In the latest set of stories at www.flyingcircusofphysics.com, I include several photos of the Millennium Bridge, which spans the Thames River in London, connecting the vicinity of Saint Paul's Cathedral (seen in the background here) with the Tate Modern museum. When the bridge was opened with great celebration to mark the new century, the first surge of people crossing it caused it to sway and gallop so much that some of the people had trouble keeping their footing. The engineers who designed the bridge were both stunned and embarrassed.

The physics is quite interesting because there was a feedback system at play. As the pedestrians walked, they pushed not only down but also sideways. At first all these pushes were randomly timed but once the density of pedestrians passed a critical point, the oscillations were large enough that many pedestrians could walk only by walking in step with the oscillations. That meant that many of them began to march in synchrony along the bridge, which caused the oscillations to build up even more, which caused even more pedestrians to march in synchrony, which ... well, you get the idea.

The bridge was closed while a solution to the oscillations was found. At the FCP web site I show some of the "mass dampers" that were now attached to the bridge to offset the pushing by the pedestrians as they walk along the bridge.

(Photo by Jearl Walker, www.flyingcircusofphysics.com)