Millennium Bridge, London

An architectural design competition was organised in 1996 by Southwark council and RIBA Competitions. The winning entry was an innovative "blade of light" effort from Arup Group, Foster + Partners and 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 consists of three main sections of , , (north to south) with a total structure length of ; the aluminium deck is 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. ==Construction==

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, and the City of London and London Borough of Southwark granting planning permission. Construction began in late 1998, and the main works were started on 28 April 1999 by Monberg & Thorsen and Sir Robert McAlpine. The eventual cost was £18.2 million (£2.2m over budget), primarily paid for by the Millennium Commission and the London Bridge Trust. ==Opening==

The bridge opened on 10 June 2000, one month late. Unexpected lateral vibration due to resonant structural response caused the bridge to be closed on 12 June for modifications. Attempts had been made to limit the number of people crossing the bridge, which led to long queues but were ineffective to dampen the vibrations. Closure of the bridge only two days after opening attracted public criticism as another high-profile British Millennium project that suffered an embarrassing setback, akin to how many saw the Millennium Dome. The vibration was attributed to a then under-researched phenomenon whereby pedestrians crossing a bridge that has a lateral sway have an unconscious tendency to match their footsteps to the sway, exacerbating it. This is different from the well-understood problem of vertical sway, which is why troops stop marching in stride together as a unit when crossing such a bridge. An example is London's Albert Bridge, which has a sign dating from 1873 warning marching ranks of soldiers to break step while crossing. ==Resonance==

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; the maximum sway was around . On the day of opening, the bridge was crossed by 90,000 people, with up to 2,000 on the bridge at a time. Resonant vibrational modes due to vertical loads (such as trains, traffic or pedestrians) and wind loads are well understood in bridge design. In the case of the Millennium Bridge, because the lateral motion caused pedestrians to directly participate with the bridge, the vibrational modes had not been anticipated by the designers. When the bridge lurches to one side, the pedestrians must adjust to keep from falling over, and they all do this at the same time. The effect is similar to soldiers marching in lockstep, but horizontal instead of vertical. The risks of lateral vibration in lightweight bridges are well known. 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. Other bridges which have seen similar problems are: • Auckland Harbour Bridge, with a lateral frequency of 0.67 Hz during a 1975 demonstration. • Birmingham NEC Link bridge, with a lateral frequency of 0.7 Hz. Mitigation Engineers at Arup, the company that designed the bridge, conducted research into the unexpected oscillation, which they called 'synchronous lateral excitation'. The first laboratory studies used pedestrians on moving platforms at the University of Southampton and Imperial College London. Later in 2000, one span of the bridge was instrumented and tested with crowds of up to 275 people. Additionally, 52 tuned mass dampers add inertia to control vertical movement. The work took from May 2001 to January 2002 and cost £5 million. After a period of testing, the bridge was reopened on 22 February 2002 and, since that date, has not been subject to significant vibration. In spite of the successful cure, the "wobbly bridge" (sometimes "wibbly-wobbly") epithet remains in common usage among Londoners. File:Millennium Bridge decking underside.jpg|Chevron dampers File:Bridge vert mode shock.jpg|Vertical to ground dampers File:Bridge_horiz_mode_shock.jpg|Pier dampers File:Moving End of Pier Damper of Millennium Bridge, London.jpg|Moving end of pier damper File:London Millennium Bridge - Damper beneath deck, north side - 240404.jpg|Tuned mass damper Cable resonance An artistic expression of the higher-frequency resonances within the cables of the bridge were explored by Bill Fontana's Harmonic Bridge exhibition at the Tate Modern in mid-2006. This used acoustic transducers placed at strategic locations on the cabling of the Millennium Bridge and the signals from those transducers were amplified and dynamically distributed throughout the Turbine Hall of the Tate by a programme which Fontana entered into the sound diffusion engine of the Richmond Sound Design AudioBox. ==Repair==

In October 2023, the bridge was closed for three weeks to allow repairs to its surface, undertaken by FM Conway. The bridge closed on Saturday 14 October and reopened late on 5 November. ==Millennium Inclinator==

A short inclined lift, known as the Millennium Inclinator, is next to the northern end of the Millennium Bridge. It was opened in December 2003 to allow pedestrians to surmount the steep slope (13.6°) of Peter's Hill from the riverside to the entrance to the Millennium Bridge without using the alternative flight of steps. The lower end of the lift is on Paul's Walk next to the Thames, and the top end is further up Peter's Hill on the terrace which is level with the deck of the bridge. The lift carriage was originally powered by an electric traction motor, manufactured in Italy by Maspero Elevatori, with a speed of and a maximum capacity of . Axis Elevators installed the new equipment. Following a redevelopment of the area in 2021 the lift was closed and will be replaced by a platform lift. ==In popular culture==

• The bridge is featured in the opening scenes of the film version of Harry Potter and the Half-Blood Prince, where the bridge collapses following an attack by Death Eaters. • The bridge appears in the 2014 Marvel Cinematic Universe film Guardians of the Galaxy during the climactic battle on Xandar. • The bridge appears in the 2017 Taylor Swift music video for "End Game" from her sixth studio album, Reputation (2017). ==See also==