# When troops march across bridges....

**URL:** <https://boards.straightdope.com/t/when-troops-march-across-bridges/166994>\
**Category:** Factual Questions\
**Created:** [April 7, 2003, 4:01am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994 "2003-04-07T04:01:17Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Hermann\_Cheruscan](https://avatars.discourse-cdn.com/v4/letter/h/b782af/32.png) [@Hermann\_Cheruscan](https://boards.straightdope.com/u/Hermann_Cheruscan)\
**Post date:** [April 7, 2003, 4:01am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994/1 "2003-04-07T04:01:17Z")

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What is the term that they use for the vibrations that are caused when troops are marched across bridges? I understand that troops are ordered to route step across bridges, because marching could cause some sort of harmonic disturbance and cause the bridge to collapse. Does anyone know anything about this?

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**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\
**Post date:** [April 7, 2003, 4:07am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994/2 "2003-04-07T04:07:55Z")

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You used the term - if the troops march in unison they can create an acoustic energy wave that may be, in its wavelength, a harmonic of the bridge, by virtue of the bridge’s length.

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**Author:** ![Ice\_Wolf](https://avatars.discourse-cdn.com/v4/letter/i/dfb087/32.png) [@Ice\_Wolf](https://boards.straightdope.com/u/Ice_Wolf)\
**Post date:** [April 7, 2003, 4:08am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994/3 "2003-04-07T04:08:05Z")

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[Resonance.](http://science.howstuffworks.com//bridge5.htm)

> [@](#):
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> When an army marches across a bridge, the soldiers are often told to “break step.” This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart.

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**Author:** ![Gairloch](https://avatars.discourse-cdn.com/v4/letter/g/d2c977/32.png) [@Gairloch](https://boards.straightdope.com/u/Gairloch)\
**Post date:** [April 7, 2003, 4:19am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994/4 "2003-04-07T04:19:42Z")

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“What is the term that they use for the vibrations that are caused when troops are marched across bridges?”

Darn. Resonance (or any equivalent term) is the right answer, but I wanted to say ‘Fear.’

Reason is only the companion of emotion, after all . . .  
Gairloch

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**Author:** ![UncleBill](https://avatars.discourse-cdn.com/v4/letter/u/d26b3c/32.png) [@UncleBill](https://boards.straightdope.com/u/UncleBill)\
**Post date:** [April 7, 2003, 11:26am UTC](https://boards.straightdope.com/t/when-troops-march-across-bridges/166994/5 "2003-04-07T11:26:12Z")

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Resonance is related to Constructive Interference, if my memory serves.