Case study 2: Single- Single-span highway overbridge

Carriageway Carriageway

Hardshoulder Central resevation Hardshoulder

Above: Figure 18 Single-span highway overbridge layout

adopted for Case Study 2.

Right: Figure 19 Section through modular precast bridge.

2100

Precast edge unit In-situ stitch concrete In-situ infill concrete

Void CL

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 fi rst 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 span-to-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.

6 Construction programme and costs

A summary of the costs is given in Table 6.

Item Modular precast Steel-composite

Span lift SPMT

Substructure £80k £80k £70k

Superstructure

Deck (excluding formwork) £150k £150k

Equivalent formwork costs

● Job specifi c £45k £115k

● Capital repayment £10k £205k £10k £275k £250k

Finishes £80k £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).

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.

Table 6 Cost summary for single-span overbridge.

6.3.2 Programme comparison

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 con-struction 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.

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.

6.3.3 Summary

7 Conclusions

7. Conclusions

The bridge market in the UK is set to grow steadily over the next fi ve to ten years. The use of steel-composite construction is almost unique to the UK, and it is believed that a well-engineered and competitive concrete option has the potential to gain considerable market share. The modular precast bridge system which this publication addresses aims to re-establish 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 specifi c bridge sites and the demands of the project programme. The con-struction 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.

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 signifi cant cost and/or programme savings over a steel-composite alternative, as well as being more more elegant, effi cient 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 fi nanced over fi ve 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 benefi ts 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.

References

References

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.