At first, the bridge was still. Then it began to sway,
just slightly.
Then, almost from one moment to the next, the wobble
intensified. And suddenly, people were walking like
tentative ice skaters: planting their feet wide,
pushing out to the side with each step. Left, right,
left, right, in near-perfect unison.
The synchrony was utterly unintentional. But it was
those unchoreographed footfalls, says Strogatz, that
were responsible for turning a $32 million design
triumph into a very embarrassing engineering quandary.
The bridge was closed almost immediately.
Strogatz, who has studied the collective behavior of
biological oscillators from neurons to fireflies,
describes each of the factors that contributed to the
bridge's swaying in his paper. Cornell graduate
student Daniel Abrams is one of the paper's
co-authors.
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.
http://www.news.cornell.edu/stories/Nov05/Strogatz.millennium.lg.html