SEISMIC ANALYSIS OF RCC BUILDING USING THE RESPONSE SPECTRUM METHOD

Abstract

This study presents the seismic analysis of a multi-storey reinforced cement concrete building using the response spectrum method as described in IS 1893 (Part 1):2016. A large part of India lies in seismic zones III, IV and V, and a considerable number of residential and commercial buildings in these regions are framed structures of moderate height. The behaviour of such buildings during an earthquake depends on their mass, stiffness and the way these properties are distributed over the height of the structure. The study is intended to show how a dynamic analysis procedure captures this behaviour more realistically than a simple static estimate of lateral force. A G+10 storey RCC moment-resisting frame building with a regular plan of 20 m by 16 m is modelled in ETABS for the purpose of this work. The building is assumed to be located in seismic zone III on medium soil, and it is intended for residential use. Beams, columns and slabs are proportioned using preliminary sizes that are common in practice, and M25 concrete with Fe500 reinforcement is adopted. Dead loads and imposed loads are applied in accordance with IS 875 (Part 1) and IS 875 (Part 2), and the floor slabs are modelled as rigid diaphragms so that the lateral forces are shared between frames according to their stiffness. Modal analysis is carried out first to obtain the natural periods, mode shapes and modal mass participation of the building. The design acceleration spectrum for medium soil is then applied in both principal horizontal directions, and the modal responses are combined using the Complete Quadratic Combination method. The base shear obtained from the response spectrum analysis is compared with the base shear from the equivalent static method, and the dynamic results are scaled up where required so that they are not less than the static value, as the code demands. The results are presented in terms of storey displacement, inter-storey drift, storey shear, overturning moment, and the bending moments and axial forces in critical members. The maximum inter-storey drift ratio is checked against the limit of 0.004 times the storey height specified in IS 1893. Beams and columns are then designed according to IS 456:2000 using the load combinations prescribed for earthquake loading, and the reinforcement obtained is checked for ductile detailing requirements of IS 13920:2016. The study finds that the response spectrum method gives a more rational distribution of lateral forces over the height of the building, particularly in the upper storeys where the contribution of higher modes becomes noticeable. The storey drifts obtained remain within the permissible limit for the adopted member sizes, and the column design is found to be governed mainly by the earthquake load combinations. The work demonstrates that dynamic analysis using commercial software is a practical tool for designers, and it can be extended in future to irregular buildings, soft storey configurations and time history analysis.