How Marching Soldiers Can Make a Bridge Collapse

Suspension bridges are an engineering marvel, allowing access across water in all parts of the world. But they have their drawbacks. They must be somewhat flexible to avoid breaking apart, but if they move too much, they will break apart anyway. And strangely, a bridge full of pedestrians moving slowly puts more weight on a bridge than normal vehicle traffic. When soldiers march across a suspension bridge in unison, they can cause a bridge to sway rhythmically, called a "mechanical resonance." The Broughton Suspension Bridge in Manchester, England, was built in 1826. But in 1831, a unit of soldiers marched across the bridge in cadence, causing it to sway more than usual. They found this amusing and marched even harder. Then the bridge fell in. No one was killed in that incident, but the British army afterward enacted a policy to break cadence when crossing a bridge. Then in 1850, a French bridge fell under the same circumstances, killing 226 people!
The effect of marching on a suspension bridge became known, but it doesn't have to be a military unit marching in cadence to produce the effect, as other bridges have failed due to the natural cadence of crowds experiencing swaying while crossing a bridge. Read about this effect and the harrowing examples of what can happen at Amusing Planet.