The Millennium Bridge, a steel suspension construction for pedestrians crossing the River Thames, links Bankside with the City of London. The bridge alignment is such that a clear view of St Paul’s south façade is presented from across the river, framed by the bridge supports.
Londoners nicknamed the bridge the ‘Wobbly Bridge’ after participants in a charity walk on behalf of Save the Children to open the bridge felt an unexpected, and, for some, uncomfortable, swaying motion on the first two days after the bridge opened in 2000. The bridge was closed later that day, and after two days of limited access the bridge was closed for almost two years while modifications were made to eliminate the wobble entirely. It was reopened in 2002.
The design of the bridge was the subject of a competition in 1996. The winning entry was an innovative ‘blade of light’ project from Arup, Foster and Partners and Sir Anthony Caro. Due to height restrictions, and to improve the view, the bridge’s suspension design had the supporting cables below the deck level, giving a very shallow profile.
The bridge has two river piers and is made of three main sections of 81 metres, 144 metres and 108 metres (north to south) with a total structure length of 325 metres; the aluminium deck is 4 metres wide. The eight suspension cables are tensioned to pull with a force of 2000 tonnes against the piers set into each bank — enough to support a working load of 5000 people on the bridge at one time.
Construction began in late 1998 and the main works were started on 28 April 1999. The bridge was completed at a cost of £18.2M (£2.2M over budget), primarily paid for by the Millennium Commission and the London Bridge Trust. It opened on 10 June 2000 (two months late).
Unexpected lateral vibration (resonant structural response) caused the bridge to be closed on 12 June for modifications. Attempts were made to limit the number of people crossing the bridge. This led to long queues but dampened neither public enthusiasm for what was something of a white-knuckle ride, nor the vibrations themselves. The closure of the bridge only two days after opening attracted public criticism of it as another high-profile British Millennium project suffered an embarrassing setback, akin to how many saw the Millennium Dome. The wobble was attributed to an under-researched phenomenon whereby pedestrians crossing a bridge that has a lateral sway have an unconscious tendency to match their footsteps to the sway, thereby exacerbating the sway. The tendency of a suspension bridge to sway when troops march over it in step was well known, which is why troops are required to break step when crossing such a bridge.
The bridge’s movements were caused by a ‘positive feedback’ phenomenon. The natural sway motion of people walking caused small sideways oscillations in the bridge, which in turn caused people on the bridge to sway in step, increasing the amplitude of the bridge oscillations and continually reinforcing the effect.
The lateral vibration problems of the Millennium Bridge are very unusual, but not entirely unique. Any bridge with lateral frequency modes of less than 1.3 Hz, and sufficiently low mass, could witness the same phenomenon with sufficient pedestrian loading. The greater the number of people, the greater the amplitude of the vibrations. For example, Albert Bridge in London has a sign dating from 1973 warning soldiers to break step while crossing. After extensive analysis by engineers, the problem was fixed by the retrofitting of 37 fluid-viscous dampers (energy dissipating) to control horizontal movement and 52 tuned mass dampers (inertial) to control vertical movement. The bridge has not been subject to significant vibration since. In spite of the successful fix of the problem, the affectionate ‘wobbly bridge’ epithet remains in common usage amongst Londoners.