Walking to the beat bad for bridges

Scientists apply ideas from mathematical biology used to describe collective oscillations of neurons, fireflies to explain why London's Millennium Bridge swayed on opening day. Study may give engineers a way to estimate how much damping needed to stabilize footbridges.

Scientists have developed a model to predict wobbling bridges, like the high-profile problem with London's Millennium Bridge.

The 320-metre-long steel suspension bridge spans the River Thames, connecting London's financial district to Bankside, south of the river.

Large numbers of people started walking across the bridge at its opening on June 10, 2000. The structure began to sway laterally or wobble, which engineers call "synchronous lateral excitation."

The $32 million US bridge was designed to withstand weight and height, but did not account for human nature. People fall spontaneously into step with the swaying of bridges, said Steven Strogatz, a professor of theoretical and applied mechanics at Cornell University.

His research focused on the effect of pedestrians who fell into step with the bridge's vibrations to counteract the sway.

The problem was, the natural frequency of the bridge closely matched that of human walking, compounding the problem, the researchers said.

Strogatz and his colleagues applied ideas used to describe other oscillations such as in neurons and fireflies.

"Our approach should help engineers to estimate the damping needed to stabilize other exceptionally crowded footbridges against synchronous lateral excitation by pedestrians," the researchers wrote in Thursday's issue of the journal Nature.

By combining vibration calculations and crowd dynamics, the researchers found it can take as few as 160 people to spark a wobble. It's estimated there were as many as 2,000 on the bridge at one time at the opening.

If tests show the analysis is correct then engineers may be able to use it to prevent such problems at the design stage.

The bridge was closed for 20 months until giant shock absorbers solved the problem at a cost of $8.9 million US.