Tuesday, 27 January 2015

How to survive earthquakes the smart way

Since the maximum casualties happen when buildings collapse, certified quake-proof constructions are the need of the hour.

Earthquakes are admittedly the most disastrous of all the natural calamities, bringing about large-scale destruction in a matter of a few seconds. In recent years, Latur and Osmanabad districts of Maharashtra witnessed an earthquake of magnitude 6.2. Striking on September 30, 1993 in the early morning hours, the tremblor razed to the ground several villages like Killari, Sastur and Rajegaon, leading to the loss of over 10,000 lives. The reason was that the houses were built with boulders brought from t he villagers’ fields and stacked one upon another with only mud in between to act as a binder.

Republic Day in 2001 in the town of Bhuj in Kutch saw a very strong quake of magnitude 8.0 wreaking havoc in many villages like Anjar. An estimated 25,000 people perished in the disaster. Here again only blocks of sandstones were used in construction with mud to act as mortar, though Bhuj was known to be highly seismic.

In between, Jabalpur in Madhya Pradesh on the banks of the Narmada, was hit by an earthquake on May 22, 1997, very similar to the Latur event, but the casualty figure was only 59 due to the proper construction of the buildings, part of the city being an army cantonment. It is clear, therefore, that only appropriate construction of houses will minimize the death toll because collapsing parts of buildings are in fact responsible for the casualties, people running out in panic getting caught below the falling blocks. If the structures stay put where they have been erected, no damage will occur.

Quake-resistant dwellings:
How then to make our dwellings resistant to earthquakes? It is nothing impossible and in fact techniques of earthquake-resistant construction were known even in the 1930s in undivided India. The Railway Bungalow on the Lytton Road in Quetta (now in Pakistan) survived the Quetta Earthquake of 1935 since it was built using such techniques as reported by the late Dr W D West, a renowned seismic geologist of the Geological Survey of India. An ordinary building nearby collapsed completely.

Earthquakes occur when pressures that have been accumulating in the rocks of the earth’s crust over a long period get suddenly released when their ultimate bearing strengths are exceeded and they cannot withstand the stresses any longer. The rocks then fail by rupturing, releasing the pent-up energy and that is when the earth literally quakes, sending different kinds of vibrations or waves. Those which pass through the surface at a great speed tend to topple the buildings which are not built to withstand the enormous force. The release of the built-up pressure is so instantaneous that destruction is brought about within ten or fifteen seconds. It is this suddenness that makes the earthquakes the deadliest of all the disasters.

Methods of construction:
Several methods of quakeproof construction of all types of structures are readily available today in the Standard Specifications of Building Codes formulated by experts and carried in government publications. These techniques of ‘aseismic designing’ as they are called, introduce a high degree of flexibility in the buildings and make them capable of withstanding the preassures of earthquakes.

Seismic structures do not break or crack in the middle. The buildings are made to behave like a ‘monolith’ — a single entity — with all the storeys acting together and putting up a united front. In the simplest design, this is accomplished by literally tying up the roofs, walls and floors of the different storeys using thin steel liner plates, about 5 cm wide and a centimeter thick in the form of diagonal cross-bracing during construction. Needless to mention that high quality cement concrete with proper reinforcements have to be used.

Other precautions:
The other precautions that need to be taken are:
  1. Avoiding differential loading of the buildings in the various floors. This is important because each structure is designed for a certain ‘dead’ load and ‘live’ load and if these are disturbed by adding huge ornamental features or installation of heavy machinery the response of the buildings to the earthquake vibrations will be drastically affected. Even the balconies need to be kept clutter free. In other words, they should never be used as dumping grounds.
  2. Buildings have to be imparted/reinforced with natural periods of oscillation (that are quite different from the oscillation periods of earthquakes) and this can be achieved through carefully planned constructions. Both the periods of oscillation should not coincide because it has been established that if they do then a total collapse is eminent. The natural periods of the earthquakes can be known from the seismograms and building vibrators help us to understand the likely natural periods with which the buildings will oscillate. Finally, it will be safe to avoid constructing skyscrapers in seismic zones because their stability cannot be guaranteed even with the best of earthquake-resistant designs. This has been proved through research.

No comments:

Post a Comment