A century-old bridge over the Machhu river collapsed in Morbi, Gujarat, killing 134 people.
• CCTV footage shows some people trying to move the suspension bridge from side to side just before the Morbi bridge in Gujarat collapsed
New Delhi: What is a Suspension Bridge? How does it transfer the load to the ground? These were the questions in the minds of many on Monday, a day after a century-old suspension bridge on the Machhu river in Gujarat's Morbi collapsed, in which 134 people died.
Just before the bridge collapsed, CCTV footage showed some people trying to move the suspension bridge from side to side.
Saptdeep Sarkar, a structural engineer working with Delhi-based Engineers India Limited, answers these questions.
"The deck of the suspension bridge, which bears the weight of the people moving on it in addition to its own weight, is by means of vertical suspensions that are made of ductile materials and subjected to tension," he said.
"These suspenders are then attached to the main cable which is also tensile and subjected to tension.
"The main cable, which extends between points connected to the bridge and is supported intermediately by vertical main column piers, has two functions. The first is to support the vertical suspension and the second is to maintain the verticality of the piers." Told.
Overall, the weight of the deck, including people's traffic, is transferred to the ground through vertical piers, which are in compression, the government explained.
So, at what point is this type of bridge most vulnerable? Quite a lot, it turns out.
"To begin with, the main cables, suspenders and the points where the suspenders connect to the bridge deck are all vulnerable to failure," the government said.
“In fact, the main cable requires the most attention in terms of regular checks and maintenance during the service period of the bridge. This is because it is subjected to constant fatigue loads during its service,” he said.
Fatigue load refers to continuously subjecting a material to some force until it develops a crack.
Referring to the suspension bridge at Morbi, the government said its cables are of highly ductile material and it should have lasted longer despite the deliberate deflection by those seen in the video (CCTV footage).
"Furthermore, the bridge that failed was recently renovated, so fatigue failure can be safely ruled out as a possible cause," the government said.
Then what was it that gave way?
The structural engineer said the connection points where the vertical suspension meets the bridge's deck are the next in line to pay attention to.
"It is through these points that the load of the deck and people is simultaneously transferred to the suspenders. The suspenders, in turn, transfer the load to the main cable," he elaborated.
"As I understand it, the dynamic loading created by the synchronous and deliberate movements of people on the bridge caused excessive deflection from the suspension joint to the approach of the bridge deck," he said.
On transmission of forces on the ground, the government further stated that the main cables are in slope and hence the tension in the cable at the pier junction has a horizontal and a vertical component.
"While the horizontal component of this force is accomplished through tension by cables attached at the two ends, the vertical component creates a compression in the pier," he said.
"Failure occurs when structural integrity is compromised at any of these points described," he concluded.
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