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?

(In reply to Simple ... by Larry)

I agree with Larry that trying to avoid creating a resonant frequency is the reason for the command of free step.
If a group of people steps in the same rhytm, they add energy to the vibration. Try to see it as a kid on a swing, if you keep pushing it at the time it starts its new swing, the swing will always get larger.  If the marching is done in the right frequency, the bridge will go up and down in increasingly large amplitude.  The exact frequency is the natural frequency of the bridge.  In modern bridges, this is avoided by connenctig different parts of the bridge with dampeners that do not transmit the vibrations.  The different parts also have their own (different) natural frequency.
The Tacoma Narrow bridge collapse Larry refers to was induced by wind and it is doubted that a group of soldiers could collapse a bridge by walking over it.  They would probably have to stay on the bridge in an effort to destroy it on purpose. 
Though in 1850, in Angers (France) a bridge collapsed, killing more then 220 soldiers.  Also here wind seems to have played an important role, as did corrosion on bridge cables.
The Osteroy bridge (Bergen Norway) was shut down (in 2000) from time to time because of heavy swingings, I have been on the bridge when it swinged (swung, swong?) and can assure you: it's frightening enough to start reading on the subject.  Now they solved it by installing a sort of spoilers.

Posted by Hugo on 2005-04-16 19:06:15