FEW REASONS OF FAILURE OF EARTHQUAKE-PROOF HOUSES

It is commonly believed that some houses could be made earthquake proof. The reality however is that by taking up precautions, the earthquake resistance of the house is increased finitely, to make them resist quakes of specific magnitudes. These houses, too, may fail once they face quakes having more intensity than their design took care of. Following are the few reasons why earthquake proof houses may fail:
Vibrations
The movement within the earth’s crust manifests itself in the form of waves that reach earth’s surface and cause vibrations in structures. The structures fail and collapse under the action of these vibrations. These vibrations may be in horizontal direction, in vertical direction or combination of both, which generally is the case. The vertical component of seismic force creates repeated changes in the weight of structures while the horizontal component induces. These forces get commanded in each cycle.
Inertia Force
Houses collapse as a result of inertial forces. Under the action of earthquake the lower portions of the structures that are in direct contact with the ground tend to vibrate more; whereas due to inertia the upper portions of the structures tend to remain static. The resultant stresses build up fast with the increased frequency of vibrations leading to failure of the structures.
Poor Quality of Material
The Lature, Gujarat, India disaster came about when huge stone masonry walls gave way under the earthquake bringing down with them the roofs that were overlaid with thick clay. The walls and roofs were heavy; the masonry was badly made with round stones. The houses were not ‘engineered’ units, but more an assemblage of materials. The mud mortar used was weak and could not provide enough cohesion to sustain the walls on that fateful night. The huge mass of construction material led to a massive disaster.
Dead Load
The magnitude of the inertial forces increase as directly as the weight of the houses. The heavier the house, heavier is the destructive force. One important consideration in making a house more earthquake resistant is to reduce its mass and making it as light as possible.