A cement and concrete industry publication
Modular Precast Concrete Bridges A state-of-the-art report Technical Guide No. 11
Acknowledgements The Concrete Bridge Development Group is particularly pleased to acknowledge the work of Simon Bourne of Benaim in preparing this report and to Benaim for providing photographs and diagrams for inclusion in this publication: CBDG acknowledges financial support from The Concrete Centre in the production of this publication. www.concretecentre.com
Published for and on behalf of the Concrete Bridge Development Group by The Concrete Society Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 (0)1276 607140 Fax: +44 (0)1276 607141 www.concrete.org.uk CCIP-028 Published December 2008 ISBN 978-1-904482-52-9 © Concrete Bridge Development Group Order reference: CBDG/TG11 CCIP publications are produced by The Concrete Society on behalf of the Cement and Concrete Industry Publications Forum – an industry initiative to publish technical guidance in support of concrete design and construction. CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to the Concrete Bridge Development Group. Although the Concrete Bridge Development Group (limited by guarantee) does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Group, its servants or agents. Printed by Holbrooks Printers Ltd, Portsmouth, UK
Modular precast concrete bridges
Contents List of figures List of tables Executive summary 1. Introduction 2. Market assessment 3. Modular precast concrete bridge 3.1 Highway bridge 3.2 Railway bridge 4. Structural details 4.1 Span variations 4.2 Width variations 4.3 Curvature 4.4 Skew 5. Construction options 5.1 Span lift construction 5.2 Incremental launching 5.3 Self-propelled modular transporters 5.4 Gantry 6. Construction programme and costs 6.1 Capital investment requirements 6.2 Case study 1: Three-span highway overbridge 6.3 Case study 2: Single-span highway overbridge 7. Conclusions References Appendix A. Scheme drawings Appendix B. Construction programmes
2 2 3 5 6 8 8 10 12 12 13 14 14 15 15 17 18 19 21 21 22 25 28 30 31 37
List of figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10
Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20
Completed modular precast bridge. Typical mid-span section through modular highway bridge. Typical longitudinal section through modular highway bridge. Typical mid-span section through 27 m-span modular railway bridge. Typical sections for varying span requirements. Typical span-to-depth ratios for proposed system. Typical sections for varying width requirements. A51 Grenoble overbridge. Installation of A51 Grenoble overbridge. Span lift construction sequence. (a) Installation of first span; (b) installation of final span. Incremental launching construction sequence. (a) Launching of first rib; (b) completed launch of second rib. Installation of Badhoevedorp Bridge. Lavender Road Bridge, Kent. Gantry system in use on Stanstead Abbotts Bypass. Three-span highway overbridge layout adopted for Case Study 1. Section through modular precast bridge. Section through steel-composite bridge. Single-span highway overbridge layout adopted for Case Study 2. Section through modular precast bridge. Section through steel-composite bridge.
Figure A1 Figure A2 Figure A3 Figure A4 Figure A5 Figure A6 Figure B1 Figure B2
Three-span option (20–25–20 m) – general arrangement. Three-span option (20–25–20 m) – typical details. Three-span option (35–45–35 m) – general arrangement. Typical details of bridge system 1. Typical details of bridge system 2. Three-span option (20–25–20 m) – Span lift construction. Case Study 1 – three-span layout (20–25–20 m). Case Study 2 – single-span layout (45 m).
Figure 11
List of tables Table 1 Table 2 Table 3 Table 4 Table 5 Table 6
Typical features of steel-composite overbridge structures. Typical features of precast concrete beam overbridge structures. Details of modular precast bridge system. Capital investment costs for modular bridge system. Cost summary for three-span overbridge. Cost summary for single-span overbridge.
Executive summary The bridge market in the UK is set to grow steadily over the next five to ten years due in part to the major improvements to the highways network. This publication outlines the development of a new modular precast concrete bridge system, which offers a real alternative to steel-composite construction for medium-span bridges. It is considered that this modular precast concrete bridge system has the potential to place concrete as the preferred option for medium-span (15–50 m) bridges in the UK. The system combines the best features of steel-composite, precast concrete, and in-situ concrete into a solution that can deliver the highest value at the majority of bridge locations. The system makes use of concrete shell units that are precast off site and builds upon the recent, successful and innovative use of segmental and incremental launching technology used on two projects in Ireland, namely the Taney Road Bridge for Graham Construction and the River Shannon Bridge for Irishenco Construction. The system benefits from relatively light, 2.5 m long, precast concrete shell units that can be easily transported to site for assembly. Permanent prestressing cables are then placed and are covered by in-situ concrete to provide additional composite action as well as the required protection. The focus on precast concrete elements and off-site construction also ensures that a high-quality product is constructed within a safe environment. A further benefit is that the construction methodology can be varied to suit specific bridge sites and demands of the project programme. The flexibility of the system is such that it can enable the use of incremental launching, lifting using mobile cranes, lifting using transporters and erection using a temporary gantry. This allows the contractor scope to select the construction option that most suits the location and complexities of the project. The modular system is shown to be suitable for a wide range of typical highway bridge layouts. Varying span lengths, carriageway widths, horizontal and vertical curvatures, and skew can be readily accommodated by the match-cast shell units. The system provides an elegant solution with the benefit of being relatively maintenance-free. Two typical highway overbridge layouts are shown, which allow the construction costs and programmes of the modular precast to be compared with steel-composite alternatives. The case studies used are a three-span structure with spans of 20–25–20 m and a single 45 m span. In both cases the modular precast concrete system has been shown to offer significant cost and/or programme savings over a steel-composite alternative. The modular precast concrete system only requires an initial capital investment of less than £250k. Initial studies indicate that this can be financed over five to ten bridges which, given the 80 bridges built per annum in the UK, suggests a viable return in only two to three years. The system places concrete as the future for medium-span bridges by offering notable benefits over alternative solutions, namely safer, faster, higher quality, less traffic disruption, less maintenance, more sustainable, more affordable, more buildable, more elegant and higher value.
