A suspension bridge on Machchhu river in Gujarat’s Morbi town collapsed on October 30, 2022, leaving more than 130 people dead. The 230-meter bridge, built during British rule in the 19^th century was touted by the state’s tourism website as an artistic and technological marvel. It had been closed for renovation for six months and was reopened for the public a week before the tragedy.

As per media reports, about 200 people were estimated to have been on the bridge when it collapsed. A 36-second video clip shared by the Morbi District Administration shows a large crowd of young men shaking the bridge from side to side few moments before it collapsed.

Did the bridge collapse due to overcrowding or overloading?

Such a possibility is remote!  Most bridges collapses world wide occurred while the bridges were under construction. Then what led to the collapse of this bridge?

Let us examine this tragedy in the light of similar ones.

In 1831, when a brigade of soldiers marched in step across England’s Broughton Suspension Bridge, a similar incident occurred. The marching steps of the soldiers happened to resonate with the natural frequency and the bridge broke apart, throwing dozens of men into the water. After this, the British Army issued orders that soldiers while crossing a suspension bridge must ‘break step‘ and not march in unison.

When soldiers march in unison across a suspension bridge, they apply a vibration at the frequency of their steps. If their frequency is closely match the bridge’s frequency, soldiers’ rhythmic marching will amplify the natural frequency of the bridge. If the mechanical resonance is strong enough, the bridge can vibrate until it collapses due to the movement.

Here is a simple experiment to understand mechanical resonance. Tie three pendulums of different lengths and two of the same length (B & D) to a rubber hose. Now swing one of the two pendulums of equal lengths and after a few minutes, all the other pendulums will begin to swing with the other pendulum of equal length swinging as much as the other. This is due to result of resonance as the frequency of the two pendulums with equal lengths are same and hence they resonate.

On October 04, 2022, at least 30 students were injured in an unfortunate incident after a hanging bridge fell in Assam’s Karimganj district. The horrific incident took place when 100 girls and boys of Cheragi Vidyapith High School were crossing it with all of them falling into the river.

In this case the students could have the tendency to fall in step due to drill classes and morning assembly at school. While walking, children tend to flock together and fall in step not to step on the neighbour’s foot.

In May 1999, two girls were drowned and 15 others injured when a suspension bridge across a river collapsed in Panathur, Kasargod in Kerala. The incident occurred when a group of people taking part in a funeral procession entered the suspension bridge, the bridge tilted and collapsed – again due to mechanical resonance l.

In a similar incident in February 2014, eight people died and more than 30 were injured when a suspension bridge collapsed over a dry stream in the North-Western province of Lai Chau in Vietnam. The accident happened as a group of residents walked across the bridge to bring the coffin of a local official to a graveyard.

What could have triggered the mechanical resonance in these two cases? The villagers participating in the two funerals were surely never drilled down by any Sergeant Majors. 

I believe that anyone while on a funeral procession walks slowly and is often accompanied by the drums or hymns being sung at a melancholic pace. The funeral participants tend to bunch together, mainly due to their sadness. These factors could have forced the funeral participants to march in step, without their knowledge. Another reason of marching in step could be that one does not want to step on another’s foot and the best way to avoid is to walk in step with the person in front. In both the cases, the coffin was carried by the coffin bearers with their hands. This needed the coffin bearers to walk in step.

Mechanical resonance may have been a contributing factor in the recent Morbi bridge collapse. Only a detailed investigation can bring out the actual cause.

You must have heard of singers breaking glass with their voice. They produce a frequency that resonates with the frequency of the glass. Glass wine goblets are especially resonant because of their hollow tubular shape, which is why they make a pleasant ringing sound when clinked. When the singer’s frequency resonates with glass’ natural frequency, it causes the glass to vibrate in resonance and if she sings loud enough, the glass will vibrate itself to smithereens.

The good old soldiers’ orders about crossing a suspension bridge must come into force immediately to save lives in future.