overturning or opposing forces. Where a framed building is to be erected alongside an existing building it is often necessary to use a cantilever or asymmetrical combined base foundation for columns next to the existing building so that pressure on the subsoil due to the base may not so heavily surcharge the subsoil under the foundation of the existing building as to cause it to settle appreciably. Combined foundations may rectangular, trapezoidal or combined column and wall footing. If the independent footings of two columns are connected by a beam, it is called a strap footing. A strap footing may be used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, with high bending moments and expensive.

Pier Foundations

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A pier foundation consist of a cylindrical column of large diameter to support and transfer large super-imposed loads to firm strata below. The difference between pile foundation and pier foundation lies in the method of construction. Though pile foundations transfer the load through friction and or bearing, pier foundations transfer the load only through bearing. Generally, pier foundation is shallower in depth than the pile foundation. Pier foundation is preferred in a location where the top strata consist of decomposed rock overlying strata of sound rock. In such a condition, it becomes difficult to drive the bearing piles through decomposed rock. In the case of stiff clays, which offer large resistance to the driving of bearing piles, pier foundation can conveniently be constructed Pier foundations may be of: Masonry or concrete pier or Drilled caisson

Sketches of pier foundation 2.5 Foundation failure

Foundation of a structure fails either due to collapse of the soil by failure in shear or due to unequal settlement of the different parts of the foundation or a combination of both. Contact pressure: A perfectly flexible foundation uniformly loaded will cause uniform contact

pressure with all types of soil. A perfectly flexible foundation supposes a perfectly flexible structure supporting flexible floors, roofs and claddings. Most large buildings however have rigid foundations designed to support a rigid or semi-rigid frame. In practice the contact pressure on a cohesive soil such as clay is reduced at the edges of the foundation by yielding of the clay and as the load on the foundation increases more yielding of the 47


clay takes place so that stresses at the edges decrease and those at the centre of the foundation increases as in the figure below. The contact pressure on a cohesionless soil such as dry sand remains parabolic and the maximum intensity of pressure increases with increased load.

An

understanding of the distribution of contact pressure between foundation and soil will guide the choice for the foundation type to use. The foundation of a building on a cohesionless soil for example would be designed so that the more heavily loaded columns would be towards the centre to allow uniformity of settlement over the whole building. Conversely a foundation on a cohesive soil such as clay would be arranged with the major loads towards the centre of the foundation where pressure intensity is least.

Relative settlement (differential settlement) :

Parts of the foundation of a building may suffer different magnitude of settlement due to variations in the load on the foundation or variation in the subsoil. These variations may cause distortion of a rigid or semi-rigid frame and consequent damage to rigid in-fill panels and cracking of load bearing walls, rigid floors and finishes. Some degree of relative settlement is inevitable in the foundation of most buildings but so long as this is not pronounced or can be accommodated in the design of the building, the performance of the building will not suffer. Cracks which are not visible do not weaken the building or encourage the penetration of rain. More pronounced relative settlement such as is common between the main wall of a house and the less heavily loaded bay window bonded to it may cause visible cracks in the brickwork at the junction of the bay window and the wall. Such cracks will allow rain to penetrate the thickness of the wall. To avoid this either the foundation should be strengthened or some form of slip joint be formed at the junction of he bay and the main wall. Unequal settlement or differential settlement is usually caused by: •

Weak sub-soils, such as made up grounds

•

Shrinkable and expansive soils (such as clay)

•

Frost action

•

Movement of ground water and uplift pressure

•

Excessive vibrations due to traffic, machinery etc

•

Slow consolidation of saturated clays and Slipping of strata on sloping sites.

When designing the foundation therefore, the above factors must be taken into account.

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CHAPTER THREE 3

SOLID CONCRETE GROUND FLOORS

Ground floors are floors that bear directly on the ground. The materials used are usually concrete, bricks or timber with timber sometimes resting on dwarf walls. Concrete is the name given to a mixture of particles of stone bound together with cement. Because the major part of concrete is of particles of broken stones and sand, it is termed the aggregate. The material which binds the aggregate is the cement and this is described as the matrix. Concrete

House foundations are invariably formed in concrete. It is available in a range of strengths and is usually brought onto site ready-mixed or mixed in-situ as, and when, required. What is concrete?

The word concrete is derived from the Latin word concretus, meaning grown together. It is a mixture of several constituents which behaves as a single material. In its simplest form concrete comprises cement, aggregate and water. The major constituent by weight in concrete is aggregate stone with a range of particle size from 40mm down to 0.1mm. The aggregate is a mixture of: • Coarse aggregate - naturally occurring gravel or crushed rock • Fine aggregate - sand or crushed rock. The aggregate is bound together by cement paste, a mixture of cement and water. Properties

The properties of the cement paste are extremely important and largely determine the properties of the concrete: • it must be fluid enough for some time after mixing to allow the concrete to be placed and compacted into its final shape • it must then set and gain strength so that it binds the aggregates together to make a strong material. • The mechanism by which cement sets and hardens depends on the type of cement, usually due to a chemical reaction between the cement and the mixing water.

Uses

The great advantage of concrete as a construction material is that after mixing it is a fluid (plastic) material which can be compacted into any shaped mould or formwork. This may be done on site (in 49


situ concrete), or for very high quality finishes, under factory conditions (precast concrete). When the cement paste solidifies due to the hydration reaction between cement and water it becomes a structural material. Concrete is very strong in compression. Its compressive strength makes concrete an ideal material for foundations and floor slabs and other structural elements that are mainly loaded in compression. However, the tensile strength of concrete is relatively low, about one tenth of the compressive strength. Therefore in structural elements such as beams, which, when loaded, are in compression at the top and tension at the bottom, it is necessary to use reinforced concrete. Reinforced concrete contains steel reinforcing rods, usually 20-30mm in diameter. These rods are positioned where the principal tensile stresses will occur in the structure, and then the concrete is poured and compacted around the reinforcement. Reinforced concrete is therefore a composite material, where the concrete takes the compressive forces and the reinforcing steel takes the tensile forces. 2.1 Preparation for oversite concrete

Turf and top soil is removed preparatory to building operations and a hardcore bed and oversite concrete is spread as a barrier to moisture that might rise from the ground. It is practice on building sites to first build external and internal load bearing walls from the concrete foundation upto the level of the damp proof course, above the ground, in walls. The hardcore bed and the oversite concrete are then spread and leveled with external walls. The Building Regulations 1976 requires that a continuous layer of concrete atleast 100 thick be spread over the site of all buildings within the external walls. Damp proof course should always be atleast 150 above the ground level. Concrete is used for floors laid directly on the ground (slab-on-grade) and for floors supported by the structural frames. The slab on grade floors may be poured before any other part of the building has been built or on leveled and compacted grade after the rest of the building has been erected. In the first case, side forms of wood or metal are placed, leveled, and staked, and screed strips are placed at convenient intervals to provide guides in leveling the concrete. A single course slab poured on grade after the walls have been erected is a common occurrence in industrial and commercial buildings. A reasonable procedure would be as below: 1. Backfill all ditches and trenches within the walls with granular fill or good marrum and compact thoroughly in layers of about 150.

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2. Isolate all columns from the floor slab by boxing them with square wood or metal forms, or with round fiberboard forms which should be set to level the top of the slab. 3. Set screed strips at the same elevation at convenient intervals through out the area to be concreted. Provide a key way form on each screed strip 4. Isolate the walls from the slab by fastening strips of asphalt-impregnated fiberboard or other joint material not more than 12mm thick around the walls, level with the top of the slab 5. Prepare any changes in slab thickness, as at doorways, to be as gradual as possible and at slopes of not more than 1 in 10. 6. Use a template with legs the length of the slab thickness to check the grade 7. Oil the screed strips 8. Cover the grade with a polythene moisture barrier, allowing generous lap between strips. 9. Place the reinforcement-mesh or rods as specified 10. Place the concrete as close to its final position as possible. Consolidate with an internal vibrator, especially at corners, walls and bulk heads 11. Trowel to a hard dense surface with hand or power trowel 12. Cure by covering with:- Water proof curing paper or two coats of curing compound or a layer of damp sand 13. Remove the forms around columns and attach joint material to the vertical faces of the slab and the base of the columns. Fill with concrete, edge and finish th

14. Cut control joints to a depth of atleast 1/5 of the slab thickness with a power saw every 6m to 7.5m in both directions 14. Caulk the joints with mastic joint filler 15. Cure for atleast 7days before allowing regular traffic on the surface The building regulations require that the top surface of the concrete ‘is not below the level of the surface of the ground or paving adjoining any external wall of the building. It would of course be possible to make the site concrete 450 thick; in this instance so as to bring its top surface to dpc level, but this would be unnecessarily expensive method. Instead, what is known as hardcore is usually spread first to raise the level of the concrete. It should be noted that it is not considered good practice to spread the soil excavated from foundation trenches over the site of buildings so as to raise the level of the site concrete, even though would appear a reasonable procedure.. The excavated soil will have been broken up in digging and would need quite thorough ramming to avoid sinking. 51


3.2 Elements of solid concrete ground floors •

Sub-grade

•

Sub-base; this is constructed from selected materials to form a leveled, smooth working platform on which to construct the slab. On very good sub-grades such as gravels, the sub base may be omitted.

•

Filling:The materials to be used for filling should have a high permeability to minimise upward movement of water. It most cases hardcore is used

•

Slip membrane-damp proof membrane is to minimise/prevent dampness rising to the concrete slab, It also reduces internal friction between the concrete slab and the sub-base

•

Prevents lost of concrete

Slip membrane

Materials for the slip membrane are polythene, mastic asphalt, building paper and where the water table is high use tanking, mastic asphalt and bituminous felt Concrete slab

This is the main concrete structural element forming the floor and may be from plain or reinforced concrete. This shall depend on the site conditions, loading or function of the structure Wearing surface Screed

This is the finishing of mortar done on the oversite concrete. Screed done immediately after casting is termed as monolithic. The advantage is that it safes time and bonds better with the oversite concrete, cheaper interms of the materials and has no joint Concrete slab can be cast in portions to minimise shrinkage and subsequent cracking. There is long strip method of casting and chequered method The bays refer to divisions of the slab interms of the widths and lengths. This also makes the working easier Control joints: Control joints to minimise cracking due to expansion and contraction should always

be allowed for which are usually inform of longitudinal and transverse joints. It is of advantage to make these joints coincide with the bay lengths and widths

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dpc oversite concrete

in 150

slip membrane hardcore{sub-base}

st 600 filled material

sub-grade( can be imported materials)

Figure 2.1 Section through a solid ground concrete floor Hardcore

This is the name given to the infill of materials such as broken bricks, stone or concrete, which are hard and do not readily absorb water or deteriorate. This hardcore is spread over the site within the external walls of the building to such thickness as required to raise the surface of the site concrete. The hardcore should be spread it is roughly level and rammed until it forms a compact bed for oversite concrete. The thickness is usually from 100 to 300. The hardcore bed serves as a solid working base for building and as a bed for the oversite concrete. If the material for the hardcore is hard and irregular in shape they will break the capillarity which would make dampness to weaken the oversite. It is important the materials are kept clean and free from old plaster or clay which in contact with broken bricks or gravel would present a ready narrow capillary path for moisture to rise. The materials for hardcore should also be chemically inert and not appreciably affected by water. The materials commonly used for hardcore are: bricks or rubble, concrete rubble, gravel and crushed hard rock, chalk, pulverized fuel ash, blast furnace slag, colliery spoil etc

Blinding

Before the concrete is laid it is usual to blind the top surface of the hardcore. The purpose of this is to prevent the wet concrete running down between the lumps of stones or bricks as this would make it easy for water to rise up by capillarity through the concrete. To blind or seal the top the top surface of the hardcore a thin layer of very dry coarse of clinker or ash can be used. A weak mix of concrete of 1:4:8 cast to 50 can also be used.

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Damp proof membrane

The model Health Bylaws of 1936 required concrete oversite as a barrier to moisture rising from the ground. Concrete is to some degree permeable to water and will absorb moisture from the ground. A damp oversite concrete slab will be cold and draw appreciable heat from rooms it it is to be maintained at an adequable temperature. A damp oversite concrete slab may cause damage and deterioration in moisture sensitive floor finishes such as wood. On building sites that retain moisture due to a high water table and on sloping sites where water may run down to the building and wherever the site concrete is likely to be damp, it is good practice to used proof membrane under, in or on the site concrete. The damp proof membrane may be on top, sandwiched in or under the concrete slab. The materials for dpm are: hot pitch or bitumen, mastic asphalt or pitch mastic, polythene sheets, tar, rubber emulsion etc. Damp proof Courses (dpc)

The function of dpc is to act as a barrier to the passage of moisture or water between the parts separated by the dpc. The movement of moisture or water may be upwards in the foundation of walls and ground floors, downwards in parapets and chimneys or horizontal where a cavity wall is closed at the jambs openings. Dpc should always be at a minimum of 150 above the finished ground level or 150 above the splash apron The materials for dpc are: Flexible dpc

Lead, copper, bitumen dpc, polythene sheets, Semi -Rigid dpc: mastic asphalts Rigid dpc: slates, bricks, etc

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CHAPTER FOUR 4

WALLS

4.1 Introduction

A wall is a continuous, usually vertical, solid structure of bricks, stone, concrete,, timber or metal thin in proportion to its length and height which encloses and protects a building or services to divide buildings into compartments or rooms. Walls are defined as external or internal to differentiate functional requirement and as load bearing or non load bearing to differentiate structural requirements. Load bearing walls carry imposed loads such as the floors and roof loads in addition to their own weights. 4.2 Types of walls

1. Solid walls: Solid or masonry walls are constructed of blocks of bricks, stone or concrete laid in mortar with the blocks laid to overlap in some form of what is called bonding or as a monolithic(one piece) 2. Framed walls: A framed wall is constructed from a frame of small sections of timber, concrete or metal joined together to provide strength and rigidity, over both sides of which or in between the members of the frame are fixed thin panels of some material to fulfill the functional requirement of a particular wall

Sketches

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4.3 Functional Requirements

1. Stability: Stability is how firmly fixed a wall is. The stability of a wall may be affected foundation movement, eccentric loads, lateral forces (wind, rain and earth quake) and expansion due to temperature and moisture changes. Intersecting walls and piers buttress and improve the stability of straight walls against overturning and irregular profile walls have greater stability than straight walls because of the buttressing effects. The common methods are: chevron or zigzag, square irregular wall and serpentine wall (e.g Lugogo stadium) 2. Strength: A wall should be designed to safely support its own weight, wind loads and the loads imposed by floors and roofs. The strength of a wall depends on the material used and the wall thickness. The thicker the wall the greater the load it can carry. Building regulations 1976 set out the minimum thickness of walls in relation to height, length and loads it can carry( Barry 1984, the Construction of Buildings Vol 1) 3. Exclusion of rain: This will depend to some extent on its exposure to wind. A measure of exposure is the “driving rain index’ which is the product of the annual average rainfall and the average wind speed divided by 1000. The actual exposure of a building will depend on its site and will be affected by proximity to coast, lakes, elevation of the site, height of the building and proximity of other buildings all of which should be taken into account. The behavour of a wall in excluding wind and rain will depend on the nature of the materials used in the construction and how they are put together 4. Durability: A block wall of sound bricks, stone or blocks laid in mortar suited to the characteristics of the material and designed with due regard to the exposure of the wall t driving rain and with sensible provision of damp proof courses around doors and windows and to parapets should be durable for the anticipated life of the majority of most buildings and require little if any maintenance and repair. 5. Fire resistance: The resistance of the elements of a structure to collapse, flame penetration and heat transmission during a fire is expressed in periods of from 1.5 to 6hours. Various periods of resistance are called for depending on the size, nature and occupancy of the building so that notional periods of resistance to fire of the elements of the buildings are assumed to be sufficient for the safe escape of the occupancy during fire.

