London's Millennium Bridge had to close two days after opening because it wobbled

The London Millennium Footbridge, a sleek steel suspension bridge for pedestrians across the Thames, opened on June 10, 2000, linking the City of London with Bankside and St Paul's. On opening day, as thousands crossed, the bridge began to sway alarmingly from side to side. Londoners promptly nicknamed it the 'Wobbly Bridge.' It was closed two days later, and stayed shut for almost two years while engineers worked out what had gone wrong and how to stop it.

The cause was a phenomenon engineers call synchronous lateral excitation. As the bridge swayed very slightly sideways, people walking on it instinctively adjusted their footing to keep their balance — and in doing so they stepped in time with the sway. Each synchronized footfall pushed the bridge a little further, which made more people synchronize, in a self-reinforcing loop. The more people on the bridge, the stronger the effect; on opening day up to 2,000 were on it at once, and the sway reached about 70 millimetres.

Vertical bounce — the kind that makes soldiers break step on a bridge — was well understood, but this sideways, crowd-driven version had not been properly anticipated for a lightweight bridge like this one. The fix, developed with the engineering firm Arup, was to fit dampers that absorbed the lateral and vertical energy. After full-scale tests with hundreds of volunteers, the bridge reopened in February 2002, steady at last. The whole episode cost about £18.2 million, roughly £2.2 million over budget.