All suspension bridges are prone to vibration and swaying caused by moving traffic and wind. These vibration are not a problem as long as they are not in resonance with the bridge’s natural frequency. However, if these vibrations fall in harmony, like the forced steps of marching soldiers in unison, the swaying of the bridge can intensify. If the frequency of their steps closely matches the bridge's natural frequency, the soldiers' rhythmic marching will amplify the vibrations on the bridge causing structural failure.
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Broughton Suspension Bridge.
Source: amusingplanet.com