Seismic Performance of Mass Irregular RC Framed Structures

Abstract

Stiffness irregularities are among the most critical factors affecting the seismic performance of reinforced concrete (RC) framed buildings. Abrupt reductions in stiffness—caused by weak storeys or sudden loss of lateral resistance—distort the uniform distribution of seismic forces along the building height. This results in concentration of stresses, localized deformations, and a significant increase in inter-storey drift, often leading to soft-storey failures. In contrast, regular buildings with uniform stiffness demonstrate more predictable dynamic behaviour. The present study investigates the seismic response of RC framed buildings with stiffness irregularities and compares their performance with that of regular frames. A six-storey (G+5) RC frame is modeled and analyzed using the Response Spectrum Method (RSM) in compliance with IS 1893 (Part 1): 2016, implemented through ETABS software. Key seismic response parameters—lateral displacement, storey drift, base shear, and fundamental time period—are evaluated across different seismic zones of India.

Introduction

Structural irregularities play a critical role in determining how buildings respond to seismic forces. Earthquakes generate dynamic, rapidly reversing loads that depend heavily on a structure’s mass, stiffness, and geometry. When buildings contain irregularities—whether in plan or elevation—these discontinuities disrupt the uniform flow of forces, leading to stress concentration, increased inter-storey drift, torsional effects, and in severe cases, partial or total collapse. As moderate to major earthquakes continue to occur worldwide, the importance of designing structures in compliance with seismic codes (such as IS 1893–2016) has become increasingly significant. Strengthening irregular buildings to effectively dissipate energy and limit lateral displacement is now a central concern in earthquake engineering.

Structural irregularities are broadly classified into vertical irregularities and plan (horizontal) irregularities.

• Vertical irregularities arise from sudden changes in mass, stiffness, strength, or geometry across the height of a building, disrupting the seismic load path and increasing vulnerability.

• Plan irregularities occur when the building layout is asymmetric or contains discontinuities such as openings, re-entrant corners, or changes in diaphragm stiffness, leading to torsion and uneven deformation.

The main types of plan irregularities include:

1. Torsional Irregularity – Occurs when lateral displacement at one end of a floor exceeds 1.2–1.5 times that at the opposite end, causing twisting of the structure.

2. Re-entrant Corners – Found in L-, T-, H-, and similar plan shapes; these corners attract high stress due to discontinuity of mass and stiffness.

3. Diaphragm Discontinuity – Present when floor diaphragms have large openings or abrupt changes in stiffness, reducing their ability to transmit seismic forces effectively.

4. Out-of-Plane Offsets – Occur when vertical elements such as walls or frames are not aligned across storeys, breaking the load path.

5. Non-Parallel Systems – Arise when vertical resisting elements are not parallel to principal axes, often resulting in significant torsion.

Common vertical irregularities include changes in stiffness such as soft storeys, where a storey exhibits less than 70% of the stiffness of the storey above (or less than 80% of the average stiffness of the three storeys above). These irregularities disrupt the continuity of the seismic load path, making the structure more susceptible to damage during strong ground motion.

Conclusion

The seismic response of Regular Frame (RF) and Stiffness Irregular Frame (SIF) buildings was studied using the Response Spectrum Method across all seismic zones of India. The major conclusions are: 1) The SIF consistently exhibits larger lateral displacements than RF, with roof displacement being 30–35% higher in Zone V, reflecting increased global flexibility due to stiffness discontinuity. 2) The response amplification from Zone II to Zone V is sharper in SIF compared to RF, indicating that stiffness-irregular systems are more sensitive to seismic intensity escalation. 3) Unlike RF’s smooth drift distribution, SIF develops sharp drift peaks at the irregularity level, which may lead to localized damage and cracking in critical storeys. 4) Peak drift values in SIF are 35–40% higher than RF in higher seismic zones, making drift control a key design requirement for stiffness-irregular frames. 5) The base shear demand in SIF is about 20–25% higher than RF, with abrupt shear variations above the discontinuity, indicating the need for stronger foundations and critical member detailing. 6) Overall, SIF frames are more vulnerable than RF, as stiffness discontinuity amplifies displacements, drifts, and shear irregularities. In high seismic zones, SIF requires careful stiffness balancing, ductile detailing, and drift-control measures to ensure codal compliance and safety.

References

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Copyright

Copyright © 2025 G. Pravalika, Sri. K. Dileep Chowdary. 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.