A Review: Effect of Floor Height Variation on Seismic Performance of Reinforced Concrete Buildings Using ETABS

Abstract

Rapid urban growth has increased the demand for multistorey reinforced concrete (RC) buildings in Indian seismic zones, where architectural requirements often introduce vertically irregular floor heights for parking or commercial podium levels. Floor to floor height directly governs lateral stiffness and mass distribution, so variations in storey height can create soft storey mechanisms that amplify seismic demand compared with regular, uniform height buildings. This review examines the effect of floor height variation on the seismic performance of RC frame buildings analysed using ETABS under IS 1893:2016 provisions. Published studies on uniform buildings and structures with increased ground floor or podium heights are synthesised, focusing on changes in fundamental period, design base shear, storey drift profiles, and concentration of demand in flexible storeys. Results consistently show that soft storey and podium configurations exhibit longer periods, reduced overall base shear, and significantly higher inter storey drifts at the tall storey than comparable uniform height frames, increasing the risk of plastic hinge formation and non structural damage. The review highlights typical height ratios that trigger soft storey behaviour, discusses code recommendations for vertical irregularity, and summarizes ETABS modelling practices for representing height variation in routine design. Research gaps are identified in performance based evaluation and retrofitting strategies for existing soft storey buildings. The paper provides practical guidance for designers to recognize unsafe height configurations early in planning and to adopt stiffness balanced layouts or strengthening measures so that architectural floor height demands remain compatible with seismic performance objectives for RC buildings in Indian conditions.

Introduction

Rapid urbanization in India has led to a rise in multistorey reinforced concrete (RC) buildings, often with varying floor heights due to architectural requirements like podiums, commercial ground floors, or double-height lobbies. Such vertical irregularity alters lateral stiffness and mass distribution, creating soft storey mechanisms where seismic drift and damage concentrate in taller, more flexible floors. Studies using ETABS and IS 1893:2016 provisions consistently show that taller ground or podium floors increase the fundamental period, reduce base shear, and amplify inter-storey drift at irregular levels, even when member sizes satisfy strength requirements. Soft storey configurations are particularly vulnerable, requiring additional lateral load-resisting elements, gradual stiffness transitions, or bracings to mitigate risk.

Indian seismic codes (IS 1893, IS 456, IS 875) recognize vertical irregularity and prescribe drift limits and detailing checks, but provide limited quantitative guidance on acceptable height ratios and stiffness transitions. Comparative ETABS modeling of regular (uniform height) versus height-irregular (tall podium/soft storey) buildings shows that irregular structures exhibit longer periods, slightly reduced base shear, higher roof displacements, and concentrated drift at taller storeys, highlighting the importance of addressing vertical stiffness irregularity in design.

The study focuses on mid-rise RC frames, using response spectrum analysis to quantify effects of floor height variation on seismic performance. Key parameters evaluated include fundamental period, base shear, storey displacements, inter-storey drift, and shear distribution. Results are expected to confirm that height irregularity increases local vulnerability despite potentially lower global base shear, emphasizing the need for careful modeling, enhanced detailing, and mitigation strategies in podium or soft storey configurations.

Conclusion

The review of floor height variation studies and the proposed ETABS methodology show that changing storey height significantly alters the seismic performance of mid rise RC buildings, even when plan layout, materials, and member sizes remain unchanged. Introducing an increased ground floor height to create a podium or soft storey configuration lengthens the fundamental period and tends to reduce total design base shear compared with a uniform height building, but it also increases overall lateral flexibility, roof displacement, and—most critically—inter storey drift at the tall storey. These effects confirm that floor height induced vertical irregularity is a key driver of soft storey behaviour and must be explicitly evaluated rather than assumed to be benign. For the G+10 RC frame considered, the expected response comparison between uniform height and podium configurations suggests that drift based criteria and local demand at the ground storey govern safety more than global force levels. While the height irregular model may satisfy base shear requirements of IS 1893:2016 due to its longer period, drift at the podium level can approach or exceed the 0.004h limit, implying a need for enhanced stiffness through larger columns, shear walls, or bracing, or for special ductile detailing to prevent brittle soft storey failures. Accordingly, architects and structural engineers should coordinate early in the design process to limit abrupt height jumps, control stiffness irregularity, and, where tall commercial or parking storeys are unavoidable, provide targeted strengthening and capacity design at the critical levels. Overall, the study concludes that regular uniform height buildings offer inherently more favourable seismic performance, while podium and soft storey configurations require careful ETABS modelling, drift checks, and refined detailing under IS 1893 and IS 456 to achieve comparable safety. The synthesized literature also highlights research gaps in defining quantitative limits for acceptable floor height ratios, evaluating performance based retrofitting strategies for existing soft storey buildings, and extending current code provisions to give clearer guidance on vertical irregularity in mid rise Indian RC construction, which future work should address.

References

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Copyright

Copyright © 2025 Sagar Dule, Dr. Mahesh Raut. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.