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Comment by throwaway_isms | Hacker News Reader

Obligatory bridge collapse fact: there have been a few historical bridge collapses resulting from walking/stepping/marching which causes vibrations in the bridge that create a feed back loop further syncing the stepping intensifying the vibrations ultimately leading to the failure. Soldiers are supposed to break stride when crossing bridges to prevent collapse.

Some people have a fear of the heights of these bridges, I have an irrational fear of vibrational collapse.

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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.

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...

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

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

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

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.

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

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

Back in the 80s the city of SF encouraged people to walk in the GG Bridge to celebrate its 50th anniversary.

Almost destroyed the bridge and killed everyone on it.

I feel like I remember that, but might be confusing it with another bridge incident. I remember there being elephants in the celebration, significant “waves” in the bridge and some pretty serious injuries...but again I may be confusing it with another incident.

Edit: watched video, GG flattened out. Not sure what I am remembering and maybe mixing up a few bridge “disasters” and collapses into one.

I looked it up and it seems that's not what happened.

_While there were some injuries, there were no fatalities and the bridge was not damaged._

_The weight of the 300,000 people flattened the normally arched roadbed, although bridge engineers later said there was never any real danger._

Source: https://www.sfgate.com/local-donotuse/article/Golden-Gate-br...

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