Millennium Bridge, London – Wikipedia, the free encyclopedia
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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.[6] 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.
The lateral vibration problems of the Millennium Bridge are very unusual, but not entirely unique.[7] 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. For example, Albert Bridge in London has a sign dating from 1873 warning marching ranks of soldiers to break step while crossing.[8] Other bridges which have seen similar problems are:
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