Why the Millennium Bridge wobbled

Science Blog:

The problem, says Strogatz, was one of crowd dynamics as much as engineering. The bridge surpassed standards for withstanding weight and wind. Every nonhuman element had been tested.

Instead of focusing on the structure, Strogatz examines the strange phenomenon of people unknowingly working together, simply by walking.

The military has known for years that troops marching in step can create enough vertical force to destroy a bridge. It is standard practice for soldiers to break step at every bridge crossing.

But the Millennium Bridge problem is not quite the same, says Strogatz. In this case, the movement was lateral, not vertical. More importantly, the people were just pedestrians. No one was trying to walk in step; pedestrians did so only to accommodate the bridge's movement under their feet.

But which came first, the bridge's movement or the synchronous strides? And what set the whole thing off?

“It's a chicken-and-egg problem,” says Strogatz. “That's what our paper explains.” From the beginning, the bridge had two factors working against it: It was by design a flexible structure, and its natural frequency is close to that of human walking. From there, Strogatz says, all it needed was a relatively small crowd to spark the wobble.

“If the people are initially disorganized and random, if a few of them get into sync by accident, the bridge would become unstable,” he says. With a certain critical number of pedestrians, the wobbling becomes marked enough to force everyone into stride — thus compounding the problem.



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