SEISMIC PERFORMANCE OF REGULAR AND IRREGULAR RCC BUILDINGS WITH EFFECT OF DIFFERENT SEISMIC ZONES ON RCC BUILDINGS

Abstract

Reinforced cement concrete framed buildings form the major part of residential and commercial construction in India, and a large portion of the country lies in regions that are exposed to moderate or severe earthquake shaking. Many of these buildings are not regular in plan or elevation. Re-entrant corners, setbacks, open ground storeys for parking, and unequal distribution of mass or stiffness are common because of site constraints and architectural requirements. Such irregularities change the way a building responds to ground motion and can concentrate damage in a few members or storeys. The present study examines how regular and irregular RCC buildings behave under seismic loading and how that behaviour changes as the building is located in different seismic zones. A G+7 storey RCC moment-resisting frame with a symmetric rectangular plan is taken as the regular reference model. Three irregular models are derived from it with the same floor area and member sizes: an L-shaped plan with a re-entrant corner, a vertically irregular frame with a setback at the upper storeys, and a frame with a soft ground storey created by removing the infill walls at plinth level. All four models are prepared in ETABS and analysed for Seismic Zones II, III, IV, and V as defined in IS 1893 (Part 1):2016, so that sixteen analysis cases are obtained in total. Dead and live loads are applied according to IS 875 (Part 1) and IS 875 (Part 2). Both the equivalent static method and the response spectrum method are used. The response parameters compared are fundamental time period, base shear, storey displacement, inter-storey drift, storey stiffness, and torsional irregularity ratio. Member forces in the critical columns and beams are extracted, and the frames are designed according to IS 456:2000 with ductile detailing requirements of IS 13920:2016, so that the change in steel quantity with zone and configuration can also be observed. The results show that base shear and lateral displacement increase directly with the zone factor, so that a building in Zone V attracts roughly 3.6 times the design base shear of the same building in Zone II. For the same zone, the irregular models show higher displacement and drift than the regular model. The soft storey model shows a sharp jump in drift at the ground storey, and the L-shaped model shows significant torsional response at its re-entrant corner. The setback model shows a concentration of drift and member forces at the storey where the plan area changes. The study concludes that irregularity becomes a more serious concern as the seismic zone becomes more severe, and that configurations which may be acceptable in Zone II need careful treatment such as shear walls, stiffer ground storey columns, or separation joints in Zones IV and V. The comparison is intended to help students and practising designers understand the combined effect of configuration and seismicity at the planning stage itself, before member sizes and reinforcement are fixed.