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Why Do Soldiers Break Their Stride When Crossing A Bridge?

Soldiers always march in unison. According to studies, synchronized movement give soldiers a certain amount of confidence. But did you know armies need to break their stride when they are about to cross a bridge?

A suspension bridge collapsed in 1831 due to the unison marching of a group of soldiers. Since then the British Army would break their stride when they are about to cross bridges.

Why Do Soldiers March in Unison?

Militaries take pride in presenting their confidence and discipline through their synchronized parades. This is practiced all over the globe, and it has always been immensely satisfying to watch. But why is this practiced in the first place?

Researches show that when soldiers march in unison, it intimidates their enemies and gives them a confidence boost. In this study, soldiers who were asked to walk in unison and carry out the task well would often judge those who didn’t.

If you know the other party in your assessment is weaker than your party, if there is a conflict of interest or a provocation, you’re much more likely to agress against them.

Daniel Fessler

According to Fessler, humans find synchronicity satisfying and rewarding. Whether performing rituals or doing the wave at football games, this behavior is also observed in animals. A perfect example of this is when dolphin groups jump and twirl together. This is how they win conflicts with other groups who are not in unison. (Source: Live Science)

What Happened at the Broughton Suspension Bridge?

In April 1831, the Broughton Suspension Bridge in England collapsed after a brigade of soldiers marched stepped across it in unison. According to the onlookers, the bridge just broke apart, and several soldiers fell into the water.

After this incident happened, the British Army released a new protocol when it came to crossing long bridges. Soldiers would need to break their stride and not march in unison to prevent accidents like that from happening. (Source: Live Science)

Why Did the Incident Occur?

When soldiers march in unison across a particular structure, force is applied upon their step frequency. This force is matched with the bridge’s frequency. The soldier’s synchronic marching amplifies the vibration, and when the mechanical resonance is strong enough, the bridge may collapse.

In general, structures like buildings and bridges seem immovable. But the fact of the matter is, natural frequency within these structures will amplify vibration. (Source: Live Science)

Did A Similar Incident Happen Again?

In the year 2000, when the Millennium Bridge in London opened to the public, several people packed onto the bridge in a matter of moments. There was a slight vibration that unfortunately knocked off people from the bridge itself. This definitely alarmed everyone involved in the unveiling.

Many pedestrians fell spontaneously into step with the bridge’s vibration, inadverently amplifying them.

Elizabeth Howell

The engineers who built the bridge insist that the Millennium Bridge was never in danger of collapsing. But it was closed for several years while energy-dissipating dampers were installed to minimize the vibration. (Source: Live Science)

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