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?

    Thing is, when vibrations are synchronised, they actually amplify each other's magnitudes. A single person walking across the bridge will not create any tangible vibration of the bridge itself. However, when soldiers march, consider the fact that they tend to hit the floor very hard and also hit them at more or less the same timing. This causes the vibration to be amplified, causing the bridge to also vibrate in a way that could cause its collapse. This effect is somewhat very much more significant if the frequency of the vibration is the same as the bridge's natural frequency, as the previous posters have rightly stated. However, even if this is not the case, the bridge, if not strong enough, may collapse anyway.

When people do cheers, they do them in unison so the resulting volume of the cheer will be extremely high. However, this is not due to the fact that their voices are at the natural frequency of air, but that their voices are all at the same frequency. Similarly, if enough soldiers much hard enough, synchronised enough and if the bridge was made out of sonorous material like wood or steel, the bridge would be adversely affected, as seen in the millenium bridge incident. Non-metal materials like wood would break apart, and metal materials would bend and start swaying according to the soldiers' footsteps.

Theoretically concrete bridges will never be affected by soldiers marching in step.

Posted by Jack Lim on 2006-03-02 03:49:54