TWO BRITISH universities have been commissioned by Millennium footbridge designer Ove Arup to carry out emergency research into pedestrian forces on bridges.
Excessive sway on London's £18M landmark structure led to its closure only three days after opening two weeks ago (News last week). Engineers have since been racing to identify the cause.
But Arup chairman of engineering Tony Fitzpatrick said that no solution for the sway problems could be finalised until the company knew exactly what forces were being exerted on the bridge when large numbers of pedestrians were crossing.
'No-one has ever actually measured these forces before, ' Fitzpatrick said. 'We have commissioned Imperial College and the University of Southampton, who are working up two separate lines of investigation, to get the data we need.'
Pedestrian induced sway was suspected almost from the start, as first revealed in NCE. Engineers viewing a video of the opening day crowds noticed large numbers 'locked in' to the bridge's horizontal movement, feeding lateral forces into the deck. Fitzpatrick said this phenomenon was almost unknown and virtually undocumented.
'Since the event we have discovered that a Japanese footbridge suffered from similar problems on its opening day in 1992, ' said Fitzpatrick. 'The client insisted on almost complete confidentiality. The problem was solved with tuned mass dampers and the only reference to it appears in very obscure literature.'
The Japanese professor who carried out the analysis and a German expert on resonance in lightweight bridges from the University of Munich have also been brought in to advise.
Full scale dynamic testing has started on the crossing. One university team will use a standard sway table, said Fitzpatrick, while another will design, build and instrument a small bridge deck and march pedestrians across it.
'We need to know what the forces are, at what level of sideways movement pedestrians start walking in phase, and what the effect of increasing pedestrian numbers is, ' he said.
Results so far, he added, had validated Arup's original model.
Twenty engineers are now working on the problem. Possible solutions include tuned mass dampers, cheaper but heavier tuned 'slosh' dampers - liquidfilled tanks suspended beneath the bridge - and off the shelf visco-elastic dampers.