#490 Why do soldiers break stride on a bridge?

Why do soldiers break stride on a bridge? So that the vibrations don’t make the bridge collapse.

This rule was introduced by the British Army in 1831. On 12^th April 1831, the 60^th Rifle Corps were returning from a military exercise and they crossed over the Broughton Suspension Bridge, which had been built five years before. 74 men were marching across the bridge and they noticed that it began to vibrate in time with their marching. They thought it was funny and started to time their marching to the bridge’s vibration. They had almost crossed the bridge when it collapsed, throwing the men into the river. Nobody was killed, but there were a lot of broken bones. Following this, the British Army passed the rule that soldiers had to break stride when they crossed a bridge. A later investigation showed that the bridge had been poorly made and was an accident waiting to happen, but the soldiers hastened its demise. However, the fact that they caused that bridge to collapse made the army change their rules and possibly saved other soldiers.

So, why? What happened to that bridge? The reason the bridge collapsed is due to something called mechanical resonance. This is also the reason why an opera singer can make a glass shatter just by singing at it.

Everything in the world around us looks like it is perfectly still. For example, the phone or the computer you are reading this on don’t look like they are moving, but they actually are. The molecules in the glass of your screen, or the molecules in the plastic or the metal of the case, are all vibrating on a molecular level. The only way you can stop them vibrating is to remove all of the energy from them, and the only way to do that is to chill them to absolute zero, which is impossible. This vibration is the natural frequency of something. All atoms vibrate at a slightly different frequency because they have a slightly different amount of energy.

Resonance is where you add energy to a system, but you time it just right so that each time you add energy to the system it replaces the energy that the system would naturally lose and adds to the energy already in the system. An excellent example of this is a swing on a playground. Imagine your daughter has jumped on the swing and wants you to push her. Once you get the swing moving you time your pushes so that the energy you add to the swing increases its energy just at the point where the swing would naturally start to lose energy. If you push too soon or too late, the swing will lose power. The swing is moving at one frequency and when you add energy at that exact frequency, the sum of the energy increases.

This is what happened to the bridge. The bridge has a natural frequency. The soldiers marching across the bridge were, by pure chance, marching at the exact same frequency. The energy they were putting into the system was at the perfect frequency to increase the energy that was already in the bridge until the energy increased to such a level that the bridge shook itself to pieces. No manmade object, apart from perhaps a block of solid steel, is completely structurally perfect. Everything we make has weak points, be they joints, welds, or nuts and bolts. The vibrations in the bridge increased until one of these weak points gave way and the bridge fell down.

You can make a glass shatter with the same principle. Every solid has an elastic limit. This is the point where enough stress has been applied to the object that it cannot return to its original shape. Some objects, rubber for example, have a very high elastic limit and some objects, glass for example, have a much lower one. Every glass has a natural frequency, which is the amount the atoms are vibrating at rest. You can find this frequency by flicking the glass. The ringing sound it will make is the frequency. If you can sing a loud enough note at this frequency, you can make the air molecules around the glass vibrate at exactly the same frequency. At the point where the glass molecules are about to lose energy, the air molecules put more energy into the system and they vibrate more. At some point, the elastic limit is exceeded and the glass shatters. This is resonance. And this is what I learned today.

Sources

https://www.warhistoryonline.com/war-articles/broughton-suspension-bridge-collapse.html?chrome=1

https://www.wearethemighty.com/military-life/soldiers-marching-over-bridges-myth/