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"Bridging Data" -- The Discovery Files

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Georgia State University researchers have found a potential safeguard against dangerous wobbling of pedestrian bridges. The key factors: biomechanically inspired models of pedestrian response to bridge motion and a mathematical formula to estimate the critical crowd size at which bridge wobbling begins.

Credit: NSF/Karson Productions

Audio Transcript:

You sway me.

I'm Bob Karson with the Discovery Files, from the National Science Foundation.

If you've ever been on a large pedestrian bridge, you know that a little sway is normal. But sometimes that gentle sway can turn into a full-out violent, oscillating, lurching, dangerous wobble. Like the Squibb Park Bridge in Brooklyn that started bouncing side to side as people crossed. Closed for three years for repairs. (Sound effect: metal gate closing) The 32-million-dollar London Millennium Bridge had to close on its first day as large crowds started it swaying.

Safer bridge design is getting a boost thanks to a study led by Georgia State University. It found that the dangerous wobbling of pedestrian bridges could be reduced by using (Sound effect: crowd of pedestrians) a biomechanically-inspired computer model of pedestrian effects on bridge motion and a mathematical formula to estimate the critical crowd size at which dangerous bridge wobbling begins.

Engineers have believed that bridge sway gradually grows with crowd size. The new study says no -- wobble starts suddenly after crowd size reaches a certain number, and increases from there. The study provides specific guidelines for determining that number, which varies bridge to bridge. Currently, collective pedestrian behavior is not accounted for in U.S. codes.

(Sound effect: pedestrian crowd) As we walk around, few of us are thinking about how we interact with the structures we traverse. (Sound effect: single footsteps) I'm just glad someone is crossing that bridge before we come to it.

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