THREE UNRECORDED cases of 'wobbly bridge syndrome' were cited by London's $25M Millennium Bridge structural engineer Arup in a spirited defence of its design philosophy at a crowded Royal Academy of Engineering meeting in London last month.
An audience of more than 300, including 32 professors of engineering, were shown a recently discovered 1970s film of a Maori demonstration on the Auckland Harbour road bridge in New Zealand. Under the influence of around 4,000 demonstrators the 1960s steel box girder bridge was seen to sway from side to side in much the same manner that led to the closure of the Millennium Bridge only three days after it opened in June last year.
Arup Millennium Bridge team leader Tony Fitzpatrick also cited the footbridge linking the UK's National Exhibition Centre in Birmingham to the nearby railway station and the Groves suspension bridge in Chester as previous cases of what Arup has dubbed 'synchronous lateral excitation'.
Despite intensive interrogation from the floor, Fitzpatrick and colleagues Arup director Pat Dallard and associate director Roger Ridsdill Smith maintained that the sway problem on the Millennium Bridge had nothing to do with the crossing's ground breaking design.
Instead, they insisted that the wobble was simply a factor of the density of pedestrians on the crossing on its opening day, and that any structure with a lateral frequency below 1.3Hz was vulnerable if pedestrian densities were high enough.