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Millennium Bridge: the moment of truth

News

On a dark evening in January more than 2,000 volunteers tramped across London's Millennium Bridge in a final test of its retrofit. Dave Parker was one of them.

THEY CAME from structural engineer Arup and architect Foster & Partners, from the London Borough of Southwark and the Corporation of London, from HSBC and from offices near the northern end of the crossing.

Nearly 4,000 volunteered, but only 2,000 were accepted, and for that one night, Wednesday 30 January, tickets for the Millennium Bridge test were the hottest in town.

Most wore their complimentary Arup woolly hats, some had babies on their backs, some needed canes to help them walk.

There was a town crier in full fig and the lady mayoress of Southwark.

Public relations staff with headsets rampant and aggressive clipboards held cryptic radio conversations, polite security men patrolled the access points and bemused members of the public hovered on the fringes. Even the press, with a very few exceptions, were safely corralled at the northern end of the bridge.

In front of the Tate Modern gallery, the volunteers assembled in their compound under damp and gloomy skies. Their mood seemed to be one of relaxed anticipation rather than feverish excitement. After all the publicity, 18 months of debate, this was to be the real moment of truth. And they would be part of it.

Observers from WS Atkins and Cambridge University took their places in the control cabin, where the readouts from the 24 accelerometers scattered across the structure would give the final word on its reaction to the thousands of marching feet.

It was the lateral forces from massed pedestrian footfalls that had caused the embarrassing lateral sway on the June 2000 opening day. Urgent Arup-funded research soon confirmed that if there is a large enough number of pedestrians walking across a relatively lightweight structure, enough of them will be marching in step to exert a significant rhythmic lateral force on it.

Should the structure have a natural frequency of response close to 1Hz, it will begin to sway. Then the unexpected factor comes into play.

As the structure moves, it becomes uncomfortable to walk out of step with the frequency of sway. So more and more people start walking in step, exerting larger and larger lateral forces, creating larger and larger displacements.

On opening day, the sway never reached dangerous levels, but many were disconcerted.

Since then, a mixture of high performance viscous dampers and tuned mass dampers have been fitted to the bridge. A week earlier, around 700 volunteers from Arup had failed to elicit any unwelcome response from the upgraded structure. Now it was time for the big one.

The town crier clanged his bell with practised Úlan and the marshalls gave a detailed briefing.

Then off we set. As the leading groups were led at a brisk pace out on to the southern span, some movement could be felt, but it was random, insignificant and easy to ignore. The middle and northern spans were even less responsive. For Arup, it must have been an unimaginable relief.

And what now? Anyone who has crossed the Millennium Bridge will already know what a special experience it is. Now millions more can share that experience over the coming decades.

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