London's Millennium Bridge wobbled because walkers fell into step with it
On 10 June 2000, some 90,000 people crossed London's sleek new footbridge over the Thames, and it began to sway sideways alarmingly. Two days later it shut for nearly two years. The culprit was not wind or weak steel but the pedestrians themselves, unconsciously adjusting their stride to the movement and making it worse.
The bridge links Bankside, near Tate Modern and the Globe, with the City of London below St Paul's Cathedral, and its supports frame a view of the cathedral's south front from across the river. A 1996 competition picked a blade of light design by Arup, Foster + Partners and the sculptor Anthony Caro. To keep the profile low, its suspension cables run below the deck rather than above it. It spans 325 metres, and building it cost £18.2 million, £2.2 million over budget.
Engineers already knew about vertical bouncing, which is why soldiers break step on bridges; a sign on Albert Bridge from 1873 still tells marching troops to do so. Sideways sway was less studied. When a light bridge nudges walkers slightly to one side, they all shift their weight to keep their balance at the same moment, pushing it further. Arup named this feedback loop synchronous lateral excitation. With up to 2,000 people on the deck at once, the sway reached about 70 millimetres. Any sufficiently light bridge with a sideways frequency below 1.3 hertz is vulnerable, and Auckland Harbour Bridge showed similar behaviour during a 1975 demonstration.
Stiffening the structure would have spoiled its looks, so Arup retrofitted 37 viscous fluid dampers, including V-shaped braces under the deck, plus 52 tuned mass dampers to control vertical motion. Fitting them took eight months, starting in May 2001, and added £5 million to the bill. Before reopening in February 2002, the fix was tested by 700 Arup staff walking across.
Londoners still call it the Wobbly Bridge. It collapses in the opening of Harry Potter and the Half-Blood Prince, an anachronism since the story is set in 1996, before it was built.
Source: Millennium Bridge, London
