Foster and Partners
The Millennium Bridge is the first new bridge across the river Thames in London since Tower Bridge opened in 1894, and it is the first ever designed for pedestrians only. The bridge links the City of London near St Paul's Cathedral with the Tate Modern art gallery on Bankside.
The bridge opened initially on Saturday 10th June 2001. During this day and the next two days a huge amount of people crossed the bridge producing a sway movement that was far greater than expected; in order to fully investigate and resolve the issue the decision was taken to close the bridge on 12th June 2001.
It was re-opened on 27th February 2002. During that interval, extensive investigations and modifications were carried out to resolve the sway of the bridge. Research indicated that the sway had been caused by the accumulative sideways movement generated by large numbers of people crossing the bridge together. The solution involved installing dampers under the deck and between the deck and the river piers. This has provided an excellent solution as it does not detract from the aesthetic impact of the bridge as originally designed.
Foster and Partners' architectural concept for the bridge was developed in close collaboration with sculptor Sir Anthony Caro and Arup engineers. The concept was to create a structure of minimum intervention; a 'ribbon of steel' across the river. This has been achieved with a very shallow suspension bridge consisting of two 'Y' frames and eight cables, four each side. The lightweight deck, four meters wide, passes between these two sets of cables and is supported by structural arms which connect onto the cables at eight-meter intervals. The cables dip below the deck level at the mid-span point, so that the bridge does not impede views of London from further up and down the river. The lighting has been incorporated into the structure and is activated by photo-cells at dusk, transforming the structure into a 'blade of light'.
Ordinarily bridges across the River Thames require an Act of Parliament. For this bridge that was avoided by the Port of London Authority granting a licence for the structure obtaining planning permissions from the City of London andLondon Borough of Southwark. Construction began in late 1998 and the main works were started on 28 April 1999 by Monberg & Thorsen and Sir Robert McAlpine. 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).V
Unexpected lateral vibration (resonant structural response) caused the bridge to be closed on 12 June 2000 for modifications. Attempts were made to limit the number of people crossing the bridge. This led to longqueues and 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 theMillennium 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 was temporarily closed on 18 January 2007, during the Kyrill storm due to strong winds and a risk of pedestrians being blown off the bridge.
Underside of bridge from Southbank
The bridge's movements were caused by a 'positive feedback' phenomenon, known as synchronous lateral excitation. 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. On the day of opening the bridge was crossed by 90,000 people, with up to 2,000 on the bridge at any one time.
Resonant vibrational modes due to vertical loads (such as trains, traffic, pedestrians) and wind loads are well understood in bridge design. In the case of the Millennium Bridge, because the lateral motion caused the pedestrians loading the bridge to directly participate with the bridge, the vibrational modes had not been anticipated by the designers. The crucial point is that when the bridge lurches to one side, the pedestrians must adjust to keep from falling over, and they all do this at exactly the same time. Hence the situation is similar to soldiers marching in lockstep, but horizontal instead of vertical.