Calcutta, March 4: A launch service that was never launched in Kestopur canal denied an extra leg to the flyover deck that collapsed yesterday, multiple accounts suggest.
A pier or pillar in the middle of the 40-metre deck slab could have made the structure more stable but the final design did not include such a support following objections from the irrigation department, officials of Mackintosh Burn and others said today.
Several engineers of Burn and the designer firm — Consulting Engineering Services (CES) — said that an additional pier was among the initial design options as it would have distributed the load of vehicles evenly.
“The final design, however, did not include the additional pier as a senior irrigation department official present at initial consultative meetings on bridge design raised objections. The additional pier would have come on Kestopur canal, running parallel to VIP Road. We were asked to construct the piers 6 metres away from the Kestopur canal,” said a Burn engineer.
“Several Hidco officials also echoed the irrigation official’s views, saying they had plans to run a launch service along the canal and a vertical column would come in the way,” the engineer added.
The cause of yesterday’s collapse — or what tore apart at least two bearings, which act as shock absorbers — has not yet been established. It cannot be said with certainty that an additional column could have prevented the fall of the arched stretch.
But engineers said that the omission of a pier meant that the curved structure, around 22 feet from the ground, had to rest on two piers, nearly 40 meters apart.
According to a Burn engineer, a 40-metre linear deck slab can comfortably rest on two vertical pillars at its two ends. “But the stability of a curved deck requires reinforcement in the middle. In this case, we had to sacrifice that as there was no option,” he added.
Curved sections of flyovers are tricky not just for speeding vehicles but also for structural engineers tasked with building them, according to civil engineers who say land constraints sometimes make curved sections unavoidable.
“We prefer to have flyovers and bridges as less curvy as possible,” said Nilanjan Mitra, an assistant professor of civil engineering at the Indian Institute of Technology, Kharagpur, who specialises in concrete structures and computational mechanics.
Mitra said the challenges that curved sections of bridges or flyovers pose are evident even with children’s building blocks. “Place a heavy-enough load on one side of a curved segment, and it flips over. You don’t see that when you place a load on a linear segment,” he said.
Mackintosh Burn has built flyovers in Lake Gardens, Kidderpore, Durgapur and elsewhere. But the company took up a new technical challenge in eastern India when it attempted to place a 40-metre curved deck slab on two pillars at either end. The gap between the two pillars below the curved slab was unusual.
The option of somewhat straightening out the curve —by shifting the flyover closer to VIP Road — was closed because of multiple reasons.
“The approach from the airport side has the canal on the left side while VIP Road is on the right. Adjacent to the road is a 30kV CESC cable line…. On the other side of the bridge, we had to avoid the Ultadanga slums,” said Sambhu Banerjee, general manager, Mackintosh Burn.
Asked about the additional pier in the canal, Subhas Naskar, irrigation minister in the then Left government and RSP MLA, said: “I was the minister of the department between 2006 and 2011. This never came up…. I was never asked for my opinion or a decision on this.”
However, a senior government official said the proposal for the pier was rejected during Naskar’s time following several communications. Work was stalled for months in early 2009.
“It was Naskar’s department that turned it down. That department does this whenever a water body is involved. Around 2009, the idea of the pier was dropped because of the irrigation department’s objections,” the official recalled.
An engineer said the constraints had forced the builder and the designing company to barely meet the Indian Roads Congress’s (IRC) specification on the minimum radius for an arched deck slab.
“According to IRC guidelines, the minimum radius for an arched deck slab should be 60 metres as any length shorter than this results in sharp turns, making it difficult for motorists. Normally, we try to keep a longer radius to allow a smoother curve. But in this case, we had to settle for 60.8 metres (just above the minimum requirement) because of space constraints,” said a CES official.
The engineering load experienced by a curved section is slightly different from the load experienced by a linear section. The only solution to this, according to IIT’s Mitra and others, is additional safety factors, or what engineers call “redundancies”.