Why did the Tacoma Narrows Bridge collapse?

  A marching band can create rhythmic vibrations even when it isn't playing music. Sometimes you can feel the ground víbrate from marching feet. Do you know why bands and military units don't march in step across suspension bridges?

Tacoma Narrows Bridge collapse

  The rhythm of the marching can cause the bridge to víbrate. If the frequency of the vibration caused by the marching matches the natural fre­quency of the bridge, resonance can occur. Res­onance can cause the bridge to move up and down until its structure weakens and collapses.
English cavalry troops marching across a foot-bridge in 1831 caused the bridge to collapse when they marched in rhythm with the natural frequency of the bridge. Winds can also cause vibrations in suspension bridges. On November 7, 1940, vibrations caused by a mild gale produced resonance in the Tacoma Narrows Bridge in the state of Washington. In a few hours, the flexible suspension bridge was torn apart and collapsed. Engineers now design suspension bridges with heavier, less-flexible roadbeds that aren't affected by wind.