Science Fair Project Encyclopedia
London Millennium Bridge
The London Millennium Footbridge is a pedestrian-only steel suspension bridge crossing the River Thames in London between the existing Southwark Bridge and Blackfriars Bridge, linking Bankside with the City. It was the first new bridge across the Thames in London since Tower Bridge in 1894 and it is owned by the Corporation of London.
The south end of the bridge is near Globe Theatre, the Bankside Gallery and Tate Modern, the north end near St Paul's Cathedral. The bridge alignment is such that a clear view of St Paul's south facade is presented from across the river, framed by the bridge supports, thus providing one of London's most photogenic views of the cathedral.
The design of the bridge was decided by a competition organised in 1996 by Southwark council. The winning entry was a innovative "blade of light" effort 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 81m, 144m and 108m (North to South) with a total structure length of 325m; the aluminium deck is 4m wide.
Construction began in late 1998 with the main works beginning on April 28, 1999 by Monberg Thorsen and McAlpine. The bridge was completed at a cost of £18.2m (£2.2m over budget) and opened on June 10, 2000 (2 months late) but unexpected lateral vibration (resonant structural response) caused the bridge to be closed on June 12 for modifications. The movements were produced by the sheer numbers of pedestrians (90,000 users in the first day, with up to 2,000 on the bridge at any one time). The initial small vibrations encouraged the users to walk in synchronisation with the sway, increasing the effect. This swaying motion earned it the nickname the Wobbly Bridge.
Resonant vibrational modes have been well understood in bridge designs following the failure of the Tacoma Narrows Bridge. However this was the first time a bridge had displayed this type of pedestrian excited lateral motion. As such the motion was not anticipated by the computational analysis of the bridge prior to construction. It is often thought that the unusually low profile of the suspension cables contributed to the problem, but an analysis by the structural engineer, Arup, shows that it can occur in any suspension bridge which happens to have the appropriate resonant frequencies. After extensive analysis, 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. This took from May 2001 to January 2002 and cost £5m. After a period of testing the bridge was successfully re-opened on February 22, 2002.
- A PDF paper from Arup, discussing the engineering and resonance of the bridge.
- A paper from Taylor Devices Inc on the dampers retrofitted to the bridge
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