The bridge that wobbled

London’s first new bridge for a century, designed by the architect Norman Foster and the engineers at Arup, closed as soon as it opened. Deyan Sudjic, in Domus 847 April 2002, on what went wrong and how it was put right. #domus1000

Deyan Sudjic, Quando i ponti tremano. Foto Julian Anderson
#domus1000, on newsstands in March, is the occasion to get back to the archive for some relevant articles of the editors that guided the magazine from 1979 until now. Deyan Sudjic has been Domus editor from 2000 to 2003.

 

Last month, when the most famous wobbling bridge in the world finally reopened, I looked back at a text that I wrote about London’s new footbridge for the Observer, the British Sunday newspaper, that appeared somewhat embarrassingly on the day the bridge was shut just hours after its first official opening. ‘The great thing about the seven-minute walk from Bankside to St. Paul’s across London’s first new bridge in a century, as compared with a trip around the London Eye, say, is that you are actually going somewhere. ‘The bridge has the effect of not just changing how a key part of the river looks, but also how it functions. For the first time it’s possible to walk directly from the trading floors and corporate headquarters of the north bank to the ducking-and-diving, south-of-the-river world that is still at the heart of Southwark, despite the epidemic of urban regeneration triggered by the Tate. ‘Negotiating the bridge is also an intensely physical experience. Under a thunderous gun-metal-grey sky, there is the unmistakable gamy tang of lowtide London mud in the air. You can feel the beginnings of rain in the breeze and the slight bouncing judder of aluminium underfoot’.

Ah yes, the slight bouncing judder. On the day before publication, too late for newspaper deadlines, Roger Risdill-Smith, the young Ove Arup engineer who had been involved with every aspect of the project since a famous paper-napkin sketch of a ‘blade of light’ had so impressed Norman Foster, was standing on the bridge. As it began to fill with people, he noticed the first sign of movement more serious than a bouncing judder. ‘It happened quite fleetingly, but you could tell that it wasn’t just a judder; it was a resonant movement. I thought, “That’s interesting”, but as the numbers thinned out, the bridge calmed down again’. Then at lunchtime, as crowds swarmed on from both ends, the bridge really started to move. All 690 tons of its steel-and-aluminium deck began to sway left and right like a giant executive toy, so much so that pedestrians, suspended above the Thames on slender steel cables, began to clutch at handrails and throw themselves against the motion to stay upright. As they did so, the swings became increasingly violent.
The bridge that wobbled
Foster called the bridge a blade of light. It represents a collaboration between architects, engineers, and the sculptor Antony Caro

Risdill- Smith mentally reviewed all the calculations, all the safety assessments, all the wind-tunnel tests, even the giant hydraulic tank in Canada used to measure the bridge’s resistance to water. None of them had predicted anything like this. This was simply not supposed to happen. All of his experience as an engineer told him that the bridge ought to be stable, and that it was certainly safe. But even he must have seen that image of the great Tacoma Narrows suspension bridge shaking itself to pieces in 1940 flashing through his mind. The police promptly cleared and closed the bridge, giving the experts time to work out what had gone wrong and how to fix it. As engineering disasters go, the great wobbly bridge fiasco might seem pretty tame. Nobody suffered so much as a bruise. But to see such a highprofile design closed down so soon after opening was a humiliation for Arup, the fiercely proud engineers who have made everything from the Sydney Opera House to the Hong Kong and Shanghai Bank tower possible. What made their embarrassment so irresistible to the more pedantic of their fellow engineers, who rushed to make judgements about what had happened, was that they seemed to have brought it on themselves.

The word was that they had allowed Foster to push them into flouting the sensible limits of design. ‘Of course this bridge was going to wobble, just look at it’ became the consensus. With its lowslung outriggers and its very flat profile, the ‘blade of light’ looked like no other bridge. It was presented as a piece of typical, too-clever-byhalf arrogance. Stray too far from the tried and trusted and you’ll get into trouble was the subtext of much of the debate. But according to Tony Fitzpatrick, who lead the Arup team struggling to find a solution, ‘It’s not what is different about the bridge that caused the problem. Every issue about the bridge that was innovative worked perfectly. What hit us on the back of the head was that bit of the bridge that was the same as every other long-span bridge. We assumed that it would work like other bridges have until now; that was the mistake’.

