November 10, 2005

Researchers Link Crowd Behavior, Bridge Swaying

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It is every little boy’s dream to have a superpower. But when architect Sir Norman Foster grew up, he attempted to turn this fantasy into reality. With his design of the Millennium Bridge, built over the River Thames in London to commemorate the year 2000, he wanted to create something moving and powerful.

“Lord Foster remembered reading Flash Gordon comic books. When he came to an abyss, he would shoot a blade of light and run across,” said Prof. Steven Strogatz, theoretical and applied mechanics. “He wanted to give London something thin and beautiful.”

However, this stunning bridge meant to unify London and stand for English pride had a serious flaw – it swayed.

“The bridge opened on a nice, sunny Saturday,” Strogatz said. “Tens of thousands showed up for this spectacular bridge opening. People started walking on it, and within a few minutes, it started to shake. It was shaking so much that people were scared.”

The question of what caused the carefully-engineered bridge to sway intrigued Strogatz and Daniel Abrams grad. They viewed footage from the opening that showed the people on the bridge stepping in synchrony. The two collaborated, with others, to answer what caused the crowd synchrony and the bridge to sway. The approximately one-page article titled “Crowd synchrony on the Millennium Bridge” ran in the science journal Nature on Nov. 3.

The swaying of the bridge was unanticipated by the engineers, and many felt it was an embarrassment.

“It was a big fiasco,” Strogatz said. “The queen came to open it. It was the pride of London; it was built by English people. When it did not work, everyone was mad and embarrassed and pointed fingers.”

Some accused the Danish engineering firm Arup, which was hired to execute Norman’s design, of making the mistake.

“Arup assumed people would not be synchronized,” Strogatz said. “Historically, it is not a problem, but on this bridge, it was a problem. So, people are attacking the engineers when they can’t be blamed because this is not really common.”

The synchronized footsteps on the bridge are what first attracted Strogatz to the Bridge.

“I’d been interested in things that get in sync for a long time, how systems get themselves organized spontaneously,” he said. “There is this tendency for order to emerge in nature. When I first heard about the bridge wobbling and people walking in step, it is the kind of thing I’m interested in.”

There are instances in the past of synchronized footsteps causing a bridge to break; however those circumstances differ greatly from those of the Millennium Bridge.

“There is an instance in the 1800s of cavalry in step, and this caused them to break a bridge,” Strogatz said. “But soldiers are different. They are trained to march. The people on the Millennium Bridge were grandmothers, people out for a stroll, not soldiers. We have to explain what got people in step.”

After an initial interest, and a mention of the bridge in his 2003 book, Sync: The Emerging Science of Spontaneous Order, Strogatz decided he wanted to know more.

“I never felt like I understood the way a mathematician wants to understand,” Strogatz said. “I thought maybe someone should try to what happened and no one did.”

This curiosity led Strogatz to take a semester-long sabbatical with his family in Copenhagen, Denmark. He invited Abrams along with him so that they could research the mystery of the bridge swaying together. Strogatz said that at first, the problem intimidated him because many people were involved and it is difficult to find an equation for people. Within a couple weeks, the men made great progress in their quest to understand the swaying, and Abrams had created a computer program simulating the bridge. However, there were still some obstacles.

“We tried e-mailing a guy from Arup because their publication sucked,” Abrams said. “Their paper was not clear; they did not want it to be in the news so they did not send a lot of data. But our paper would have been really good if they had said they would work with us and would have given us data.”

Collaborator Edward Ott, a professor of physics and electrical engineering at the University of Maryland and a former Cornell professor, began researching the problem separately when he first heard Strogatz speak about it. They ended up collaborating on the paper to find the cause of the swaying and the crowd synchrony.

“It involves an interaction of things that are all different and behave in different ways,” Ott said. “We have models of people walking as oscillations. They put force on the bridge, which causes the bridge to oscillate, which causes people to oscillate, and the oscillation gets bigger and keeps going. It was a bad way to start the millennium. ”

Ott is also working with Strogatz to come out with a second, more technical volume that will be published in the Physical Review.

“One of the things that came out of this was that it is possible to treat this theoretically without knowing the dynamics of people walking, because not that much is known,” Ott said.

Archived article by Bekah Grant
Sun Contributor