The danger of always marching in lockstep

If you’ve ever been part of an organization that marches — not just the military but even marching bands and re-enactors (sort of) — you know what it means to march in lockstep.  Everyone steps in precision to a standard cadence (usually 120 steps/min, 28 in/step) and one person takes a step with his left leg, so do the other members of the group. We practice this constantly and we are always striving for perfect step.  EXCEPT in one instance – crossing a bridge.  Why not on a bridge? To understand this, we have to look to a couple of marching disasters from the 19^th Century.  Yes, people got hurt marching in the 1800’s.  

Let’s start with the

Albert Bridge, which crosses the Thames in London.  On 12 April 1831, the 60th Rifle Corps, a detachment of 74 men were returning to their barracks in Salford by way of the Albert Bridge.  As the detachment crossed the bridge, the soldiers, who were marching four abreast, felt the bridge begin to vibrate in time with their footsteps. Finding the vibration amusing, some of them started to whistle a marching tune and stamp out their footfalls causing the bridge to vibrate even more. The head of the column had almost reached the Pendleton side when they heard “a sound resembling an irregular discharge of firearms”.  One of the iron columns supporting the suspension chains on the Broughton side of the river fell towards the bridge, carrying with it a large stone from the pier to which it had been bolted. The corner of the bridge, no longer supported, then fell 16 or 18 feet into the river, throwing about forty of the soldiers into the water or against the chains. None of the men were killed, but twenty were injured, including six who suffered severe injuries including broken arms and legs, severe bruising, and contusions to the head.

A similar event happened in France on the Basse-Chaîne Bridge over the Maine River in Angers, France on 16 April 1850.  Soldiers frequently used the bridge but when one battalion crossed during a powerful thunderstorm their bodies acted as sails, further catching the wind. Survivors reported that they had been walking as if drunk and could barely keep themselves from falling, first to one side and then to the other. As usual in crossing that bridge, the soldiers had been ordered to break step and to space themselves farther apart than normal. However, their efforts to match the swaying and keep their balance may have caused them to involuntarily march with the same cadence, causing a resonance. As the oscillation increased, the upstream anchoring cable on the right bank broke in its concrete mooring with a noise like “a badly done volley from a firing squad”. The adjacent downstream cable broke a second later, and the right-bank end of the deck fell, making the deck slope very steeply and throwing soldiers into the river. Many of those who fell were saved by their fellow soldiers who had not yet crossed and by residents of Angers who came to the rescue, but a total of 226 people died.

So, ALWAYS break step on a bridge.  This is not some weird antipathy toward your sergeant who will be screaming all the while.  Its physics. Structures like bridges and buildings, although they appear to be solid and immovable, have a natural frequency of vibration within them.” That’s usually not a problem but when something comes along that adds more vibrations at the same frequency, like a column of marching soldiers, mechanical resonance kicks in. That “standing wave” then applies repeated force to the anchoring structures, sort of like hammer blows from a bridge sized hammer, until those structures fail.  While these failures are rare, they are also typically quite dramatic.