The bridge that wobbled
Components for the bridge were manufactured all over Europe, then brought to London to be assembled on site
What Arup eventually discovered was a previously misunderstood phenomenon. Once the number of pedestrians on a bridge reaches a critical mass, their footsteps start to make it move, and the more that they react to that movement to stay upright, the more the bridge shakes. It could potentially affect any pedestrian bridge over a given length. Arup believes that there are hundreds of such bridges around the world. All bridges are liable to move. Their weight and structure keep them still to a certain extent, but get enough people walking across and all the natural damping is cancelled out. Then the next few footsteps will set it wobbling violently. It’s not a gradual effect – it’s all or nothing. ‘Put a thousand people on the bridge, and it will seem to be OK. Put another hundred on it, and it starts to wobble’, says Fitzpatrick. Arup discovered a number of bridges that had suffered from the problem, and every one of them looked utterly different.
There was a high-level suspension bridge in Tokyo, completed in the 1980s, that had a makeshift damping system retrofitted after its opening, and a 100-year-old steel-truss bridge in Canada that never moved a millimetre in its entire life until a fireworks display on its centenary attracted so many pedestrians that it began to wobble. Then there was the Pont Solferino, a graceful arched footbridge across the Seine in Paris that stayed closed after its opening, ostensibly because its surface was slippery. Why had none of these cases forewarned Arup? Fitzpatrick’s answer is that they were never reported to the authorities who write the codes that bridge designers must follow. Reopening the bridge has cost Arup £5 million to cover the expense of fitting a pair of X-shaped braces under each of the structural bays under the bridge, along with 37 viscous dampers – the kind of large shock absorbers you find on a truck – and 50 tuned mass dampers that absorb the energy triggered by pedestrians. It has all been done deftly enough for the additions to look like part of the original design.
The bridge is the product of an unusually diverse group of people. Norman Foster was the architect. Ove Arup were the engineers. But it was the sculptor Anthony Caro who brought the team together to win the competition for the commission, beating, among others, Frank Gehry and Richard Serra. They have created a bridge that is a beautiful object, lithe and delicate, but that still manages to acknowledge the magnitude of the task facing its builders. Spanning a fifth of a mile of water with just two supports is no party trick. But the bridge doesn’t want to boast about it either. Rather than pumping up the structure like a bodybuilder on steroids, they have given the bridge muscles that ripple under the skin, like a sleek racehorse. ‘Teamwork’ is one of those touchyfeely words that architects love to use but that seldom describe the reality of warring egos and competing claims for credit. Here, for once, it actually reflects what happened.
The bridge that wobbled
The bridge was originally conceived as part of the strategy to convert the former Bankside power station into the Tate Modern. It was seen as a way to encourage pedestrian visitors to cross the Thames from St Paul’s cathedral, directly opposite

Without the engineers, the bridge would not stand up. Without the architects, there would never have been the manic attention to detail that treated the design of ever bolt head and weld as a major architectural issue. Without Foster’s design skill and highly effective championing of the bridge against doubters, it would never have been built. And without Caro? In its present form it’s hard to see Caro’s direct involvement, but without him it would have been an entirely different bridge. He remained fully involved with the project, even when the original design, which would have borne his signature more clearly, was modified in response to the competition judges’ request for a less sculptural design.

‘People ask me, “Which part of the bridge is yours?”’, says Caro. ‘And of course I can’t point to any single aspect of it. But that isn’t really the issue. The idea of a sculptor, an architect and an engineer working together is what really makes this bridge different, and I was there when all the important decisions were made. Asking a sculptor to do a handrail at the last moment is the old-fashioned way of working. We did it the newfashioned way, and that was wonderful. I’ve learned that architects think differently from us artists. We can say things they wouldn’t dare to, like “Why not try it upside-down?” We think from an unexpected standpoint. We don’t take practicality into account. On the other hand, they can tell us quite a bit about scale’.

The solution is a flat suspension bridge consisting of a deck and two piers in the river. It may not look like a suspension bridge – there are none of the characteristic towers to hold up the cables – but that’s what it is. Rather than rise above the bridge deck, the cables are grouped on either side of the structure like the outriggers of a canoe, barely rising above the level of the main bridge structure. What made the bridge such a demanding design task is that every aspect is on display, visible from every direction. This meant that every screw fixing, every weld, every bolt connection needed to be considered in detail, not just as a practical functional issue but also as an aesthetic one. There is no room for camouflage. Everything is visible; nothing is hidden. It is an aesthetic conception distilled to its essentials. The bridge is as beautiful as its smallest bolt head and as the springing curves of its suspension cables. It achieves a balance between the solidity of its graceful piers and the lightness of the aluminium deck, and it has the curious effect of feeling as if it has always been there.
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