New Study Challenges Popular Explanation for London’s Infamous "Wobbly Bridge"

upstart writes:

New study challenges popular explanation for London's infamous "Wobbly Bridge":

London's Millennium Bridge is notorious for its "wobble" when it first opened in June 2000, as thousands of pedestrians streamed across. Londoners nicknamed it "Wobbly Bridge." The accepted explanation has been that the swaying was due to a weird synchronicity between the bridge's lateral (sideways) sway and pedestrians' gaits-an example of emergent collective phenomena.

But that explanation turns out to be a bit more complicated, according to a recent paper published in the journal Nature Communications. "This [old] explanation was so popular, it has been part of the scientific zeitgeist," said co-author Igor Belykh, a mathematician at Georgia State University. "Our work shows that very tiny vibrations from each person walking can get amplified significantly." People adjust their footsteps to keep their balance in response to the wobble, which only makes things worse. Eventually the bridge becomes unstable.

[...] But that original explanation was incomplete. "The initial impulse a lot of researchers had when looking at this problem was that it was about collective behavior," Varun Joshi, a biomechanical engineer at the University of Michigan, told Ars. "This was based on the presence of multiple pedestrians and the apparent synchronization between them, as observed in videos. However, data collected from actual bridges showed a lack of synchronization in many cases. This led to a lot of experimental work studying individual human response to shaken treadmills, looking for a 'negative damping effect' from individuals. The hope was that the scaled effect of negative damping (even without any adaptation to the presence of other people) would explain the phenomenon."

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