A In September 1996, a competition was organized by the Financial Times in association with the London Borough of Southwark to design a new footbridge across the Thames, which at that time would be London's only pedestrian bridge and the first new crossing on this part of the Thames. The news quickly spread nationwide and worldwide through TV stations and radio networks. The competition attracted over 200 entries. 7In the end, architect Norman Foster, sculptor Sir Anthony Caro as well as Arup (an engineer team) took the top prize. The Millennium Bridge represents a creative collaboration between architecture, art and engineering. As a key element in London's pedestrian infrastructure, this bridge has created new routes for Londoners and encouraged new life on the embankment alongside St Paul's.
B On 10th June 2000, the bridge was opened to the public. It is estimated that 100,000 people crossed it that day, and there were up to 2,000 people on it at any one time. Initially the bridge remained stationary, but it gradually began to sway. Although a few visitors raised the concern when they reached the other end of the bridge, the organizer was not aware of how serious the safety problem was until it began to wobble more noticeably when a large group of people were crossing. The wobble became so marked that people needed to stop walking to regain their balance, and even grab the handrails for support. Quickly, quite a lot of people realized that the bridge was not safe for walking. 1-8The decision to immediately limit the number of people crossing was taken, but even so the deck movement was sufficient to make people uncomfortable and to raise concern for public safety so that on 12th June the bridge was closed until the problem could be solved.
C Embarrassed, the engineers searched out a video recording which showed the center span oscillating about 3 inches side to side each second. 9The engineers first thought that winds might be exerting excessive force on the many large flags and banners be decking the bridge for its gala premiere. 2-9However, they discovered that the pedestrians themselves had a significant impact. 2Normal human activities like jumping, running, swaying and even walking can create horizontal forces which cause excessive dynamic vibration in a lateral direction on the bridge. As they felt the bridge moving, the pedestrians would naturally have adjusted their walking style to the same rhythm as the bridge. These altered footsteps would have made the motion worse - something like when four people in a boat all stand up together. As more pedestrians copied the movements of the bridge, the dramatic swaying increased, as seen in the video.
D 3The engineers realized that in order to reduce the movements of the bridge, the impact of the movement of pedestrians needed to be quantified and related to the motion of the bridge. 10Although there were some descriptions of this phenomenon in existing literature, none of these actually quantified the force. They realized that there was no quantitative analytical way to design the bridge against this effect. The Engineering Designer, Ove Arup, launched a research program and invited a number of universities and research organizations to have input.
E 11One research method used at the University of Southampton involved a pedestrian walking on the spot on a small shake - table. Tests undertaken by Imperial College included pedestrians walking along a 7.2-meter platform which was shaken laterally at various frequencies and amplitudes. 4-11Unfortunately, each test had its limitations. 12The Imperial College test could only capture seven or eight footsteps, and the Southampton tests, although monitoring more footsteps, couldn’t examine normal forward-walking. Neither test could investigate any influence of other people in the crowd on the behaviour of the individual being tested.
F 14The laboratory testing resulted in information which allowed the design of a custom retro-fit to begin. Unfortunately, the limitations of the tests became clear, and it was felt that the only way to replicate the exact conditions of the bridge was to carry out real tests involving pedestrians, on the bridge itself. These tests, undertaken by the Arup engineers, could include factors not available in a laboratory test. The first test was held in July 2000 with 100 participants. 5-14The result was used to refine the load model for pedestrians. Another series of pedestrian-tests involving 275 pedestrians was held in December 2000. The aim of these tests was to further validate the assumptions made by the engineers, and to undertake a load test to provide information in order to design a prototype damper installation.
G 6-14Aside from greatly limiting the usage of the bridge, only two feasible options to improve the situation were found by researchers. 13-14The first was to increase the stiffness of the bridge to move all its lateral natural frequencies out of the range so that it could be excited by the lateral footfall forces, and the second option was to improve the bridge's damping to reduce the resonant response.