Introduction 1
1. Introduction A review of the highway market in the UK indicates that growth will be strong over the next ten years. Currently in the UK there is greater use of steel-composite construction but this approach is not generally reflected elsewhere in the world. This technical guide describes the development of a modular precast bridge system that has the potential to re-establish concrete as the preferred option for medium-span bridges, in the range 15–50 m spans. The system combines the best features of steel-composite, precast beam, in-situ and segmental schemes into a solution that can deliver the highest value at the majority of bridge locations. This guide gives an overview of the new system, which has been developed by Benaim following the recent, successful and innovative use of segmental technology on two alternative design projects in the Republic of Ireland, namely the Taney Road Bridge for Graham Construction and the River Shannon Bridge for Irishenco Construction. Both projects exhibited the ease of construction while also reducing cost and programme time. Experience suggests that the innovations developed from these bespoke schemes have the possibility to reduce the current steel-composite dominance on highway bridges in the UK.
5
2 Market assessment
2. Market assessment In the years immediately following the publication of the Department for Transport’s Tenyear plan in 2000(1), investment in the highway sector was sluggish. More recently, however, funding has returned to the highway sector, illustrated by an increase in spending by the Highways Agency in Major improvements in the strategic road network(2) and confirmed by the publication of The future of transport: A network for 2030(3) by the Department for Transport in 2004. Bridge structures form an important element in the majority of highway schemes, and hence it is expected that the bridge market will increase in line with this overall investment. The responsibility for the construction of highway bridges in the UK lies with the Highways Agency (HA), local and regional authorities and private developers. Determining the total number of typical bridges constructed in any given year is a difficult task due to the lack of a central database of information. Two different approaches were adopted to allow a best estimate. First, the HA website and the Glenigan construction information database (the latter compiled for local/regional authorities and private developers) were used to estimate the number of bridges constructed, based on data from recent years. The other method used the HA’s business plan for expenditure on major improvements to the network for 2006/07 assuming that one bridge is built for every £10 million spent. The results from these studies indicated that, on average, around 80 typical bridges per year are built in the UK. A typical bridge, for the purposes of this Guide, carries about 1000 m2 of deck area. Concrete is the material of choice for the majority of short-span bridges. However, the use of precast concrete beams in medium-span structures has declined in recent years with steel-composite construction controlling this portion of the market. The oftenperceived advantages and disadvantages associated with steel-composite and precast concrete bridge types are presented in Tables 1 and 2 respectively. Table 1 Typical features of steel-composite overbridge structures.
Advantages Low risk
Work at height
Fast
Aesthetics
Good quality
Maintenance
Buildability
Material economy
Good traffic management Labour economy Low technology
6
Disadvantages
Market assessment 2
Table 2 Typical features of precast concrete beam overbridge structures.
Advantages
Disadvantages
Low risk
Work at height
Good quality
Aesthetics
Buildability
Less efficient
Good traffic management Durability Sustainability Labour economy Low technology
A comparison of the two construction types would suggest that they are similar, but with precast beams offering a better overall solution than steel-composite due to environmental sustainability and whole-life durability. It is believed that this new modular precast concrete bridge system with its well-engineered solutions, enhanced value, lower maintenance and improved aesthetics will offer a real alternative to steel-composite construction. The cost and programme time of the system are very competitive when compared with steel-composite solutions (see Chapter 6). The system also offers the potential to form a pleasing, sustainable solution that will benefit clients and customers, both over the life of the bridges and through their delivery in a safe and efficient manner.
7
3 Modular precast concrete bridge
3. Modular precast concrete bridge The modular precast system outlined in this publication consists of relatively light, 2.5 m long, precast shell units that are precast off site and can be easily transported. On site, the units are assembled, stressed together and in-situ concrete placed. The combined units are positioned using launching or lifting technology, dependent on the site characteristics. Figure 1 shows a completed three-span modular precast bridge. Figure 1 Completed modular precast bridge.
3.1 Highway bridge
Figures 2 and 3 show typical sections through the modular bridge. Where the bridge is constructed using the Span lift method (see Chapter 5), the wall thickness of the precast unit is reduced in order to minimise the weight to be lifted.
Figure 2 Typical mid-span section through modular highway bridge.
3650 carriageway
2100 footway
3650 carriageway
2100 footway
Precast edge unit
6 No. 19/15 mm profile cables
8
.
200
.
1500
. 100
. 2 No. 19/15 mm launch cables
CL
200 Temporary prestressing bars 2 No. 32 Dia. Precast concrete shell approx. 2500 long
1000
1500 250
1000 250
1500
In-situ infill concrete
Modular precast concrete bridge 3
Figure 3
2500 typical uit
Typical longitudinal section through modular highway bridge.
2000 to 3000 range C L
Launch cable ducts
1500 typical
C L
400 Precast unit
Diaphragm
In-situ infill
Profiled cable ducts
Glued, match-cast or 25-50 thick grouted/ concrete joint
The system is broken down into its constituent components and explained in detail in Table 3. Reference should also be made to the illustrative drawings given in Figures A1–A6 of Appendix A. Table 3 Details of modular precast bridge system.
System component
Advantage
Precast, segmental shell units formed as a single rib and top slab
Reduces the required mould technology normally associated with box forms High-quality components constructed off site
Units that are around 5 m wide and 2.5 m long
Easily transported to site (12–15 t each)
Units that can be varied in width (4–6 m)
Variety of road widths catered for (4–40 m)
Units that can be varied in depth (1–3 m)
Variety of spans catered for (12–50 m)
Units that can be varied in length (2–3 m)
Variety of bridge lengths catered for (up to 150 m)
Units that can be match-cast and glued with epoxy resin
Easily assembled on site Vertical and horizontal curvature easily accommodated High-quality joint between adjacent shells
Units that can be positioned using a number of methods, i.e. incremental launching, Span lift, transporter or gantry
Construction methods that can be adopted to suit site characteristics and programme requirements
Shell units that are then infilled with in-situ concrete, which houses all the internal prestressing cables
Provides continuous ducts and protection to the cables (in accordance with Concrete Society TR47, Durable post-tensioned concrete bridges(4))
A deck that is then prestressed internally from end to end with profiled cables that carry all the load effects
Efficient use of structural materials
A deck that can be made fully integral with bankseat Reduces future maintenance costs by eliminating all abutments and fully monolithic with intermediate piers bearings and expansion joints
The use of the precast segmental process allows high-quality, accurate and factoryproduced units to be created in a safe and efficient manner. The use of precast shell units also allows the in-situ infill concrete to be poured in small bays, thus eliminating most issues associated with early-age thermal and restraint effects. In addition the use of the shell units eliminates the complex technology needed for box moulds.
