Why do soldiers break step in marching over a bridge?
To avoid stressing the bridge excessively… if they march in step, there’s a chance that their steps will coincide with the resonant frequency of the bridge and cause possibly dangerous amplified shaking of the whole structure. EDIT = = = = = = = = = = = On a related, note, here’s the famous collapse of the Tacoma Narrows bridge due to resonance caused by wind… it’s commonly cited as one of the great engineering disasters of modern times, in which the designers failed to consider resonance as a potential source of failure.