Explained: What causes a bridge collapse?

In the last few years, the world has witnessed several such incidents. While sometimes, these led to the death of workers, at times, the collapse ended up killing hundreds of civilians.    

Hyderabad: At least 141 people died when a pedestrian suspension bridge collapsed in Gujarat on Sunday. The 230m (754ft) bridge, which was built during the British rule in the 19^th century, had been reopened just a week ago after repairs.

In the last few years, the world has witnessed several such incidents. While sometimes, these led to the death of workers, at times, the collapse ended up killing hundreds of civilians.

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But what causes a bridge collapse? A bridge, though can be designed to support vast amounts of weight, the vibrations associated with them must be controlled. While bridges appear to be solid structures, they are exposed to the effects of vibration.

Most people jump or sway on a pedestrian bridge to make it bounce. In case the force applied to the bridge is at a frequency which matches the bridge’s natural frequency, the vibration within the bridge will be amplified in a phenomenon called mechanical resonance.

In a situation where the mechanical resonance is strong enough, the vibrations can cause a bridge to collapse from the steady movement.

In most cases, the resonance frequency of the bridge lowers as the span increases and also due to large displacement amplitude vibrations. Thus, it is important to understand the resonant frequency of the bridge.

Similar collapses around the world 

• In the year 1940, Tacoma Narrows Bridge, the third biggest suspension bridge in the world, collapsed when wind induced vortex shedding coincided with the bridge’s own natural frequency.
• The Millennium Bridge across the River Thames in London provides another example. It collapsed on its opening itself as thousands of people walking caused the bridge to vibrate, following which the pedestrians fell into step with these vibrations, amplifying them and causing oscillations resulting in the bridge to sway from side to side.

According to Xi engineers, vibration can also be used to assess the performance and condition of a bridge. Changes in vibration frequency can inform when the condition of cables, hangers, and decks have detreated or changed.

Meanwhile, earthquakes, flooding, and high winds can all contribute to bridge collapses.