Essential Architecture-  London

Millenium Footbridge

architect

Arup Associates, Norman Foster and Sir Anthony Caro

location

crossing the River Thames in London, England, between the existing Southwark Bridge and Blackfriars Bridge, linking Bankside with the City

date

2000

style

High-Tech Modern

construction

steel

type

Bridge
 
  Painting- Image copyright Doug Myers.
 
 
 
 
 
 
The London Millennium Footbridge is a pedestrian-only steel suspension bridge crossing the River Thames in London, England, 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 and maintained by Bridge House Estates, a charitable trust overseen by the Corporation of London.

The south end of the bridge is near Globe Theatre, the Bankside Gallery and Tate Modern, the north end next to the City of London School below 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.

Design

The design of the bridge was the subject of a competition organised in 1996 by Southwark council. The winning entry was an 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. The eight suspension cables are tensioned to pull with a force of 2,000 tons against the piers set into each bank — enough to support a working load of 5,000 people on the bridge at one time.

History

The bridge from St Paul's after opening: long queues formed as attempts were made to limit vibrations
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 bridge was on the route of a major charity walk and it was an exceptionally fine day. The initial small vibrations encouraged (or even obliged) the users to walk in synchronisation with the sway, increasing the effect even when the bridge was comparatively lightly loaded at the beginning of the day. This swaying motion earned it the nickname the Wobbly Bridge.

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 three days after opening attracted public criticism, as another high-profile British millennium project suffered an embarrassing setback, akin to how many saw the Millennium Dome.

Resonant vibrational modes have been well understood in bridge designs following the failure of the Tacoma Narrows Bridge. However not much notice had been given to pedestrian excited lateral motion. This motion was caused by the human reaction to small lateral movements in the bridge, which set up a driven harmonic motion in the bridge. 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 bridge, suspension or otherwise, which happens to have the appropriate resonant frequencies and is subjected to large crowds. 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 22 February 2002, and as yet there have not been any noticeable severe vibrations.

References
Reaney, Patricia (Nov. 6, 2005). "Why the Millennium Bridge wobbled". New Sunday Times, p. F20.


links

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