Synchronized Steps Caused the London Millennium Bridge to Wobble?

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When the London Millennium Bridge opened on June 10, 2000, the small sway of the bridge caused the pedestrians to synchronize their steps in hope of off-setting the swaying. Instead of achieving such intension, the synchronized steps of the people caused such heavy oscillations that the bridge had to close down until dampers were put in 2 years later. After the incident, Steve Strogatz, a mechanics professor at Cornell University, attributed the wobbling to the synchronization of the people. Strogatz believed that because the bridge was designed to be flexible, any unintended synchronization of a small group of people could have triggered the wobbling. Then the oscillations were intensified when people started syncing their steps intensionally to accommodate the swaying, setting off a cataclysmic accident.
Bernard Feldman, a writer for Physics Today, however, believes Strogatz is wrong. Feldman argues that the frequency of the lateral oscillation of bridges tend to be around 0.5 Hz whereas the average frequency of walking is 1.0 Hz. Therefore it is unlikely that synchronized footsteps could not have intensified the wobbling of the London Millennium Bridge. Although Feldman cannot provide an explanation for the wobble, he hopes that future research will debunk the misconception concerning the swaying of the bridge.


Photo Courtesy of FreeFoto.com

-CR

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