Any engineer will tell you that any type of span, roof, floor, bridge etc. if it is hit with rhythmic waves of vibration will develop a sympathetic sway or bending movement that will take out pretty well anything holding it together. Enough weight of bodies with feet moving in a synchronized rhythm will just start the floor joists a bouncing.
I was in the Sugar Bowl in 87 when Alex Van Halen set up a wave form by rolling the low drums. The concrete literally went up and down a half a foot. I thought he was gonna kill us all.
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.
“...any type of span, roof, floor, bridge etc. if it is hit with rhythmic waves of vibration will develop a sympathetic sway...”
Like the Tacoma Narrows bridge “Galloping Gertie” in 1940.
The movie footage is still amazing.