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Seismic steps to safer structures

The Sunday shocker that has shaken the confidence of high-rise residents underscores the need for prospective flat buyers to ensure that the structure they want to invest in is reasonably earthquake-proof.

Realtors are supposed to follow the Bureau of Indian Standards (BIS) guidelines, which take into account a zone’s vulnerability to quakes, while preparing the design of a structure.

All buildings in Calcutta must comply with BIS guidelines prepared for Seismic Zone 3 (moderate risk), to which the city belongs. The guidelines are based on “extensive research” on how a structure should be built keeping in mind the historical seismic data available for the zone.

Nothing should happen to a structure complying with the guidelines if the tremors are within the limit derived from the historical seismic data.

But how can a prospective buyer be sure that a structure has been built following the guidelines? Under the Calcutta Municipal Corporation rules, a building plan can be sanctioned only if it is vetted by an empanelled structural engineer.

“A prospective buyer must demand a copy of the structural engineer’s undertaking from the promoter,” said a source in the realty trade. “The buyer must also ask the developer whether the design has been ‘tested’ under different conditions.”

“Calcuttans must be on their guard,” said Keya Mitra, a professor at Besu’s architecture department. “There is no room for complacency just because the city is in Zone 3. Calcutta is very much vulnerable to quakes.”

To drive home the point, she cited Ahmedabad’s fate in the 2001 earthquake. “Ahmedabad is in the same zone as Calcutta but it still suffered widespread devastation. More than 20,000 people died as countless RC (reinforced concrete)-framed structures crumbled,” Mitra said.

The Gujarat quake measured more than 7 on the Richter scale and was epicentred around 240 km from Ahmedabad.

Mitra also called for ending the unsafe practice of keeping the ground floors of high-rises open for parking. “Across the world such buildings are known to be more prone to quake damage. Heavy overhangs — upper storeys jutting out — are another source of danger for buildings.”

Are the builders of “modern” high-rises in the city aware of the city’s vulnerability? Rahul Saraf, the owner of Forum Projects that is building a “structural marvel” called Atmosphere on the Bypass, said his company had put the design under various simulated tremors to ascertain its stability.

“We approached a consultant who put the design under a cocktail of earthquakes through computer simulation to know if the structure can withstand seismic disturbances,” Saraf said. But such simulated tests are expensive and not available locally.

Some promoters vouch for the Universal Building Code of 1997, which they claim is followed globally and is more stringent than the BIS rules. Sushil Mohta, the director of South City and owner of Merlin Group, said the four towers on Prince Anwar Shah Road have been built following the “universal code”.

Harsh Neotia of Ambuja Realty, whose Siliguri project has been affected by Sunday’s quake, said time has come “to put our mind to design, practise and execute drills for earthquake. We are planning to do so soon”.

Adherence to one safety code or the other, however, can’t be a guarantee against destruction. “The guidelines are based on historical data. Is there any guarantee that the next tremor won’t cross that?” asked structural engineer Hirak Roy.

BIS norms for quake-proof building

Structural configuration: Size, shape and structural system are so designed that the load is properly spread out and collapse is prevented

Lateral strength: The strength should be such that maximum lateral (horizontal) force created during a quake can be resisted

Stiffness: The structure must be stiff enough to counter lateral load generated during a quake

Ductility: The structure should resist large-scale deformations even during a severe quake

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