This was the issue with the "Millenium" bridge in London, renamed unofficially as the "wobbly" bridge.

Because of the nature of it's construction people would over time start to sync their steps, the more this happened the more the bridge swayed and the more people synced their steps.

It was fixed by adding dampers.

https://en.wikipedia.org/wiki/Millennium_Bridge,_London

On the note of bridge resonance, one can not forget the Tacoma Narrows Bridge: https://www.youtube.com/watch?v=j-zczJXSxnw

I was talking to a colleague today and I made that example I remembered from my physics course done on the Halliday-Resnick textbook https://en.wikipedia.org/wiki/Fundamentals_of_Physics

If anything, the way that bridge wobbles before collapsing is pretty assuring - it’s unlikely I’d enter a bridge even remotely as unstable!

I think I’ve read something similar about the ex-dictator in Albania, who would place engineers into bunkers they've constructed and then fire on them to see if they hold

EDIT: found a source, seems to be a true story

https://www.balkanride.com/2017/03/07/albanias-concrete-bunk...

It’s like dogfooding.

https://en.wikipedia.org/wiki/Angers_Bridge : Soldiers create resonance on a brdige, making it collapse.

There is actually little evidence here that a resonance excited by marching soldiers caused this bridge to fall. The article says that, as usual, the soldiers were instructed to break step when crossing. The article cites a powerful thunderstorm, the effect of strong gusts being 'caught' by the massed soldiers on the bridge, and corrosion of the cables at their anchorages, as likely causes. A survivor reported that the soldiers were stumbling around as if drunk, and could barely stand. There was some speculation that the motion of the bridge caused the soldiers to respond in a coordinated way that exacerbated the swaying, but lurching from side to side is hardly marching.

Contrary to what the grandparent post says, few bridges are susceptible to being brought down by marching.

I recall that we always had to break stride when marching across bridges, but I also recall that this has long been debunked, a properly designed bridge is in no danger at all from people marching across it in step.

There was that British bridge, but it was suspension:

https://en.m.wikipedia.org/wiki/Broughton_Suspension_Bridge

It is easy to see how a suspension bridge would resonate fairly easily. Also see the Tacoma Narrows bridge.