Map of the week – Why troops “break step” when crossing a bridge.

Broughton Bridge was a suspended-deck suspension bridge built in 1826 to span the River Irwell between Broughton and Pendleton, now in Salford, Greater Manchester. It was one of the first suspension bridges constructed in Europe.
broughton
On 12 April 1831, the bridge collapsed, reportedly owing to mechanical resonance induced by troops marching over the bridge in step. A detachment of 74 men from the 60th Rifle Corps were returning to their barracks in Salford by way of Broughton bridge marching four abreast. At first the bridge begin to vibrate in time with their footsteps. Finding the vibration a pleasant sensation some of them started to whistle a marching tune, and they began to “humour it by the manner in which they stepped”, causing the bridge to vibrate even more. As a result a bolt in one of the stay-chains snapped, causing the bridge to collapse at one end, throwing about 40 of the men into the river. None of the men were killed, but 20 were injured, including six who suffered severe injuries including broken arms and legs. As a result of the incident, the British Army issued an order that troops should “break step” when crossing a bridge. A requirement that is still in place to this day.