Engineering (5)
At last - a bridge you can cross
After a shaky start, the Millennium Bridge is undergoing major surgery. Here,
its creators reveal what went wrong and why the blade of light won't wobble
when it reopens
Deyan Sudjic
Sunday March 11, 2001
The Observer
It was just after 9 am on a sunny Saturday last June that the Millennium Bridge first began to
wobble. Roger Ridsdill-Smith, the young engineer who had been involved with every aspect
of the project since his famous paper napkin sketch of a blade of light had so impressed
Norman Foster and Anthony Caro, was waiting for the start of a sponsored walk. As the
bridge began to fill with people, he spotted the first sign of movement. 'It happened quite
fleetingly, but you could tell 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 lunch time when the bridge finally opened to the public; the crowds swarmed on from
both ends. And that's when the bridge really started to move. All 690 tons of its steel-andaluminium deck began to sway left and right like a giant, executive desktop toy, so much so
that pedestrians, suspended above the Thames on slender steel cables, began to clutch at
handrails to steady themselves, and throw themselves against the sway, to stay upright. As
they did, so the swings began to get increasingly violent.
Words: 1339
Flesch Reading Ease: 58.8
Flesch Kincaid Grade Level: 9.3
Deyan, Sudjic. “At last – a bridge you can cross.” Observer 11 Mar. 2001
http://observer.guardian.co.uk/review/story/0,,449639,00.html
picture: http://community.webshots.com/photo/297123731/415607225ANZrAj
Ridsdill-Smith and the police working on crowd-control looked at each other. In his mind,
Ridsdill-Smith went back over all the calculations, all the safety assessments, all the windtunnel tests, even the giant hydraulic tank in Canada used to measure resistance to water.
None of them had predicted anything like this. This was simply not supposed to be
happening.
All 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 an image of the great Tacoma Narrows
suspension bridge near Tacoma, Washington in 1940, shaking itself to pieces, flashing
through his mind.
'We asked ourselves, "Is it dangerous?" You could see that the movement was self-limiting.
Beyond a certain point, it simply becomes impossible to get any more people on the bridge.
As it gets fuller and fuller, people stop moving and the effect subsides, long before structural
safety limits are reached. We knew it wouldn't fall down, but suppose granny takes a tumble
and breaks a hip?'
The next day, Nicholas Serota lent the bridge his security staff from the Tate to limit the
numbers crossing at any one time. But the wobble didn't go away. The Millennium Bridge
Trust hesitated for 24 hours before making the decision to close it altogether.
As engineering disasters go, the great wobbly bridge fiasco might seem pretty tame. But to
see such a high-profile 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 possible. They could build skyscrapers and nuclear-power
stations all over the world, but here, in their own backyard, they were conspicuously failing to
make a simple footbridge stand still long enough for people to stroll across it.
What made their embarrassment so irresistible to the more pedantic of their fellow engineers,
who rushed in to make judgments about what had happened, was that they seemed to have
brought it on themselves. The word was that they had allowed Foster, an architect determined
to make his mark on the bridge, push them into flouting the sensible limits of design. 'Of
course this bridge was going to wobble, just look at it,' was the consensus. With its low-slung
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-by-half arrogance. Stray too far from tried-andtested traditional designs and you'll get in trouble was the subtext of much of the debate.
To the enormous relief of both Arup and Foster, it turns out that the striking looks of the bridge
have nothing to do with the causes of the wobble. Arup's analysis has been checked by two
independent engineers and three university departments.
'It's not what is different about the bridge that caused the problem,' says Tony Fitzpatrick, who
has lead the Arup team struggling to find a solution. 'Every issue about the bridge that was
innovative was properly researched and it worked perfectly. What hit us in the back of the
head was that bit of the bridge that was the same as every other long-span bridge. We
assumed it would work like other bridges have until now; that was the mistake, but there has
to be a first time for everything.'
What Arup eventually discovered was a previously misunderstood phenomenon. Once the
number of pedestrians on a bridge passes 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 effect any pedestrian bridge over a given length. Fitzpatrick
Words: 1339
Flesch Reading Ease: 58.8
Flesch Kincaid Grade Level: 9.3
Deyan, Sudjic. “At last – a bridge you can cross.” Observer 11 Mar. 2001
http://observer.guardian.co.uk/review/story/0,,449639,00.html
picture: http://community.webshots.com/photo/297123731/415607225ANZrAj
believes that there are least 70 such bridges in Britain alone, with hundreds more around the
world.
'We are not ashamed,' says Fitzpatrick, who is presenting Arup's findings in a guidance note
to John Prescott's DETR. 'We want the full story to be out there. We are very clear about what
we have done; we are happy for others to judge us. Any conspiracy to keep quiet about the
bridge only makes us look worse. We have had only one thing on our agenda - to get the
bridge open. I just want to be allowed on the bridge to fix it.'
As soon as the bridge closed, Arup began talking to the world's footbridge experts in Japan
and Germany. At the same time, they did urgent checks to see if the bridge had been built
exactly as they had specified - it had - and then they looked yet again at their own
calculations to see if there was any elementary mistakes in the numbers. There weren't.
All bridges are liable to move. Their weight and structure keep them still to a certain extent,
but get enough people walking across a bridge and all the natural damping is cancelled out;
then the next few footsteps will set it wobbling violently. It's not a gradual effect. Arup found
it's all or nothing. 'Put 1,000 people on a bridge and it will seem to be OK. Put 1,100 and it
starts to wobble,' says Fitzpatrick.
Arup discovered a number of bridges which had suffered from the problem, and every one of
them looked utterly different. There is a high-level suspension bridge in Tokyo, completed in
the 1980s, that had a makeshift damping system retrofitted after its opening; a 100-year-old
steel truss bridge in Canada that never moved a millimetre in its entire life, until the day of the
firework display on its birthday that attracted so many pedestrians that it began to wobble.
Then there was was the Pont Solferino, a graceful arched footbridge across the Seine which
stayed closed after its opening, ostensibly because its surface was slippery. This, too, turned
out to be the victim of wobbly-bridge syndrome. Fitzpatrick hints at a cover-up. 'If the
engineers involved with some of these cases had been as open about their bridges as we are
being now, the problem would have been resolved long ago.'
The best news from Arup's point of view was the footbridge linking the National Exhibition
Centre in Birmingham to its neighbouring railway station with all the grace and subtlety of a
sledgehammer. This was a design clearly untroubled by the least vestige of aesthetic
ambition. Yet it, too, suffered from wobbly-bridge syndrome. The avant-garde was off the
hook.
Why had none of these cases forewarned Arup? Fitzpatrick's answer is that they were never
reported to the people who write the codes that bridge designers must follow.
Words: 1339
Flesch Reading Ease: 58.8
Flesch Kincaid Grade Level: 9.3
Deyan, Sudjic. “At last – a bridge you can cross.” Observer 11 Mar. 2001
http://observer.guardian.co.uk/review/story/0,,449639,00.html
picture: http://community.webshots.com/photo/297123731/415607225ANZrAj