9
3 Modular precast concrete bridge
The use of the launching or lifting processes allows safe and easy construction, minimising disruption to traffic and considerably reducing the amount of work at height over the road, which can generally be kept open throughout; when closures are required, they are short and compact. The deck can be fully aligned and formed before the launch or the lift, enhancing the speed and quality of construction. The modular system can cater for a variety of deck configurations as detailed in Chapter 4. In addition, the construction methodology can be varied to suit the site characteristics and project requirements. This is discussed in greater detail in Chapter 5. Chapter 6 looks at the cost and programme implications for two typical bridge layouts. On both counts the proposed modular system is shown to be very competitive compared with a steel-composite solution.
3.2 Railway bridge
Figure 4 shows how the modular bridge system can be adopted to suit the UK railway environment. The main longitudinal ribs are located to the side, thus permitting a shallow construction depth.
Figure 4 Typical mid-span section through 27 m-span modular railway bridge.
Structure gauge at zero cant
In-situ stitch
In-situ infill
Approx. 2500 long precast unit
The characteristics and benefits of the system are similar to those of the highway bridge option, i.e. a buildable and economic structure with a clean aesthetic and simple details, which is robust to impact, and has low maintenance and enhanced safety through its offsite and off-line construction. The construction methodology can also be varied to suit specific bridge sites and demands of the project programme.
3.2.1 Medium-span bridges
10
There are a number of construction options available, which are discussed further in Chapter 5. Those of particular merit for medium-span (15–40 m) modular railway bridge structures are as follows:
Modular precast concrete bridge 3
Launched method The deck is built, as a kit of parts, off-line from the railway, and from
the road over which it will cross. From this off-line platform, it is assembled, stitched and prestressed, and then launched in its complete form over the roadway to a position alongside the existing bridge, where it becomes supported on slide tracks. From here, the new concrete bridge would then be slid sideways during a possession period. This whole process significantly reduces the work at height over the roadway. Sliding method The deck is assembled adjacent to its final position, either at road level during a medium-term road closure, or on a gantry at high level. The deck is then stitched and prestressed, and during the possession period jacked onto slide tracks and slid sideways. Self-propelled modular transporter (SPMT) method The deck is fully assembled and prestressed off line. The entire deck is then transported and lifted into position during the possession period.
3.2.2 Short-span bridges
Shorter-span bridges in the range 10–15 m comprise the majority of railway underbridges in the UK. The construction options for such bridges are similar to those for medium-span bridges. However, the lower deck weights (of around 150 t for a double-track structure) also permit the use of a large mobile crane that would be capable of lifting the entire structure into position during limited possession time.
11
4 Structural details
4. Structural details This chapter examines the ability of the modular system to cope with a variety of bridge layouts and configurations. Reference should also be made to the illustrative drawings given in Figures A1–A6 in Appendix A.
4.1 Span variations
The modular precast system is adaptable to varying span requirements. Figure 5 shows how the depth of the shells can be varied to allow different spans to be achieved. Preliminary calculations have been undertaken for typical single-span, two-span and three-span highway overbridge layouts as part of this design study.
Figure 5 Typical sections for varying span requirements.
C L
1000 1500
Minimum section: Spans 15-25 m
C L
Precast units
Launch cables
In-situ stitch
1500 2000
In-situ infill Profiled span cables Typical section: Spans 25-35 m
CL
2000 3000
Maximum section: Spans 35-50 m
For typical spans between 25 m and 35 m, the section depth would vary between 1.5 and 2.0 m. The minimum depth for shorter spans is 1.0 m, increasing to a maximum of 3.0 m for longer spans of between 35 and 50 m. Figure 6 shows the range of span-to-depth ratios for typical schemes.
12
Structural details 4
Figure 6 Typical span-to-depth ratios for proposed system.
3.5 Max depth 3.0
Depth (m)
2.5
2.0
1.5 Min depth 1.0 Maximum (1:15) Typical (1:17) Minimum(1:20)
0.5
0
0
10
20
30
40
50
Span (m)
4.2 Width variations
The modular precast system is also adaptable to varying width requirements. Figure 7 shows how the width of the bridge deck can be changed through a variation in unit width and/or varying the width of the central in-situ stitch.
Figure 7 Typical sections for varying width requirements.
4000
1000
4000
CL
Narrow deck: 9 m
5000
1500 C L
5000
Typical deck width: 11.5 m
6000
2000 C L
6000
Wide deck: 14 m
13
4 Structural details
4.3 Curvature
4.4 Skew
The modular precast units can also be match-cast, thereby allowing any curvature to be achieved. Match-casting ensures that units are easily assembled on site and provides a high-quality joint between adjacent precast units.
Figure A5 in Appendix A shows a typical three-span highway overbridge with skews of 7.5°, 15° and 30°. The modular system can cater for skew by: forming a skew end with the precast end units, which can be readily achieved by using
an additional stop end in the mould; spanning square between the abutments – a slight increase in span is required but the cost of additional material is minimal; spanning skew between abutments with cast-in-situ end sections, which are poured with the integral abutments.
14
Construction options 5
5. Construction options A number of easily constructible options have been developed with the purpose of identifying the most suitable option for the UK highway market.
5.1 Span lift construction
Span lift construction is an attractive method of construction that has been developed as part of this study. This method provides a more efficient structural form while utilising traditional UK bridge construction skills, i.e. heavy lifting technology. As noted in Chapter 3, where a bridge is constructed via the Span lift method, the wall thickness of the precast unit is reduced in order to minimise the weight to be lifted. Figure A6 of Appendix A details the typical construction sequence and shell units. The construction sequence for a typical single-span bridge is as follows: Shell units are transported to site and aligned at road level close to the bridge site. Permanent external prestress is applied to the shell units alone. The complete 5 m-wide rib is lifted into position using a large mobile crane. Infill concrete is poured into ribs. Further permanent prestress is applied to the composite section. Adjacent ribs are stitched together. Parapet edge beams and finishes are applied.
