Seismic Performance of RC Framed Structures with Vertical Single Setback Irregularity

Abstract

The presence of structural irregularities has been identified as one of the major factors contributing to the poor seismic performance of reinforced concrete (RC) framed buildings. Vertical setback irregularities, in particular, introduce abrupt discontinuities in stiffness and mass distribution, which amplify seismic demands and increase the risk of local and global failure modes. This paper investigates the seismic behaviour of RC framed buildings with a single vertical setback irregularity and compares their response with that of a regular frame of identical plan dimensions and height. A six-storey RC frame is modelled and analysed using the Response Spectrum Method (RSM) in compliance with IS 1893:2016 (Part-1), with simulations performed in ETABS software. The study covers all seismic zones of India to capture the influence of varying seismic intensities on building response. Key seismic response parameters—lateral displacement, storey drift, and base shear—are extracted and compared between regular and irregular configurations. The results reveal that vertical setback irregularity significantly modifies the seismic demand. While the regular frame exhibits uniform and predictable response patterns, the setback frame records increased lateral displacement and storey drift near the setback levels, with localized peaks that may approach or exceed codal drift limits. Base shear distribution is also found to be non-uniform, with the setback frame showing higher concentration at critical floors. These findings highlight that even a single vertical setback can alter the seismic performance substantially, underlining the importance of considering such irregularities in design and codal provisions. The outcomes of this study provide valuable insights for structural engineers and policymakers to enhance the seismic safety of irregular RC buildings in India.

Introduction

Structural irregularities significantly affect a building’s behavior under seismic loads. Earthquakes generate dynamic forces that interact with a structure’s mass, stiffness, and geometry, making irregular buildings more vulnerable. Irregularities can be vertical (setbacks, soft storeys, floating columns) or horizontal/plan-based (re-entrant corners, torsional irregularities, diaphragm discontinuities), often leading to stress concentrations, higher inter-storey drifts, torsion, and potential partial collapse. Modern earthquake-resistant design, guided by codes like IS 1893–2016, aims to enhance structural resilience and prevent catastrophic failures.

Types of Structural Irregularities:

1. Vertical Irregularities: Abrupt changes in geometry, mass, stiffness, or strength along the building height. Examples include:

   • Stiffness irregularity (soft storey): A storey with significantly less lateral stiffness than the one above.

   • Mass irregularity: A storey with more than 150% mass of the storey below.

   • Vertical geometric irregularity: Setbacks or re-entrant corners causing discontinuities.

   • In-plane discontinuity of vertical elements: Offsets in lateral force-resisting members.

   • Weak/extreme weak storey: Storeys with lateral strength below 80% (or 65%) of the one above.

2. Horizontal/Plan Irregularities: Variations in the floor plan or lateral system that affect force distribution. Examples include:

   • Torsional irregularity: Unequal horizontal displacement across a floor.

   • Re-entrant corners: L-, T-, H-, or +-shaped projections causing stress concentration.

   • Diaphragm discontinuity: Openings or sudden changes in diaphragm stiffness.

   • Out-of-plane offsets: Misaligned vertical elements along the building height.

   • Non-parallel systems: Asymmetrical or triangular floor layouts leading to torsion.

Seismic Response:
Irregularities disrupt the load path for seismic forces, concentrating stresses, amplifying drifts, and increasing torsion. Vertical setbacks and discontinuities can cause partial or total failure if not properly accounted for. Ensuring continuous load transfer and proper connection of all elements is critical.

Objectives of the Study:

1. Analyze seismic performance of RC framed buildings with a single vertical setback.

2. Compare irregular and regular RC framed buildings of similar size and height.

3. Evaluate key seismic parameters: lateral displacement, storey drift, and base shear across different seismic zones in India.

4. Conduct Response Spectrum Analysis (RSA) using ETABS per IS 1893:2016.

Conclusion

The seismic response of Regular Frame (RF) and Vertical Single Setback Frame (VSSF) buildings was studied using the Response Spectrum Method across all seismic zones of India. The major conclusions are: 1) Structural responses increase consistently with seismic zone severity; values in Zone V are approximately 2.5–3 times higher than in Zone II, confirming the significant influence of seismic zoning. 2) VSSF exhibits 30–35% higher roof displacement than RF in Zone V due to stiffness discontinuity above the setback, which reduces global resistance and amplifies sway. 3) RF records 5–10% higher storey drift compared to VSSF, as the uniform mass and flexibility of RF generate larger inter-storey deformations, whereas the reduced seismic mass of VSSF limits inertia forces despite its geometric irregularity. 4) Base shear is consistently higher in RF (by about 8–12%) compared to VSSF, primarily because the larger seismic mass in RF produces greater inertial forces, while VSSF, with reduced effective mass, transfers comparatively lower shear to the foundation. 5) VSSF is vulnerable to excessive roof displacements from stiffness irregularity, but its lower mass results in comparatively reduced drift and shear. 6) The non-equivalent areas and mass reduction in VSSF play a critical role in explaining these trends, and must be considered while interpreting the results.

References

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Copyright

Copyright © 2025 Avanapu Sri Hari, Smt. B. Prudvi Rani. 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.