Can Span Overcome Its Shaky Start?


It was supposed to be a "blade of light" into the 21st century. Adored by architecture critics and eagerly awaited by Londoners, the Millennium Bridge opened last summer with great celebration. But when a hundred thousand people crossed the bridge at the inauguration, it swayed wildly from side to side.

The blade dulled, the light dimmed--the bridge closed after just two days.

A year later, the Millennium Bridge is still closed, much to the embarrassment of the all-star British crew that built it: architect Norman Foster, sculptor Anthony Caro and engineering firm Ove Arup & Partners of Sydney Opera House fame.

The footbridge that cost about $25 million to build will cost $7 million more to fix. And no one will say exactly where the money is coming from, or if any public funds are being spent on the repairs.

The engineers insist that the amendments--a series of fluid-filled "shock absorbers"--won't affect the look of the sleek minimalist structure.

Made of 690 tons of aluminum and stainless steel, the bridge spans 350 yards across the Thames, linking the area near St. Paul's Cathedral on the north with the new Tate Modern art gallery on the south. One of several cultural projects built for 2000, it is central London's first new bridge since 1894 and its first pedestrian crossing built since Roman times.

The bridge was greeted with rave reviews as a feat of architecture that combined form and function with stunning grace. "A work of magical beauty," the Evening Standard newspaper called it the day before it opened.

Architecture critic Jay Merrick wrote in the Independent on Sunday newspaper: "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."

But on opening day, the bridge's engineers watched with disbelief as revelers strolled across their creation. Or tried to. As the bridge rocked from side to side, many people lost their balance and grasped at the rails in order to stand straight.

The pedestrians leaned the opposite direction to compensate for the movement. The more they leaned in time with the rocking bridge, the wider its swing grew. The Independent newspaper reported at the time that "the opening of the Millennium Bridge has been as clumsy as a drunk trying to build a bed out of driftwood."

Engineers say the ultramodern design of the bridge wasn't responsible for its sway. They say it moved because of a previously undocumented phenomenon called "synchronous lateral excitation"--large numbers of people exerting sideways force at the same time.

But doesn't every playground gang know about swaying footbridges?

It's not that simple, insist the engineers. "Only half a dozen people in the world knew about this effect, and they kept it to themselves," said Tony Fitzpatrick, chairman of Arup's Americas Division and a lead engineer on the Millennium Bridge.

Engineers also point out that this is not the same phenomenon as soldiers needing to break step while crossing a bridge. That's a problem with vertical vibration, as opposed to walkers reacting to the movement of a bridge and causing lateral vibration.

Arup engineers say their bridge isn't the first to experience the lateral movement. Another footbridge, the Pont Solferino in Paris, opened in 1999 and also began to sway when a crowd crossed. It was closed immediately. But Arup says it couldn't learn from the Solferino incident because "there was no suggestion in any literature on the Solferino opening day that this could be a generic problem that could affect other bridges."

The engineers are quick to list other bridges that suffered the sideways swing. They cite the 50th anniversary of the Golden Gate Bridge in 1987 and a protest on the Auckland Harbor Bridge in New Zealand in 1975, both events where foot traffic caused swaying. But those were car bridges closed to traffic for special occasions. The Millennium Bridge was a footbridge that would have to support many pedestrians. It had to be fixed. Arup was held accountable.

Arup says that no formal report documents the synchronous lateral excitation that causes a bridge to sway. But plenty of anecdotal evidence exists to say that a bridge with a lot of people walking across it can rock from side to side.

In 1987, the San Francisco Chronicle reported: "The Golden Gate Bridge moved several feet lower and began swaying from side to side under the weight of the 300,000 or so people who managed to get on the bridge at any given point during the morning. . . . Engineers became concerned but concluded that the bridge and the people were in no danger."

Fitzpatrick says his engineering team did everything possible to build a good bridge. "String me up by my fingertips if you can show me where we were wrong," he said in a telephone interview from his San Francisco office.

He says he's determined that Arup's findings be made public so that other bridges don't suffer the movement--and the humiliation--that his did. Arup engineers say they're confident that their repairs will tackle the problem.

The bridge should reopen at the end of the year, but Arup isn't giving a firm date. No plans are yet in the works for another round of festivities.

Copyright © 2019, Los Angeles Times
EDITION: California | U.S. & World