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Hacked off

Letters

The mindless hacksaw attack on the M60 stayed girder footbridge (NCE 11 January) unfortunately demonstrates the special vulnerability of stay cables to terrorist attack.

This is particularly evident on footbridges with their readily accessible small diameter cables.

Being a bridge designer with two structures attacked by the IRA (Staples Corner in 1992 and DLR 1996) the talk of protective metal sleeves reminded me of a stayed girder road bridge we proposed to be built in Northern Ireland during the Troubles.

Our clients were concerned that terrorists could drive onto the bridge and lean out of the window to plant a small parcel of adhering plastic explosive on the cable.

We took the problem to explosives experts and they recommended the use of 3m-4m lengths of metal ducts around the cables at deck level.

This would provide a protective air gap that would signicantly reduce any explosive effect on the cable. The bridge was accordingly detailed with these steel ducts, described as 'anticorrosion sheaths'.

The bridge was eventually built, but with a non-stayed composite girder deck.

However, concern was reected in later stayed girder bridge designs by ensuring that the deck girders were strong enough to prevent collapse if any two stay cables were destroyed.

Brian Pritchard, 44 Denmans Lane, Lindeld, Haywards Heath, West Sussex RH16 2JR

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