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6. Thermal properties: To maintain reasonable and economic conditions of thermal comfort in buildings, wall should provide adequate insulation against excessive loss or gain of heat, have good thermal storage capacity and the thermal face of walls should be at reasonable temperature. For insulation against loss of heat, light weight materials with low conductivity are more effective than dense materials with high conductivity whereas dense materials have better thermal storage capacity than light weight materials 7. Resistance to sound transmission and sound absorption: Sound is transmitted as a airborne sound and impact sound. Airborne sound is generated as cyclical disturbances of air from the source of the sound with diminishing intensity. Impact sound is caused by contact with a surface, as for example, the slamming of a door or foot steps on the floor. The most effective insulation against airborne sound is a dense barrier such as a solid wall which absorbs the energy of the airborne sound waves. The heavier and more dense the barrier the material, the more effective it is in reducing sound. For reasonable reduction of airborne sound between dwellings one above the other, a concrete floor is necessary. It should however, be noted that the more dense the material the more readily it will transmit impact sound. Absorbent materials inform of carpets will cushion the impact sound. Use of acoustic tiles and curtains should be encouraged to prevent reflected sounds in smoothly painted rooms and walls 4.4 Methods for constructing walls for buildings

Walls are constructed in different forms and of various materials to serve several functions. Exterior walls protect the building interior from external environmental effects such as heat and cold, sunlight, ultraviolet radiation, rain and snow, and sound, while containing desirable interior environmental conditions. Walls are also designed to provide resistance to passage of fire for some defined period of time, such as a one-hour wall. Walls often contain doors and windows, which provide for controlled passage of environmental factors and people through the wall line. Walls are designed to be strong enough to safely resist the horizontal and vertical forces imposed upon them, as defined by building codes. Such loads include wind forces, self-weight, possibly the weights of walls and floors from above, the effects of expansion and contraction as generated by temperature and humidity variations as well as by certain impacts, and the wear and tear of interior occupancy. 57


Modern building walls may be designed to serve as either bearing walls or curtain walls or as a combination of both in response to the design requirements of the building as a whole. Both types may appear similar when complete, but their sequence of construction is usually different. Bearing-wall construction may be masonry, cast-in-place or precast reinforced concrete, studs and sheathing, and composite types. The design loads in bearing walls are the vertical loading from above, plus horizontal loads, both perpendicular and parallel to the wall plane. Bearing walls must be erected before supported building components above can be erected. Curtain-wall construction takes several forms, including lighter versions of those used for bearing walls. These walls can also comprise assemblies of corrugated metal sheets, glass panels, or ceramic-coated metal panels, each laterally supported by light subframing members. The curtain wall can be erected after the building frame is completed, since it receives vertical support by spandrel beams, or relieving angles, at the wall line. Masonry walls are a traditional, common, and durable form of wall construction used in both bearing and curtain walls. They are designed in accordance with building codes and are constructed by individual placement of bricks, blocks of stone, cinder concrete, cut stone, or combinations of these. The units are bonded together by mortar. Reinforced concrete walls are used for both strength and aesthetic purposes. Such walls may be cast in place or precast, and they may be bearing or curtain walls. Some precast concrete walls are constructed of tee-shaped or rectangular prestressed concrete beams, which are more commonly used for floor or roof deck construction. They are placed vertically, side by side, and caulked at adjacent edges. Stud and sheathing walls are a light type of wall construction, commonly used in residential or other light construction where they usually serve as light bearing walls. They usually consist of wood sheathing nailed to wood or steel studs, usually with the dimensions 4× 2in. (5 × 10 cm) or 6× 2in. (5 × 15 cm), and spaced at 16 in. (40 cm) or 24 in. (60 cm) on center—all common building module dimensions. The interior sides of the studs are usually covered with an attached facing material. This is often sheetrock, which is a sandwich of gypsum between cardboard facings. Composite walls are essentially a more substantial form of stud walls. They are constructed of 58


cementitious materials, such as weatherproof sheetrock or precast concrete as an exterior sheathing, and sheetrock as an interior surface finish. Prefabricated walls are commonly used for curtain-wall construction and are frequently known as prefab walls. Prefabricated walls are usually made of corrugated steel or aluminum sheets, although they are sometimes constructed of fiber-reinforced plastic sheets, fastened to light horizontal beams (girts) spaced several feet apart. Prefab walls are often made of sandwich construction: outside corrugated sheets, an inside liner of flat or corrugated sheet, and an enclosed insulation are fastened together by screws to form a thin, effective sandwich wall. These usually have tongue-and-groove vertical edges to permit sealed joints when the units are erected at the building site by being fastened to framing girts. Glass, metal, or ceramic-coated metal panel walls are a common type of curtain wall used in highrise construction. They are typically assembled as a sandwich by using glass, formed metal, or ceramic-coated metal sheets on the outside, and some form of liner, including possibly masonry, on the inside; insulation is enclosed. Tilt-up walls are sometimes used for construction efficiency. Here, a wall of any of the various types is fabricated in a horizontal position at ground level, and it is then tilted up and connected at its edges to adjacent tilt-up wall sections. Interior partitions are a lighter form of wall used to separate interior areas in buildings. They are usually nonbearing, constructed as thinner versions of some of the standard wall types; and they are often designed for some resistance to fire and sound. Retaining walls are used as exterior walls of basements to resist outside soil pressure. They are usually of reinforced concrete; however, where the basement depth or exterior soil height is low, the wall may be constructed as a masonry wall. The word brick is used to describe a small block of burned clay of such size that it can be conveniently held in one hand and is slightly longer than twice its width. The standard brick is 225x112.5x65,which 10mm mortar joint becomes 225x112.5x75, to BS 3921 part 2 Materials from which bricks are made are –Clay, -Concrete and soil

Brick Wall Construction Types of Bricks

59


Commons ;These are bricks which are sufficiently hard to safely carry the loads normally

supported by brick work, but because they have a dull texture or poor colour they are not in demand for use as facing bricks. Any brick which is sufficiently hard and has reasonably good shape and of moderate price may be used as a common brick. Facings; This is by far the widest range of bricks as it includes any brick which is sufficiently hard

burned to carry normal loads is capable of withstanding the effects of rain, wind, spot and frost without breaking up and usually have pleasant appearance Engineering bricks; these are made from selected clay which have been carefully prepared by

washing and crushing. They are heavily moulded and carefully burned so that the finished brick is very solid and hard and capable of safely carrying much heavier loads than other types of bricks. They are mainly used for carrying exceptionally heavy loads, for bricks piers and general engineering worker strength of bricks >50N/mm2. Semi-Engineering bricks: These are harder than most ordinary bricks but not as hard as

engineering bricks, and hence carry less loads than engineering bricks and are cheaper . Hollow, perforated and special bricks: Cellular and perforated bricks are lighter than solid blocks

and the cells and perforations facilitate drying and burning. The saving in clay and consequent reduction in weight is an advantage in non-load bearing walls but does not significantly improve thermal insulation in external walls. Cellular bricks are laid with the cells or hollows downwards and perforated bricks should be laid so that the mortar does not fill perforations

Sketches Prosperities of Bricks Hardness –Any brick with good compression strength, reasonable resistance to saturation by rain

water and reasonable resistance to the disruptive action of frost should be hard burned. Simple test is to hold one brick in one hand and give it a light tap with a hammer. A dull ringing sound is a sign of bad brick.

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Compressive strength- This is the only property of bricks that can be determined accurately. The

compressive strength of bricks is found by crushing 12 bricks individually until they fail or crumble. The pressure required to crush them is noted and the average compressive strength of the 2

brick is stated as Newton per mm of surface area required to ultimately crush the bricks. The strength varies from 3.5N/mm2 for soft facing bricks to 140N/mm2 for engineering bricks Absorption- Much scientific work has been done to determine the amount of water absorbed by

bricks and the rate of absorption. The amount of water a brick will absorb is a guide to its density and therefore its strength in resisting crushing but is not a reasonable guide to its ability to weather well. the term “weather well” describes the ability of the bricks in a particular situation to suffer rain, frost and wind without loosing strength, without crumbling and to keep their colour and texture. Frost resistance- In chimney stacks and parapet walls where brickwork suffers most rain saturation

and there is a likely hood of damage by frost . Parapet walls and Chimney stacks and garden walls should be built of sound, hard burned bricks protected with coping, cappings and dpcs Efflorescence- Clay bricks contain soluble salts that migrate in solutions in water, to the surface of

brickwork as water evaporates to outside air. These salts will collect on the face of brick work as an efflorescence [flowering] of white crystals that appear in irregular unsightly patches. It is most pronounced in parapet and chimney walls. Sulphate attack on mortar and rendering –when brick work is persistently wet as in foundations,

retaining walls parapet, chimneys, soluble sulphate in bricks and mortar may with time crystalise and expand and cause mortar and rendering to disintegrate. To minimize this effect, bricks with low sulphate content should be used. Bonding

In building a wall of bricks or blocks it is usually to lay the bricks in some regular pattern that each brick bears partly upon two or more bricks below itself . The bricks are said to be bonded, meaning they bind together by being laid across each other as in the sketch below. Types of bonds

1-Strecher, 2-English, 3-Flemish, 4- Double English bonds, 5- Double Flemish bonds Mortar for brick work

Mixtures of cement and sand, Cement, lime and sand, Lime and sand and clay Usually in ratios of 1:3 for external walls and 1:4 for internal walls Pointing

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The word painting is used to describe the filling of the mortar joints in the external faces of brickwork. Brick work is pointed for two reasons: 1) To ensure that all horizontal and vertical mortar joints in external brickwork are solidly filled with mortar to make them water tight 2) For decorative reasons

Sketches Cavity walls

If instead of building a solid wall with bricks or blocks bonded along the length and into the thickness of the wall, two separate skins are built with an air space or cavity between, the result will be a wall with better resistance to the penetration of rain. Walls built in this way with two skins or leaves separated by a 50 wide air space, or cavity have been used for many years and are very satisfactory. The height to which such a wall can safely be built must be limited because in effect the wall is no more stable than each skin as there is no bonding into the thickness of the wall . The vertical stability of the two walls can be and always is improved by building metal ties across the cavity in such a way that the ends of the ties are bedded in the horizontal mortar joints of each skin. The cavity at base to the ground level can always be filled with concrete as mortar usually get accumulated there or a solid one and half brick wall can be constructed. Wall Ties: There are 3 patterns of metal ties in common use the galvanized iron twisted tie , the

double triangle tie and the galvanized wire butterfly ties

62


The usual intervals at which ties are built into cavity walls are 900 horizontal and 450 vertically and 250 near openings: The purpose of cavity is to prevent rain penetrating to the inner skin and to improve the insulation of the wall. Openings in Brick and Block walls Jamb: The term Jamb is derived from the French word ‘jambe’ meaning leg. It is because the brick

work on either side of the opening acts as like legs which support brickwork over the head of the opening. Reveal: The term reveal is used to describe the thickness of the wall revealed by cutting the opening

and the reveal is a surface of brickwork as long as the height of the opening. Closing of cavities at opening

The cavity is closed either with brick or block and a continuous strip of bitumen impregnated felt, lead-cored felt, or strip of lead or copper is sandwiched in the cavity at the jabs of the opening. Around door openings with wood frames, a strip of vertical dpc material is tacked to the back of the wood frame or the lead or copper fixed to the back of the wood frame as in the sketches below:

Lintels

This the is the name given to any single solid length of concrete, steel, timber or stone built in over an opening to support the wall over it. The ends of the lintels must be built into the bricks or block work over the jambs as to convey the weight carried by the lintels to the jambs. The area of the wall on which the ends of the lintel bears is termed as its bearing ends .The wider the opening the more weight the lintel has to support and the greater it bearing at the ends must be so as to transmit the load it carries to an area capable of supporting it. Read and make notes on Arches

63


64


CHAPTER FIVE 5

ROOFS

The primary purpose of a roof is to protect a building’s interior, but it may also be used to contribute to a building’s exterior appearance. The completed roof consists of several components, including the roof frame, roof deck, vapour barrier, insulation, water proof roofing material, flashing and drains, construction and control joints In the design of a roof, a number of factors are considered .e.g.: weather, appearance, height, area, and style of the frame. A roof may be constructed as a flat roof from a timber, metal or concrete framed platform which is either horizontal or inclined up to 10degrees to the horizontal, or as a pitched roof with one or more slopes pitched at more than 10 degrees to the horizontal. Some of the examples of pitched roofs are: Symmetrical pitch, asymmetrical pitch, mono-pitch with trussed rafters, and mono-pitch with slopping soffit, butterfly roof, and lean to roof.

lean-to asymmetrical pitch

mono-pitch with sloping soffit

symmetrical pitch

butterfly roof mono-pitch with trussed rafters

Figure 1 Sketches of the different pitched roofs 5.2 The functional requirements of a roof are:

Stability Strength 65


Exclusion of wind and rain Durability Fire resistance Thermal properties

Stability

A roof is constructed to support the dead load of the roof structure and its covering, insulation and internal finishes, snow loads and pressure or suction due wind without undue deflection or distortion. The dead load can be calculated from unit weight of materials set out in BS 648. Snow loads are assumed from average snow falls. The pressure of wind on a roof will depend on the exposure, height and shape of the roof and the surrounding buildings. Wind blowing across a roof will tend to cause pressure on the wind ward side and suction on the opposite side of the building. The stability of a flat roof depends on the adequate support from walls or beams and sufficient depth or thickness of timber joist or concrete relative to spans, and the assumed loads to avoid gross deflection under load.

