CROSS Safety Report
Quality of design and construction of a major bridge structure
This report is over 2 years old
This report concerns the design and installation of the bridge bearing zones on a new prestressed precast concrete viaduct.
Key Learning Outcomes
For civil and structural design engineers:
Where specialist systems such as bridge bearings are used, it is beneficial to have a close working relationship with the supplier from the earliest opportunity to ensure design requirements are met
Be aware of the importance of site levels and gradients and how localised overstressing of elements can occur if they are not properly considered or detailed
For the construction team:
It is good practice to review the proposed construction sequence to ensure it can be safely achieved on site
Consider engaging with specialist suppliers such as bridge bearing suppliers to seek advice on the proper installation of their products
Toolbox talks and workshops with the site crew on the correct installation of the products can help ensure construction is carried out in accordance with the design
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The reporter had concerns about the design and installation of bridge bearings and the main prestressed precast concrete beams.
Bridge bearing design problems
Anchorage of bottom tensile reinforcement was insufficient which may compromise shear capacity at the bearing location. A bearing schedule issued before construction showed ultimate vertical load on the bearing of around 3,000kN. The schedule issued after the bridge was constructed, and design had been challenged, showed this vertical load to have been reduced to around 2,000kN. This was explained by more refined computer modelling.
There was insufficient edge distance at the back of the elastomeric bearing. The code calls for minimum of 45mm not including any construction tolerances. Zero tolerance was provided because the bearing size increased once detail design by a specialist supplier was finalised.
Insufficient edge distance between bearing and front face of a supporting pier. Similar issue to point two above. Concerns over effective load transfer in this area.
The designer did not consider bridge gradient. All bearings were designed to sit level on a pier bearing shelf and to be in place before the main girder installation. However, the main precast concrete girders were installed on a small gradient. This resulted in a smaller contact area and possibly localised overstressing of bearings.
Incorrect construction sequence was provided in the design documentation. The construction sequence called for the bridge beams to be positioned directly on top of elastomeric bearings with a thin layer of epoxy grout. However, as the bridge beam was installed on a small gradient, full contact surface between bridge bearing and precast concrete girder could not be achieved. This resulted in localised bearing overstressing and damage.
Bridge bearing construction problems
The contractor followed an incorrect construction sequence without question
The specialist bearing supplier was not engaged to provide installation advice
Full contact surface between bearing and bridge superstructure was not achieved. This resulted in localised bearing damage.
In conclusion, the reporter questions the competency of those involved including the designer, the detailer of the beams, the category 3 checking engineer, and indeed the client for signing off the detailed design. Many of the personnel, he says, were not trained or experienced in bridge construction.
Intensive design checking and remedial works required
The bridge entered service following a period of intensive design checking and remedial works on site. The design life of bearings and their failure mode in the future is however, according to the reporter, uncertain.
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Surely the real problem here was of dis-aggregating the design and there being no one controlling mind. Presumably the Cat 3 check was done before the project went to site. It is a common problem that design and build contractors take designs and amend them to suit sub-contract suppliers offers, or worse, to suit commercial pressure to reduce costs. Because of the chasm in the industry between designers and site, and with no effective supervision, site personnel are becoming more and more likely to have insufficient design experience to know when this is safe or, worse still, when to refer such matters to a designer. This will only be solved when a single designer signs off every detail including any change howsoever caused from start to finish of a project.
Expert Panel Comments
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It is worrying that we should come across such a large and complex structure being, apparently, so poorly designed and executed. Especially so at a time when we are all so focused on quality of design and construction and after there have been recent major bridge failures.
The importance of competency in the industry
Key factors in avoiding such issues are to use experienced Chartered Engineers for both design and construction, and to have regular and effective communication between all parties.
Key factors in avoiding such issues are to use experienced Chartered Engineers for both design and construction, and to have regular and effective communication between all parties
Importantly, is this an unusual case? New bridges in the UK are designed and built in accordance with robust procedures; lessons having been learned from past failures.
Competency of all concerned is a critical component of such processes and the Hackitt report Independent Review of Building Regulations and Fire Safety stressed the importance of this having found competency to be ‘patchy’ in the building industry.
How is competency measured, achieved and maintained?
There are currently several groups led by the Construction Industry Council (CIC) looking into how competency is defined, measured, achieved, and maintained. Competent personnel at several levels with the organisations who created this bridge should have detected the potential problems and acted before the components came together on site.
Experienced designers know that attention to detail is vital if satisfactory and safe designs are to be realised. Hence skilled design (i.e., not just analysis) requires the ability to think ahead within the iterative design process and ‘claim enough space’ at the outset for the final design to be achieved.
The correct selection of bearings and the transfer of designs into detailed drawings are critical and should highlight issues such as bearing tolerances. The reporter says that the issues here were generally resolved but they should not have arisen in the first place.