This method is suited to both single- and multi-span bridges. In the case of a multi-span bridge, cast-in-situ segments are required at the pier locations, prior to the installation of full-length continuity prestressing cables. The method is more attractive to the UK industry as it employs well-known technology in the form of large mobile cranes. Mobile cranes of up to 1000 t capacity are now available, which allow a 35 m rib of around 125 t weight to be lifted at a radius of over 30 m. More widely available, and more easily rigged, mobile cranes of 800 t capacity would allow the same 35 m rib to be lifted at a radius of about 25 m. As the initial prestress is applied to the shell section alone, its structural efficiency is significantly enhanced, thus reducing the total amount of prestress required. The method also avoids the need for investing in launching/gantry equipment, thus reducing the initial capital investment cost, and the cost required in constructing temporary props. A similar method has recently been used during the construction of a bridleway overbridge on the A51 in Grenoble by Vinci (Figure 8). Although it differs significantly in terms of its section, this scheme is very similar with regard to the construction method.
15
5 Construction options
Figure 8 Cross-section of A51 Grenoble overbridge.
140
120 1640
2100
Figure 9 Installation of A51 Grenoble overbridge.
The 190 t bridge was constructed at road level parallel to the road, and then lifted and turned through 90° during a road closure (Figure 9), in a similar vein to that proposed for the modular precast bridge solution.
16
Construction options 5
a) Installation of first span
b) Installation of final span
Figure 10 Span lift construction beginning and end stages.
Figures 10(a) and 10(b) show a graphic representation of part of the sequence of construction for the Span lift scheme. It is believed that this method offers great potential, and the associated cost and programme implications are investigated further in Chapter 6.
5.2 Incremental launching
The concept of the modular bridge system began with incremental launching as the envisaged construction methodology, and it is thought that this method should generally be the preferred option at most multi-span locations. Figure A2 of Appendix A details the typical construction sequence. The major advantages associated with launched construction include safety, due to minimal work at height over the carriageway; minimal traffic disruption, due to the smaller number of short closures that are required; and a higher quality because of the off-line construction. This system can be developed with or without the need for temporary propping. However, the use of temporary propping reduces the length of the launching nose and the amount of central prestressing steel required. The construction sequence for a typical multi-span bridge is as follows: Shell units are transported to site and aligned behind the abutment. Units are stressed together longitudinally with prestressing bars. Infill concrete is poured into ribs. Adjacent ribs are stitched together transversely. Permanent launching prestress is applied to the composite section. The complete bridge deck is pushed into position using launching jacks and a nose,
possibly using temporary props. Further permanent prestress is applied to the launched section. Parapet edge beams and finishes are applied.
17
5 Construction options
a) Launching of first rib
b) Completed launch of second rib
Figure 11 Incremental launching construction sequence.
Figures 11(a) and 11(b) give a graphic representation of part of the sequence of construction for the launched scheme. This method is very suitable for many bridge types/locations, especially multi-span structures, and the associated cost and programme implications are investigated in parallel with that of the Span lift method in Chapter 6.
5.3 Self-propelled modular transporters
Self-propelled modular transporters (SPMT) are beginning to be used in the UK on bridge projects. SPMT allow very large loads to be lifted and moved in a controlled manner. The four- and six-axle line units can be assembled into any combination to allow the required load to be moved. The units offer 360° steering, hydraulic suspension and electronic control of individual wheel units. For the modular precast bridge system, the SPMT are believed to be most suitable for moving large or unusual single-span structures. They offer improved safety conditions as the bridge is constructed off-line and then transported to site and lifted in place during a single road closure. Their use would also be very suitable for the railway sector. Recent examples of SPMT in use on bridge projects include the Badhoevedorp Bridge in the Netherlands and the Lavender Road Bridge in Kent, UK In the case of the Badhoevedorp Bridge (see Figure 12), a 120 m-long deck weighing 3300 t was lifted and moved into position in a two-hour period.
18
Construction options 5
Right: Figure 12
Installation of Badhoevedorp Bridge. Photo: Mammoet
Far right: Figure 13
Lavender Road Bridge, Kent. Photo: Birse/ALE-Heavylift
Figure 13 shows the movement of the 1200 t Lavender Road Bridge deck in Kent during the widening of a road. The bridge was moved from its original piers, stored off-line and then repositioned on new piers 9 m to the east of the original location. Although the SPMT have a number of significant advantages, they can be expensive for use on typical highway bridge projects. For example, the cost of installing a 45 m singlespan modular bridge (weight of the entire deck approximately 1200 t) would be in the region of £100 000. With a total superstructure cost of only around £250 000, the SPMT method is costly when compared with other methods (see Chapter 6) but there will be situations where project requirements could favour this option.
5.4 Gantry
The modular precast bridge system is also suited to construction using temporary gantries. In this method, the gantry is used to support the shell units as they are aligned in their final position. The shells are then stressed to support the infill concrete, which is cast in-situ. Further permanent prestress is then applied. This option is suited to both single- and multi-span structures, and requires lighter cranage for the single shell units, which weigh only 12–15 t each. The technology is well known and in certain circumstances may offer the most economical method of construction even though an investment in a gantry system is required. However, the method requires most of the operations to be undertaken at height, unlike some of the other construction options. Figure 14 shows an example of an under-slung gantry system in use on the Stanstead Abbotts Bypass in the UK.
19
5 Construction options
Figure 14 Gantry system in use on Stanstead Abbotts Bypass. Photo: Benaim
In the early 1990s, a precast segmental ‘channel bridge system’ was developed by Jean Muller in France as a new concrete cross-sectional shape for freeway overpasses. This was further developed in the USA in the mid-1990s. The system is of relevance to this report as it employed an under-slung gantry to position the precast units, although the units were solid and internally prestressed.
20
Construction programme and costs 6
6. Construction programme and costs Two typical highway overbridge layouts have been chosen to allow the modular precast concrete construction costs and programmes to be compared with a steel-composite alternative. The rates used in the study have been determined through experience on similar projects and are therefore believed to offer robust estimates. For the purposes of this Guide, costs have not been attributed to ‘lane closures’. Such costs can vary widely dependent on the road location and can only be considered on a bridge-specific basis. The construction programmes have also been developed through recent experience on similar projects. However, they serve mainly as a comparative tool and exact details will differ from contractor to contractor, at different locations.
