This is also why military troop formations are never, ever marched in stride across bridges. For almost two centuries, troop leaders have been taught that the rhythmic motion of a formation marching in unison can destroy a bridge, and to order their formations to break stride into "route step" with independent footfalls.
Yep, I remember route step over the bridge at the Navy recruit training center, otherwise known as “boot camp” at San Diego in ‘62. Next door to the Marine camp in those days as you no doubt know. They used to tell us that if we thought we had made a mistake joining the Navy we could cross the fence and see how we liked the Marine corps.
...the soldiers, who were marching four abreast, felt it begin to vibrate in time with their footsteps. Finding the vibration a pleasant sensation some of them started to whistle a marching tune, and to “humour it by the manner in which they stepped”, causing the bridge to vibrate even more.[8] The head of the column had almost reached the Pendleton side when they heard “a sound resembling an irregular discharge of firearms”.[8] Immediately, one of the iron columns supporting the suspension chains on the Broughton side of the river fell towards the bridge, carrying with it a large stone from the pier to which it had been bolted.
https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge