A Study on Seismic Reliability of Multi-Storeyed Buildings with Complex Plan and Vertical Irregularities
Buildings with re-entrant corners, projecting wings, and asymmetric plan forms are increasingly common in contemporary urban construction, yet such configurations depart from the uniform mass and stiffness distribution on which conventional seismic design provisions are based. This study evaluates the seismic reliability of four six-storey (G+5) reinforced-concrete special moment-resisting frame buildings possessing L-, C-, T-, and cruciform (+) plan shapes, each incorporating an open ground storey. Three-dimensional models with equal floor area and comparable structural quantities were developed in ETABS and analysed for Seismic Zone V in accordance with IS 1893 (Part 1): 2002, using both the Equivalent Static Method (ESM) and the Response Spectrum Method (RSM). Lateral displacement, storey drift, storey shear, torsional response, storey stiffness, modal periods, and overturning moment were compared across the four configurations. The equivalent static results were closely grouped across models, differing by only 8-9%, whereas the dynamic (RSM) results varied far more widely - up to 47% between configurations - showing that plan shape has a much stronger influence on true dynamic behavior than on code-based static estimates. The C-shaped plan (Model 2) consistently produced the largest dynamic displacement, drift, and base shear, identifying it as the most seismically demanding configuration, while the L-shaped plan (Model 1) developed the largest torsional moment, concentrated in members adjacent to its re-entrant corner. All models satisfied the permissible storey-drift limit of IS 1893 (Part 1): 2002. The results demonstrate that reliance on equivalent static analysis alone can substantially understate the seismic demand on irregular multi-storeyed buildings, and that response spectrum analysis, combined with explicit torsional checks at re-entrant corners, is essential for the reliable seismic design of complex plan-irregular structures.