6.1 Capital investment requirements
Standardisation of the construction process requires investment in a ‘kit of parts’ that would allow the modular system to be used at a variety of locations. The essential components of the kit required to cast and align the shell units on site are: two moulds within a casting shed unit transporter/low-loader cranes to lift approximately 12–15 t units temporary packs and jacks to align the units prestressing bars to hold the units together.
As noted in Chapter 5, the system can be constructed using a number of techniques. For the Span lift and SPMT options, the remaining construction cost is covered by a specialist subcontractor. In the case of the incrementally launched option, investment in further items is required, namely: launching nose possible temporary props and foundations jacking equipment launch bearings, jacks and lateral guides.
The capital investment costs for the modular bridge system are outlined in Table 4. Table 4 Capital investment costs for modular bridge system.
Item
Cost
Moulds (2No.)
£80k
Low loader
£25k
Prestressing bars
£5k
Launching nose
£25k
Intermediate props
£10k
Pulling beams
£5k
Strand & jacks
£5k
Launch bearings
£5k
Total
£160k
21
6 Construction programme and costs
Typical precast segmental schemes using box sections have been shown to be economic for bridge deck areas of more than 10 000 m2. However, a study of the capital investment costs for this modular bridge system indicates that 5000 m2 of bridge deck would be required for it to be cost-effective, i.e. about five to ten typical highway bridges. Therefore in the following cost comparisons, 10% of these capital investment costs has been added to the cost of the modular bridge options, as a capital repayment. With approximately 80 bridges under construction in the UK each year, it is believed that the market is large enough to make the recovery of this investment figure attainable in the short to medium term. This is based on the premise that of these 80 bridges, approximately half would be suitable for the proposed system, of which a single team could expect to bid for 20 and win a minimum of 25%, i.e. five bridges per year. A return on investment cost could therefore be expected within a rather short period of two to three years.
6.2 Case study 1: Threespan highway overbridge
Figure 15 shows the three-span highway overbridge chosen for the study. The bridge has spans of 20–25–20 m, typical of overbridges crossing both carriageways of a two-lane dual carriageway.
Figure 15 Three-span highway overbridge layout adopted for Case Study 1.
Figure 16 shows the cross-section of the proposed modular precast bridge. The shell sections are 1.5 m deep giving a span-to-depth ratio of approximately 17. Two construction options were investigated for this bridge type. The first considered the Span lift option, which offers the advantage of using more traditional UK construction skills. Construction via two stages of incremental launching was also investigated, as shown in Figure A2 of Appendix A. Figure 17 shows the cross-section of the steel-composite bridge adopted for this study. A ladder deck layout was chosen as it has been shown to be the most economic form in recent projects. The main girders are 1.25 m deep with a 250 mm deck slab, also giving a span-to-depth ratio of 17. It is proposed that the bridge is constructed by traditional means, i.e. the main beams lifted in position, followed by the cross-girders, with the in-situ slab cast on permanent, participating formwork.
22
Construction programme and costs 6
Figure 16 Section through modular precast bridge.
2100
3650
3650
2100
Footway
Carriageway
Carriageway
Footway
CL
Figure 17 Section through steel-composite bridge.
2100 Footway
3650 Carriageway
3650 Carriageway
2100 Footway
1250
23
6 Construction programme and costs
6.2.1 Cost comparison Table 5 Cost summary for three-span overbridge.
A summary of the costs is given in Table 5. Item
Modular precast Span lift
Substructure
Steel-composite Launched
£80k
£80k
£70k
£185k
£235k
Superstructure Deck (excluding formwork)
£140k
£140k
£29k
Equivalent formwork costs ●
Job specific
£45k
●
Capital repayment
£10k
£195k
£16k
Finishes (including abutments)
£175k
£175k
£175k
Preliminaries
£225k
£220k
£235k
Discounted whole-life maintenance
£35k
£35k
£50k
Total
£710k
£695k
£765k
Percentage saving
8%
10%
Monetary saving
£60k
£75k
The modular precast system shows itself to be more economic by offering a saving of between 8 and 10% when compared to the steel-composite option. As expected, the major cost savings are attributable to the reduced material costs for the superstructure. Substructure costs are only marginally higher for the concrete option. The launched option in this comparison is shown to be cheaper than the Span lift option, due mainly to the relatively high crane hire costs required. However, a reduced initial investment is required for the Span lift option, thereby reducing the potential exposure of the contractor. This comparison indicates that both the Span lift and launched methods are viable construction options.
6.2.2 Programme comparison
6.2.3 Summary
24
The construction programmes for each of the construction options are shown in Figure B1 of Appendix B. The total construction times for the modular bridge with launched construction and the composite steel option were found to be very similar, at around 33 weeks each, with the modular Span lift construction offering a three-week benefit in construction time.
For the case of a typical three-span highway overbridge, the modular precast bridge has been shown to offer a well-engineered solution at a lower cost and some benefits in construction time compared with a steel-composite bridge.
Construction programme and costs 6
6.3 Case study 2: Singlespan highway overbridge
Figure 18 shows the single-span highway overbridge chosen for the study. The bridge has a span of 45 m, which is typical for bridges over widened motorways where piers in the central reservation are avoided due to construction difficulties close to passing traffic.
Hardshoulder Central resevation Hardshoulder Carriageway Carriageway
Above: Figure 18
Single-span highway overbridge layout adopted for Case Study 2. Right: Figure 19
2100
3650
3650
2100
Footway
Carriageway
Carriageway
Footway
Section through modular precast bridge. Precast edge unit
In-situ stitch concrete
In-situ infill concrete
CL
Void
Figure 19 shows the cross-section of the proposed modular precast bridge. The shell sections are 3 m deep giving a span-to-depth ratio of 15. Two construction options were also investigated for this bridge type. The first considered the Span lift option. An alternative option of constructing the entire bridge off-line and lifting it in place with an SPMT was also investigated. Figure 20 shows the cross-section of the steel-composite bridge adopted for this study. A ladder deck layout was chosen as it has been shown to be the most economic form in recent projects. The main girders are 2.25 m deep with a 250 mm deck slab, giving a spanto-depth ratio of 18. It is proposed that the bridge is constructed by traditional means, i.e. the main beams lifted in position, followed by the cross-girders, with the in-situ slab cast on permanent, participating formwork.