Strength: The strength of a roof depends on the characteristics of the materials from which it is

constructed and the way in which they are put together in the form of a platform or some form of triangulated frame.

Exclusion of wind and rain: A roof excludes rain through the material with which it is covered;

varying from the continuous impermeable layer of asphalt covering that can be laid horizontal to exclude rain, to the small units of clay tiles that are laid overlapping down slopes so that rain runs rapidly to the eaves. In general the smaller the units of roof covering, such as tiles or slate, the greater the pitch or slope to exclude rain that runs down in the joints between the tiles onto the back of another tile or slate lapped under and so on down the roof. Impermeable materials such as asphalt and bitumen that are laid without joints can be laid flat and sheet metals such as lead and copper that are joined with welts can be laid with a very shallow fall.

Durability: This depends largely on the ability of the roof covering to exclude rain. Persistent

penetration of water into the roof structure may cause decay of timber, corrosion of steel or disintegration of concrete 66


Fire resistance: A roof and its covering should have adequate resistance to damage by fire, and

against spread of flame for escape in fire, for the periods of from thirty minutes to six hours. Thermal properties: The materials of roof structures and roof coverings are generally poor

insulators against transfer of heat and it is usually necessary to use some material which is a good insulator, such as light weight boards, quilts or loose fill to provide insulation against excessive loss or gain of heat. Insulating materials may be applied to the underside or the top of flat roofs or between the joists of timber flat roofs. Rigid materials such as wood wool, that serves as roof deck and insulation are laid on top of the roof and non-structural materials at ceiling level or on top below some form of decking. It is of good practice to fix insulating materials at ceiling level in timber flat roofs, so that there can be cross ventilation between the joists from permanent vents, to limit condensation risks as required by building Regulations 1981. Vapour barrier: Insulating materials are effective against transfer of heat to the extent that they

retain still air between fibres, in granules or in minute spaces. When this light weight materials absorb water they lose their insulating properties as water enters the air spaces, and water is not a good insulator. Precaution must be taken, therefore, to prevent moisture or water saturating the insulation either through the roof covering or from humid warm air from inside the buildings. As a barrier to humid warm air from inside the building, an impermeable vapour barrier should be fixed between the warm air side and the insulation. This vapour barrier takes the form of a sheet of bitumen, polythene, or aluminium that is impermeable to moisture. 5.3 Flat Roofs

A flat roof by definition is any roof with a slope of less than ten degrees. The simplest roof to construct is a flat roof, framed in wood, steel, or reinforced concrete. Factors considered in the choice of material to use for structural frame work. •

Cost

•

Size (span)

•

Availability of materials and equipment

•

Working Space

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Timber flat roof construction:

Timber flat roofs consists of soft wood timber joist of 38 to 50 thick and from 75 to 225 deep placed on edges from 400 to 600 apart with the ends of the joists built into or onto or against brick walls and partitions. Strutting between joists: Solid or herringbone strutting should be fixed between the roof joists.

When timber is seasoned it shrinks, and timber such as roof joists, which is not cut on the radius of the circle of the log does not shrink uniformly. The shrinkage will tend to make the floor joists twists, or wind, and to prevent this

4 0 0 - 6 0 0

solid strutting

75 - 225

100 x 75 wall plates

Figure 2 timber strutting is used. Herringbone strutting consists of short lengths of 50 x 38 softwood timber nailed between the joists as shown in the illustration below. The other method of strutting termed solid strutting consists of short length of timber of the same section as the joists which are nailed between the joists in a line or staggered as in the figure below. This is not usually so effective a system of strutting as the herringbone system, because unless the short solid lengths are cut very accurately to fit to the sides of the joists they do not firmly strut between the joists. Note: Ceiling noggings can also be used in place of strutting.

Usually one set of struts is used for joists spanning up to 3.6 and two for joists spanning more than 3.6. A single set of struts is fixed across the roof at mid span.

Roof deck/boards: Boards which are left rough surfaced from the saw are usually employed to

board timber flat roofs and is called rough boarding and are usually 19 thick cut with square. For 68


good work tongued and grooved boards are often used as the plain edged boarding may shrink and twist out of level as they dry. Chip boards may also be used in lieu of them to maintain a level roof deck End support of joists: Roof joists are normally supported on timber or metal wall plates. Wall

plates serve to distribute the roof loads uniformly over the walls and Provides a level bed for the roof joists. Where there is a parapet wall, the end of the joists can rest on the inner walls of cavity walls or on metal hangers. 1 9 m m t im b e r b o a r d in g

roof joist

100 x 7 5 w a ll plate or m etal plates

Figure 3

1 9 m m tim b e r b o a r d in g

roof joist

1 0 0 x 7 5 w a l l p l a te o n b r ic k c o r b e l

Figure 4

wall plate resting on the inner wall of a cavity wall construction The ends of roof joists are sometimes carried on brick corbel courses, timber plate and corbel brackets or on hangers.

69


1 9 m m t im b e r b o a r d in g

roof joist

1 0 0 x 7 5 w a l l p l a te o n steel corbel brackets b u i lt in a t 7 5 0 c e n t re s l

Figure 5. Timber joists on wall plates supported by steel corbel brackets The ends of roof joists built into solid brick walls should be given some protection from dampness by treating them with a preservative. Timber joist may be built into a solid external wall if the wall is thick enough to prevent penetration of moisture to the joist ends and where the wall is protected externally with slate or tile hanging.

cavity insulation carried upto joint roof insulation

bituminus felt on boards on insulation

timber joists 100 x 75 wall plate cavity wall Figure 6. joists end on cavity walls

Timber firring: Flat roofs are usually constructed so that the surface has a slight slope or fall

towards rainwater outlets. This slope could be achieved by fixing the joists to a slight slope but the 70


ceiling below the roof would then also be sloping. It is usual to provide a sloping surface to the roof by means of firring pieces. These consist of either tapered lengths of softwood nailed across the joists or varying depth lengths of softwood nailed across the joists. Tapered firring is used for roofs covered with chipboard or wood wool slabs and the varying depth firring for boards laid parallel to the slope of the roof so that variations in the level of the boards do not impede the flow of rainwater down the shallow slope. As an alternative to firring, some insulating boards are cut or made to a slight wedge section to provide the necessary fall to a roof.

varying height firring pieces nailed across joists tapered firing piece nailed to top of joists

75 - 225

joists 100 x 75wall plates 4 0 0 - 60 0

Roof joists

Figure 7. Timber firring

Thermal insulation: A timber flat roof provides poor insulation against loss or gain of heat as most

of the materials used are poor insulators. Any material that is to be a good thermal insulator must have a great number of tiny air spaces in it as it is the air trapped in these spaces that acts as the thermal insulator. Insulating materials are manufactured in the form of boards, slabs, quilts or loose fill and when used with timber roofs the boards and slabs are fixed on the joists under the boarding or on the underside of the joists. Quilted materials are usually laid between or over the joists and dry fill between the joists. Reinforced concrete roofs

Reinforced concrete roofs have a better resistance to damage by fire and can safely support self weight, wind/rain pressure. The resistance to fire required by building regulations for most offices, large blocks of flats, factories and public buildings is greater than can be obtained with a timber roof. A reinforced concrete roof will usually span the least width between the external or external walls and internal load bearing walls and will be supported on walls and partitions. 71


Monolithic Reinforced concrete roof

The word monolithic means one stone and is used in buildings to described one unbroken mass of any material. A monolithic concrete roof is one unbroken solid mass of concrete cast-in-situ and reinforced with mild steel reinforcing bars. To support the concrete while it is still wet and plastic, and for seven days after it has been placed, a temporary centering has to be used (form work). This takes the form of rough timer boarding or steel sheets, supported on timber or steel beams and post. The steel reinforcement is laid out on top of the centering and raised 15 above the centering by means of small blocks of fine concrete (spacers) which are tied to the reinforcement bars with wires. The wet concrete is then placed and spread on the centering, and is compacted and leveled off. It is usual to design the roof to span the least width of the building and two opposite sides of the concrete are build into walls incase of parapet walls.

asphalt on screed

concrete roof

cavity wall

insulating lining fixed to sofit of roof and inside of wall for intermittent heating

Figure 8. Reinforced concrete roof

Centering: The temporary timber board or sheet steel support for monolithic concrete floor or roof

is termed centering. Reinforcement of concrete: The steel reinforcing bars are cast into the under side of the roof with

15 of concrete cover below them to prevent the steel rusting and to give it some protection incase of fire. The thicker the concrete cover to the reinforcement the greater the resistance of the roof to fire. The duty of determining the amount of reinforcement to use in a concrete roof is done by Engineers usually Structural or Civil Engineers. When the engineer designs a reinforced concrete roof, he usually calculates the amount of steel reinforcement required for an imaginary strip of roof 300 72


wide spanning between the walls, as though the roof were made up of strips of 300 wide concrete beams placed side by side. Because the centering required to give temporary support to a monolithic concrete roof tends to obstruct and delay building operation below the roof, the most common concrete roof used today are the “self centering” concrete. Self-centering concrete roofs:

These are constructed with precast reinforced concrete slabs which are cast in the manufacturer’s yard and are delivered to the building site where they are hoisted to the level of the roof and placed in position. Once in their positions they require no support other than the bearing of their ends on beams or walls. Advantages of self centering concrete roofs: •

Concrete has good quality since it is done under strict specialized supervision

•

It is faster to complete roofing as the roof slabs can be ordered for in advance.

•

There is no much interference of the activities below the roof.

Disadvantages: Difficulty in hoisting where there is no enough space The joints sometimes leak when not well finished

Thermal insulation: A reinforced concrete roof provides poor insulation against loss or gain of

heat and some material which is a good thermal insulator should be incorporated in the construction of the roof or a light weight concrete slab be used. One way of doing this is to used light weight aggregate instead of sand when screeding. It is the screed which provides the slope for the rain water to run off the roof. The light weight aggregate in common use are foamed slag, pumice and vermiculite. These materials are porous and it is the air trapped in the minute pores of the material which at once make them light in weight and good thermal insulator. Foam slag: This is formed by spraying water on molten slag which is poured off molten iron in

blast-furnaces. The water causes the slag to expand into a porous light weight mass. The slag is crushed into small particles used for screed which greatly improves on the thermal properties of the concrete roof. The thickness of the screed is usually 25mm. This is a cheap material to use.

73


Pumice: This is a rock of volcanic origin which is usually porous, light weight, and a good thermal

insulator. It is crushed into small particles and used for screeding though usually expensive and hence not commonly used. Vermiculite: This is a micaceous mineral which consists of fine layers of materials closely packed.

When it is heated the fine layers open out and gases are trapped in the many spaces between the expanded layers.

It is very light in weight and most commonly used today because of its

effectiveness in thermal insulation. Any of the rigid, light weight insulating boards may be used to improve the thermal insulation of a concrete roof fixed either on top or below the concrete roof. The most convenient place for the insulating board is on top of the concrete roof, under the roof covering. By insulating the concrete roof from out side air, concrete roof can act to store heat in continuously heated buildings (winter). 5.4 Flat roof coverings:

The materials used to cover flat roofs are: Built-up bitumen felt, mastic asphalt and the non ferrous sheet metals, lead, copper, zinc and aluminium. Built-up bitumen felt:

This is one of the cheapest and most commonly used roof coverings for flat and shallow roof slopes. The roof is built with three layers of bitumen roof felt. The three types of base materials used for bitumen roofing are: fibre, asbestos and glass fibre, the material of the base being felted and impregnated with bitumen. The surface of the under layer is finished with fine mineral granules so that the bitumen does not bond in rolls and the exposed layers are finished with a mineral particle finish. The method of fixing is based on the nature of the roof surface to which it is being applied. The felt is laid across the roof with 50 side lap and 75 end laps between sheets. Glass fibre based felts have excellent dimensional stability, are non-absorbent and will not rot. Normally used for very good quality works Asbestos based felts have good resistance to damage by fire, good dimensional stability and are used as a base layer for fire resistance and for good quality work for both under layers and exposed layers.

Timber boarded roofs: On a timber board or chip board roof surface with the insulation either

under the boards or at or over the ceiling level, the first under layer of felt is nailed to the boards 74


either at 150 centres both across and along the roof, or at 50 centres along the laps of sheets and 150 centres elsewhere. The wider centre of nailing is considered adequate for fixing. The second underlayer is then bonded to the first in hot bitumen spread by mop or brush on the first underlayer, and the top, or exposed layer, likewise bonded to the second underlayer with the joints between sheets in each layer breaking joint.

