How suspension bridges work, and what could have happened in Gujarat’s Morbi

Source: The post is based on the article “How suspension bridges work, and what could have happened in Gujarat’s Morbi” published in Indian Express on 1st November 2022.

What is the News?

The bridge that collapsed in Gujarat’s Morbi has killed at least 134 people.This bridge was a suspension bridge.

What is Suspension Bridge?

The suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders.

The basic structural components of a suspension bridge system include stiffening girders, two or more main suspension cables, and towers and anchorages for cables at either end of the bridge. 

The main cables are suspended between the towers and are connected to the anchorage or the bridge itself. The vertical suspenders carry the weight of the deck and the commuter load on it.

The design ensures that the load on the suspension cables is transferred to the towers at the two ends, which transfer them further by vertical compression to the ground by way of the anchorage cables.

All of this balancing has to happen within the permissible weight restrictions for the bridge, given that the deck is hanging in the air, supported by the two sets of cables.

The suspension cables are the mainstay of carrying the load and ensuring that buckling does not happen. But this is subject to two preconditions: there must be no overloading and no excessive swaying.

What are the possible reasons for the collapse of Morbi Bridge?

Weak Structure: One or two suspension cables of the bridge give way and the bridge breaks and hangs before the rest of the structure collapses. It is a slower process. But the sudden collapse, in this case, suggests that most or all the suspension cables were weak or corroded.

Overcrowding: Overcrowding or very excited crowds jumping on the bridge can have a disproportionate impact on the bridge.

Vertical Cables snapped: The job of the vertical cables in a suspension bridge is to transfer the weight of the deck, by tension, to the twin suspension cables that run horizontally between the two anchorages on either end, which, in turn, transfer the tension to the towers and, through them, to the ground by way of cables whose ends are anchored.

– In Morbi, the vertical cables seem to have snapped entirely from the deck at one end of the bridge, sending part of the unsuspended deck and those on it plunging into the river.