Seismic Performance of RC Framed Structures with Equal and Single Setbacks on Both Sides

Abstract

Structural irregularities significantly influence the seismic behaviour of reinforced concrete (RC) framed buildings, often making them more vulnerable than regular frames. A single vertical setback provided symmetrically on both sides introduces abrupt changes in geometry and stiffness along the building height. While such a configuration maintains balance in plan, it interrupts the continuity of the load path and modifies the dynamic characteristics of the structure. These discontinuities amplify seismic demands at the setback transition levels and alter the overall response under earthquake excitations. The present study investigates the seismic performance of RC framed buildings with single and equal setbacks on both sides and compares their behaviour with that of regular frames of similar plan area and height. The analysis is carried out using the Response Spectrum Method (RSM) in accordance with IS 1893:2016 (Part 1), with ETABS software used for modelling and simulation. Key response parameters, namely lateral displacement, storey drift, and base shear, are evaluated for different seismic zones of India. The results highlight that even symmetrical setback configurations can induce non- uniform seismic demand, leading to increased drift and stress concentrations at transition levels compared to regular buildings. These findings emphasize the importance of incorporating setback effects in seismic design considerations to ensure the safety and stability of RC framed buildings.

Introduction

Structural irregularities—both in plan and elevation—significantly influence the seismic behavior of buildings. Earthquakes impose dynamic, reversal forces that interact with a structure’s mass, stiffness, and geometry, making irregular structures particularly vulnerable to higher stresses, torsion, and potential partial collapse. Modern earthquake engineering emphasizes designing structures to safely dissipate energy, control lateral displacements, and prevent catastrophic failures, as codified in standards like IS 1893–2016 (India).

Classification of Structural Irregularities

1. Plan (Horizontal) Irregularities:

   • Torsional irregularity: Uneven horizontal displacement causing torsion; exists if one end of a floor moves >1.5 times the other.

   • Re-entrant corners: L-shaped, T-shaped, or similar geometries causing stress concentration.

   • Diaphragm discontinuity: Abrupt changes or openings >50% in horizontal diaphragms affecting lateral force transfer.

   • Out-of-plane offsets: Vertical elements shifted out of alignment, disrupting lateral load paths.

   • Non-parallel systems: Vertical elements not symmetric or aligned with principal axes, increasing torsional effects.

2. Elevation (Vertical) Irregularities:

   • Stiffness irregularity (soft storey): A storey with <70% stiffness of the above storey or <80% average stiffness of three storeys above, prone to collapse.

   • Mass irregularity: A storey with >15% more effective mass than adjacent storeys, leading to higher inertial forces and reduced ductility.

   • Setbacks and geometric discontinuities: Sudden changes in height or shape affecting force distribution and load paths.

Seismic Implications

• Irregularities cause stress concentrations, higher inter-storey drifts, torsion, and uneven load distribution.

• Continuous and robust load paths are essential to transfer seismic forces from all elements through diaphragms to vertical resisting components (columns, shear walls, frames) down to the foundation.

• Real buildings are rarely perfectly regular; both plan and elevation irregularities often combine, increasing vulnerability during earthquakes.

Conclusion

The seismic response of Regular Frame (RF) and Vertical Equal & Single Setback Frame (VESF) 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 Sanapala Sai Kumar , 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.