Dry insulation boards: Rigid preformed insulation boards may be used as insulation and the

surface for bitumen felt roofing on a timber board or chipboard covered roof and on metal and timber roof decking. Many of the rigid, dry insulation boards, except expanded polystyrene, are suitable for the direct application of bitumen felt roofing. The insulation boards are laid on an underlay of self finished roofing felt that serves as a barrier against warm air from the room below. The underlayer of felt may be nailed, or partially or fully bonded on hot bitumen to the boards. The insulation board is then partially or fully bonded to the felt underlay and the roof finish of three layers of glass fibre, asbestos or asbestos first layer and felt fibre layers is then fully bonded to the insulation. Concrete screed finish: Cement screeds and particularly light weight aggregate screeds on concrete

roofs take time to thoroughly dry out and may absorb rain water so that it is likely that some water will be trapped in the screed once bitumen felt covering has been applied. The heat of the sun will then cause this water to vaporize and the vapour pressure will cause the felt roofing to blister, crack and let in water. To relieve this water vapour pressure, it is practice to use a venting layer of felt on wet screeded roofs. This perforated layer of felt is laid dry on the screed and the three layers of felt are then bonded to it. The venting layer allows water vapour to be released through vapour pressure releases at abutments and verges of the roof.

Parapet walls and abutments: The bitumen felt roofing should be turned up 150 against parapet

and abutting walls, over an angle fillet as shown in the sketch below and either the damp-proof course turned down over the upstand of the felt roofing or a separate flashing dressed over the upstand.

75


copping with a dpc under

non-ferrous sheet metal flashing built into wall and dressed over upstand of roofing felt

3-layers of roofing felt bitumen felt dpc

angle fillet

timber boards on isulation board & vapour pressure barrier

Figure 9. Parapet wall Eaves and Verges: Either the bitumen felt roofing may be dressed over gutters with a welt or a

separate non-ferrous drip may be used. Similarly, either the felt or a separate flashing may be used at verges.

f e l t t u r n e d o v e r v e r g e f i ll e t a n d n a i le d t o f a s c i a

3 - l a y e r s o f b it u m e n r o o f in g f e lt o n t o p o f 2 5 s c r e e d

concrete roof

cavity w a l l fi ll e d u p t o r o o f l e v e l w i th i n s u l a t io n

Figure 9. Treatment at eaves and verges Mastic asphalt: This is a mixture of naturally occurring material which is soft, has a low melting

point and is an effective barrier to penetration of water. Asphalt is manufactured either by crushing natural rock asphalt and mixing it with natural lake asphalt, or by crushing natural limestone and mixing it with bitumen whilst the two materials are sufficiently hot to run together. The heated asphalt is run into moulds in which it solidifies as it cools. Solid blocks of asphalt are heated on the building g sites and the hot plastic material is spread over the surface of the roof in two layers breaking joint to a finished thickness of 20mm. as it cools it hardens and forms a continuous, hard water proof surface. 76


Parapet walls: External walls of buildings are raised above the level of the roof as a parapet wall

for the sake of the appearance of the building as a whole. Parapet walls should not be built above the roof level higher than six times the least thickness of the parapet wall for the sake of stability as they are free standing. To prevent rain water from saturating parapet walls, it is essential that it should be covered or capped with some non-absorbent material. Natural stone, concrete, and bricks are some of the materials used for capping. Parapet wall d.p.c: It is good practice to build a continuous horizontal d.p.c into brick parapet wall

at the junction of the roof covering, upstand or skirting with the wall. In stone capping similarly rain water usually penetrate through the cracks and saturate the wall below. If frost occurs the parapet wall may be damaged, therefore it is good practice to build in a continuous layer of dpc of bituminous felt, copper or lead below the stone. Parapet to cavity walls: The construction of a parapet built on a cavity wall is usually somewhat

different from that built on a solid wall. An external wall built with a cavity to prevent rain penetrating the wall and it is logical to continue the cavity to at least the top of the roof, so that the cavity protects roof timber or concrete built into or against the wall. The cavity should always be continued to the level of the asphalt skirting. non-ferrous sheet metal flashing built into wall and dressed over upstand of roofing felt

copping with a dpc under

3-layers of roofing felt

weep holes Cavity gutter of felt or metal

angle fillet flat roof

Figure 10 Thermal insulation: For effectiveness the thermal insulation of a wall must be continuous for the

height of the wall upto the insulation in the roof. Where a cavity lining or fill is used in a cavity wall it must be carried up atleast to the roof insulation.

77


Sheet metal roof coverings

Sheet metal is used as a covering because it gives excellent protection against wind and rain; it is durable and lighter in weight than asphalt, tiles or slates. The four common sheet forms used are; lead, copper, zinc and aluminium. Properties of metals which make them to be used as either a flat roof or pitched roof covering are: Lead: This is a heavy, comparatively soft metal with poor resistance to tearing and crushing hence

has to be used in thick sheets as a roof covering. It is malleable and can easily be bent and beaten into quite complicated shapes without damage to the sheets. Lead is quite resistant to all weathering and can last upto 100 years. Copper: This is a heavy metal with good mechanical strength and malleable. Because of its

mechanical strength this metal can be used in quite thin sheets as a roof covering. Like lead, copper can be beaten and bent into complicated shapes. On exposure to atmosphere a thin layer of copper oxide forms which is tenacious, non-absorbent and prevents further oxidation of the copper below. Copper is quite weather resistant and last as long as lead. Zinc: It is one of the lighter metals with good mechanical strength but not so malleable and

normally brittle. Zinc sheet is liable to damage in very heavily polluted industrial atmospheres and should not be used there. The useful life of zinc as a roof covering is between 20 to 40 years. Aluminium: This is one of the lightest metals with moderate mechanical strength and is as

malleable as copper. It is resistant to all weathering agents. On exposure to atmosphere a film of aluminium oxide forms which is dense and tenacious and prevents further corrosion. Aluminium as a roof covering has a useful life intermediate between zinc and lead. Bitumen and asphalt have replaced the above metal roof covering because of their low initial cost, although metal roof covering is becoming more common because of their use for low pitched roofs, architectural designs (fashion). Joint sheets:

The sheets of metals have to be fixed to the roof and jointed to allow for expansion and contraction without tearing. Three types of jointing have been developed which successfully joints the sheets, keeps out water and allows for expansion and contraction. All metal sheets are laid to a fall or slope on roofs so that water runs off. The longitudinal joints are usually in form of a roll. Rounded timber battens some 50 square are nailed to the roof and the edges of the sheets are either overlapped or covered at these timber rolls. The joints across or 78


transverse to the fall of the roof are always formed as a small step called a drip. The purpose of the drip is to accelerate the flow of rain water running down the shallow slope of the roof Upstand and apron: Where there is a parapet wall around the roof or where the roof is built up

against a wall, the sheets are turned up against the wall about 150 as an upstand. The top of these upstands are not fixed in any way so that the sheets can expand without restrain. To cover the gap between the upstand and the wall strips of sheets, the sheets are tucked into a horizontal brick joint, wedged in place and then dressed down over the upstand as an apron flashing. Rain water gutters: If the flat roof is surrounded on all sides by parapet walls it is necessary to

collect the rain water falling off at the lowest point of the roof. A shallow timber framed gutter is constructed and is lined with sheets. The gutter is constructed to slope or fall towards one or more rain water outlets. The gutter is usually made 300 wide and is formed between one roof joist, spaced 300 from a wall, and the wall itself. Sketch how it is done. Eaves gutter: Where the roof has no parapet walls as for copper roof covering where the beauty of

the roof covering is of importance, the run off rain water is discharged into an eaves gutter as in the sketch. It is practice to drain the water from the gutters into down pipes which discharges the water into reserve tanks or into storm water channels Draw the sketch Sheet metal covering to concrete roofs: Bitumen and asphalt have been the cheapest roof

coverings on concrete roofs but they have a useful life of some twenty years only as a result sheet metals are sometime preferred. The sheet metal is jointed and fixed to a concrete roof in the same way as a timber roof. The wood rolls are secured to the concrete by screwing them to splayed timber battens set into the screed on the concrete or by securing them with bolts set in sand and cement in holes punched in the screed as shown below.

Roofing felt: It is essential that sheet metal be laid on a continuous layer of roofing felt laid on the

surface of the concrete roof. The felt enables the metal to contract and expand freely and prevents it tearing on any sharp projections in the surface of the concrete roof

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5.5 Pitched Roofs

A pitch roof has one or more roof slopes at a pitch or slope of more than 10

0

to the horizontal. The

most common roof shape is the symmetrical pitch roof pitched to a central ridge with equal slopes.

Hip

Ridge Verge board

Hipped end

Gable end with a vent Eaves

Figure 11. Illustration of a pitched roof with a hip and a gabled end

The traditional roofing materials like slate and tiles can only be successfully fixed on to a surface inclined at atleast 25degrees to the horizontal. The construction method is to slope the surfaces by pitching the rafters on either sides of the ridge piece with the rafters bearing on the wall plate. This is the simplest roof because each pair of rafters acts like two arms pinned at the top and is called a couple. Precautions should be taken on the span as the weight of the roof tends to spread the rafters of a couple roof and over turn the supporting walls. In the traditional pitched roof form, timber ties are nailed to the foot of pairs of rafters to prevent them spreading under the load of the roof. The ties may also serve to support the ceiling frame. The other approach is to use timber ties nailed to the foot of pairs of rafters to prevent them spreading under the load of the roof and is termed a closed couple roof A modification of the close couple roof is the collar roof, where the ties are fixed between pairs of rafters one third the height of the roof up from the wall plate. The advantage here is that the roof may extend up into the part of the roof

80


h

3.5max

4.5max

couple roof

5.5max

collar roof

closed couple roof

Figure 12. 5.6 Trussed rafters

A trussed rafter is a triangular roof frame of rafters, ceiling joists and internal webs joined with spiked connector plates and assembled in a factory. A trussed rafter uses upto 60% less timber than a comparable traditional pitched roof and requires less on site labour as most of the members are brought to the site and assembled or to be assembled only. Timber- framed pitched roofs are usually constructed with trussed rafters and are the most economical and convenient way of framing pitched roofs. Trussed rafters are fabricated from stress graded timbers, accurately cut to shape and assembled and joined with steel connector plates. Much of the preparation and fabrication of these trussed rafters is mechanized, resulting in accurately cut and finished trusses that are delivered to site ready to be lifted and fixed as a roof frame. The connector plates are made from carbon steel which is stamped out so that the teeth protrude. The connector- plates are machine pressed to form strong rigid joints and is used where the joints are butt joints. If the members overlap one another, split rings and bolts are used to connect them. The split rings are set in circular grooves cut in the meeting places and a bolt through the assembly holds the two together tightly. Trussed rafters are erected and nailed to a timber wall plate, bedded on the external walls, at centres to suit the roof covering. 2 0 0 x 3 2 - 5 0 r id g e b o a r d

Collarpiece

1 0 0 x 5 0 t ie s a n d s t r u t s 1 0 0 - 1 5 0 r a f te r s o n wa ll plates

1 5 0 x 5 0 t ie b e a m s at 400-600 centres

100 x 75 w all plate f i tt i n g i n t o t h e b i r d m o u t h on the rafter

A t y p ic a l tr u s s e d r a f t e r

81


Figure 13 Trussed rafters Size of roof timbers

Rafters are usually 38 – 50 thick and 100 – 150 deep and are spaced at from 400 to 600 centres. The depth of rafters and the centres at which they are fixed depends on the type and weight of the roof covering they have to support and their unsupported length. In addition to the dead weight of the roof covering, such as tiles or slates, the rafters have to be able to resist the pressure of wind. Collars are usually 44 thick and are usually as deep as the roof rafters. The ridge board is usually 25 – 38 thick and so deep that the whole depth of the splay cut ends of rafters bear on it. Eaves: This is a general term used to describe the lowest courses of the slates or tiles and the timber

supporting them. The eaves of most pitched roofs are made to project some 150 to 300 beyond the external face of walls and in Uganda they are as wide as 600. This gives some protection to walls and enhances the appearance of buildings. Eaves can also be finished flush with the wall. The roof coverings drains into an eaves gutter fixed to the fascia boards. The soffit of projecting eaves can be finished closed with boards, sheets or plastered ceiling or it can also be left open. 125x50 rafters

50x25 bracket nailed to rafter to support soffit

125x50 tie beam or ceiling joists

225x25 fascia

19 soffit board making a closed eaves FIGURE 14 DETAIL OF CLOSED EAVES CONSTRUCTION

5.7 Purlin or double roof:

A purlin is a continuous timber fixed horizontally under the roof rafters to give the support between the ridge and the wall plate. The purlin is in turn supported by means of timber struts which bear onto a load bearing partition or fixed onto the tie beams resting on the wall plates. It will be seen that the purlins support the rafters mid-way between the ridge and the eaves and are supported by struts at intervals of about 1.8 along their lengths. Where the roof slope is long, more than a line of purlin should be provided corresponding to the struts. Collars fixed every fourth rafter serve to 82


brace the roof and provide a secure fixing for the purlins which bear on them. The size of the purlins depends on the weight of the roof and their unsupported length between the struts. With struts not more than 1.8 apart a 125 x 50 purlin is used for most rafters. Collars of the same section as the roof rafters are fixed to every third or fourth rafter. Struts are usually 75 square in section. The foot of the strut is fixed to a timber wall plate bedded in mortar on the load bearing partition. Incase of terrace buildings the purlins can be made to rest on the diving walls, this also helps to prevent the spread of fire from one house to the other. In this Case the diving wall should be taken up to the under side of the roof covering or even through to form a parapet wall. Timber trusses

A strongly constructed purlin roof depends for support on the load bearing partitions conveniently placed and these partitions often restrict freedom in planning the rooms of the building. A method of constructing pitched roofs so as to avoid the use of struts to support the purlins, and load bearing partitions to support the struts, is to use timber trusses. The word truss means tied together and a timber roof truss is a triangular frame of light section timbers fixed together. The timber trusses span between external walls and are spaced about 1.8m apart and they serve to support the purlins which in turn support the roof rafters. The timbers of the truss are bolted together and to make the connections rigid galvanized iron timber connectors are bolted between each two timbers at connections. The strength of the trusses derives mainly from the rigidity of the connections. To reduce the quantity of timbers used, the ceiling rafters are given support by means of hangers and binders. The hangers are nailed to the purlins and to these are nailed horizontal binders to which the ceiling joists are nailed or secured with metal plates. The timber connectors have opposed teeth which when firmly bolted between the timbers prevents any scissor movement between them. Timber trusses have largely been superseded by trussed rafters for most domestic buildings.

