It is very dangerous and banned for a battalion of soldiers to cross a bridge marching in step. Indeed the bridge can enter into a resonance, a physical phenomenon that makes the bridge wave and may lead to its destruction.
In 1850, the Angers Bridge, also called the Basse-Chaîne Bridge collapsed, due to a resonance phenomenon.
226 soldiers died during the collapse of this bridge. We do not know with certainty if it's the soldier's step or the gusts of wind that caused the resonance. It was probably a mix of both, even if the phenomenon was known at the time, and troops were forbidden to march in step while crossing a bridge.
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Soldiers stationed in the region frequently used the bridge, and two battalions of the same regiment had crossed earlier that day. The third battalion arrived during a powerful thunderstorm when the wind was making the bridge oscillate. When the soldiers began to cross, they gave the wind still more purchase. Survivors reported that they had been walking as if drunk and could barely keep themselves from falling, first to one side and then to the other. As usual in crossing that bridge, the soldiers had been ordered to break step and to space themselves farther apart than normal. However, their efforts to match the swaying and keep their balance may have caused them to involuntarily march with the same cadence, contributing to the resonance. In any case, the oscillation increased. At a time when the bridge was covered with 483 soldiers and four other people (though the police had prevented many curiosity seekers from joining the march), the upstream anchoring cable on the right bank broke in its concrete mooring, three to four meters underground, with a noise like "a badly done volley from a firing squad". The adjacent downstream cable broke a second later, and the right-bank end of the deck fell, making the deck slope very steeply and throwing soldiers into the river. Many of those who fell were saved by their fellow soldiers who had not yet crossed and by residents of Angers who came to the rescue, but a total of 226 people died.
The failure was attributed to dynamic load due to the storm and the soldiers, particularly as they seem to have been somewhat in step, combined with corrosion of the anchors for the main cables. The cable anchorages at Angers were found to be highly vulnerable, as they were surrounded by cement, which was believed to rustproof them for the indefinite future. However, the wire strands separated from their cement surrounds. This allowed water to penetrate and corrode the wires.
- Source 1: Etymonline Online Etymology Dictionary