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Re: [Phys-l] Why soldiers break step on small bridges, and not so small.


On 2011, Jul 26, , at 17:38, William Robertson wrote:

For what it's worth, the Mythbusters addressed this issue and made a
fairly good case that it would be difficult for soldiers to get a
regular bridge (not a flimsy footbridge) to execute normal modes of
vibration by marching in step. The skyscraper story is definitely
believable, as extensive measures are taken to prevent winds from
making skyscrapers sway out of control. A brief article intended for
the layperson on the subject: http://www.nsta.org/publications/search_journals.aspx?keyword=skyscrapers&journal=SC

Bill


Not worth much (Using the word "flimsy" makes it un-falsefiable.)



Wiki. has an article on soldier caused bridge failures, to which I referred unsuccessfully. I try again. (third time -- phys-l owner please note.)

"As usual in crossing that bridge, the soldiers had been ordered to break step and to space themselves farther apart than normal. However, their efforts to match the swaying and keep their balance may have caused them to involuntarily march with the same cadence, contributing to the resonance. In any case, the oscillation increased. At a point when the bridge was covered with 483 soldiers and four other people (though the police had prevented many curiosity seekers from joining the march), the upstream anchoring cable on the right bank broke in its concrete mooring, three to four meters underground, with a noise like "a badly done volley from a firing squad".

So you are partly correct.

If:

http://en.wikipedia.org/wiki/Angers_Bridge

doesn't work Google Angers.

here's the other:

http://home.messiah.edu/~barrett/video2.htm

bc thinks the myth busters should be time machined to Angers, France, 1850 April 16 on the bridge.

OTOH: Broughton:

http://books.google.com/books?id=USWV3PP3b08C&printsec=frontcover&dq=Differential+Equations+and+Their+Applications:+An+Introduction+to+Applied+Mathematics#v=onepage&q=broughton&f=false

Searched Broughton.


http://en.wikipedia.org/wiki/Broughton_Suspension_Bridge





On Jul 26, 2011, at 12:23 AM, Bernard Cleyet wrote:

Again, because Links were broken.



Scientist: Tae Bo workout sent skyscraper shaking – This Just In -
CNN.com Blogs


bc


p.s. It is a "real" problem:

Angers Bridge - Wikipedia, the free encyclopedia

p.p.s Anyone seen the film loop of a demonstration of the normal
modes of a foot bridge?

This is all I could find -- is the one I remember.


Resonant Bridge Video Clips






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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l