83

200 x 32-50 ridge board

100-150 rafters on wall plates 150x50 purlin 125 x50 collar llar & 75 x 75 strut to every 3rd or 4th rafter

150 x 50 tie beams/ceiling jois joists ts at 400-600 centre tres

100 x 75 wall plate fitting into the bird mouth on the rafter

Load bearing partition

FIGURE 15: A typical trussed rafter for span upto 7.5

200 x 32-50 ridge board

75x38 tie

all joints in truss bolted through timber connectors 75 x 50 hangers

100x38 rafters at 400-600 c/c

150x50 purlin

150 x 50 tie beams/ceiling joists joists at at 400 400-6 -600 00 centre centres s

75x50 strut

125x50 ceiling binders 100 x 75 wall plate fitting into the bird mouth on the rafter

FIGURE 16: A typical trussed rafter for span upto 8.0

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75x38 tie

galvanised steel gang-nail connector plates at all joints

100x38 rafters at 400-600 c/c 150 x 50 tie beams/ceiling jo is ts at 40 0- 60 0 ce nt re s

75x50 strut

Figure 17: A typical trussed rafter for for span upto 12.0 and pitch pitch from 15 to 40 deg.

5.8 Hipped roofs

The most economical way of constructing a pitched roof is to form it with two slopes with gable ends. But a simple gable end roof sometimes looks clumsy due to the great area of tile or slate covering and this can be avoided by forming hipped ends to the roof. The hipped ends are pitched at the same slope as the main part of the roof and the rafters in the triangle of the hipped end are pitched up to a hip rafter. The hip rafters carry the ends of the cut rafters in the hipped ends and those of the main roof slopes. The hip rafter is usually 38-50 thick and 200 to 250 deep. The cut ‘jack rafter’ are nailed each side of the hip rafter. Because the hip rafter carries the ends of several jack rafters it tends to over turn the walls at the corner of the building where it bears on the wall plates and to resist this, angle tie should always be fixed across the angle of the roof. The angle ties are usually 100 x 75 timber and are either firmly bolted to or dovetail housed into the top of the wall plates some 600 from the corner of the building. 200x 38 ridge board e n d o f h i p p e d r a f te te r s c u t & n a i l e d t o r id id g e b o a r d

2 0 0 x 5 0 h i p r a f te te r b e a r i n g on the wa ll plate

1 2 5 x 5 0 j a c k r a f te te r s

1 2 5 x 5 0 c e i lil i n g j o i s ts ts

1 0 0 x 7 5 w a l l p l a te te load bearing wall

F i g u r e 1 8 : h ip p e d r o o f c o n s tr u c tio n

85

Roof Ventilation

The limited capacity of air to hold moisture in the form of water vapour increases with temperature. When the temperature of air falls, so does its capacity to hold moisture and the surplus moisture is given up in the form of condensation. The air inside heated buildings usually contains more water vapour than outside air and so has a higher vapour pressure which creates a vapour drive from the area of high pressure inside the building to the out side through the material of the roofs, so that warm moist air will penetrate the ceiling and insulation of roofs and condense on cold surfaces inside the roof space due to condensation which will cause corrosion of fixings and decay of timber. To prevent an excessive built-up of moisture from condensation inside roofs, a cross ventilation of roof spaces by vents not less than 0.3% of the roof plan area is required. This is done by fixing ventilators either in the soffit of overhanging eaves incases of hipped roofs or on the gable ends incase of gabled ends

Lamella Roof construction

A lamella roof is a curved roof similar in shape to one formed by the use of bowstring trusses, but without the use of frame work of webs and lower chords found in truss roof. It does however; provide clear spans of great width. It is formed by framing together a series of intersecting arches made up of relatively short members called lamellas. They are made of 50100 material (steel or concrete), 3.6m to 4.9m long, beveled, bored with two holes at each end, and bolted together. A reinforced concrete lamella roof may be erected over a curved form made the width of the building and the depth of one bay carried over movable scaffolds. The erection of the formwork is begun from both sides at the sill and completed at the centre. The horizontal thrust developed in this roof must be taken care of b y tie rods, wooden ties, buttressed walls or wall columns. The usual length of individual members is 3.6, 4.2 or 4.9 with arch spacing of approximately 1.2, 0

1.36 or 1.5 respectively. The angles between the intersecting lamellas should not exceed 45 and 0

should preferably be between 38 and 40

0

Decking must be applied directly over the framework of the roof. Folded plate roofs: A folded roof is another roof in which the roof slab has been formed in thin,

self supporting structure, usually made either of wood or concrete. A concrete roof of this type can be made with precast panels or may be cast-in-situ. The rest of the construction is like for flat roofs except in this case they are pitched and folded. 86

Lamella roof

lag screw plate

tie rod

bolt

Lamella joint Figure 19: construction of a lamella roof.

Figure 20: Folded plate roof construction Pitched Roof Coverings

The traditional covering for pitched roofs, plain clay tiles and natural slates, are much less used than they were because they are comparatively expensive and the majority of pitched roofs of new buildings are covered with single lap concrete til es and mangalore tiles. The small unit pitched roof coverings are single lap tiles, plain tiles and slates. Single lap tiles

These are so shaped that they overlap the edges of adjacent tiles in each course. The overlap prevents water entering the roof between adjacent tiles and in consequence the tiles can be laid with a single end lap. The advantage of single lap tiling is that its weight per unit area is up to 40% less than that of plain tiling. Plain tiles: These are flat rectangular roofing units of size 265 by 165 with holes for nailing and

nibs for hanging to batten. These tiles are laid double lap down the slope of the roof because water running between the open joints between adjacent tiles runs on to the back of a tile double lapped under the joint. A plain tile roof is generally heavier than a comparable single lap tile roof.

87

Concrete roofing tiles: These are extensively used today as a substitute for good quality clay

tiles. They are manufactured from a mixture of carefully graded sand, and Portland cement which is compressed in a mould and painted in different colours. Clay roofing tiles : Clay can be excavated, moulded and burned without any expensive or

elaborate machinery and for years clay roofing tiles have been used in Uganda. There are hand made clay tiles and machine pressed clay roofing tiles. Hands made roofing tiles are not so good and usually have a lot of defects. Mangalore clay roofing tiles (Uganda clay roofing tiles)

These are the single lap clay tiles. They differ from ordinary single lap tiles in that one or more grooves exist in the vertical edges of the tiles. The tiles are machine pressed during the manufacturing. They are hung on softwood battens of 50x38 and weighs 40kg per unit roof area. Each unit has a weight of 2.5kg and there are 15 pieces in a square metre. The side laps are usually 50 and the end laps are adjustable with a minimum of 62. Mangalore tiles are of size 400x230. In Uganda roofing timber is supplied in sizes of 150x50, 100x50, 100x 75, 75x50 and 250x 25 and 4.2m long. Roof trusses.

The trusses for mangalore roofing tiles consists of principal rafters of double pieces of 100x50 at 1.8m centres with common rafters of the same size to that of the principal rafters in between at a spacing of 600 centres. The main tie/ tie beams or ceiling joists are of 150 x 50 and the purlins of the same size are used to transfer the loads from the common rafters to the principal rafters. The tie beams are fixed to the legs of the principal rafters at the same centering. Struts and ties are from 100 x50 timbers and the struts serves to transmit the load from the purlins to the tie beams and onto the wall plates which are of size 100x75 38x200 ridge piece 100x50 tie 100x50 principal rafters at 1.8m c/c made of 2pcs of rafters 150 x 50 tie beams/ceiling joists at 1 .8m cen tres

100x50 strut

100x75 wall plate

Figure 21: A typical principal trussed rafter 88

100x50principal rafter

8 - 3 3 3 1 2

38x50 timber battens laid over plain sheets

150x50tie beam 100x75 wall plate

plain galv. sheet metal

Figure 22: Detail of laying mangalore roofing tiles

Traditionally battens were laid on polythene supported by chicken wire mesh due high cost of metal sheets. This was meant to prevent water escaping through the numerous joints to the inside of the roof. Today the cost of galvanized plain sheets of lower gauges have come down and with the coming up of many industries they are readily available in the local markets and as a result most roofs in Uganda are now covered with plain iron sheets underneath the battens to receive tiles. Battens of usually size 50x38 are fixed using wire nails at a margin of 312 to 338. The tiles ate then hooked on the battens starting from down t he eaves moving up the slope of the roof to the ridge piece. Ridge: Any one of the four standard sections of clay ridges may be used to cover the ridge.

Ridges are usually laid using mortar. It is economical to first pack the broken pieces of the tiles around the ridge piece before applying the mortar. Hips: Hips are laid the same way like the ridges. However to prevent the tiles from slipping

down the hip a galvanized iron or wrought-iron hip iron is fixed to the hip or fascia.

Read about: Roofing slates, Pan Tiles, Spanish tiles, and Italian tiles especially the la ying.

Timber Pitched Roofs in Sheet Metal Coverings

Various methods are used to make roofing sheets, two basic types are generally made: corrugated and flat. Galvanized steel, aluminium and galbestos are all used to make corrugated 89

roofing sheets of varying width, depth and pattern of corrugation and allowable span, depending on the gauge and material used. Corrugated sheet metal roofing sheets are normally supported on wood or steel purlins properly spaced according to the gauge of the metal and the roof load involved. Manufactures normally give tables of unsupported length of the sheets depending on the gauges to guide roofers in spacing the purlins. There are two common laying orders for roofing sheets. Laying should start at the leeward end of the building so that side laps will have better protection from wind driven rain. The top edges of eave sheets should extend atleast 38 beyond the back of steel purlin and 75 beyond the centre line of timber purlins. At side laps where edge corrugation of adjacent sheets is opposite in direction, the under lapping side should finish with an upturn edge and overlapping side with a down turned edge. Sheets should extend atleast one corrugation over the gable and there should be 75 of over hang at the eaves. End laps between sheets should generally be 150 and side laps of 1.5corrugations but they may be increased to 225 and two corrugations for extreme conditions Special nails with a ring or screw-type shank should be used for fastening corrugated sheets to wood purlin. Nails should be driven at the top of corrugations, but care must be taken not to drive them so far as to flatten the corrugation, thus preventing the next sheet from fitting properly. Sheets are fastened to steel purlins with stain less self-tapping screws and aluminium washers. Steel Roof Trusses

Mild steel is much stronger than timber, it is more fire resisting and its sections can be readily assembled to for comparatively simple connections. It is principally for these reasons that mild steel is now employed extensively for roof trusses of small and medium spans and its supersede of timber as a material for trusses of large span. Steel for trusses of open (unceiled) roofs of certain buildings, well designed for large spans with light weight members and satisfactory appearance, chiefly because of the small size of the members and the simple joints are commonly in use. Mild steel trusses must be painted periodically to prevent rusting. A steel truss like the built-up truss is a triangulated structure. The principle rafters are prevented from spreading by connecting their lower ends by a tie and strut and ties are provided at intermediate points to afford adequate bracing. The struts should be kept as short as possible. The centre line principle is adopted through out and thus the point of attachment of each purlin

90

coincides with the intersection of the axes of truss members. Secondary stresses such as bending moments in the rafters are thereby avoided. All the members of a modern metal roof truss are mild steel, and most, if not all should be of angles. Angles effectively resist both compression and tension stresses; they can be conveniently attached and the manufacturing process is more economical. Struts consist of either single or double angles and the main consist of either one or two angles placed back to back. Until comparatively recently, it was a common practice to use single or double flat bars for the main tie, as they were suitable for resisting tension stresses, however, owing to wind pressure and the abnormal strain imposed during the transportation and the erection of trusses, members may be subjected to changes of stresses and flats will not resist compression. Flat main therefore tend to become buckled. .if a ceiling is to be provided, ceiling joints can readily be fixed to a main tie of double angles and this is an additional reason why they should be used instead of flats. 5.9 Connections

The members of a truss are connected together normally by means of: a. Bolts and thin plates called gussets b. welding c. rivets ( not in common use) The pitch of rivets is the distance between their centres and should not be less than 2.5 times the diametre of the bolts. The maximum pitch should not exceed 32t or 300mm. the size of the bolts depends upon that of the members to be connected, thus 16mm diametre bolts are commonly employed for angles and flats up to 60mm wide and 20mm diametre bolts for larger members. When making a joint, a number, even if subjected to a small stress, should be connected to a gusset by at least two bolts. If a member consists of double angles, gussets are always placed between them.

Support to the trusses

Sound concrete pads of sufficient thickness and area must be provided to give reliable and level bearing for the end of the truss and to receive the steel fixing bolts. The bolts are called ragged bolts or ragged lewis bolts.

Fixing the roof coverings

Purlins are laid across the rafters to support the sheeting or tiles/slates (battens). The purlins can be from timber members or metal angles or zed sections. The spacing of the purlins will depend 91

on the roof loading, the type of roof covering used and the spacing of the mild steel roof trusses. Manufactures will recommend maximum centres appropriate to the roof coverings. Traditionally a hook bolt was used to fix the sheeting, but this presented problems with water proofing at the top of the bolts. Today an Oakley clip is fixed and adjusted inside the roof and ensures a satisfactory water seal.

100x50 timber purlin

1 80x80x6

rafter L

1 65 x 50 x 6 strut L 1 65 x 50 x 6 ties L

1 65 x 50 x 6main tie L

shoe

6mm gusset

Figure 24: Typical trusses of upto 6m span and upto 3.7 c/c 175x100 purlin

12mmdia.coach bolt 1 4 0

rafter

strut

1 80x80x6L.rafter

150x 150x10L.cleat 150long 1 x60x6.L. Tie 65

0 0 2

Figure (a)section thru the strut

80X80X10 L.cleats welded to bearing plates

150 X140X328Pad stone

Figure (b)section thru the pad stone

92

150x 150x10L.cleat 150long

1 5 0

oakley clip

twin angle rafter

Zed purlin 1 4 0

strut

0 0 2

Figure 26: showing the fixing of the roof covering on the zed purlin

Figure 25: details of a

steel truss connection to the strut and a section thru the pad stone.