25
6 Construction programme and costs
Figure 20 Section through steel-composite bridge. 2100 Footway
3650 Carriageway
3650 Carriageway
2100 Footway
CL
6.3.1 Cost comparison Table 6 Cost summary for single-span overbridge.
A summary of the costs is given in Table 6. Item
Modular precast Span lift
Substructure
Steel-composite SPMT
£80k
£80k
£70k
£275k
£250k £80k
Superstructure Deck (excluding formwork)
£150k
£150k
Equivalent formwork costs ●
Job specific
£45k
●
Capital repayment
£10k
£115k £205k
£10k
Finishes
£80k
£80k
Preliminaries
£185k
£215k
£200k
Discounted whole life maintenance
£15k
£15k
£25k
Total
£565k
£665k
£625k
Percentage saving
10%
(6%)
Monetary saving
£60k
(£40k)
The modular precast bridge constructed by the Span lift method is the most economic option, giving a saving of 10% of the total cost when compared to the steel-composite alternative. Constructing the modular precast bridge using the transporter is the more expensive option due to the high cost of the SPMT. However, the small premium in cost of 6% can be offset by the improvement it offers in construction time (see next section).
6.3.2 Programme comparison
26
The construction programmes for each of the construction options are shown in Figure B2 of Appendix B. The total construction times for the modular bridge construction options were found to be somewhat shorter than composite steel, within the accuracy achievable at this stage.
Construction programme and costs 6
In particular, constructing the modular precast bridge using a transporter has the potential to save approximately three weeks programme time when compared with the steel-composite alternative. In addition, this option has many safety advantages, as the majority of construction work takes place away from the roadway. Constructing the modular precast bridge using the Span lift method was also shown to be faster by around two weeks compared with the steel-composite option.
6.3.3 Summary
For the case of a typical long, single-span highway overbridge, the modular precast bridge has been shown to offer a well-engineered solution at a lower cost than a comparable steel-composite bridge when constructed using the Span lift method. The construction time has been shown to be very competitive for this method, being marginally shorter when compared with the steel-composite solution. Constructing the modular bridge using an SPMT has been shown to be slightly more expensive than the traditional steel-composite solution while offering a saving on programme time of approximately three weeks.
27
7 Conclusions
7. Conclusions The bridge market in the UK is set to grow steadily over the next five to ten years. The use of steel-composite construction is almost unique to the UK, and it is believed that a wellengineered and competitive concrete option has the potential to gain considerable market share. The modular precast bridge system which this publication addresses aims to reestablish concrete as the preferred option for medium-span bridges. The modular system has been shown to be suitable for a wide range of typical bridge layouts. Varying span lengths, carriageway widths, horizontal and vertical curvatures, and skew can be readily accommodated by the match-cast shell units. The focus on precast elements and off-site construction ensures a high-quality product is constructed within a safe environment. Importantly for both the developer and contractor the construction methodology can be varied to suit specific bridge sites and the demands of the project programme. The construction options available include the following: Span lift method High-quality, low-maintenance precast components. Suited to single- and multi-span structures. Two-phase stressing technique reduces quantity of prestress steel required. Utilises existing skills within UK construction sector. Cheaper than steel-composite alternative for both typical single- and three-span bridges (Case Studies 1 and 2), see section 6.2 and 6.3. Faster than steel-composite alternative for both typical single- and three-span bridges (Case Studies 1 and 2), see section 6.2 and 6.3. Safer – less work to be carried out at height than with steel-composite alternative. Traffic – some disruption but similar to steel-composite. Incrementally launched method High-quality, low-maintenance precast components. Suited to multi-span structures. Utilises newer technology within UK construction sector. Cheaper than steel-composite and Span lift alternatives for a typical three-span bridge (Case Study 2), see section 6.3. Similar speed to steel-composite alternative. Safer – minimum work to be carried out at height, especially if temporary props are avoided. Traffic – minimal as possessions required during launching only.
28
Conclusions 7
Self-propelled modular transporter (SPMT) method High-quality, low-maintenance precast components. Suited to single-span structures. Utilises emerging skills within UK construction sector. More expensive than steel-composite alternative. Faster than steel-composite alternative for a typical single-span bridge (Case Study 1), see section 6.2. Safer – minimum work to be carried out at height. Traffic – single possession required for all bridge works. Gantry method High-quality, low-maintenance precast components. Suited to single- and multi-span structures. Utilises newer technology within UK construction sector. Requires additional work to be carried out at height. Costs, programme times and traffic management remain to be investigated. For the two typical case studies presented in this guide, the modular system has been shown to offer significant cost and/or programme savings over a steel-composite alternative, as well as being more more elegant, efficient and robust and requiring less maintenance. The modular system requires an initial capital investment of less than £250k. It has been shown that this can be financed over five to ten bridges which, given the 80 bridges built per annum in the UK, would seem to generate a return in only two to three years. The system offers a large number of benefits over alternative solutions, namely: safer – typically much less work at height and more factory-based work faster – simple, repetitive cycles and easy detailing more buildable – known low-technology solutions minimal traffic disruption – less disturbance to the road users higher quality – factory-based and off-site construction lower maintenance – no exposed steel, joints or bearings more efficient sections – optimisation of quantities aesthetically pleasing – clean, simple proportions, forms and details more sustainable – for the future of us all higher value – achieved on all aspects.
29
References
References
30
1.
DEPARTMENT FOR TRANSPORT. Transport Ten year plan 2000, DFT, London, 2000.
2.
HIGHWAYS AGENCY. Major improvements in the strategic road network, The Highways Agency, London.
3.
DEPARTMENT FOR TRANSPORT. The future of transport: A network for 2030. DFT, London, 2004.
4.
THE CONCRETE SOCIETY. Durable post-tensioned concrete bridges. Technical Report 47 (second edition) The Concrete Society, Camberley, 2002.
1200
1
1:50
SECTION A - A
Monolithic piers (reinforced concrete) refer to drawing No 11725/011 for details
CL
Benaim drawing ref.: 11725/10-C
Three-span option (20–25–20 m) – general arrangement.
