An earthquake is the vibration, sometimes violent to the earth’s surface that follows a release of energy in the earth’s crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption or even by a manmade explosion. The dislocation of the crust causes most destructive earthquakes. The crust may first bend and then the stresses exceed the strength of rocks, they break. In the process of breaking, vibrations called seismic waves are generated. These waves travel outward from the source of the earthquake along the surface and through the earth at varying speeds depending on the material through which they move. These waves can cause disasters on the earth’s surface.
No structure on the planet can be constructed 100% earthquake proof; only its resistance to earthquake can be increased. Treatment is required to be given depending on the zone in which the particular site is located. Earthquake occurred in the recent past have raised various issues and have forced us to think about the disaster management. It has become essential to think right from planning stage to completion stage of a structure to avoid failure or to minimize the loss of property. Not only this, once the earthquake has occurred and disaster has taken place; how to use the debris to construct economical houses using this waste material without affecting their structural stability.
HOW EARTHQUAKE RESISTANT CONSTRUCTION IS DIFFERENT?
Since the magnitude of a future earthquake and shaking intensity expected at a particular site cannot be estimated with a reasonable accuracy, the seismic forces are difficult to quantify for the purposes of design. Further, the actual forces that can be generated in the structure during an earthquake are very large and designing the structure to respond elastically against these forces make it too expensive.Therefore, in the earthquake resistant design post yield inelastic behavior is usually relied upon to dissipate the input seismic energy. Thus the design forces of earthquakes may be only a fraction of maximum (probable) forces generated if the structure is to remain elastic during the earthquake. For instance, the design seismic for buildings may at times be as low as one tenths of the maximum elastic seismic force. Thus, the earthquake resistant construction and design does not aim to achieve a structure that will not get damaged in a strong earthquake having low probability of occurrence; it aims to have a structure that will perform appropriately and without collapse in the event of such a shaking.
Ductility is the capacity of the structure to undergo deformation beyond yield without loosing much of its load carrying capacity. Higher is the ductility of the structure; more is the reduction possible in its design seismic force over what one gets for linear elastic response. Ensuring ductility in a structure is a major concern in a seismic construction.
The records of various earthquake failures reveal that unsymmetrical structure performs poorly during earthquake. The unsymmetrical building usually develops torsion due to seismic forces, which causes development of crack leading to collapse of a structure. Building therefore should be constructed rectangular and symmetrical in plan. If a building has to be planned in irregular or unsymmetrical shape, it should be treated as the combination of a few rectangular blocks connected with passages. It will avoid torsion and will increase resistance of building to earthquake forces.
IS code recommends that as far as possible entire building should be founded on uniform soil strata. It is basically to avoid differential settlement. In case if loads transmitted on different column and column footing varies, foundation should be designed to have uniform settlement by changing foundation size as per code conditions to have a loading intensity for uniform settlement.
Raft foundation performs better for seismic forces. If piles are driven to some depth over which a raft is constructed (raft cum pile foundation), the behaviour of foundation under seismic load will be far better. Piles will take care of differential settlement with raft and resistance of structure to earthquake forces will be very large.
Provision of band:
IS code recommends construction of concrete band at lintel level to resist earthquake. The studies revealed that building with band at lintel level and one at plinth level improves load carrying of building to earthquake tremendously. It is suggested here that if bands are plinth level, sill level, lintel level and roof level in the case of masonry structure only, the resistance of building to earthquake will increase tremendously. Band at sill level should go with vertical band and door openings to meet at lintel level. Hold fast of doors can be fitted in their sill band. In case of earthquake of very high intensity or large duration only infill wall between walls will fail minimizing casualties and sudden collapse of structure. People will get sufficient time to escape because of these bands.
Arches and domes:
Behavior of arches has been found very unsatisfactory during earthquake. However domes perform very satisfactory due to symmetrical in nature. Arches during earthquake have tendency to separate out and collapse. Mild steel ties if provided at the ends, their resistance can be increased to a considerable extent.
These are the worst affected part of any building during earthquake. Studies reveal that this is mainly due to differential displacement of connected floors. This can be avoided by providing open joints at each floor at the stairway to eliminate bracing effect.
Beam column joints:
In framed structures the monolithic beam column connections are desirable so as to accommodate reversible deformations. The maximum moments occur at beam-column junction. Therefore most of the ductility requirements should be provided at the ends. Therefore spacing of ties in column is restricted to 100mm centre and in case of beam strips and rings should be closely spaced near the joints. The spacing should be restricted to 100mm centre to centre only near the supports. In case of columns, vertical ties are provided; performance of columns to earthquake forces can be increased to a considerable extent.Steel columns for tall buildings ie buildings more than 8 storey height should be provided as their performance is better than concrete column due to ductility behavior of material.
Mortar plays an important role in masonry construction. Mortar possessing adequate strength should only be used. Studies reveal that a cement sand ratio of 1:5 or 1:6 is quite strong as well as economical also. If reinforcing bars are put after 8 to 10 bricklayers, their performance to earthquake is still better. Other studies have revealed that masonry infill should not be considered as non-structural element. It has been seen that in case of column bars are provided with joints at particular level about 600-700mm above floor level at all storey should be staggered. It may be working as a weak zone at complete floor level in that storey.