Why do soldiers break their steps while passing over a bridge?

Answer 1

In theory, a number of people marching in step can set up vibrations that will cause a bridge to collapse. It may only be an urban legend, but our DI said it was true, so we always "broke step" when crossing the bridge from Recruit Island to the main base.

On 12th April 1831 about 70 men from the 60th Rifle Corps, had been carrying out exercises on Kersal Moor and were returning to their barracks by the way of a suspension bridge across the River Irwell in Lower Broughton. As they crossed the bridge it began to vibrate with their step and as the vibrations increased with more soldiers on the bridge, the suspension chains snapped and the bridge collapsed, throwing the men into the water. As it was, the river level was quite low and there were no major injuries and no lives were lost. It was never clear whether the accident was a result of the vibration caused by the marching men or inferior engineering, however, since then the order has been given to break step whenever the military cross a bridge.