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Standby dampers calm fears of Dublin cable stay bridge wobble

NEWS

SPECIAL ANTI-WOBBLING features are being designed into a new railway bridge in Dublin by engineers keen avoid a repetition of the debacle which has kept London's Millennium Bridge closed for almost two years.

Fears of vibration affecting the Taney Bridge in south Dublin have prompted consultant Roughan O'Donovan to include provision for dampers should they be needed.

Although the cable stayed concrete deck structure which will carry one of the light rail Luas lines is entirely different to Arup's Blade of Light over the River Thames, it too could fall foul of complex vibration effects.

Roughan O'Donovan chief bridge engineer Keith Wilson played down the risk, saying:

'The genereal view is that it won't occur. But if it does, we're not going to be in a position saying 'We've got another wobbling bridge - what are we going to do about it?'' Wobbling - or 'structural excitation' as described by the Millennium Bridge engineers - could be induced in a cable stayed bridge by a combination of wind, rain and train.

Small protruding ribs fabricated in the sheaths covering the bridge cables will stop vibration effects from rain, said Wilson. 'But vibration could occur from wind vortex shedding or turbulence, while a passing vehicle could induce a resonant frequency.'

Six dampers on the main 108.5m span can be fitted should the bridge, which also has a pedestrian walkway, start to vibrate excessively.

Because modelling or prediction of the complex interaction of the wind and train-induced vibrations is impossible, engineers will have to wait until the bridge is in service before deciding if dampers are needed.

Dampers can be installed within 2m high, 600mm diameter anti-vandal steel tubes which house the cables at deck level. 'They are like brake pads, and can be clamped to the cables, ' said Wilson. This will prevent excessive movement from being transmitted from deck to cables.

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