Soldiers shouldn’t march across a bridge.
A structural member vibrates in response to normal loads and impacts, in the manner of a plucked guitar string. The natural or resonant frequency of an object is the time it takes to complete one cycle of movement (fully back and forth or up and down) upon disturbance.
When a force acts repeatedly on a structural member, and at a rate that matches its natural frequency, the member’s response is enhanced with every cycle. The effects range from loud humming (such as when vibrations from a building’s mechanical equipment coincide with a beam’s natural frequency) to uncomfortable oscillation to occasional collapse. Many relatively small earthquakes have induced significant damage when their wave frequency has matched that of affected buildings. In 2000, thousands of pedestrians celebrating the opening of the London Millennium Footbridge inadvertently induced oscillation when their walking rhythms matched the structure’s natural frequency. As they swayed in response to the unanticipated movement, they inadvertently increased it. The bridge was closed following the event and the structural system was repaired.
