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Vandals are biggest threat to footbridges

MOVEMENT TRIGGERED deliberately by vandals is a bigger threat to the new generation of lightweight footbridges than the involuntary marching in step which shut London's Millennium Bridge, a leading engineer claimed this week.

University of Sheffield department of civil and structural engineering vibration engineering section head Dr Aleksandar Pavic was commissioned by Millennium Bridge engineer Ove Arup to help solve the excessive horizontal sway problems which led to the £18M structure's closure in June.

He said: 'There is a growing realisation throughout the world that vandal loading is an increasing problem. This covers both vertical and horizontal motion - and modern grandstands could be as vulnerable as the latest lightweight footbridges.'

Sheffield's 11m by 2m prestressed concrete 'footbridge simulator' will shortly be used to gather data on vandal loading, Pavic confirmed.

Up to 10 volunteers will be asked to do their best to generate excessive deflections while the loads are monitored by accelerometers.

But Pavic said that earlier work on crowd behaviour suggested there could be problems for the volunteers. 'When we got volunteers to try to march in step with a metronome, only 50 per cent could manage it for more than a few minutes.'

He added that Japan's Professor Fujino, who first researched the synchronised marching problem, found that no more than 20 per cent of people on a swaying bridge could actually co-ordinate movements.

However, about 20 Arup engineers were videotaped just after the bridge shut deliberately swaying together on the southern span and generating significant lateral movement. Earlier this month 100 volunteers were used in investigations (NCE last week).

An Arup spokeswoman confirmed this week that it was not ready to present its findings to a meeting between the Millennium Bridge Trust, Southwark Council and the Corporation of London today.

'We still intend to deliver our final conclusions by the end of August or the beginning of September, ' she said.

The Corporation of London also confirmed this week that it had appointed consultant WS Atkins as specialist adviser on the project.

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