London's bridge ascendant

London’s Millennium Bridge reopened on February 22, 2002, with its designer, Lord Foster, in attendance, but without its original, notorious wobble. Described by Foster as a "blade of light," the Millennium Bridge was closed just two days after it first opened in June, 2000, because it shimmied and shook uncomfortably.

Two years ago, throngs of pedestrians on the 1050-foot- (320-meter-) long bridge, which arcs between St. Paul’s Cathedral on the north bank of the Thames River and the Tate Modern Gallery on the south bank, helped send the bridge into its swaying state.

More than 150,000 people crossed the bridge that first day, and the engineering firm Arup did not anticipate that the pedestrians would fall into an "unintentional synchronization of walking," according to Tony Fitzpatrick, Arup's chief engineer for the project.

As the 13-foot- (4-meter-) wide bridge swayed, pedestrians compensated by adjusting their gait, which magnified the movement. Officials first tried to limit the number of pedestrians on the bridge, but when that did not seem to help they decided to close the span to allow engineers to study the problem.

Bridge as Art

With or without its bounce, the Millennium Bridge is a breathtaking symbiosis of architecture, art, and engineering. Norman Foster worked closely with sculptor Anthony Caro and Arup to create a thin ribbon of stainless steel and aluminum, raised on just two Y-shaped concrete pylons 36 feet (11 meters) above the Thames.

"Lateral suspension" is used for support, provided by eight laid steel cables, four on each side of the bridge, that gently drape between the pylons and tie back to the river banks. The pedestrian walkway rests on steel transverse arms that hang on the cables. Viewed from any angle, or from either bank, the effect is elegantly spare.

When it opened in 2000, the Millennium Bridge was the first new river crossing in central London since the completion of the landmark Tower Bridge, farther east, in 1894.

Stabilizing the Bridge

The Millennium Bridge was built at a cost of more than £18 million, and the cost of corrections (which have been borne by Arup) are estimated at an additional £5 million. Movement has been mitigated using two techniques: mass dampers and shock absorbers.

An early proposal was to add more supporting pylons, which would have ruined the buoyant esthetic that Foster sought. Thankfully, the dampers and shock absorbers do their job without being easily visible, taking away nothing from Foster’s "blade of light."

The viscous dampers are installed in the central span between the deck and the transverse arms. These X-shaped stiffeners counter lateral movement. Tuned mass dampers with springs are placed between the underside of the walkway and the steel transverse arms below it. A total of 37 viscous dampers and 54 tuned mass dampers were placed across the span.

This past January, with corrections in place, more than 2000 local architects and engineers were enlisted to test the bridge while it was monitored for movement. They walking briskly across it, then slowly, then stopped and started at mid-span.

On the day of its public reopening, crowds once again thronged to the Millennium Bridge. Walking from St. Paul’s to the Tate and back again, I detected not the slightest sway — the bridge seemed rock solid.

The bridge's commissioning pains remind us how important and still potentially unpredictable human factors are in any type of engineering for people. Now well-adjusted after its unexpected period of "beta-testing," the radically slender construction is settling into the London scene.

Do the successful corrections mean the bridge has lost part of its charm? Is it now a Tower of Pisa without its lean? I don’t believe so. The Millennium Bridge and its initial troubles have just reminded us that even with the assurance of technology, we also need humility to glide on a blade of light.

(via Architecture Week)