An earthquake is the vibration of the Earth's surface caused by waves emanating from a disturbance in the Earth's crust, resulting in the release of energy. It manifests as a sudden and transient motion or series of motions of the Earth's surface, originating from underground disturbances that disrupt the elastic equilibrium of the Earth's mass, spreading in all directions.
Reasons for High Casualty During Earthquakes:
Urbanization has rapidly increased, leading to the construction of many multi-story buildings due to escalating land costs. However, adherence to seismic building codes is often lacking. Municipal bye-laws govern construction, but seismic provisions are not consistently incorporated. Non-enforcement of thorough checks exacerbates vulnerabilities, with even simple, ordinary designs escaping scrutiny.
General Guidelines for Earthquake Resistant Design
Drift:
Drift refers to the maximum lateral displacement of a structure concerning its total height or relative inter-story displacement. Non-structural elements and non-seismic structural members are prone to damage due to drift. Higher lateral stiffness reduces the likelihood of damage. The maximum storey drift under specified design lateral forces should not exceed 0.004 times the storey height.
Separation between adjacent units or buildings:
Buildings or units with separation joints between them should be sufficiently distanced to prevent damaging contact during deflection towards each other. This distance should be equal to a specified factor times the sum of calculated storey displacements of each unit.
Soft storey:
A soft storey has less lateral stiffness compared to the storey above. Special measures are necessary to enhance the lateral strength and stiffness of soft storeys, especially in structures like stilt buildings. Dynamic analysis, including the effects of infills and inelastic deformations, is recommended for such buildings.
Foundation:
Structures in seismic zones should avoid foundations vulnerable to significant differential settlement. Interconnected ties are necessary for spread footings or pile caps, capable of carrying additional axial forces due to ground shaking.
Projections:
Vertical and horizontal projections such as tanks, towers, parapets, chimneys, and balconies must be designed and checked for stability against seismic forces. Overhangs and heavy masses at the top of buildings should be avoided.
Shape of the building:
Very slender buildings and asymmetrical shapes should be avoided to minimize torsion and extreme corner forces during earthquakes. Buildings should be situated away from steep slopes and built on filled-up soil.
Damping:
Damping, the dissipation of kinetic and potential energy from vibrating structures, is crucial for controlling amplitude. Proper damping reduces vibration amplitudes, ensuring structural stability. The Indian standard IS 1893 (Part I) – 2002 outlines seismic design considerations, with more than 60% of India's area classified as earthquake-prone.
Adhering to comprehensive seismic design guidelines is imperative for mitigating earthquake risks in urban environments. Proper construction techniques, incorporating seismic provisions, rigorous checks, and adherence to building codes, are essential for ensuring structural resilience and minimizing casualties during seismic events.