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Cable rejig could stop the sway

Letters

In 1972, I worked for Freeman Fox on the design of two long span pipe suspension bridges in Pakistan which had lateral and vertical suspension cables. I have also spent many years designing dynamic marine structures.

It is well known that back span configuration is important in suspension bridge design and multi-span suspension bridges always have an anchorage to the ground at the middle of alternate spans. If this is not done the bridge is a mechanism with nothing to stop one span rising when the adjacent one is pressed down.

With the Millennium bridge the designer has tried to stretch the south back span so that it almost becomes a second suspended span. The problem is that there is not enough rake in the backspan cables to prevent coupled motion of the main and back span: the designer has unwittingly defied the basic principals of suspension bridge design. The mass dampers (NCE 6 July) will not act to remove the offending mode of vibration because there is no shear movement in the zones proposed for the mass dampers.

I think that the only solution is to re-rig the main south cables to a similar geometry to those on the north side with a new anchorage point just outboard of the south river wall, or cross the cables just outboard of the riverwall and use the existing anchorage points but swap the north cable to south side and the south to the north anchorage. This work would entail fitting new, shorter lateral fishbones to the South back span and an anchorage to the ground on the bank parapet to bring the cables down below the soffit of the deck where they cross it. This is where the mass dampers could be useful because the relative movement between the deck and ground at the south river wall is probably the greatest.

RW Brewerton (M), managing director, Natabelle Technology, rwb@natabelle.demon.co.uk

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