Some of the simplest physical concepts carry hidden within them potentially horrific implications. Consider the idea of resonance. Mechanical systems have natural frequencies of oscillation; for example, the strings of a guitar are each designed and adjusted to have their own preferred frequency of vibration. Normally we let these systems vibrate freely, but if we apply a force to them at a frequency matching that of one of their natural vibrational modes, the amplitude of vibration increases dramatically: we have matched the resonant frequency. The most obvious example of this is a playground swing: by pumping our legs and arms at the right time, we can build up a huge oscillation from nothing.
Suspension bridges also have their own natural modes of vibration, and this has led in the past to deadly consequences. Soldiers marching in step produce a strong driving force, and if their pace matches a resonance of the bridge, they can produce a tremendous oscillation that can bring down the structure.
One of the earliest examples of this was the 1831 collapse of the Broughton Suspension Bridge in England. On April 12 of that year, a detachment of 74 men were returning to barracks over the bridge. Marching four abreast, they noticed the bridge swaying in time with their motion; by the time the first of them had reached the other end, they heard “a sound resembling an irregular discharge of firearms.” One of the iron columns supporting the suspension chains had snapped, causing that end of the bridge to collapse, dropping 40 men into the river. None were killed, but several were severely injured.
(The Broughton Bridge rebuilt, in 1883, via Wikipedia.)
It was immediately recognized that the regular march step of the soldiers had contributed to the collapse, and soon afterwards the British Army ordered that soldiers should break step while crossing a bridge. This, however, was not enough to prevent another more disastrous collapse, this time in France.
(The Anger Bridge, before its collapse. Via Wikipedia.)
On April 16, 1850, a battalion of French soldiers were crossing the Anger Bridge during a severe thunderstorm that had already caused the bridge to sway back and forth. Though the soldiers knew to break step in crossing, the severe motion of the structure caused the soldiers to adjust their gait to stay standing: in essence, the oscillation of the bridge inadvertently forced the soldiers to match its motion. At a point when 483 soldiers and 4 others were on the bridge, an upstream anchoring cable broke with a sound like “a badly done volley from a firing squad.”
226 people died when they were cast into the river. More would have died if not for the rescue efforts of the locals and fellow soldiers.
(The Angers Bridge, after the collapse. From Wikipedia.)
It should be noted that neither the Angers Bridge nor the Broughton Bridge collapsed entirely due to the actions of the soldiers. Both bridges were determined to have maintenance and engineering problems that made collapse inevitable; the marching exacerbated existing weaknesses.
The Angers Bridge tragedy had significant implications: France abandoned the use of suspension bridges for two decades afterwards. Awareness of the possible dangers led to more rigorous inspection regimes on future construction, as well.
It is hard to imagine such a simple act as walking having such dire consequences. However, little actions, working in unison, can great powerful forces under the right — or in these cases wrong — circumstances.