Why are army troops not allowed to march in steps while crossing the bridge?

Explanation

`(i)` Soliders are not allowed to march on a bridge. The reason behind this is resonace.
`(ii)` If the frequency of soliders marching on a bridge, equals the frequency of vibration of bridge, then the displacement of the bridges movement will be maximum resulting what we call as as Resonance.
`(iii)` Due to this resonance the bridge may get broken or get dameged.