The background.
The failure of four steel box girder bridges during construction caused quite a furore. The use of such girders in bridge construction had been a fairly recent phenomenon, partly because the mathematical analysis was quite daunting before the advent of electronic computers. Their popularity came at a time when the post-war surge in bridge building was getting underway. They required less steel for the construction of a bridge deck than the truss or the plate girder that had been the popular choice previously.

A cantilever built using box girders poses problems for the designer. The stresses will be distributed between the various constituent plates of the boxes depending upon the accuracy of the individual elements and the precision of the fixings involved. Distortions of an individual plate could cause havoc especially during the construction of a cantilever and they are difficult to spot and to eliminate. Try to imagine how a box girder would crumple if tested to destruction by the continual adding of new units, i.e. more weight, to the unsupported end of the cantilever.

The government’s response.
In 1970, the government responded to the box girder crisis by setting up an advisory committee under the chairmanship of mathematician, Dr. (later Sir) Alec Merrison, then Vice Chancellor of Bristol University, to investigate the circumstances of the previous failures and to make recommendations for the safe re-construction of the Milford Haven bridge and the safe completion of the Avonmouth Bridge (under construction at the time). The committee also advised changes to the design rules and on the procedures to be adopted for future applications. As part of this process, a major programme of research was initiated to study the behaviour of the various components of a box girder and to consider how the mathematical analyses, then in circulation, might be improved.

The committee delivered its final report in 1974 after the results of the research studies became available. It concluded that the primary causes for two out of the four failures were inadequate organisational procedures and poor communications. It provided an improved set of rules for the design of box girders, incorporating the findings of the latest research. It also stressed the need for the introduction of an independent check on methods of erection and on the design of temporary works, together with the need for clearer distinctions between the respective responsibilities of the Engineer and the Contractor,

It is true to say that only one of the four box girder failures was strictly the result of lack of technical knowledge of steel box girder behaviour, although that was not apparent when the failures originally occurred. In three cases, the primary cause of failure was human error during construction and can arguably be attributed to the rapid growth of the design and construction programme out-stripping the supply of suitably qualified and experienced engineers to adequately supervise these works.

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