The tide turns for the bridge

A year since its shaky opening, the 'Blade of Light' is still closed. But it has done design good, says Jay Merrick

It was hailed as the Blade of Light, a footbridge spanning the Thames between St Paul's Cathedral and the Tate Modern art gallery that combined architecture, engineering and sculpture with brilliant finesse.

But a year ago, the bridge was closed after just three days when it was found that the structure swayed alarmingly from side to side as people crossed. The £14m bang-cum-whimper was media feeding-frenzy material ­ a bridge too far for the dream team that conceived it.

Why, when the designers included the architect Lord Foster and the engineers Ove Arup, did the bridge develop a dramatic lateral movement as the crowds streamed across it? How could such a renowned partnership be responsible for such a disaster?

But perhaps the apparently scandalous shimmy was A Good Thing for architecture.

The Millennium Bridge is a striking example of a growing phenomenon in high-tech architecture ­ design crossover, in which engineers' structural solutions are increasingly dictating the form and detail of buildings. The bridge is remarkable because there's very little of it, and in architecture less usually means more ­ much more ­ in terms of calculations, selection of materials and risk. Foster wanted something flat and thin, and Arup delivered a conceptual form that would span the Thames with no obvious support apart from two slim piers.

The Blade of Light was conceived as a bridge supported by super-tensioned cables, carried almost horizontally on vestigial outriggers, and anchored on the banks of the Thames. The result: a minimalist structure whose "sprung" dynamics, in wind or under load, were hugely complex. When it was unveiled last June ­ even if it did wobble too much ­ it looked stunning.

Arup will pay the £5m fix-it bill without a quibble because the bridge is essentially engineering in the raw. Arup's difficulty was that it had embarked upon an extremely high-profile project that pushed the technical envelope to the limit. Similar "ribbon bridges" exist, though not in the same configuration. This made it difficult to rehearse any problems.

"How do you design a load-test for a bridge type that doesn't exist?" asks Roger Ridsdill Smith, Arup associate director. "The unusual facet of this bridge that uncovered the problem was its central point ­ and its population on a sunny day." The Millennium Bridge Trust project director, Malcolm Reading, says Arup suggested letting 300 at a time on the bridge ­ but 250,000 turned up and couldn't be held back.

"The biggest hindsight learning," he admits, "is to have tested things before it was opened. We could have run thousands over the bridge."

Robert Benaim, a highly respected British engineer and bridge designer, said the bridge's physical position put severe pressure on its designers to deliver something stunning. "There's a feeling that the bridge was perhaps conceived a bit too boldly."

But the Blade of Light's "failure" and the mass of research it has generated should make innovative lightweight bridges safer in the future. A year on, the bridge's wobbly start may well turn out to be a good thing after all. As Lord Foster says: "Some projects are close to being research that stretches the boundaries, or that consolidates something that was once considered radical."

This coming week a major exhibition on his work opens at the British Museum. Exploring The City: The Foster Studio illuminates just how "Fosterian" a city London is becoming ­ his practice is designing the headquarters for the Greater London Authority, a new residential complex, Albion wharf, the new Wembley Stadium, and the masterplans for the redesigns of Trafalgar Square and Parliament Square.

Foster has been driven by a profound curiosity about how things work ­ "the way a place feels, the quantifiable and the unquantifiable" ­ since he paid his way through architecture school at Yale because nobody would give him a grant.

The curiosity that propels Foster will drive hordes of people on to the re-honed Blade of Light in six months' time. When it reopens in December, fitted with 91 dampers weighing more than 700 tons, hundreds of thousands will again want to cross it ­ to see if it still wobbles.