Figure A1
Precast concrete shell
First stage insitu infill concrete
Second stage insitu infill concrete
2
A
Hard strip
Verge 1000
A
Carriageway
7360
B
Hard strip
1:00
ELEVATION
B
Central reservation
Hard strip
1000 2500 1000
25000
3650 Carriageway
6 No 19/15mm profile cables
CL
1000
CL
1:50
1:50
SECTION C - C
200
250
1500 250
1000
2100 Footway
2
3650
1
Carriageway
SECTION B - B
8 No 19/15mm anchorages
Precast concrete shell approx. 2500 long
1500 Temporary prestressing bars 2 No 32 Dia
Insitu infill concrete
Precast edge unit
Footway
2100
Hard strip
1000 Verge
2 No 19/15mm launch cables
Carriageway
7360
20000
C
C
200 100 1500
20000
Appendix A
Appendix A. Scheme drawings
31
32
1:500
Banaim drawing ref.: 11725/11-B
Three-span option (20–25–20 m) – typical details.
Figure A2
7
Temporary piers & foundations
TYPICAL CONSTRUCTION SEQUENCE FOR 3 SPAN BRIDGE
Cast integral abutments Complete finishes
6
Complete launch
Pour infill concrete Couple launch cables Stress launch cables
Launch over props Pack units and align form joints Temporary stress
Pour infill concrete Stress launch cables Attach nose
Remove nose Cast monolithic pier tops Stress profiled cables Remove props
5
4
3
2
Pack units and align form joints Temporary stress
Temporary piers & foundations
Approx 8 No 32φ prestressing bars cast in ducts to pier head
1st stage in-situ infill
CL
Final 1250 x 1250 pack Initial 1000 x 500 pack
1500 x 1500 pier head
In-situ infill Glued, match-cast or 20-50 thick grouted concrete joint
Precast unit
2000 to 3000 range CL
PLAN
Launch cables in continuous ducts
1:10
JOINT DETAIL
Profiled cable ducts Glued, match-cast or 20-50 thick grouted concrete joint
Launch cable ducts
Typical precast unit = 121 (range = 81 - 201)
Glued, match-cast or 25-60 thick grouted / concrete joint
LONGITUDINAL SECTION
400 diaphragm
2500 typical unit CL
1:50
1:50
TYPICAL JOINT / UNIT DETAILS FOR 1500 DEEP SECTION
Profiled cables in continuous duct
1:20
PIER HEAD DETAIL
4 No 100 jacks
1000 x 300 launch bearing CL
CL
BUILT IN PIER HEAD DETAILS FOR 1500 DEEP SECTION
Bar couplers
Stressed launch cables
Profiled cables
2 No 32φ temporary prestressing bars
CROSS SECTION
150φ typical
1
2nd stage in-situ infill
1500 stitch
T16-150 crs
CL
Asphaltic plug joint
Fall
Expansion joint
Drain channel and pipe
1800 x 1000 inspection gallery
1:50
OPTIONAL JOINTED ABUTMENT
Bank seat abutment
CL
800 thick end diaphragm wall
1:50
PROPOSED INTEGRAL ABUTMENT
Mild steel continuity reinforcement
In-situ end diaphragm wall
1:20
CONCRETE JOINT BETWEEN UNITS
T12-150 crs CL
Bank seat abutment
Mortar bed to precast units
Rubber CR pot bearing
Precast units
In-situ infill
Final concrete pack refer to detail
LONGITUDINAL SECTION
CL
Appendix A
1:50
TYPICAL PIER SECTION A - A
monolithic piers (reinforced concrete)
9 No 19/15mm profile cables
CL
7300 Carriageway
Benaim drawing ref.: 11725/12-B
Three-span option (35–45–35 m) – general arrangement.
Figure A3
Precast concrete shell
First stage insitu infill concrete
1
Second stage insitu infill concrete
2
A
A
Verge
3300 Hard shoulder
B
B
11000 Carriageway
1:200
ELEVATION
Central reservation
3100
45000
3300 Verge
Precast concrete shell
1500
500 1000
200
260
3650 Carriageway
1:50
TYPICAL ANCHORAGE SECTION C - C
12 No 19/15mm anchorages
CL
1:50
TYPICAL MID-SPAN SECTION B - B
9 No 19/15mm profile cables
760 Dia. void
CL
7300 Carriageway
3650 Carriageway
insitu infill concrete
Precast edge unit
2100 Footway
Hard shoulder
3 No 19/15mm launch cables
11000 Carriageway
35000
2
1500
1
260
1000 500
2100 Footway
C
C
200 100
35000
Appendix A
33
2760
MAXIMUM SECTION: SPANS 35-50m
CL
TYPICAL SECTION: SPANS 25-35m
In-situ infill
1:50
TYPICAL SECTIONS FOR VARYING SPAN REQUIREMENTS
Profiled span cables
In-situ stitch
Benaim drawing ref.: 11725/13-B
Typical details of bridge system 1.
Figure A4
Launch cables
CL
MINIMUM SECTION: SPANS 15-25m
CL
Precast units
1000 1500 - 2000
34 2000 - 9000
WIDE DECK: 14m
CL
2000
TYPICAL DECK WIDTH: 11.5m
CL
1500
NARROW DECK: 9m
CL
1000
5000
4000
6000
1:50
TYPICAL SECTIONS FOR VARYING WIDTH REQUIREMENTS
6000
5000
4000
0
0.5
1.0
1.5
2.0
2.5
No of beams
10
14 19
14
20
24
22
30
30
Overall deck width (m)
9
4 6
40
36
T10-160 crs
T12-150 crs
20
30 Span (m)
40
50
TYPICAL SPAN TO DEPTH RATIOS
10
Min. depth
Minimum (1:20)
Maximum (1:15 Typical depth (1:18)
1:20
TYPICAL DETAILS OF CENTRE STITCH
CL
T10-150 crs 1000 stitch
CL
1500 stitch
OPTIONS FOR INCREASING DECK WIDTH USING A NUMBER OF ADJACENT BEAMS
5
4
3
2
1
Appendix A
Depth (m)
- 1500
20000
20173
20000
30
o
80000 24860
SCALE 1:500
25000
23094
20000
SCALE 1:500
LAYOUT 3 BRIDGE WITH 30 DEGREE SKEW END SHELL UNITS CAST SKEW WITH IN-SITU SECTION
23094
o
25000
7.5
67500 25216
LAYOUT 1 BRIDGE WITH 7.5 DEGREE SKEW END SHELL UNITS CAST SKEW
20000
Benaim drawing ref.: 11725/14-A
Typical details of bridge system 2.
