Modifications designed to stop the severe swaying of London鈥檚 Millennium Bridge were tested on Tuesday. The wobbles closed the bridge just three days after it opened in June.
Three hundred volunteers from the consulting engineers Arup strolled up and down the footbridge to test the effect of new dampers strapped beneath the structure.
鈥淭he tests serve the purpose of verifying the design that we鈥檝e put forward to fix the problem,鈥� says Arup鈥檚 Pat Dallard, structural advisor for the bridge.
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Since the bridge closed, Arup engineers have sought to identify why the bridge swayed so much when crowds walked over it. They discovered that slight sideways movements in the bridge caused people to fall into step that then exacerbated the swaying.
Damping down
Arup engineers believe the solution is to sling 拢5 million of specialised damping equipment beneath the walkway. 鈥淲e鈥檙e using viscous dampers to damp the lateral movement. They are just like shock absorbers in a car,鈥� says Dallard. So-called 鈥渢uned mass dampers鈥� are also used to damp out unwanted vertical movement.
鈥淲hat we have on the bridge at the moment is about a fifth of the damping we鈥檒l have in the final solution,鈥� says Dallard. 鈥淲e鈥檙e putting on about a fifth of the people to see if it performs as we鈥檝e predicted it should.鈥�
In tests, Dallard ordered the Arup volunteers to walk up and down the centre span of the bridge which has been fitted with two viscous dampers and one tuned mass damper. By sending people across in groups, the engineers were able to assess how the bridge moved under different sized crowds.
Sensors attached to the bridge supplied data on how much it was wobbling. Arup believe that at its maximum loading of two people per square metre, the bridge should move sideways a maximum of only two centimetres.
If the tests vindicate Arup鈥檚 solution, work could begin to fix the bridge early next year. 鈥淚t鈥檚 the client鈥檚 decision,鈥� says Dallard. As yet, no-one has agreed to pay for the extra work.
