Impact of earthquake on buildings

AN EARTHQUAKE is said to be caused when mountain ranges are formed, when the plates on which continents float grind against each other. The quaking or shaking of earth causes a movement on the ground on which a structure is situated causing it to vibrate. The vibrations may be resolved in any three perpendicular directions.

The horizontal motions are greater by about ten times than the vertical directions and more destructive. Depending upon the depth of focus, the distance from epicentre and the strata on which the structure stands, the magnitude of earthquake and the intensity of vibration vary.

Charles Richter, a seismologist of California, devised a logarithmic scale measuring from one to ten to record the magnitude and the amount of energy released at the focus.

Enough progress has not been made in the understanding of the geological configurations. The type, place and occurrence of intensity of earthquake cannot be predicted reliably. Astrological predictions have been in vogue in many countries, however they are like speculation. The prediction is based on the Lagna at the time of occurring an eclipse if aspected by the Mars in transit when the energy stored is released.

Scientific studies began about four decades back. It was propounded that earthquakes can be forecast based on changes in the ground surface like ``dilatance,'' a process of swelling.

Work on earthquake prediction in our country is being done at the School of Research and Training in Earthquake Engineering at Roorkee and Indian Meteorological Department.

Different zones

In India, more than 50 per cent of the land area falls under active seismic zones. Scientific knowledge has not progressed to reliably predict earthquake in issuing the warning to save lives. Therefore it is necessary to construct buildings to resist seismic forces.

India has been divided into different zones for determining the seismic forces and maps showing zones liable to earthquake of severe or moderate or mild nature are available.

Some of the provisions which will help resist the seismic forces include:

The maximum height of the building should not exceed 27 metres.

A building of U or L shape is preferable.

All corners of the building should be braced and other parts should be firmly tied together so that the whole structure will tend to move as a unit.

Long walls without any cross walls, large openings or many openings in masonry walls are to be avoided as these will have less shear-resisting capacity.

If all walls of a building behave as one unit, its lateral strength will be more and will resist lateral forces caused due to induced vibrations.

In stone masonry walls, small stones are often used and the empty space in between is also filled with smaller stones. They do not have sufficient bond and bulge out during earthquake.

Steel and wood are ductile materials and with the use of such ductile materials, the structure will resist earthquake forces better.

Vertical steel mat be provided near the openings and at the corners and junctions of the walls which will help in resisting tensile forces and able to deform and bend without collapse.

Parapets and cantilever projections exceeding 75 cm are to be avoided.

Symmetry in the arrangement of cross walls will help the centre of gravity to be as low as possible.

Adjacent building should be separated by sufficient distance to prevent hammering of one another or alternatively they should be rigidly interconnected.

The sub grade with loose fine sand or clay is considered unsuitable for resisting earthquake in buildings. By providing a 0.9 metre bed of sand or gravel the amplitude transmitted is reduced due to the cushioning effect.

In case of staircase the steps should be rigidly connected on either side of the walls and provision of cantilevered staircase should be avoided.

Weak binding materials

Properly designed and well-constructed buildings will resist earthquake and withstand shock, maybe with economically justified repairable damage without any collapse. Weak binding materials like mud and mortar will cause more damage than cement and sand mortar.

Improper bonding between walls, unfilled joints between bricks, walls constructed out of plumb, structures not cured well and poorly maintained buildings are the weakness of buildings and these types of buildings will get damaged more.

Structures cannot be expected to be designed to have all necessary strength to resist severe earthquakes. However buildings should be built in safety against collapse and save human lives.

Many publications are now made available by organisations such as the International Associations for Earthquake Engineering, Indian Society of Earthquake Technology, Bureau of Indian Standards, Rajiv Gandhi Foundation, Department of Building Engineering and Management, School of Planning and Architecture, New Delhi, University of Roorkee, which include specific guidelines for earthquake resistant buildings which recommend various features to be incorporated in building design and construction and will reduce the chances of collapse due to earthquake.

In constructing buildings, priority is to be given to the aspects of safety and utility compared to other aspects of finish with extravagance.

C. H. GOPINATHA RAO

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