June 10, 2000: A London Bridge Is Swaying Hard

2000: London’s new Millennium Bridge opens, and starts swinging side to side, inducing seasickness. Engineers have to shut it down two days later for two years of repairs. The innovative design by architect Sir Norman Foster and Partners, sculptor Sir Anthony Caro, and engineers Ove Arup & Partners won a 1996 competition to build a […]

millenium_bridge1

__2000: __London's new Millennium Bridge opens, and starts swinging side to side, inducing seasickness. Engineers have to shut it down two days later for two years of repairs.

The innovative design by architect Sir Norman Foster and Partners, sculptor Sir Anthony Caro, and engineers Ove Arup & Partners won a 1996 competition to build a new footbridge across the Thames to mark the year 2000. The 1,082-foot bridge connects the Tate Modern Gallery (housed in the refitted Bankside power station) on the South Bank with the neighborhood just downhill from St. Paul's Cathedral on the north.

The 13-foot-wide aluminum deck is held up by four cables on each side. The 5-inch-diameter cables are suspended from short towers that rise diagonally from the bridge piers. The cables sag a shallow 7.5 feet over the 472-foot length of the center span and are therefore highly tensioned.

The bridge presents an elegant design to those viewing it and those on it. But when a thousand people started crossing it on opening day, the Millennium Bridge started swaying side to side. They grabbed for the security of the 4-foot-high stainless steel railings. Some people felt seasick.

Despite the appearance of an amusement-park ride, about 80,000 to 100,000 people crossed it on the first day. The dignified Millennium Bridge, opened by none other than Her Majesty Queen Elizabeth II, got an immediate and unregal nickname: the Wobbly Bridge.

Embarrassed engineers closed the bridge to the public on June 12. Videotape showed the center span swaying about 3 inches side to side every second. The engineers first thought that brisk winds might be exerting excessive force on the many large flags and banners bedecking the bridge for its gala premiere.

Close study showed that not to be the case. Like all modern bridges, it was designed to sway and flex a little with the loads it carries. But it had been modeled on bridges for motor traffic, and pedestrians walk one foot at a time, exerting more lateral force on the structure.

What's more, people unconsciously react to the slight motion by walking in time with it, and thus falling into step with one another. This synchronous lateral excitation reinforces the motion, making it worse.

Further study led engineers to a double solution to dampen the initial motion and thus circumvent the amplified problem. They installed viscous dampers (like automobile shock absorbers) under the deck and around the piers to control lateral motion. They also put in tuned mass dampers, similar to the giant movable, inertial weights that are installed atop skyscrapers to reduce wind and seismic forces. The bridge weights are tuned to a specific frequency and and strung from key structural locations.

The engineers employed hundreds of temp workers to walk across the bridge in lockstep to test the possible solutions and to place and adjust the dampers. Eventually they got it tuned just so, and the Millennium Bridge reopened Feb. 27, 2002.

The bridge initially cost 18 million pounds (or $34 million in today's U.S. money), but the studies and mods cost another 5 million pounds ($9 million today).

But, unlike some oscillating bridges we know, it's still there.

Source: Cambridge University Dept. of Engineering, Urban75
Photo: jonlarge/Flickr

See Also: