Lord Foster sat quietly to one side as four Ove Arup engineers presented their initial analysis of the cause of the swaying Millennium Bridge last week. Tony Fitzpatrick, the engineer at Arup responsible for bridge, was bullish: 'We know a lot of things that it isn't and we're fairly certain that we know what it is.'
Preliminary findings indicate that the impact of people walking on the bridge had exacerbated slight wind movement in the structure. Effectively, the bridge was designed anticipating pedestrians walking normally across the bridge - using random strides. However, any movement in the bridge, which is usual on a suspended structure, had caused the public to alter their walking patterns to help them balance and to compensate for the rocking motion. Walking in time with the bridge amplified the rocking motion.
Emphasising that the bridge was safe, Tony Fitzpatrick said that he would 'put my kids in the middle of it right now'. Another Arup engineer, Pat Dallard, said: 'Our method of modelling was not correct . . . synchronous movement is not accounted for [in Arup's calculations].' Recognising the potential liabilities, Fitzpatrick interrupted to say that their 'maths was valid' and that this type of problem 'could not have been predicted'.
Arup was also keen to show that it was carrying out the post mortem 'in a spirit of openness', so that others might learn the lessons and that it could not be accused of hiding anything. Blaming the lack of shared information within the profession, Fitzpatrick stated that 'knowledge not circulated is not known knowledge', and suggested that the firm's calculations and modelling had far exceeded the existing codes and guidance on structures of this nature. The only other similar case occurred in Japan in 1989, and, even though Professor Fujino of Tokyo has been invited over to advise the design team, no-one present knew where the other bridge was located.
No-one was prepared to admit the extent of costs incurred in the research so far, but Arup did not believe that it was at fault. David Bell, chairman of the Millennium Bridge Trust, said that costs were not being considered at the moment and that the main thing was to get the bridge secure and reopened. Decisions on liability and costs would be looked at once the full extent of the remedial work was established, he said. Presenting a state of the art defence may absolve the design team from potential liability. The investigations will take at least another six weeks and remedial works will mean that the bridge remains closed to the public for five months.
To reduce the resonant frequency of the bridge requires stiffening the existing structure or damping the movement. Stiffening would involve more cables and piers, which would detract from Foster's vision of a 'ribbon of steel'. Curiously, when asked if the engineers had ruled out additional supports, Fitzpatrick said: 'It depends what you mean by ruling it out.' Pat Dallard confirmed that, had the bridge been designed to cater for vehicular traffic, this problem might never have arisen.
The preferred solution is to insert massive dampers across the underside of the bridge or to hang counterweighted dampers to counteract the movement of the bridge. Further work is required, but Fitzpatrick confirmed that after remedial works, the bridge 'will be a comfortable suspension bridge, but it will still be lively'.
Lord Foster was circumspect: 'In this industry, we always have to take the long view. If you accept that progress is about stretching the boundaries, then you have to be prepared for setbacks.' He admitted that the problem had caught them out, but that 'dampers are very discreet and there is no chance of the bridge being compromised'.