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23. All Aboard and Synchronized

Steven StrogatzSteven H. Strogatz, Theoretical and Applied Mechanics, did an analysis to explain why London’s Millennium Bridge, hailed by engineers as an “absolute statement of our capabilities at the beginning of the twenty-first century,” swayed uncontrollably on its opening day as pedestrians crossed it. When the 320-meter–long lateral suspension bridge across the Thames River connecting London’s financial district to Bankside opened on its first day, thousands of pedestrians were there to walk across it. At first, the bridge was still. Then it began to sway slightly. The wobble intensified. People began walking tentatively, planting their feet widely apart with each step in left-right synchronized unison. Strogatz explained the problem as one of crowd dynamics, as much as engineering. Every nonhuman element had been tested, surpassing standards for withstanding weight and wind. Strogatz examined the problem as unintentional synchrony—the phenomenon of people unknowingly working together, simply by walking. The $32-million design fiasco closed the bridge immediately in 2000, and the bridge reopened in 2002 outfitted with 91 dampers to absorb lateral and vertical oscillations, with modification costs of $8.9 million. Strogatz’s theory could help engineers solve this type of problem before they begin to build.

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