An earthquake is vibration of earth surface by waves emerging from the source of disturbance in the earth by virtue of release of energy in the earth’s crust. It is essentially a sudden and transient motion or series of motions of the earth surface originating in a limited under ground motion due to disturbance of the elastic equilibrium of the earth mass and spreading from there in all directions.
Reasons for High Casualty During Earthquakes:
- Urbanization is rapidly increasing and due to increase in land cost, many multi storied buildings are being constructed.
- Code is not mandatory.
- Construction as such is governed by municipal bye-laws.
- Seismic provisions are not incorporated.
- Non enforcement of elaborated checks proper ways.
- No checks even for simple ordinary design.
General Guidelines for Earthquake Resistant Design
Drift:It is the maximum lateral displacement of the structure with respect to total height or relative inter-storey displacement. The overall drifts index is the ratio of maximum roof displacement to the height of the structure and inter-storey drift is the ratio of maximum difference of lateral displacement at top and bottom of the storey divided by the storey height. Non structural elements and structural non seismic members primarily get damaged due to drift. Higher the lateral stiffness lesser is the likely damage. The storey drift in any storey due to minimum specified design lateral force with partial safety factor of unity shall not exceed 0.004 times the storey height.
Separation between adjacent units or buildings:Two adjacent buildings or two adjacent units of the same building with separation joint in between shall be separated by distance equal to the amount R times the sum of the calculated storey displacements as specified above of each of them to avoid damaging contact when the two units deflect towards each other.
Soft storey:Soft storey or flexible storey is one in which the lateral stiffness is less than 70% of that in the storey above or less than 80% of the average lateral stiffness of the three storeys above. In case of buildings with a flexible storey such as ground storey consisting of open spaces for parking i.e. stilt buildings, special arrangements are need to be made to increase the lateral strength and stiffness of the soft storey. For such buildings, dynamic analysis is carried out including the strength and stiffness effects of infills and inelastic deformations in the members particularly those in the soft storey and members designed accordingly. Alternatively, the following design criteria are to be adopted after carrying the earthquake analysis neglecting the effect of infill walls in other storeys. When the floor levels of two similar adjacent buildings are at the same elevation levels, factor R can be taken as R/2. a) The columns and beams of the soft storey are to be designed for 2.5 times the storey shear and moments calculated under seismic loads specified. b) Besides the columns designed and detailed for calculated storey shears and moments, shear walls placed symmetrically in both directions of the building as far away from the centre of the building as feasible to be designed exclusively for 1.25 times the lateral storey shear calculated.
Foundation:The use of foundations vulnerable to significant differential settlement due to ground shaking shall be avoided for structures in seismic zones-III, IV & V. individual spread footings or pile caps shall be interconnected with ties except when individual spread footings are directly supported on rock. All ties shall be capable of carrying in tension and in compression an axial force equal to Ah/A times the larger of the column or pile cap load in addition to the otherwise computed forces where Ah is the design horizontal spectrum value.
a) vertical projections:Tanks, towers parapets, chimneys and other vertical cantilever projections attached to buildings and projecting the above roof shall be designed and checked for stability for 5 times the design horizontal seismic co-efficient Ah. In the analysis of the building, the weight of these projecting elements will be lumped with the roof weight.
b) horizontal projections:All horizontal projections like cornices and balconies shall be designed and checked for stability for 5 times the design vertical co- efficient equal to 10/3 Ah. These increased design forces either for vertical projection or horizontal projection are only for designing the projecting parts and their connection with the main structures. This means that for the design of main structure such increase need not to be considered.
Shape of the building:Very slender buildings should be avoided. Large overhangs and projections attract large earthquake forces. Heavy masses like large water tanks, etc., at the top shall be avoided. Small water tanks, if provided, should be properly connected with the framing system. Building should be sufficiently be away from steep slopes. It should be built on filled up soil. Asymmetry should be avoided as they undergo torsion and extreme corners are subjected to very large earthquake forces.
Damping:Damping is the removal of kinetic energy and potential energy from a vibrating structure and by virtue of which the amplitude of vibration diminishes steadily. Some vibrations are due to initial displacement or initial velocity. Due to damping, these vibrations decay in amplitude.
- When there is harmonic applied force and its period is nearly equal to the natural period of the structure. The vibration will grow from zero displacement and velocity. Damping limits the vibration maximum amplitude.
- More damping less is the amplitude.
- Negative damping may arise while the vibration is small, followed by positive damping at large amplitude vibrations. The code adopted for design of multistoried buildings considering seismic forces is IS 1893 (part I) – 2002. more than 60% area of India is earthquake prone. According to IS 1893 (part I) – 2002, India is divided into several zones to their magnitude of intensities.