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I recently stumbled across this article "Why are soldiers ordered to break their marching steps while crossing a bridge?" (The Times of India) where a claim is made:

When soldiers march in three files over a bridge, they generate a rhythmic oscillation of sine waves on the bridge. At a certain point, the bridge would start oscillating to the same rhythm as that of the marching steps.
This oscillation would reach a maximum peak when the bridge can no longer sustain its own strength and hence collapses. Therefore, soldiers are ordered to break their steps while crossing a bridge. (my emphasis)

Is it true that column of marching soldiers has to break their rhythm while crossing a bridge to prevent its collapse?

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Note to the would be answerers: Mythbusters is not a scientifically valid source. They did try it here, but we would like answers based on something more reputable. –  Sklivvz Dec 23 '12 at 20:33
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Personally I suspect that this is more of an issue for old, damaged, field-expedient and intended-for-foot-traffic bridges than for modern bridges intended for vehicular traffic. –  dmckee Dec 23 '12 at 22:05
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I can say that we marched across plenty of modern bridges in the ARMY. In a combat environment a bridge is a vulnerable point so you break formation and spread out to reduce the number of casualties should the bridge be taken out while crossing. –  Chad Dec 25 '12 at 4:49
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It depends on the bridge. Albert Bridge in London is known to resonate at marching pace and has signs ordering troops to break step –  Henry Dec 25 '12 at 10:27
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I know it happens sometimes that the floor of concerts break of that reason. People jump up and down to the rhythm of the music. I remember my physics teacher in high school explaining it, that the force increased by quite a bit than if people just was standing on it, and that they doesn't always calculate for the increase of force when people jump in rhythm. –  Wertilq Apr 24 '13 at 16:54
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1 Answer

up vote 37 down vote accepted

Certain types of bridge are susceptible to resonance effects even when marching soldiers are not involved. One example is London's Millennium Bridge which was closed shortly after opening because low-frequency vibrations in the bridge were causing large groups of pedestrians to simultaneously shift their weight and reinforcing the oscillation. Dampers were fitted.

All 690 tons of its steel-and-aluminium deck began to sway left and right like a giant, executive desktop toy, so much so that pedestrians, suspended above the Thames on slender steel cables, began to clutch at handrails to steady themselves, and throw themselves against the sway, to stay upright. As they did, so the swings began to get increasingly violent.

Report in The Guardian

Soon after the crowd streamed on to London's Millennium Bridge on the day it opened, the bridge started to sway from side to side: many pedestrians fell spontaneously into step with the bridge's vibrations, inadvertently amplifying them.

Crowd synchrony on the Millennium Bridge - Nature 438, 43-44 (3 November 2005)

other bridges – with completely different structures to the Millennium Bridge – have also moved laterally under large crowds. An example of this is the Auckland Harbour Road Bridge which has a box-girder structure. This bridge was filmed vibrating laterally at 0.7 Hz when a large crowd of demonstrators walked over it in 1975.

This is particularly significant because the Auckland Harbour Bridge is a large road bridge with a conventional structure.

Stabilising the London Millennium Bridge


There are reports of small suspension bridges collapsing partly due to resonance effects

Broughton Suspension Bridge

Angers Bridge Catastrophe


There's at least one PhD Thesis on the subject


bridges should be crosssed at a slow gait, and marching men should break step.

The Examination & Repair of Bridges WITH PRINCIPLES RELATING TO THEIR DESIGN, Captain C. O. SHERRILL, Corps of Engineers Instructor, Department of Engineering, 1909

companies of soldiers must break step when crossing bridges due to the risk of creating large motions at resonant frequencies.

EVALUATION OF CONDITION OF LAKE SUPERIOR REGULATORY STRUCTURE SAULT STE. MARIE, MICHIGAN. 1981

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The Tacoma Narrows brinde famously failed due to a wind driven resonance. –  dmckee Dec 23 '12 at 22:01
    
@dmckee, yes, Moisseiff, the lead designer of the 1940 Tacoma Narrows Bridge, called it the "most beautiful" bridge in the world, but the disaster effectively ended his career. He died just three years after the failure the bridge. Happy Christmas, and do not forget to upvote the question! –  Carlo_R. Dec 23 '12 at 22:52
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+1 for a good answer but is there any evidence that military organizations actually change their marching behavior to avoid this kind of resonance (as the actual question is asking)? –  Michael Edenfield Dec 25 '12 at 4:01
    
@Michael: Yes - see addendum to answer. –  RedGrittyBrick Dec 26 '12 at 18:18
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Actually every structure has an inherent resonance (based on its structural design). It's a common concept in earthquake engineering. e.g. tall buildings often have higher resonance. There's also more than one in complex structures, e.g. multistorey buildings. –  Lela Dax Dec 27 '12 at 13:22
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