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All large footbridges face Millennium Bridge sway problem

TRIALS ON London's infamous Millennium Bridge are said to have shown that any large lightweight crossing is at risk from the 'opening day sway' problem that led to the £18M structure's closure last year.

Structural designer Arup carried out trials for prototype damper installation before Christmas (NCE 23 November 2000).

This week Arup engineers told NCE that as a result of the trials they had developed a 'magic formula' that could accurately predict when crowds of pedestrians would 'lock in' to the lateral movements of a bridge.

Arup director Pat Dallard said that lock-in was determined by crowd density. 'People are effectively negative dampers. If there are enough of them the total negative effect will overcome the bridge's inherent damping and lock-in will occur.'

During the pre-Christmas trials large numbers of Arup volunteers were marched across individual spans of the bridge.

Video recordings clearly show what Arup said was the most surprising feature of lock-in - the sudden onset of the phenomenon as crowd numbers increase.

With 156 volunteers circulating on the northern span no lateral movement was visible. Ten more volunteers joined the crowd and within a few seconds the bridge began to sway violently. The test was abruptly terminated.

Arup project engineer Roger Ridsdill Smith said: 'If any large crossing hasn't developed this problem so far it's because there haven't been enough people on it. During the opening day we calculated that there was a peak of 1.3 people on every square metre. Very few other bridges have approached this sort of density - but they could.'

According to Arup, lock-in is related only to natural dynamic frequencies, not to structural type. Other discoveries were the lower than expected frequencies at which lock-in could occur and the absence of any signs of vertical lock-in.

An array of tuned mass dampers designed to damp out vertical lock in was included in Arup's retrofit proposals. Arup chairman Bob Emmerson confirmed that they would still be retained as an extra precaution.

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