Millennium Bridge, London


The Millennium Bridge, officially known as the London Millennium Footbridge, is a steel suspension bridge for pedestrians crossing the River Thames in London, linking Bankside with the City of London. It is owned and maintained by Bridge House Estates, a charitable trust overseen by the City of London Corporation. Construction began in 1998, and it initially opened in June 2000.
Londoners nicknamed it the "Wobbly Bridge" after pedestrians experienced an alarming swaying motion on its opening day. The bridge was closed later that day and, after two days of limited access, it was closed again for almost two years so that modifications and repairs could be made to keep the bridge stable and stop the swaying motion. It reopened in February 2002.
The bridge is located between Southwark Bridge and Blackfriars Railway Bridge. Its southern end is near the Globe Theatre, the Bankside Gallery, and Tate Modern, while its northern end is next to the City of London School below St Paul's Cathedral. The bridge's alignment is such that a clear view of St Paul's south façade is presented from across the river, framed by the bridge supports.

Design

The design of the bridge was the subject of a competition organised in 1996 by Southwark council and RIBA Competitions. The winning entry was an innovative "blade of light" effort from Arup Group, 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,, and 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 on the bridge at one time.

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, primarily paid for by the Millennium Commission and the London Bridge Trust.

Opening

The bridge opened on 10 June 2000, two months 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.
Vibration 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, exacerbating it. The tendency of a suspension bridge to sway vertically when troops march over it in step was well known, which is why troops are required to break step 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 70mm. 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 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:
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.
They concluded that making the bridge stiffer, to move its resonant frequency out of the excitation range, was not feasible as it would greatly change its appearance. Instead, the resonance was controlled by retrofitting 37 viscous fluid dampers to dissipate energy. These include 17 chevron dampers – long V-shaped braces under the deck panels – to control lateral movement, 4 vertical to ground dampers to control lateral and vertical movements, and 16 pier dampers to control lateral and torsional movements. Additionally, 52 tuned mass dampers add inertia to control vertical movement. The work took from May 2001 to January 2002 and cost £5M. After a period of testing, the bridge was reopened on 22 February 2002 and has not been subject to significant vibration since. In spite of the successful cure, the affectionate "wobbly bridge" epithet remains in common usage among Londoners.

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 museum 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.

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 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. It was primarily installed for use by those who cannot easily manage the steep steps, such as people with disabilities and parents with push chairs.
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. However, by 2010 the City of London Planning and Transportation Committee decided that the level of service was unacceptable, because the inclinator was frequently out of service due to mechanical breakdowns and vandalism. So the Committee agreed it would be replaced at a cost of up to £750,000 in time for the 2012 Summer Paralympics.
A major renovation project was undertaken in 2012, and the funicular was reopened in time to be used by people attending the Thames Diamond Jubilee Pageant of 3 June 2012, which took place about a month before the 2012 Summer Olympics. The new lift was manufactured by the company Hütter Aufzüge of Glinde, Schleswig-Holstein, Germany, a lift manufacturer which was taken over by the Otis Elevator Company in 2013. Installation was by Axis Elevators.

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