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CHAPTER 6

SIX

Windows and Doors

6.2 Windows

A Window is an opening formed in a wall or roof to admit daylight through some transparent or translucent material fixed in the opening. The primary function of a window is served by a sheet of glass fixed in a frame in the widow opening. This is a simple widow and is termed a dead light window because no part can be opened. As the window is part of the wall, it should serve the functional requirements of a wall like exclusion of wind and rain, act as a barrier to excessive transfer of heat and sound and should be fire resistant in the same way just like the surrounding wall and roof. The function material of a window, glass, is efficient in admitting day light and excluding wind and rain but is a poor barrier to the transfer of heat, sound and the spread of fire. The traditional window is usually designed to ventilate rooms through one or more parts that open to encourage an exchange of air between inside and outside. It is important to separate windows from ventilations so that the window may be made more effectively wind and weather tight and ventilation can be more accuratel y controlled. 6.3 Functional requirements

The primary function of a window is: admission of light. The secondary functions are: a view and ventilation. The functional requirements of a window as a component part of a wall or roof are: •

Strength and stiffness.

•

Exclusion of wind and rain

•

Thermal insulation

•

Sound insulation

•

Fire resistance

Strength and stiffness

A window should be strong enough when closed t o resist the likely pressures and suctions due to wind, and when open be strong and stiff enough to resist the effect of gale force winds on opening lights. A window should be sufficiently strong and stiff against pressures and knocks due to normal use and appear to be safe, particularly to occupants in high buildings. A window

94

should be securely fixed in the wall opening for security, weather tightness and the strength and stiffness given by fixings. Exclusion of wind and rain

Air tightness: to conserve heat and avoid cold draught it is good practice to design windows so that there is little unnecessary leakage of air. Air movement through closed windows may occur between the window frame and the surrounding wall, through cracks between glass and the framing, through glazing joints and more particularly through clearance gaps between opening lights and the window frame. Leakage around window frames, around glass and through glazing joints can be avoided by care in design, construction and maintenance. The flow of air through windows is caused by changes in pressure and suction caused by wind and may cause draughts of in ward flowing cold air and loss of heat by excessive inflow of cold and outflow of warm air. It is to control this air movement that systems checks rebates and weather stripping are used in windows. Exclusion of rain : Penetration of rain through cracks around opening lights, frames or glass

occurs when rain is driven on to vertical windows by wind so that the more the window is exposed to driving rain, the greater the likely wood of Rain penetration. The performance of windows in excluding rain is tested in the laboratory by throwing water in droplets, from horizontally mounted jets, in a band some 50 deep at the head of the test window so that water runs down the window face. To minimize the penetration of driven rain through vertical windows the followings should be done: •

Set the face of the window back from the wall face so that the projecting head and jamb will to some extent give protection by dispersing rain

•

Ensure that external horizontal surfaces below openings are as few and as narrow as practicable to avoid water being driven into the gaps.

•

Ensure that there are no open gaps around opening lights by the use of lapped and rebated joints and that where there are narrow joints that may act as capillary paths there may be capillary grooves.

•

Restrict air penetration by means of weather stripping on the room side of the window so that the pressure inside the joint is the same as that outside; a pressure difference would drive water into the joint

•

Ensure that any water entering the joints be drained to the outside by open drainage channels that run to the outside.

95

Weather stripping: In modern window design weather stripping used depends on the opening

movement of the windows, compression strips being used for hinged and pivoted opening lights and wiping sliding seals for sliding windows. The material used is resilient rubber compounds in the form of compression strips and seals or nylon pile strip. Thermal insulation

Unlike the wall around it a window, which is a component part of a wall, will affect internal thermal comfort in two ways: by its transmittance of heat and through the penetration of the radiant heat of the sun that causes solar heat gain. Heat is transferred through a wall or window by conduction, convection and radiation. Sound insulation

There is a considerable variation in the level and type of noise that different people can tolerate without discomfort. In order to establish an acceptable noise level it is necessary to assume a measure of sound level that corresponds to subjective judgment of noise. The audible frequencies of sound are from about 20Hz to 20000Hz, where Hz represents the unit hertz where one hertz is equal to one cycle per second. Noise is the general term used for the subjective judgement of level of sound that is distracting or uncomfortable and therefore unacceptable. Tolerable sound level depends on the activities of those inside particular rooms and the general background level of sound within the room. The transmission of sound through materials depends on their mass, the more dense or heavier the material the more effective it is in reducing transmission of sound. The reduction of sound transmission is termed sound insulation. Because of the thin material with which they are glazed and the necessary clearance gaps around opening lights, windows afford poor insulation against external noise. Open windows, as well as providing an obstructed path for intrusive sound, may often serve to reflect external sounds into rooms. Insulation of a glass can be done by use of thicker glass or doubling the glass. Doubling the glass by sealed double glazing is not so effective. It is advisable to use double windows wi th two separate sheets 200 to 300 apart. Fire resistance

Ordinary glass cracks and breaks within a few minutes when subjected to the heat generated by fire. To limit the spread of fire, regulations require fire breaks to windows to limit the spread of fire to adjacent buildings. Fire breaks are solid incombustible upstands or projections to windows that serve as a barrier to the spread of fire. Wired glass also limits the spread of fire as the broken glass will be held in place. Day light

96

The prime function of a window is to admit daylight for day time activities in sufficient quantity for efficient performance. The quantity of light admitted depends in general terms on the size of the window or windows in relation to the area of the room lit and the depth inside the room to which useful light will penetrate depends on the area of the window and the height of the head of the window above floor level. Good sense dictates taking the maximum advantage of this free source of illumination. The accepted minimum level of day light for performance of various activities varies from ½ for bedrooms, 1 for living rooms to 6 for drawing rooms, the figures given being the day light factor which is the percentage of day light admitted through a window from the hemisphere of unobstructed sky. In a room with windows on one long side with no external obstructions and a room surface reflectance of 40%, where the glass area is 1/5

th

0r

20% of the floor area, the average day light factor will be 4 and the minimum half of 4.

Example: Determine the size of a window in a room measuring 4x3 with a daylight factor of 6. Solution: Floor area…………………………………..12m

2 th

The average day light factor in side –lit rooms is roughly equal to 1/5 of the percentage ratio of glass to floor area. Conversely required glass area = ………………………...6x12x5/100 ………………………………. = 3.6 m

2

Window sizes say 2.4x1.5 or two windows of 1.2x1.5 Ventilation

For the comfort and well being of people it is necessary to ventilate rooms by allowing a natural change of air between inside and outside and outside or to cause a change by mechanical means. The necessary rate of change will depend on the activities and numbers of those in the room. th

The total area of ventilation for any habitable room can be calculated as 1/20 of the floor area. The size of a ventilating opening, by itself, gives no clear indication of the likely air change as the ventilating effect of an opening depends on air pressure difference between inside and outside and the size of opening or openings through which air will be evacuated to cause air flow. 6.4 Materials used for windows

The common materials for making windows are: Wood, steel, Stainless steel, aluminium, bronze, and plastics

97

Wood: The traditional material used for making windows is wood, which is easy to work by

hand or machine, can readily be shaped for rebates, drips, grooves and mouldings, has a favorable strength to weight ratio, and with good t hermal properties. The disadvantages of wood are the considerable m oisture movement that occurs across the grain with moderate moisture changes and liability to rot. The dimensional changes can make the joints to open and admit water that increases the moisture content that can lead to rot. Where windows are made of soft wood timber it should regularly be painted besides treatment with preservatives. This is to avoid rot.

Steel

Steel section windows have been in use for quite a long time and it is gaining popularity over timber windows. Steel windows often rust, and corrode there fore care must be taken by use of zinc coating or regular painting. The advantage it has is the slender sections for both frame and opening lights that are possible due to inherent strength and rigidity of the material. The disadvantages are high thermal conductivity that makes the window framing act as a cold bridge to the transfer of heat and the very necessary regular painting required to protect the steel from rusting. Aluminium

Aluminium windows are made from aluminium alloy of magnesium and silicon that is extruded in channel and box sections with flanges and grooves for rebates and weather stripping. Aluminium windows have adequate strength and stiffness with good resistance to corrosion and can also be readily welded and brazed. The advantages of aluminium windows are the variety of sections available for the production of a wide range of window types, and the freedom from destructive corrosion. The disadvantage is however the high thermal conductivity of the material which acts as a cold bridge to heat transfer and aluminium window is relatively very expensive. Stainless steel: It is made from an alloy of steel and chromium making it corrosion-resistant and

expensive. Because of its cost it is used in windows as a thin surface coating to other materials such a wood and aluminium for its appearance and freedom from corrosion. Bronze: Manganese brass is the material commonly used for bronze windows. The material is

rolled or extruded to form window sections. It has advantages of freedom from corrosion, high strength to weight ratio, and attractive colour and t exture of the material.

98

Plastics: This is the latest material to be used as windows with a particular advantage of being

maintenance free. The disadvantage of plastic is that it is less rigid than the wood or steel and does not resist heat and fire.

6.5 Window types Fixed light: A fixed light or dead light is a window opening in which one square, pane or sheet

of glass is fixed either directly to the wall structure or more usually to a frame which is in turn fixed to the wall so that no part of the window will open. Opening light: A n opening light is the whole or part of parts of a window that can be opened by

being hinged or pivoted to the frame or can slide open inside the frame. Windows with opening lights are classified in accordance with the manner in which the opening lights open inside the frame as below •

Pivoted

•

Hinged

•

Sliding and

•

Composite action

And as a broad classification as: •

Side hung, Top hung and Bottom hung

•

Horizontally pivoted and vertically pivoted

•

Vertically siding and horizontally sliding and sliding folding

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to p h u n g

s id e h u n g

h o r r iz o n t a lly p i v o t e d

b o t to m

v e r t. p iv o te d

hung

louvre

h o r r iz o n t a l ly s l i d i n g

v e r t s l id i n g

Side hung: The traditional casement consists of a square or rectangular window frame of wood

with the opening light or casement hinged at one side of the frame to open in or out. The side hung opening part of the window is termed the casement and it consist of glass surrounded and supported by a wooden frame as below with a simple one light casement, opening out.

head

hinge w in d o w fr a m e fra m e o f c a s e m e n t or sash

glass sill

post

c a s e m e n t h in g e d a t s id e t o o p e n o u t S i d e h u n g c a s e m e n t w in d o w

The traditional casement is hinged to open outward. An outward opening casement can more readily be made to exclude wind and rain than the one opening in as the casement is forced in to the outward-facing rebate in the frame by wind pressure and the outward facing rebate is more effective than the inward facing rebate. Because casement is hinged on one side, its other side tends to sink, due to the weight of the casement when it is open. If any appreciable sinking occurs the casement will bind in the window frame and in time may be impossible to open. The wider the casement the greater its

100

weight and the more likely it is to sink. It is considered wise to construct casement of widths of not more than 600. Where a window is wider than 600, you design more than one casement. A window of two casements can be designed with the casements hinged so that when closed they meet in the middle of the window. It is usually considered better to construct the window frame with vertical wood members, called mullions, to which each casement closes to avoid jamming of casements where they meet in the middle. Because a casement does not provide close control of ventilation it is common to provide small opening lights, called vent lights, which are usually hinged at the top to open out head head

ventlight open transom

hinge hinge frame of casement or sash

glass

mullion sill

glass

glass

mullion sill

casement hinged at side to open out

casements close to mullion

glass

casement hinged at side to open out

casements close to mullion with ventlights

Casement windows with vent lights are usually designed so that the transom is above the average eye level of people using the room (2.1m) for obvious reasons. The disadvantage of casement window is that the casements, ventlights, mullions, and transoms reduce the possible unobstructed area of glass and therefore day light through a window of any size and the many clearances gaps around opening casements and ventlights and frame members emphasize the problem of making the window weather tight. Wood casement windows

For years wood casement windows have been the traditional windows for small buildings. To provide adequate strength and stiffness in the frame, casements and ventlights of casement windows and to accommodate rebates for casements and ventlights and for glazing, timber of adequate section has to be used and joined. The traditional joint used is the mortice and tenon joint in which a protruding tenon, cut on the end of one section fits into a matching mortice on the other, the joint being made secure with glue and wedges as below:

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head

tenon

hinge

glass

dead light

mullion mortice

sill wedge casement hinged at side to open out

casements with a dead light on one side and o en ventli hts

style

Where mass production of wood windows is expected, combed joints are used. This involves interlocking tongues cut on the ends of members which are put together, glued and pinned. A casement window frame consists of a head, two posts (jambs) and a sill joined with mortice and tenon depending on the number of casements and ventlights.

Wood members

The members of a wood window frame are cut from 100x75 or 75x50 sawn timbers for the head, posts and mullion and from 150x75 or 100x63 for sill and transom. Similarly the rails and stiles of casements and ventlights are cut from 50x50 or 50x44 sawn timbers which are planed (wrought) and whose finished sizes are about 45x45 or 39. The sawn timbers are planed smooth (wrought) and this reduces their sizes by about 5mm on both sides. Window frames

The members of the frame are joined with wedged mortice and tenon joints. The posts of the frames are tenoned to the head and sill with the ends of the sill and head projecting some 40mm or more each side of the frame as horns. These projecting horns can be built into the wall in the jambs of openings or they may be cut off on sites if the frame is built in flush with the outside of the wall. The reason for using a haunced tenon joint between posts and head is so that when the horn is cut off there will be a complete mortice and tenon left. Fixing windows

Wood window frames are usually built in to solid walls as the walls are raised. The other method is to fix the window in position after the wall is built. Wood window frames are secured 102

in position in solid walls by means of galvanized steel cramps or lugs that are screwed to the back of the frame and built into horizontal brick or block work as the wall is raised. The spacing is the cramps should be between 300 and 450. The other approach is to do the finishing according to the sizes of the frames and use raw bolts to fix them. Casement: The four members of the casement are two stiles, top rail and bottom rail. The stiles

and top rail are cut from 50x44 timbers and the bottom rail from 75x44 timbers. The stiles and rails are rebated fro glass and rounded or moulded on their inside edges for appearance sake. The rails are tenoned to mortices in the stiles and put together in glue, cramped up and wedged Ventlights: The four members are cut from the same timbers as the stiles of the casement and

are rebated, moulded and joined in the same way as for the casement. Some standard wood casement sizes. Heights

Widths

900

600

900

1200

1500

1800

2400

1050 1200 1500

The manufactures of standard windows produce a range of standard windows. The advantage of having standard windows is in the economy of mass production. In line with the move to dimensionally co-ordinate building components and assemblies the standard range of windows may fit with such allowances for tolerances and joints as appropriate. The purpose of dimensional co-ordination is to rationalize the production of building components and assemblies through the standardization of sizes within a frame work of basic spaces into which the standard components and assemblies may fit.

