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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?

Carlo Alterego
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    **Note to the would be answerers**: Mythbusters is not a scientifically valid source. They did try it [here](http://dsc.discovery.com/tv-shows/mythbusters/videos/breakstep-bridge-minimyth.htm), 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 --- ex-moderator kitten Dec 23 '12 at 22:05
  • @dmckee yes, I agree with you, but I would add that this is more an issue for SUSPENDED bridges. – Carlo Alterego Dec 23 '12 at 22:55
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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 04:49
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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](http://www.google.com/search?site=imghp&tbm=isch&q=albert+bridge+break+step) – Henry Dec 25 '12 at 10:27
  • Your bounty claims the question has not received enough attention, but it has an answer with 18+ upvotes, and you haven't accepted it. Please explain what is missing from this answer. – Oddthinking Jan 03 '13 at 12:32
  • Hello @Odd, nothing is being missed from that answer, but I wish the question be viewed 1000 times. – Carlo Alterego Jan 03 '13 at 12:59
  • ... @Odd, do you know why the bounty field does not allow HTML tags? – Carlo Alterego Jan 03 '13 at 13:05
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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

1 Answers1

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Mechanical Resonance

Certain types of bridge are susceptible to mechanical resonance effects even when marching soldiers are not involved.

Mechanical structures usually have one or more frequencies at which some part of the structure oscillates. A tuning fork has a well-defined natural frequency of oscillation. More complex structures may have a dominant natural frequency of oscillation. If some mechanical inputs (such as the pressure of feet walking in unison) have a frequency that is close to a natural frequency of the structure, these inputs will tend to initiate and, over a short time, increase the oscillating movements of the structure. Like pushing a child's swing at the right time.

London Millenium Bridge

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.

enter image description here
Maths in the city

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

Other Bridges

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

Broughton Suspension Bridge

Angers Bridge Catastrophe

Studies

There's at least one PhD Thesis on the subject

Instructions to Soldiers

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

RedGrittyBrick
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    The Tacoma Narrows brinde famously failed due to a wind driven resonance. – dmckee --- ex-moderator kitten 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 Alterego 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)? – KutuluMike Dec 25 '12 at 04:01
  • @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. – j riv Dec 27 '12 at 13:22
  • This excellent answer could be slightly improved by directly referencing the causative phenomenon, [mechanical resonance](http://en.wikipedia.org/wiki/Mechanical_resonance). Most high school or early university physics textbooks will cover this phenomenon and incidents of it. – Brian M. Hunt Jul 09 '14 at 15:20
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    @Brian: Thanks for the suggestion. I've added some links and a brief (though inexpert) discussion of the point. I'll dig out my old textbooks later :) – RedGrittyBrick Jul 09 '14 at 15:46
  • Awesome, thanks @RedGrittyBrick - it might be worth noting that Broughton in particular was caused by synchronized marching, since that is the topic of the question. – Brian M. Hunt Jul 09 '14 at 16:24
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    It doesn't happen *often* but it *does* happen, and when it does it's a really stupid way of losing millions of dollars' worth of infrastructure plus the loss of life of people on the bridge when it collapses, and the military feels that's too high a price to pay for its parade marching pride, so they play it safe. – Shadur Feb 16 '16 at 14:30
  • Anectdotally speaking, I've seen it happen. Our company was on a jog, and as soldiers, we automatically fell into step. Crossing a welded steel truss pedestrian bridge, we cause it to start to bounce in a dangerous manner. Our LT quickly ordered a break step, and the bouncing stopped immediately. – Chris Cudmore Jul 22 '16 at 16:00