In the Military there is a long standing tradition that you do not march soldiers over a bridge in step. When marching and approaching a bridge the command of Route Step, MARCH is given. This command keeps the soldiers marching in formation, but every soldier is allowed to march at her/his own step. Although this is widely practiced in the Army, I found very few who knew the reason why.

What could have possibly prompted this tradition?

Simple harmonic motion, that is.

I think the rythmic foot pounding, if it were close to one of the natural frequencies of the bridge structure, could cause resonance and start uncontrolled vibrations in the bridge (especially a bridge where you can feel the bridge give slightly as you walk on it).

The Tacoma Narrow bridge collapsed from the wind-induced resonance on November 7, 1940.   
See:  http://www.ketchum.org/bridgecollapse.html

Also, if it is one of those rope sky bridges, everyone standing on their left foot at the same time would tilt the bridge floor way to the left, even without any resonant frequencies being involved

Edited on June 29, 2013, 2:53 am

Posted by Larry on 2005-04-16 16:13:35