Friday, March 1, 2024

On Earthquakes: When It All Shakes

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By Anang Pal Malik

(The following article by Anang Pal Malik was written for for the website and posted there on March 12, 2010. The last paragraph has been edited to make it suitable for general audience here.-Ed.)
Recently, Haiti, Chile and Turkey have been devastated by the earthquakes, flattening cities and killing thousands. India saw something similar in Gujarat, Uttarkashi and Lature earthquakes-just to recount the most recent ones to have struck the subcontinent.

As they say, earthquakes do not kill people, buildings do.

The most vivid and abiding image of the Gujarat earthquake I have, is a photo published on the front page of the TOI Ahmadabad edition of the period. It showed a policeman picking a piece of concrete from the debris of a collapsed building in Ahmadabad. The picture was captioned-Policeman collecting evidence against the builders.
After the Gujarat earthquake, quite a few builders were arrested for causing death due to negligence. Negligence it was, but the piece of the concrete from the debris of the collapsed building was no evidence. But of course that is how we work.

If a building has not been designed to withstand forces due to earthquakes, it will collapse, even if the concrete quality in the building is of the highest order, rebars are more than required and concrete sections are also in excess. Because the forces an earthquake generates are in a direction in which almost no other loads act, and therefore oversafe design in the direction of other normal loads is of no use.
An earthquake generates horizontal acceleration in the form of waves travelling outward from the epicenter, much like the waves on the surface of water after a disturbance is caused on it. The wave tries to carry the foundation of the structure with it whereas the structure tries to remain where it was, thus causing all the damage and collapse if the earthquake is of higher magnitude.

The IS codes, IS 1893:1984 Criteria for Earthquake Resistant Design of Structures and IS 4326:1993 Earthquake Resistant Design and Construction of Buildings – Code of Practice on the earthquake proof design of the building are basically based upon analysis of this horizontal force for various magnitudes of the earthquake. Computer programmes are available for very tall buildings in case of which actual forces can be computed by applying the acceleration in various directions on the building and arriving at the worst possible combination. For smaller height buildings, codes give general rules regarding arrangement of the walls, detailing at the joints and enhancing ductility etc which if followed are generally sufficient as far as earthquake proofing of the building is concerned.
If the IS code is diligently followed, the building can be constructed to withstand the plausible magnitude of the earthquake. IS 1893 is currently under revision under 5 parts, each part dealing with one class of the structure e.g. Part I revised and published in 2002 deals with General provisions and Buildings. Complete list of the codes dealing with Earthquake resistant design of various types of the structures is available at the following link.

Coming back to that policeman collecting the concrete sample, with that sample he would be able to prove nothing. The fact that IS code on earthquake has been available all the while, and if it had been followed would have resulted into an earthquake proof structure, and the fact that the buildings actually collapsed during the earthquake, was the evidence enough that the IS code was not followed out of negligence.

In Indian cities, rampant unauthorised construction carries the danger that structure may not be designed to be earthquake resistant. Or later alterations may render it vulnerable to earthquakes.

At around election time, politicians also regularise thousands of illegal colonies. Because these illegal colonies are not constructed according to plans approved by the local bodies, it is highly unlikely that they are designed to be earthquake resistant. So politicians basically regularise death traps.
It costs little extra if the building is constructed in accordance with relevant IS codes. If codes are followed, we can be almost sure that no loss of life and property will occur in case of an earthquake. But rampant lawlessness in India means that people do not follow codes even when they construct buildings for themselves, making our cities and towns highly vulnerable to earthquakes.


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