Figure A5
DETAIL 2
DETAIL 1
20173 CL
Temp bearing
Abutment
End section cast in-situ CL
Parapet edge beam
Temp bearing
Abutment
Parapet edge beam
2500
1:100
1:100
DETAIL 2
End shell unit cast skew
Precast shell units
End shell units cast skew
DETAIL 1
1850
2500
Precast shell units
20000
70000 25862
25000
20000
20706
SCALE 1:500
LAYOUT 2 BRIDGE WITH 15 DEGREE SKEW SHELL UNITS SPAN SQUARE BETWEEN ABUTMENTS
20706
Appendix A
35
36 1:500
TYPICAL CONSTRUCTION SEQUENCE FOR ‘SPAN LIFT’ BRIDGE
Cast integral abutments Complete finishes
Stich adjacent ribs together Complete sections over piers Stress full length continuity cables
Lift main span ribs into position Demobilise crane
Mobilise crane on site (Crawler crane illustrated however use of large mobile likely) Lift ribs into position
Transport precast shells to site Assemble ribs close to final bridge position Apply initial prestress to shell units
Construct piers and abutments Prepare area for deck assembly
Benaim drawing ref.: 11725/15-A
Three-span option (20–25–20 m) – Span lift construction.
Figure A6
6
5
4
3
2
1
2100 Footway
8 No 19/15mm profile cables 750
150
CL
3850 Carriageway
1000
AS SHOWN
TYPICAL DETAILS OF ‘SPAN LIFT’ BRIDGE
1:50
TYPICAL PIER SECTION
3850 Carriageway
1:50
TYPICAL END SECTION
1500
CL
1:50
250
3850 Carriageway
TYPICAL MIDSPAN SECTION
Precast concrete shell approx 2500 long
Temporary prestressing bars - 2 No 32 Dia
1500
CL
In-situ infill concrete
3850 Carriageway
In-situ infill concrete
Precast edge unit
2100 Footway
250
225 6 No 19/15mm anchorages
2100 Footway
750
2100 Footway
Appendix A
1500
Case Study 1 – three-span layout (20–25–20 m).
Figure B1
Appendix B
Appendix B. Construction programmes
37
38
Case Study 2 – single-span layout (45 m).
Figure B2
Appendix B
CONCRETE BRIDGE DEVELOPMENT GROUP
A cement and concrete industry publication
The Concrete Bridge Development Group aims to promote excellence in the design, construction and management of concrete bridges. With a membership that includes all sectors involved in the concrete bridge industry –bridge owners and managers, contractors, designers and suppliers– the Group acts as a forum for debate and the exchange of new ideas. A major programme of bridge assessment, strengthening and widening is already underway to accommodate European standards and the increasing pressures on the UK road network. The Group provides an excellent vehicle for the industry to co-ordinate an effective approach and to enhance the use of concrete. Through an active programme of events and seminars, task groups, newsletters, study visits and publications, the Concrete Bridge Development Group aims to:
Address the challenge of the national bridge programme Provide a focus for all those involved in concrete bridge design, construction and management Promote an integrated approach and encourage development of innovative ideas and concepts Promote best practice in design and construction through education, training and information dissemination Make representations on national and international codes and standards Identify future research and development needs Maximise opportunities to develop the wider and better use of concrete. Membership of the Concrete Bridge Development Group is open to those who have an interest in promoting and enhancing the concrete bridge industry. Five main types of membership are available:
Group membership for industry organisations and associations Corporate membership for contractors, consultants, suppliers and specialist service companies Associate membership for academic organisations Bridge owners for all organisations that commission, own, maintain and manage concrete bridges Individual consultants
By being representative of the whole industry, the Concrete Bridge Development Group acts as a catalyst for the best in concrete bridge design, construction, maintenance and management. PUBLICATIONS FROM THE CONCRETE BRIDGE DEVELOPMENT GROUP Integral bridges Technical Guide 1 A report of a study visit in August 1997 by a CBDG delegation to North America, sponsored by DTI (1997) Guide to testing and monitoring of durability of concrete structures Technical Guide 2 A practical guide for bridge owners and designers (2002) The use of fibre composites in concrete bridges Technical Guide 3 A state-of-the-art review of the use of fibre composites (2000) The aesthetics of concrete bridges Technical Guide 4 A technical guide dealing with the appearance and aesthetics of concrete bridges (2001) Fast construction of concrete bridges Technical Guide 5 The report of a Concrete Bridge Development Group Working Party (2005) High strength concrete in bridge construction Technical Guide 6 A state-of-the-art report (2005) CCIP-002 Self-compacting concrete in bridge construction Technical Guide 7 Written by Peter JM Bartos (2005) CCIP-003 An Introduction to Concrete Bridges A publication dedicated to undergraduates and young engineers (2006) Guide to the Use of Lightweight Aggregate Concrete in bridges Technical Guide 8 A state-of-the-art report, written by Philip Bamforth (2006) CCIP-015 Guidance on the Assessment of Concrete Bridges Technical Guide 9 A Task Group report (2007) CCIP-024 Enhancing the Capacity of Concrete Bridges Technical Guide 10 A Task Group report (2008) CCIP-036 Modular Precast Concrete Bridges Technical Guide 11 A state-of-the-art report (2008) CCIP-028 You can buy the above publications from the Concrete Bookshop at www.concrete.org.uk and please visit www.cbdg.org.uk for further publications, including free download.
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CI/Sfb
UDC 624.21.012.3
Modular Precast Concrete Bridges: A state-of-the-art report
This publication outlines the development of a modular precast concrete bridge system. This new system provides designers with a concrete solution for medium span bridges that can be used for the majority of bridge locations. The modular system incorporates easily transportable precast concrete shell units that support and combine compositely with the post-tensioned, in-situ concrete core. Guidance includes an introduction to the system followed by structural details, construction options and example costs. Scheme drawings and construction programmes for case study examples are also provided.
CCIP-028 Published December 2008 ISBN 978-1-904482-52-9 © Concrete Bridge Development Group Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey, GU17 9AB Tel: +44 (0)1276 33777 Fax: +44 (0)1276 38899 www.cbdg.org.uk
