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

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

Subject: Science and Technology

Context:

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

• The bridge that collapsed in Gujarat’s Morbi on Sunday killing at least 134 people, was a suspension bridge — a type in which the deck is hung below suspension cables on vertical suspenders.

About the Morbi Bridge-

• Also known as JhultoPul was a 230-metre-long (750 ft), 1.25-metre-wide (4.1 ft) pedestrian suspension bridge over the Machchhu River in Gujrat,, built during British rule in India in the 19th century.
•  It was inaugurated on 20 February 1879.
• The bridge is owned by Morbi municipality, which had signed a contract with a trust owned by private company Oreva— which makes digital products ranging from Ajanta clocks to battery-operated bikes — for maintenance and operations.
  • Machchhu River is a river in Gujarat, India, with its origin in the Madla hills.
  • Its basin has a maximum length of 130 km (81 mi).
  • The total catchment area of the basin is 2,515 km2 (971 sq mi).

Structure of the Suspension bridges-

• 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.
• Given that the most important load-bearing members are the main suspension cables, the entire cross-section of the main cable is the mainstay of carrying the load and ensuring that buckling does not happen.
• The buckling is subject to two preconditions: there must be no overloading and no excessive swaying.
• While beam bridges are among the simplest and oldest bridges, the reason for the enduring design of the suspension bridge is that the supporting cables running horizontally between the two far-flung anchorages provide the counterweight and effectively pass on the entire tensional force to the anchorages.
• As a result, suspension bridges can easily cross distances of well over 2,000 metres, beyond the scope of other bridge designs.
• The Morbi bridge was on the smaller side in terms of span and was pedestrian-only.

Morbi bridge collapse: What could have happened-

• The weight limit of this 19th-century bridge is not known, purported video footage from before the incident seems to suggest the bridge was swaying, possibly because of the large crowd on it.
• The sudden collapse, as seen in the videos, suggests that most or all the suspension cables were weak or corroded.
• This is possible considering that this was a very old bridge.
• Visuals from the site appear to suggest that the two towers were unaffected.
• What seems to have given way are the connections securing the vertical cables with the deck, especially on one end of the bridge.

Robustness of these bridges-

• The core design of a bridge determines how it distributes the internal forces of tension, compression, torsion, bending, and sheer.
• Suspension bridges are among the most robust structures, starting from the earliest ones made of twisted grass.
• The Golden Gate Bridge and Brooklyn Bridge in the US are examples of suspension bridges.
• India’s longest single-lane motorable suspension bridge — the 725-metre Dobra-Chanti suspension bridge built over the Tehri lake — was inaugurated in November 2020.
• Besides the suspension, bridges can be arch bridges, beam bridges, cantilever bridges, truss bridges and tied-arch bridges.