because marching of troops cause small forces(legs) to superimpose into a single large force which causes vibrations and repeated marching provides the resonance frequency and thus it may cause damage to the bridge!
Harmonics. Most bridges are strong enough to support the weight of soldiers marching in lock step, but the constructive vibrations still hasten the need for repairs. Harmonic vibrations induced by the wind brought down the Tacoma Narrows Bridge, "Galloping Gertie."
Soldiers and bands break step so as not to reinforce the fundamental vibration frequency of the bridge. For an example of wind reinforceing the fundamental frequency look up the Tacoma Narrows Bridge.
the 17th of march 1973
forward march
The Sydney Harbour Bridge opened in March 1932. In March 2015, it will have been 83 years since e bridge was opened.
The possessive form for the plural noun soldiers is soldiers'.Example: The soldiers' march took them across a river.
march
march
It is unlikely for a marching army to collapse a bridge unless the bridge was already structurally compromised. The weight distribution of marching soldiers is typically spread out enough to not generate enough force to collapse a bridge.
The story is that 20 soldiers were injured when the Broughton Bridge (Manchester) collapsed on April 12, 1831. The cause was determined to be a bolt that sheared off from the effects of mechanical resonance. The impact vibrations from the uniform step of the marching soldiers was believed responsible. In other less famous situations, the same type of physical damage has occurred to other structures.
the 17th of march 1973
The Harbour Bridge was opened on March 19, 1932. Today, the bridge is 82 years old.