Analysis of an RCC Irregular and Setback Building under Dynamic Loading Using ETABS

Abstract

Vertical irregularities such as setbacks, abrupt changes in stiffness, mass distribution, and geometric discontinuities are increasingly common in reinforced cement concrete (RCC) buildings due to architectural and functional requirements. However, these irregularities significantly influence the seismic response of structures and often lead to concentration of damage during strong ground motions. This paper presents a comprehensive review of research studies published between 2015 and 2025 on the seismic analysis of vertically irregular and setback RCC buildings subjected to dynamic loading. Emphasis is placed on analytical investigations conducted using ETABS, including response spectrum analysis, time history analysis, and nonlinear performance-based approaches. The reviewed literature highlights the impact of different types of vertical irregularities on key seismic response parameters such as natural period, storey displacement, inter-storey drift, base shear, torsional effects, and damage distribution. Comparative insights from code-based studies, experimental investigations, and seismic vulnerability assessments are also discussed. The review identifies consistent trends indicating increased seismic demand and reduced performance in irregular buildings compared to regular configurations. Furthermore, existing research gaps related to combined irregularities, nonlinear dynamic analysis, and mitigation strategies are summarized. The findings of this review aim to support researchers and practicing engineers in understanding the seismic behavior of irregular RCC buildings and in adopting appropriate analysis and design approaches.

Introduction

Rapid urbanization has led to the construction of multistorey RCC buildings with architectural features such as open ground storeys, setbacks, floating columns, and large openings. While these features improve functionality and aesthetics, they introduce structural irregularities that significantly degrade seismic performance. Past earthquakes have shown that irregular buildings are more vulnerable than regular ones due to uneven distribution of mass, stiffness, and strength, which causes stress concentrations, excessive storey drifts, torsional effects, and localized failures. As per IS 1893:2002 (Part I), irregularities are classified into vertical (soft storey, mass, stiffness, setback, infill discontinuity) and plan irregularities, with setback buildings being particularly critical because of abrupt changes in stiffness and mass.

To realistically evaluate such behavior, advanced dynamic analysis methods are required. In this study, ETABS is used to perform response spectrum analysis on sixteen G+10 RCC building models, including a regular reference model and models with four types of vertical irregularities: infill, stiffness, mass, and setback. Key seismic response parameters such as storey displacement, storey drift, storey shear, overturning moment, storey stiffness, base shear, and modal participation are evaluated in accordance with IS 1893:2002.

Results show that irregular buildings exhibit significantly higher displacements, drifts, and concentration of forces at irregular levels compared to regular buildings. Bare frames and soft-storey configurations produce the highest displacements and drifts, while fully infilled frames develop higher storey shear and overturning moments due to increased stiffness. Abrupt changes in infill, stiffness, or geometry lead to sudden variations in seismic response, making these buildings prone to damage. Overall, the study highlights the critical influence of vertical irregularities—especially setback and soft-storey conditions—on seismic behavior and emphasizes the need for detailed dynamic analysis for the safe and economical design of RCC buildings in seismic regions.

Conclusion

This study examined the seismic behavior of RCC buildings with four types of vertical irregularities—partial infill, stiffness, mass, and setback—while maintaining plan symmetry. A total of sixteen regular and irregular building models were developed and analyzed using CSI ETABS 2015. Response spectrum analysis was performed in accordance with IS 1893:2002 (Part I) for Seismic Zone V and medium soil conditions. Key response parameters, including storey displacement, inter-storey drift, storey shear, overturning moment, and storey stiffness, were evaluated and compared with those of a regular building. The results indicate that masonry infill significantly enhances structural stiffness and strength, reducing roof displacement and storey drift; however, partial infill causes abrupt drift concentrations, making such configurations undesirable in high seismic zones. Stiffness irregularity, particularly soft storeys, leads to excessive local drift and higher displacement demand, with a noticeable drop in storey stiffness at the irregular levels. Mass irregularity increases roof displacement, inter-storey drift, and base shear, thereby degrading seismic performance. Setback irregularity reduces overall stiffness and induces sudden drift variations near the setback levels, although the regular building consistently exhibits superior performance. Overall, the regular RCC building demonstrates the most favorable seismic behavior, highlighting the importance of avoiding vertical irregularities in earthquake-prone regions.

References

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Copyright

Copyright © 2026 Yogesh Ladiya, Murlidhar Chourasia, Dr. Rahul Kumar Satbhaiya. 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.