Question

Soldiers are not allowed to march to cross suspension bridges. Why?

Answer 1

Structures like bridges and buildings, although they appear to be solid and immovable, have a natural frequency of vibration within them. A force that's applied to an object at the same frequency as the object's natural frequency will amplify the vibration of the object in an occurrence called mechanical resonance.
So when soldiers march on a hanging bridge, it may have a chance to make a resonance condition .So the bridge will collapse. So to avoid that, soldiers break their stride on a bridge.