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It don't mean a thing if you know it will swing

'If it's safe, let it bounce - it'll be the biggest tourist attraction since the Leaning Tower of Pisa.' Such was the gist of just one of the many letters about London's Millennium footbridge published in the national press last week. The author might be right. Certainly, when I was on the crossing just before it closed many of those who had queued for 30 minutes or more to get on were expressing disappointment at its lack of movement. And expert observers confirm that on the infamous opening Saturday a sizeable percentage of the crowd was doing its best to increase the sway.

But there is no denying that a number of those crossing were seriously upset by the movement. Perhaps they had been reassured by the bridge designer's website, on which Ove Arup claimed it would be 'as sturdy as nearby Southwark and Blackfriars Bridges'. Research shows very clearly that tolerance of all forms of movement is linked very closely to expectation: what a human being will accept and even enjoy at a funfair will cause panic on an airliner.

I have never visited the famous Maunsell-designed 'plastic' Aberfeldy footbridge in Scotland, but I am told it bounces vertically even more than the Millennium footbridge moves horizontally. Yet golfers cross it without panic every day of the week, and count the experience as one of the highlights of their round. They expect it to bounce; they have no reason to doubt its ultimate strength, so there is no problem.

Obviously, if any structure moves too violently there is some safety risk. Elderly pedestrians or toddlers crossing very lively footbridges could fall over and hurt themselves. Engineers have to try to keep the movement of such structures under control. But if the expectation is that there will be noticeable movement, no-one is likely to panic, nobody will stampede.

It is to be hoped that Ove Arup will soon be able to explain why the £18M structure behaved so unpredictably. Coming up with an elegant and cost effective way of eliminating the worst of the horizontal movements may take a little longer.

To my mind, however, there are already two lessons to be learned. First, avoid hubris. Predicting the dynamic characteristics of complex asymmetrical highly stressed structures is still more of an art than a science, and there is no shame in admitting as much.

Second, be aware that the non-technical press believes that architects design structures - and that architects have very good reasons for encouraging that belief. Especially when the press coverage is favourable. When the brickbats start flying, however, high profile architects with media savvy are the first to duck for cover.

In stating that the problems with the bridge are 'an engineering issue', Lord Foster is of course technically correct - although it may be interesting to discover in due course just how much 'architectural' details contributed to the problem. But, as other published letters - and many communications to NCE - show, his remarks left a nasty taste. However at least the public at large now know that engineers, not architects, design bridges.

Dave Parker is technical editor of NCE

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