Hinges and fasteners

Wood casements and ventlights are hung on standard steel butt hinges or on metal offset hinges. The offset hinge is designed with the pin offset outside the window so that when the casement is open there is a gap between the hinged edge of the casement and the frame wide enough for access for cleaning the outside of glass from within t he building. Steel windows:

Steel casement windows are made either from standard Z-sections or the universal sections. Steel casements are assembled by welding the joints. 103

Standard steel casements are made from the hot rolled steel Z-sections which are used both for the frame, casement and ventlights. The section is cut to length and mitred and welded at the corners. The assembled and cleaned parts of the window are then rust proofed by the hot dip, galvanizing process in which the window parts are dipped in a bath of molten zinc. Hinges and fasteners: Steel casement windows are fitted with steel butt or offset hinges and

lever catches and stay similar to those used for wood windows, the fittings being welded to frame and casement. Fixing steel windows

Standard steel windows are usually built in to openings in solid walls and secured with buildingin lugs or ties that are bolted to the back of the frames through a slot that allow adjustment for building into horizontal brick or block courses. Window sills

It is good practice to set the outside face of widows back from the outside face of the wall in which they are set so that the reveals of the opening give some protection against driving rain. Wind driven rain which will run down the impermeable surface of the window glass to the bottom of the window should be run out from the window by some form of sill. The function of an external sill is to conduct the water that runs down from windows, away from the window and to cover the wall below the window and exclude rain from the window. The material from which the sills are made should be sufficiently impermeable and durable to perform its function during the life of the building. External sills are formed either as an integral part of the window frame, as an attachment to the under side of the window or as a sub-sill, which is in effect a part of the wall designed to serve as a sill. The materials used for the construction of window sills are: natural stone, cast stone, concrete, slates, tiles and bricks.

w e a t h e r e d w in d o w s ill w it h a th r o a t b e lo w

S e c t io n

th r o u g h

a

w in d o w

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6.6 Glass and Glazing

Glass is made by heating soda, lime and sand to a temperature at which they melt and fuse. Molten glass is drawn, cast, rolled or run onto a bed of molten tin to form flat glass. The followings types of glass are in use in buildings: Float glass, patterned glass, wired glass, toughened glass, clear sheet glass, polished plate glass, double glazing units. Glass are manufactured in thicknesses of 3 to 19mm

Wind loading

Glass should be sufficiently thick in relation to its area to safely withstand wind pressure and suction. The likely wind pressure depends on the exposure of the building and three grades of exposure are defined as sheltered, moderate and severe Glazing

The operation of fixing glass in windows, doors and openings is termed glazing. Glass must be accurately cut to size to provide an edge clearance between the edges of the glass and the bed of the rebate to allow for variations in the sash or frame and of the glass and to facilitate setting the glass in position. An edge clearance of 2mm for putty glazing and 3 for other methods of glazing for glass upto 6 thick and upto 5 for thicker glass must be made. To secure glass in the glazing rebates with the requisite edge clearance all round, setting locks are placed below the glass. The setting glass are made of pvc, hammered lead, hard nylon or hard wood from 25 to 150 long and of the same thickness as the edge clearance. The two common methods of glazing are putty and bead glazing 6.7 Doors

A door is a solid barrier to a doorway or opening that can be opened for access and closed to deny access for privacy and security and serves as a thermal, acoustic and fire barrier as part of an external wall. A doorway is an opening in a wall or partition for access and a door frame or lining is the timber or metallic or plastic frame or lining fixed in the doorway or opening to which the door closes on hinges, pivots or runners 6.8 Functional requirements of doors

The primary function of a door is to provide access. The secondary functions as a components part of a wall or partition are: 105

•

Strength

•

Shape stability

•

Privacy

•

Security

•

Thermal insulation

•

Sound insulation

•

Fire resistance

•

Exclusion of wind and rain as a part of an external wall.

Means of access

The operating characteristics of a door to serve this function depend on the weight of the door itself and the hardware such as hinges and locks and fitments such as door closers fixed to the door and frame and draught stripping which cause operating difficulties. Door(s) leaf

The traditional domestic door is of one leaf which is hinged on one side to open in one direction for the convenient entry or exit of people. Double-leaf, double swing, sliding, and sliding and folding doors are also used for both domestic and other purposes. The word leaf refers to the opening part of a door. Doors are made of timber, aluminium, steel and plastics j ust like the windows Standard Doors

The standard size of door leaf are weight 2040, width 526, 626, 726, 826 and 926 for internal doors and height 1994, width 806 and 906 for external doors and thickness 40 or 44. A door set is a standard combination of door leaf with frame or lining and hinges and furniture packed as a unit ready for fix. Wood Doors:

Wood doors may be classified as:(i)

Flush doors

(ii)

Panelled doors

(iii)

Match boarded doors.

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top rail

brace

stile

middle rail

panel bottom rail matchboarded door

panelled door

flush door

Flush Doors

The fashion in buildings has been for plain surfaces devoid of decorative mouldings that will collect dust. Hence the use of flush doors which are surfaced with sheets of hardboard or plywood fixed either to a cellular skeleton or solid core.

Cellular core flush doors:

These doors are made with cellular, fibreboard or paper core in a light softwood frame with lockage blocks covered with plywood or hardboard both sides. Skeleton frame flush doors:

In skeleton core flushed doors, a small section in timbers is constructed as illustrated above. The main members of this structural core are stiles and rails, with intermediate rails.

core strips glued together

soft wood frame

lock block

lock block

ply wood or hardboard facing glued to frame & cellular core

ply wood or hardboard facing glued to frame & cellular core

Cellular core flush door

Skeleton frame flush door

Solid core Flush door

107

Solid core flush doors:

Plywood and hardboard facings bonded to cellular or skeleton flush doors do not always remain flat and waves on their surface may be apparent particularly if the door is painted with gloss paint. A flush door with a solid core of timber, clipboard, flax board or compressed fibreboard can be used for public buildings and oth er buildings with high levels of use externally and as fire door. It has better thermal and acoustic properties than cellular core or skeleton core flush doors.

Fire doors:

The term fire is used as a general description of all doors that serve to control the spread of fire or the smoke and gases resulting from the fires in buildings. The term fire resisting is used more specifically to describe a door, together with its frame, that has resistance to collapse, flame penetration and excessive temperature rise for a stated period of time during fires. Fire check door:

This most accurately describes the function of a fire door in checking the spread of fire for a stated period of time. Smoke control doors:

This accurately describes the purpose of fitting a door solely to c heck the spread of smoke: Function

Most fires in buildings from small sources which develop quantities of smoke and other combustion products in the early stage of the fire.

Pressure differences may force smoke

through gaps around the door. As the fire develops and the temperatures rises on the effect of the heat of the fire without collapsing and be capable of serving as a barrier to the spread of excessive quantities of heat and hot gases Construction

A range of wood doors has been tested to give fire resistance from 30 to 60 minutes. These include skeleton-core flush with a plasterboard core and solid-core flush door with solid timber, compressed, straw, chipboard, flax board or compressed fibre board strips. The resistance of a door set to the spread of smoke and fire depends on the door frame and the door and its fittings. Use of Intumescent Strip:

This is a material that swells when heated by foaming and expanding. The material is used with aluminium or PVC cover strips fixed in rebates to the edges of the door or frames so that in fires the Intumescent material expands and seals the gaps between the door and frame as a barrier to the spread of smoke and fire. The seals incorporate a neoprene draught strip that serves as a 108

smoke seal in the early stages of a fire and Intumescent material acts as a seal against the spread of fire in the later stages. Hinges, locks and door closers:

For a door to be effective as a barrier to smoke and frame, it must be held securely in position on its hinges and firmly on the closed position by the latch and be self closing for the period of minutes specified for stability and integrity.

The purpose three steel hinges are generally

recommended. The latch must be strong and engage the latch plate at least 10 to maintain the door in the closed position. Panelled Doors

Panelled doors are framed with stiles and rails around a panel or panels of wood or plywood. The stiles and rails are cut from timbers of the same thickness and some of the more usual sizes of timber used are; stiles and top rails 100×38 or 100 ×50; middle rail 175×38 or 175 ×50, bottom rail 200×38 or 200×50. Because the door is hinged on one side to open, it tends to sink on the lock stile. The stiles and rails have to be joined to resist the tendency of the door to sink and the two types of joint used are a mortice and tenon joint or a dovetail joint. Mortice and tenon joint:

This is the strongest type of joint used to frame members at right angles in joinery work. The panels are usually jilted into stiles, rails and cramped after gluing and wedging around the panels. For economy and mass production dowel joints should always be used.

Panels:

Timber panels, more than 250 wide are made up from boards 150 wide that are tongued together. The term tongued describes the operation of jointing boards by cutting grooves in their edges into which a thin tongue or feather of wood is cramped and glued. Plywood:

This is made from three, five, seven or nine piles or thin layers of wood firmly glued together, so that the long grain of one ply is at right angles to the grain of the plies to which it is boarded. The most pronounced shrinkage in wood occurs at right angles to the long grain of the wood and any shrinkage of the centre ply is resisted by the outer plies, hence the odd number of plies used. Plywood does not shrink appreciably and because of the opposed long grains, it does not warp or twist. The three plywood 5 or 6.5 mm thick is generally used for door panels. Fixing panels:

This is done by fixing panel in the grooves cut in the edges of the stiles and rails. If any shrinkage of the members of the door occurs, gaps will not appear around the panels. A panel 109

set in grooves to stiles and rails with square edges may leave an unfinished look which can be modified by cutting mouldings on the edges of the members. An inferior method of fixing panels is to plat nail timber beads each side of the panel. Double swing doors

Doors are hunged to swing both ways to provide ready access to and from parts of buildings used in common by the occupants and users at points where it is convenient to provide an opening barrier, for example from halls to corridors, to provide some separation of the public and the more private parts of the building. These doors, which are liable to heavy use, are usually constructed as panelled doors with a glazed panel at eye level to prevent accidents due to simultaneous use for each side. The door leaf is hung either on double action hinges or pivoted on a double-action floor spring and top pivot Sliding and sliding folding doors

Sliding doors are designed for intermittent use to provide either a clear opening or a barrier between adjacent rooms or spaces to accommodate change use or function, and in narrow spaces to avoid the obstruction caused by hinged leaf. They are also designed for intermittent use to provide a larger opening than is practical with sliding doors, and to divide large spaces into smaller by closing back to one by opening. Matchboarded doors

Matchboarded doors are made with a facing of tongued, grooved and V-jointed boards fixe3d vertically to either ledges and braces or a frame. These doors are used for cellars, sheds and stores where the appearance of the door is not important. Ledged Matchboarded door . Matchboarding is nailed to horizontal ledges directly. The direct

nailing does not strongly frame the door which is liable to sinking and losing shape. Ledged and braced Matchboarded : This type of door is strengthened against sinking with

braces between the rails and is fixed at an angle to resist sinking on the lock edge. The braces are nailed to the boarding Framed and braced Matchboarded door : the match boarding is fixed to a frame of stiles and

rails that are framed with mortice and tenon joints with braces to strengthen the door against sinking. Hardware for doors

Hardware is the general term for the hinges, locks latches and handles for a door. Ironmongery was a term used when most of these were made of steel or iron. Examples of hinges are; pressed steel butt hinges, cast iron butt hinges, brass butt hinges, steel skew butt hinges, hook and band hinges. (Read about more) 110

matchboarding 25, T&G, V-joint both sides 150

ledge100x32

matchboarding nailed to ledges

matchboarding 25, T&G, V-joint both sides

ledge 150x25

sitle 100x50 ledge 150x25 brace 100x25

ledge 150x32

Ledged martchboarded door

ledge 150x25 ledge 150x32

Ledged & braced martchboarded door

Framed, braced & martchboarded door

Latches and locks

The word latch is used to describe any wood or metal device which is attached to a door or window to keep it closed and which can be opened by the movement of the latch operated by a handle, lever or bar, a lock is any device of wood or metal attached to a door which can be used to keep it closed by application of a loose key. Examples of locks are mortice locks, rim lock and mortice dead lock. Door frames and linings

A door frame is a surround in a door way or opening, to which the door is hung and to which it closes, which sufficient strength in itself to support the weight of the door. A door lining is a surround inside a door way or opening , as wide as the reveal of the opening, to which the door is hung and closes, which is not in itself strong enough to support the weight of the door without support from the surrounding wall or partition. Door frames and linings maybe made of wood, metal or plastic. Wood door frames A door frame consists of three or four members which are either rebated 13mm deep for the door or a wood stop 13mm is planted to the frame. A frame consists of two posts and a head member and may also have the fourth member, a threshold or sill to assist in weather exclusion.

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head 100 x 75

Ends of head project 100 as horns for building in the wall 100 posts of frame 100 x 75 rebate for door 50 13 40/44 section through the post

Dowels

Door frames that do not have a threshold or sill are often secured to the floor by a mild steel dowel, 12mm diametre and 50 long that is driven into the foot of the posts and set the concrete floor. Fixing door frames

Door frames are usually built in, which describes the operation of building walls or partitions around the frame. The frame is secured onto the walls with L-shaped galvanized steel buildingin lugs which are screwed to the back of the frames. Frames are also fixed in by screwing in through to wood plugs fitted in the walls during finishing

Threshold or sill

A wood sill to an external door is usually of some wood, such as oak, and t he sill is joined to the posts of the frame with haunced mortice and tenon joints. The sill is usually wider than the frame and is rebated for the door 13 deep for an outward opening door and grooved for water bar for an inward opening door and weathered and throated. Standard wood door frames and door sets

There are no generally available standard wood door frames and linings for standard doors. Manufacturers offer standard frames for standard doors of sections from ex. 104x64 to ex 89x64, rebated for doors with co-ordinating dimensions of frame, 900, 1000, 1200, 1500, 1800, 2100 wide and 2100 high.

