60-Second Science

Crowds Walking Can Make A Bridge Sway

A study in the Proceedings of the Royal Society explains why thousands of tourists were able to make the London Millennium Bridge sway on its opening day in 2000. Karen Hopkin reports.

[The following is an exact transcript of this podcast.]

On its opening day, 90,000 tourists traipsed across the London Millennium Bridge. That pitter patter of pedestrian feet caused the structure to visibly rock, earning it the nickname “the wobbly bridge.” Now, in the December 17th issue of the Proceedings of the Royal Society, civil engineers say they think they know what made the London bridge sway.

It had initially been assumed that the movement was caused by a portion of the pedestrians marching in lock-step. Their synchronized waddle could have caused the bridge to oscillate, leading even more of them to tread in tandem. But the engineers say that the same oscillations can be generated by people who are simply plodding along, just trying to keep their balance—no marching necessary.

Using mathematical modeling, the scientists showed that even walking on level ground people stabilize themselves by controlling where they place their feet side-to-side. So if they’re walking on a bridge that also wiggles side-to-side, in an effort to keep their balance they may shift their weight even more, making the problem worse. So next time you go to walk across the Thames, remember, the London Millennium Bridge is not falling down. But you still might want to watch your step.

—Karen Hopkin 

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