[kens]

I read the same claim on Usenet about 30 years ago and researched it. My conclusion was that soldiers break stride to avoid nasty oscillations, but I couldn't find any actual collapses. There was one bridge that was claimed as a collapse from people marching on it, but it was actually very high winds that caused the collapse.

[LeoPanthera]

Well, London's Millennium Bridge did not collapse, but it did sway alarmingly when crowds of people walking on it were forced into resonance, resulting in the immediate closure of the bridge before it could be fixed.

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

[cameronh90]

And even now people still call it the “wobbly bridge”.

[juhatl]

Apparently an incident took place in 1831 where a bridge collapsed under soldiers marching, but the Wikipedia article [1] notes that "The conclusion of the investigation was that the vibration caused by the marching precipitated the bolt's failure, but it would have failed eventually anyway".

There's more interesting information on synchronization and bridges on Veritasium's video, The Secret of Synchronization [2].

[1] https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge [2] https://www.youtube.com/watch?v=t-_VPRCtiUg

[pomian]

That video link about synchronisation is very interesting. Thankyou. Synchronization of bridges, hearts, lightning bugs, metronomes, sailing clocks. Cool

[jfk13]

The Albert Bridge in London even has signs to remind soldiers to break step:

https://www.londonremembers.com/memorials/albert-bridge-troo...

[guerrilla]

In a documentary about London's Millennium Bridge, they showed videos of other bridges collapsing due to it. Sorry I don't have a link, just saying it's out there.