SUSPENSION BRIDGES

1.Context

The bridge that collapsed in Morbi, Gujarat, was built using old, robust technology. But overloading and excessive swaying present risks to such structures.

2.About Bridge

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. Emails sent to Oreva and district administration on the specific issue of changes made to the bridge structure during the repair works went unanswered.

3. Structure of Bridge

1. 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.
2. 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.
3. 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.

4. Reasons for Collapse

 4.1.Archaic bridge-The 19th century Bridge, which had been reopened days previously after repairs spanning six months, reportedly had over 400 people on it when it collapsed.  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.

 5.1.Overcrowding- though we do not know to what extent the capacity was exceeded. Unlike aerodynamic instability caused by strong winds, which is now very well understood and is almost eliminated from modern suspension bridges, the impact of the behavior of pedestrians is not very well understood or modeled. It is, however, known that the impacts are nonlinear.

6.1.Vertical cables -

• The twin suspension cables arising from the towers, which ran along the length of the bridge and were connected to the vertical cables holding up the road deck, also seem to be in place. What seems to have given way are the connections securing the vertical cables with the deck, especially on one end of the bridge.
• 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
•  Aluminum deck-during the recent repairs, the original wooden deck had possibly been replaced with an aluminum deck, which could have involved rewiring the mechanism connecting the vertical cables with the deck, thereby likely altering the basic structure of the bridge that had endured for well over a century.

5.Role of vertical support

 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.

5.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. When Spanish conquistadors made their way into Peru in 1532, they discovered an Incan empire connected by hundreds of suspension bridges spanning deep mountain gorges

6.Examples of Suspension Bridge

7.Types of Bridges

 Bridges can be arch bridges, beam bridges, cantilever bridges, truss bridges, and tied-arch bridges.

 While beam bridges are 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 meters, beyond the scope of other bridge designs. The Morbi Bridge was on the smaller side in terms of span and was pedestrian-only.