Review on 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

Modern urban buildings often incorporate architectural and functional variations along their height, leading to vertical irregularities such as setbacks, soft or weak storeys, transfer floors, and abrupt changes in mass or stiffness. From a seismic perspective, these irregularities are critical because they disrupt uniform force distribution, alter dynamic characteristics, and cause localized concentration of seismic demands. Evidence from past earthquakes consistently shows that vertically irregular buildings are more vulnerable to severe damage than regular structures.

Extensive research has examined the seismic response of vertically irregular reinforced concrete (RCC) buildings using analytical, numerical, and experimental approaches. Studies show that discontinuities in stiffness and mass amplify inter-storey drift, increase base shear and force concentration at irregular levels, and negatively affect overall structural stability. Combined vertical and horizontal irregularities further compound these adverse effects by modifying modal participation and increasing energy demand during strong ground motion, making dynamic analysis essential for accurate assessment.

Among different irregularities, vertical setback configurations are the most commonly studied. Research indicates that both symmetric and asymmetric setbacks can lead to unfavorable seismic behavior due to sudden changes in mass and stiffness, disproving the assumption that geometric symmetry ensures better performance. Advanced studies using nonlinear dynamic and mainshock–aftershock analyses reveal that setback buildings can experience cumulative damage and drift demands significantly higher than those predicted by linear methods.

ETABS-based response spectrum and time-history analyses dominate the literature, demonstrating that irregular buildings exhibit higher storey drift ratios, altered modal mass participation, and greater displacement demands compared to regular buildings. Code-based comparisons (IS 1893 and Eurocode 8) show that design assumptions and reduction factors significantly influence predicted seismic responses, especially in high seismic zones.

Mass and stiffness irregularities, often introduced unintentionally, further worsen seismic performance, particularly when combined. Experimental and vulnerability studies confirm analytical findings, showing early damage initiation, concentration of failure at irregular storeys, and higher fragility indices for irregular buildings.

Mitigation strategies such as the strategic use of shear walls, infill walls, and integrated structural systems can improve seismic performance, but improper placement may introduce new irregularities. Overall, the literature establishes the critical impact of vertical irregularities on seismic behavior but reveals key research gaps: limited comparative studies across multiple irregularity types and heavy reliance on linear analysis. These gaps highlight the need for systematic, performance-based and nonlinear evaluation frameworks to better understand and design vertically irregular RCC buildings for seismic resilience.

Conclusion

The tabulated comparison indicates that most studies focus on vertical and setback irregularities using linear dynamic approaches. Only a limited number of investigations incorporate nonlinear dynamic or experimental methods, indicating a research gap in performance-based evaluation of irregular RCC buildings. This review paper has presented a detailed synthesis of recent research on the seismic analysis of vertically irregular and setback RCC buildings under dynamic loading, with particular emphasis on studies employing ETABS as the primary analysis platform. The literature clearly demonstrates that vertical irregularities, including setbacks, mass discontinuities, and stiffness variations, significantly alter the dynamic characteristics of RCC buildings and result in amplified seismic demand compared to regular structures. Increased storey drift, higher displacement concentration at irregular levels, and unfavorable force redistribution are commonly reported consequences of such irregular configurations. The reviewed studies indicate that linear static analysis methods are often inadequate for accurately capturing the complex behavior of irregular buildings. Dynamic analysis techniques, such as response spectrum and time history analysis, provide more reliable insight into seismic performance, while nonlinear and performance-based approaches offer a deeper understanding of damage mechanisms and collapse potential. Research findings also show that buildings with combined irregularities exhibit the most critical seismic response, emphasizing the need for careful modeling and comprehensive evaluation during the design stage. Although several mitigation measures, including the use of shear walls and infill walls, have been proposed to improve seismic performance, their effectiveness largely depends on proper placement and continuity. Despite significant progress, gaps remain in the systematic comparison of different irregularity types, the use of nonlinear dynamic analysis for setback buildings, and the assessment of cumulative damage under multiple seismic events. Future research should focus on these aspects to develop more robust design guidelines and enhance the seismic resilience of irregular RCC buildings.

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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.