Asymmetric bridge design and construction – a near miss

During the construction stage of an arch footbridge, says a reporter, a design error was identified that had been indirectly revealed by an associated error in construction. It is reported that effective correction of these errors was expensive but resolved the potential performance safety issues.

The bridge was designed to be constructed from four precast concrete arch legs, two each side which were to be cast integrally with an insitu deck to create an open spandrel arch elevation with two legs on each side supported on spherical bearings. The bridge was skew and curved in plan. The legs tapered longitudinally and, in order to aesthetically enhance the simplicity of the elevation, had skewed cross sections.

The bridge was designed using a three dimensional frame model with grillage type members representing the deck, and beam members representing each leg. Pinned supports were positioned at the deck ends and at the base of each arch leg. Reactions from the analysis model were aligned to the global axes and would therefore not be suitable as input into the bearing schedule without translation to the proposed local axes of the bearing.

The drawings being prepared for the arch springing area were given a lot of attention, but this was focused on the aesthetic and construction aspects. However, although the designer correctly extracted forces from the model, it is believed that they gave insufficient attention to the drawings being prepared and how they communicated their design. Consequently, they did not realise that the bearings were shown to be aligned parallel to the plan skew of the bridge and not perpendicular to the line of thrust from the arch legs as was his design intent. The error was compounded by the checker who suffered a similar lack of appreciation of what was shown on the drawings and agreed the design to be correct. See Figure 1. 

Whilst dealing with construction issues related to the casting of the arch legs a senior engineer noted the anomaly between what was shown on the drawings and the figures in the bearing schedule and, after investigating the design calculations, concluded that the actual lateral load on the bearing would have been some four times greater than required by the schedule. Although the bridge legs had not been erected, they had been cast.

The construction error, which involved incorrect reinforcement layout, had already necessitated the decision to recast the legs. It was at this stage that it is reported that the design error was detected. It also provided the time window to revise the leg end geometry to eliminate the undesirable component of lateral reaction and restore the original design intent. It was however necessary to undertake some challenging re-modelling of the bridge foundations that were already cast.

The discovery of the design error was fortuitous. If there had been no issues with the precasting of the concrete legs, the additional load on the bridge support would probably have remained undiscovered until the temporary works for the insitu deck were released. Under these circumstances the bearings would have experienced significantly larger lateral loads than they were designed for. This may have led to the safety or the performance of the bridge being compromised.

It is however more likely that the overloading would have manifested itself before the falsework was fully released giving an opportunity to reinstate temporary support and thus address the issue. This would, however, have left the considerable problem of restoring the bridge to a safe condition. Overall, the errors resulted in significant costs and delay to the works on site. However, the bridge was duly safely constructed and is now complete and in service.

Once the problem had been resolved, a programme of dissemination within the company of the issues involved and lessons learnt was undertaken. This improved awareness of the problems that can arise in structures with complex and unusual geometry. During the initial dissemination of the information, it became clear that others had experienced problems when designs involved bearings with non-vertical axes. This highlighted the challenges of ensuring that lessons from experience are not lost.

A further difficulty in preventing the reoccurrence of errors became apparent when considering the wider implications. It is relatively straightforward to warn of the issues of bearing alignment in precast concrete arches and to have good confidence that the same error should be avoided in future bridge designs. However, the post project reviews of this particular lesson have also emphasised the need for greater global awareness. It was evident that there was a need to identify where a similar issue could occur in a different structural form or context. Consequently, an extensive series of presentations and workshops have been organised within the company to widely disseminate the key lessons learned.