Last night I was accused of perpetuating an urban myth

It did not help that my recollection of the incident at the time was vague

This morning I went to my source material, my A Level Physics text book

The incident to which I was refering was when in 1850 200 French infantry men were killed when a bridge collapsed due to the forced vibrations brought on by the soldiers marching in step. My physics text book cited this as a reason why soldiers always break step when crossing bridges.

I suspect the detail given in my Physic Text book will still not be specific enough for my accusers.

Like when in 1850
Where
Why were they marching

Any clues as to more details of this incident

I suspect the problems surrounding the Millenium Bridge in London when it first opened were the same, i.e forced vibrations due to resonance.

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While I'm not sure about whether or not this is true (though I have heard it, and can very well believe it), I can give you another example of resonance doing bad things to bridges: the Tacoma Narrows Bridge in Washington state. I'm sure if you google that, you'll come up with a lot of information. Hope that helps you somewhat.

Eh. There's a footbridge in England (London, maybe?) where this was actually a problem. It was built for Y2K, and had a natural resonence at 1Hz. Apparently, if you had enough people walking on it, it would start to sway at 1Hz. Everyone on the bridge would compensate for the swaying, and start walking in step. Stabilizers had to be added to prevent it from falling apart.

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The Discovery Channel's Mythbusters took this on in Episode 12:
"...Jamie and Adam move on to a myth with more legs when they take a stroll across a bridge to see if the rhythm of soldiers marching together can cause a bridge to collapse..."
Video Gallery (Note they also do the egg-balance/Equinox thing.)

I was gonna mention that, though I think they're method wasn't the best way of going about it.

They weren't able to find anything either way. They were able to cause their bridge to fail, but it had nothing to do with harmonics. But they also acknowledged that their testing method wasn't adequate to determine whether this was a myth or not.

Having crossed the Millennium Bridge in London several times (it goes from the Tate Modern, a disused power station, to St. Paul's Cathedral) I was curious about this. When I've looked up obscure history of technology in Google, I haven't always got very far (eg as a minor part of an Open University course - did wrought iron rails, which weren't as strong as steel, hold back locomotive design? I still don't know!).

As it happened in France, I tried a guess of 'pont 1850 soldats' in Google - to my surprise it worked (and most of the pages are in French). I think the incident was at the Pont de la Basse-Chaîne and it seems to have stopped the construction of suspension bridges in France for some years.

This page has more - Pont Basse-Chaîne (Angers) - 1838, but my French is too limited and rusty to understand it all without a dictionary. It mentions a similar incident in Manchester in 1831.

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From what I heard, dampeners were somewhere added to fix the problem.

About soldiers breaking step on a bridge, where is Stuart when you need him?

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Can't cite chapter and verse, but I believe this one has been pretty well debunked. It is one of those conventional wisdom things about "all that weight" coming down at once.

Marching soldiers have a tradition of breaking step over bridges, but it is not based on anything but a notion. Remember, that walking men never remove their weight from the surface. Also, any harmonizing would need to match the resonances of the bridge. Try plucking a taut string at many points along its length simultaneously, and you won't find much resonance. Plus it is difficult to imagine that something the size of a bridge would have a resonance at a frequency as high as marching pace.

We still call it the Millennium Bridge the 'wobbly bridge', even though it isn't anymore.

I've been looking a bit further - both the bridges at Broughton, Manchester (1831) and at Angers (1850) were suspension bridges. There's an www.vibrationdata.com/Newsletters/March2002_NL.pdf+broughton+bridge+1831+irwell+rifl e&hl=en]html version of a pdf file[/url] in Google's cache (I couldn't access it directly) which says (after the bit about moonquakes) that soldiers marching in step may have been a factor, but not the only one causing the bridges to collapse.

As the film of the Tacoma Narrows bridge before it collapsed shows, suspension bridges can move a lot.

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"All the screens are filled with heroes and losers, but the sky's still filled with stars"
...Midnight Oil - 'Golden Age'

The Tacoma Narrows bridge collapsed partly because of wind causing vortex shedding at the same frequency as that of the brige. Modern bridges are designed to break up the vortecies before harmfull vibrations can build up and are also considrably stiffer.

The forces on the Millenium bridge had a different cause according to this item

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I haven't seen that (I'll watch it soon, now, with the link available), but a friend told me that they didn't tie the ends of the bridge down. If that's true, then the modes excited in the bridge wouldn't be the same as if it were tied down, and the experiment is invalid.

:-k :^o :-?

Did Mssrs Sears, Zemansky and Young mislead me?

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