112

Metal door frames

These are manufactured from mild steel strip pressed into one of the three standard profiles. The same profile is used for head and jambs of the frame. The three pressed steel members are welded together at angles. Two loose pin butt hinges are welded to one jamb of the frame and an adjustable lock strike plate to the other. Two rubber buffers are fitted into the rebate of the jambs to which the door closes to cushion the impact sound of the door closing. Metal door frames are built in and secured with adjustable metal building in lugs. The frames may be used externally or internally.

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CHAPTER SEVEN 7

Temporary Works

Temporary work refers to any construction activity which is not a permanent part of the structure. These are works which are carried out as a means or process of executing the permanent work. It can also be called ‘false works’. 7.2 Scaffolds

This is a temporary structure which provides access on or from which persons work or that is used to support materials or equipment. Basic requirements

Where work can not be safely done from the ground level, any part of the permanent structure, scaffolds can bear on the ground, anchored on the permanent structure from up down wards or cantilevered Objectives •

You should be able to identify the different members of a scaffold

•

Know the types of scaffolds

•

Know how to erect and strip a scaffold

Characteristics of false works •

False works are normally designed for short term loading

•

Has no visual requirement

•

Often dismantled under load

•

The materials can be second hand

•

Does not need much skill in erecting

•

Made up of numerous small components usually assembled by simple connections

•

Alterations are inevitable.

Problems associated with scaffolds: •

It often overturns due to light weight members

•

Usually unstable

•

Has a high windage surface on the top

•

Scaffolds are given scan attention

Types of scaffolds

Independent scaffold 114

Builders or put-log scaffold Towers Independent scaffolds: An independent scaffold stand on its own without getting support from

the structure being constructed.

g u a r d rail le d g e r s b o a r d e d p la t fo r m

s t a n d a r d to e

b o a rd

c ro s s

b r a c e s

transom b a s e p la te 1 5 0 x 1 5 0 s o le p la t e

in d e p e n d e n t s c a ffo l d

transom b a s e p la t e 1 5 0 x 1 5 0 s o le p la t e

b u i d e r 's s c a f f o l d

Figure 1 Independent scaffolds and Put-log scaffold

When assembling metallic props fittings and couplers are used. The members tying them horizontally are called laces. 115

Standard: This is a vertical or near vertical tube that transmits loads to the foundation (base

plate) and onto a sole plate. It is usually of size 48.3mm and 4mm thick. Ledger: It is a longitudinal tube normally fixed parallel to the face of the building in the

direction of the larger dimension of the scaffolds. It acts as a support for the put logs and transoms and frequently for the tie tubes and ledger braces. They are usually joined to adjacent standards. Braces: This is a tube fixed diagonally and with a tolerance of 25mm in 2m. They should not be

fitted to touch the grounds. It prevents lateral movements. They are fixed diagonally with respect to the vertical and horizontal members to accord stability. They are usually cross and longitudinal bracings. Tie or tie assembly

This is a system of tubes attached to anchorage on buildings or framed around part of it or wedged or screwed into it with a tie tube; used to secure the scaffold to the structure

b r id le

A

t ie

a s s e m b ly

th ro u g h a

w a ll .

A t ie a s s e m b ly w in d o w

th ro u g h a

Figure 2 Tie assembly

Care must be taken to ensure the wall above is in compression or it should have a beam to support the tie system otherwise the wall might collapse outwards. Bridle: This is a horizontal tube fixed across an opening or parallel to the face of a building to

support the inner end of a put log, transom or a tie tube. Guard rail: A member incorporated into the scaffolding system to prevent the fall of a person

from a flat form or access way. Most scaffolds should be atleast one board (300) from the building Buttresses: These are used to add on lateral rigidity and especially when the height is going

higher for independent scaffolds.

116

Ladders: These are used to access the scaffolds and should always go above the working plat

form atleast 1070mm to enhance easy climbing Lift: This is the distance from a ledger to ledger and should always be atleast 2.0 to

accommodate a person working. Bay length: This is the centre to centre of standards and are usually 1.5 to 1.5m

Nets, fans or sheets should always be incorporated to prevent particles from flying off and disrupting the pubic from normal operation around the site and also allow operatives to concentrate. Independent tied scaffolds are sometimes called double scaffolds and can be classified into t hree according to their use. The working platform: This is where workers stand to execute the works. It is where the

materials for the days work are also kept. Platforms for walkway is usually 630wide, for materials and walkway 830mmwide (4boards) and if barrows are used then add 200 to the four boards. It is usual to use 5boards Types of scaffolds

Purpose For painting or cleaning faces of buildings.

Light duty

Only one platform used at a time.

To provide upto four working platforms in use at any one time. Maximum load per General purpose

platform is 180kg/m2

Heavy duty

Has two

heavy duty

platforms& two

working platforms with maximum load of 290kg/m2 g u a r d r a il

toe board

transom

l o n g i tu d i n a l b r a c i n g ledger b a s e p l a te standard s o l e p l a te

T h i s i s c a l le d l o n g i tu d i n a l b r a c i n g . T h e a n g l e o f t h e r a k e r shou ld be at 45 de grees and u p to full height of the scaffold

Figure 3 Elevation showing the different components

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The load on the standards needs to be spread over a sufficient area of ground to avoid settlement. This is achieved by using a scaffolding fitting called a base plate and a timber sole plate as seen from the above figure. A builder’s scaffold is usually made up of a single line of standards; as a result it gets its support from the structure being erected as seen from the sketch. It is important to introduce members called laces at some intervals on the standards to reduce the effective heights. Safety considerations when using scaffolds •

After the erection, the scaffolds should always be checked for any false like missing components, settlement etc.

•

Materials should not be thrown down or up

•

Materials should be heaped close to the standards

•

If any member is to be dismantle temporarily, it should first be braced

•

The loading should always be axially done

Hoist: This is a power operated means of delivering or transporting workers and materials to the

work points A hoist is composed of: •

A gate at every platform

•

Lift

•

A cage were workers and materials are l oaded for transporting

Differences between a temporary work and permanent works Temporary works

Permanent works

Short term loading

Permanent loading

No visual beauty is required

Appearance very important

Dismantled under load

Needs skilled labour

Second hand materials

No part can be dismantled under load

Does not require skilled labour

Materials are usually first hand

Alterations are inevitable

Read about :

1. Tower scaffolds, Cantilever scaffolds, Trussed scaffolds ,Suspended scaffolding 2. Trestles 118

7.1 Formworks

Definition: A formwork is a container within which in-situ concrete is cast. The purpose is to contain freshly placed concrete until it has gained sufficient strength to resist superimposed loads, frost damage and mechanical damages. Formwork also helps to produce the desired shape and finish to the concrete member The principle of construction of formworks must revolve around the following requirements 1. Strength: Formwork must be sufficiently strong to support the loads imposed during placing and curing concrete. These loads will be dead load of the fresh concrete and the dead load of formwork. It should also contain the live loads due operatives, mechanical compaction and tools and equipment. Formworks should be tight fitted and to the required tolerance. These tolerances are usually given in the specifications for the work. Economics dictates that the shapes of the members be the same allowing the formwork to be reused. The sequence of reusing formwork is as below: I.

Position steel reinforcement or position formwork

II.

Check for alignment, plumb and tolerance

III.

Concrete member

IV.

Cure concrete

V.

Support until concrete achieves required strength

VI.

Strike formwork

VII.

Clean and re-use.

2. Durability: It should withstand adverse conditions for the expected period of use 3. Impermeability: The material should be impermeable to a void the lost of water. 4. Surface finish: The finish required to the concrete can affect the cost considerably. Formwork should be able to produce the desired finish on the surface. There is direct finishing after formwork is removed, indirect finishing where some portions of the concrete is removed to say expose coarse aggregates as desired and secondary finishing where concrete is added say by rough casting after striking formwork. 5. Cost: Cheapness must be in consistence with quality and this can be achieved by re-use of standard stock. 6. Economy: Consider the re-use value without cutting unnecessary waste. 7. Ease of fixing and striking : consider the problem of striking without damaging the concrete and the formwork. Formwork must be easy to assemble and dismantle Materials from which formworks are made:

Timber, steel, plastics, rubber, plywood, fibre glass, plaster of Paris, aluminium, iron sheets 119

Timber formwork

This is the most commonly used and is divided into hardwood and softwood. Softwood is the most commonly used for formwork because it is cheaper than hardwood. The softwoods used are kirundu, Cyprus, pines, etc. Besides being cheap they are soft and hence easily worked. Formwork to columns :

chemical releasing agent coated on sheeting

adjustible props

starter bar

50mm conc kicker

softwood framing wt of conc. + vibration transfered to sheeting

plan of column

column elevation

Figure 6.4 Column Formwork

The weight of wet concrete plus the equipment and vibration load has to be supported. Column forms are often subjected to a much greater lateral pressure than wall forms because of their comparatively small cross-section and relatively high rates of placement. It is therefore necessary to provide tight joints and strong tie support. Some means of accurately locating column forms, anchoring them at their base, and keeping them in a vertical position are also prime considerations. Where possible a clean out opening should be provided at the bottom of columns so that debris may be removed before pouring begins. Windows are often built into one side of tall column forms to allow the placing of concrete in the bottom half of the form without having to it from the top. Columns may be square, rectangular, round, or irregular, and forms may be of wood, steel or fibreboard

120

7.2 Factors which influences the pressure of concrete on the formwork: •

Density of concrete

•

Depth of concrete

•

Workability of the mix

•

Rate of placing

•

Concrete temperature

•

Height of lift

•

Section of the formwork

•

Type of cement used

Form treatment, care and removal •

In nearly all types of building construction, formwork constitutes a significant part of the cost of the building. In order to keep this cost at a minimum, forms are often made reusable, either wholly or in part. They must therefore be designed so that removal is simple and can be accomplished without damage to the form sections. Care must be taken in handling and storing these units so they will not be broken or damaged and will be available for re-use. In order to facilitate removal, form faces must be treated to prevent concrete from adhering to them. A number of materials are available for this purpose usually consisting of liquids which are to be brushed or sprayed on the form. Wooden forms must be treated to minimize absorption of water. Oil is one material used for this purpose. Form sealers which coat the surface of the form with an impervious film are also used for this type of treatment. Form removal must be carried out without damaging either the forms or the structure being stripped. Levers should not be used against the concrete to pry forms away because green concrete is relatively easy to damage.

Positioning the formwork :

The starter bars usually go through the slab, but should not be used for alignment. A concrete kicker is first cast and then the formwork is erected tight up to the kicker. Before the formwork is erected the release agent is applied either by brush or spray. To keep the formwork tight together to prevent shape distortion and loss of cement slurry a steel cramp can be used. The columns should be kept plumb and this is done by the use of steel adjustable props or pull-push props. A tie piece can also be used. A tie has the following functions: a) It fixes two sides together 121

b) Holds two parts away from each other c) Anchor one side only d) Provides anchorage for the next lift e) Resist shear stresses. There are two types of ties: Non-recoverable and Recoverable tie. A yoke can also be used to do the same thing. The weight of the concrete will depend on the rate of pour. In columns we normally fill the concrete to column full height, whereas in a wall we would fill or pour the concrete in lifts. Column formwork should always be checked for tightness, alignment and plumb prior to concreting. The steel reinforcement should always have sufficient concrete cover of atleast 20mm. Curing concrete: Concrete hardens by a process called hydration. As water evaporates voids

are created in the setting concrete. It is the extent to which these voids are filled with silicate gel that determines the strength, durability and density of concrete. As active hydration takes place in the first few hours after placing fresh concrete, it is important for water to be retained during an extended period, this is called curing. The rate of evaporation from unprotected area will be higher when; the relative humidity, Wind speed and the concrete temperature is high or not uniform

Generally concrete curing refers to the act of controlling the concrete temperature and water content in the concrete for a definite period of time after placing. The time for curing concrete depends on: a. Air temperature b. Shuttering material c. Concrete temperature d. Thermal insulation of the curing material e. Size of pour 0

Optimum concrete temperature is 20 c The length of hydration of the cement and therefore the rate of hardening of the concrete depends on temperature and moisture available. The duration of controlled curing is important.

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100x50timber bearers Formwork wall(200x25)

Brace

Cleat

Gravel

Forming top of a trench

simple footing form Form braced against excavtion

Figure6.5. Footing Forms

There are usually two factors of prime importance to consider in the construction of footings. One is that the concrete must be upto specified strength and the other that the footings be positioned according to plan. A certain amount of tolerance is allowed in footing size and thickness, but reinforcing bars and dowels must be placed as specified. Concrete is can sometime be cast against the excavation, but care must be taken that this does not give inferior results, caused by the earth absorbing water from the concrete or by pieces of earth falling into it. In cases where wall footings are shallow, lateral pressure is small and the forms are simple structures as seen in the figure above. When the soil is firm, the form can be held in place by stakes and braces. If the soil will hold stakes, the forms may be secured by bracing them against the excavation sides only. Formwork for beams

spacer form side

ledgers

kicker 100x50stud

brace (Shore) bearer

sofit prop

Figure 3.6. Section through a beam formwork

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7.3 Timbering to excavations:

This is the support given to the sides of excavation to protect soil collapsing inwards, people falling in. It includes the covering of trenches or forming a barrier to warn people. By regulation any excavation exceeding 1.5m should have the sides supported. Timbering should be inspected everyday and after every shift. Some form of record should be kept especially when using explosives for excavation.

100x100waling

100x100waling

100x100strut

closedboardingusedinloose soil

semi-closedboarding

openboardingusedin firmsoil

Figure 6. 7. The different methods of timbering

The choice of the type and size of boarding depends on the nature of the soil and depth of excavation Sheet piles

These are temporary structures used in place of timbering Advantages of sheet piling •

Higher efficiency and speed in erection

•

Driven before excavation commences

•

Re-used

•

No strutting required

Precautions taken during excavation: •

Proper access to excavation

•

Escape route/ provision be allowed

•

Ventilation

•

Warning signs should be installed

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cantilevered sheet pile

anchored sheet pile

Figure 3. 8 Sheet piling

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CE 223 NOTES 2010.2011.pdf

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