Why the wiggle in a crowd’s walk can put a wobble in a bridge

London’s Millennium Bridge [image credit: Alison Wheeler / Wikipedia]

Researchers now believe ‘that the synchrony of the crowd might not be a root cause but instead acts as a feedback effect that amplifies pre-existing small-scale wobbles’, but leave open the question of how or why the swaying starts. So to date any such resonance seems to be largely a matter of luck – or bad luck, which ideally is where testing comes in.

Some bridges could really put a swing in your step, says Science News.

Crowds walking on a bridge can cause it to sway — sometimes dangerously. Using improved simulations to represent how people walk, scientists have now devised a better way to calculate under what conditions this swaying may arise, researchers report November 10 online in Science Advances.

When a bridge — typically a suspension bridge — is loaded with strolling pedestrians, their gaits can sync, causing the structure to shimmy from side to side.

The new study “allows us to better predict the crowd size at which significant wobbling can appear abruptly,” says mathematician Igor Belykh of Georgia State University in Atlanta.

Engineers might eventually use the researchers’ results to avoid debacles like the one that befell the Millennium Bridge in London. This suspension bridge temporarily shut down just days after it opened in 2000 due to the large wobble that occurred when many people tromped across it at once (SN: 11/24/07, p. 331), necessitating costly repairs to fix the problem.

Pedestrians crossing a bridge can cause slight sideways motion of the bridge as they push with their feet. This swaying may lead to the crowd unintentionally falling into lockstep because it’s easier to go with the flow of the swinging bridge than fight it. That synchronization, in turn, creates larger and larger oscillations.

“It’s a dangerous phenomenon that could cause a bridge to collapse if it went unchecked,” says applied mathematician Daniel Abrams of Northwestern University in Evanston, Ill., who was not involved with the research.

Continued here [includes video links].