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Arup picks double dampers to stop Millennium Bridge sway

A UNIQUE combination of viscous and tuned mass dampers is expected to be revealed as the cure for London's notorious wobbly Millennium Bridge tomorrow.

The solution also involves tying down the southern span.

Details of the proposed solution are shown in drawings lodged with the Corporation of London as part of the application for planning permission to modify the structure.

These show a system of inconspicuous viscous and tuned mass dampers under a 'chevronbraced' deck, coupled with new viscous-damped diagonal bracing between deck and suspension cables at the piers.

TMDs are little more than a mass suspended below the deck and fitted with some form of friction energy absorbing device.

They oscillate out of phase with the deck, virtually eliminating one precise frequency of movement.

Viscous dampers work on the same principle as car shock absorbers.

On the southern span a set of four vertical viscous dampers will connect the underside of the deck with two new concrete bases cast in the river bank below. This appears to be the only major change to the appearance of the bridge.

The southern span differs significantly from the northern span and moved much more during the infamous opening day.

Structural engineer Arup was due to announce the preferred solution as NCE went to press, 10 weeks after it presented recommendations to client Millennium Bridge Trust (NCE 14 September).

Technical advisors to the Trust, Southwark Council and the Corporation of London, are now understood to have cleared the proposals.

The excessive horizontal sway which forced the £18M structure to close only days after its June opening triggered a massive research and investigation effort.

Teams at Imperial College and Southampton and Sheffield universities were commissioned to carry out basic research into how pedestrians and lightweight footbridges interact - especially when the bridge is very crowded.

(NCE 29 June).

The few previous records of 'opening day sway' or its equivalent have all featured large footbridges crowded with pedestrians who unwittingly start to march in step. Arup's solution is much more complex than that used for other wobbly bridges, which have usually relied on simple tuned mass dampers.

Avoiding any serious compromise to the aesthetics of the Foster-designed crossing was one of Arup's top priorities. Simple tuned mass dampers would have been very visible from the river and the Thameside walkways.

But standard viscous dampers only work effectively when movements are large.

Combining a large number of relatively small TMDs with viscous damping enables Arup to tune out a wide range of oscillation frequencies as well as keeping the retrofit as visually inoffensive as possible.

It should also make the structure much more resistant to any attempt by vandals to deliberately induce excessive horizontal motion.

Elegant solutions are rarely cheap. The cost of the retrofit seems likely to run well into seven figures. Who will eventually pay has still to be decided.

A spokeswoman for Arup refused to comment in advance of Friday's announcement, but would only say: 'Arup is confident that work to correct the problem will begin in the near future.'

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