Dynamic Performance of Stiffness Irregular RC Framed Structures against Seismic Loads

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 modelled and analysed 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. The results highlight that stiffness irregularities intensify inter-storey drift and shear concentration at specific levels, particularly in soft-storey and partially infilled configurations. These findings emphasize the necessity of accounting for stiffness discontinuities in seismic design to avoid premature failures and to ensure compliance with codal safety requirements.

Introduction

Structural irregularities play a critical role in determining how buildings respond to seismic forces. Earthquakes generate dynamic, reversible, and unpredictable loads, making irregular structures more susceptible to damage due to stress concentration, excessive drift, torsion, and potential partial collapse. As a result, modern seismic design, guided by standards such as IS 1893:2016, emphasizes robust lateral load–resisting systems, continuous load paths, and the control of deformation to enhance structural resilience.

Irregularities in buildings are broadly classified into plan irregularities and vertical irregularities, each affecting seismic performance differently.
Plan irregularities include torsional irregularity, re-entrant corners, diaphragm discontinuities, out-of-plane offsets, and non-parallel systems. These arise from asymmetrical layouts, large openings, or misalignment of lateral load-resisting elements, often leading to torsion, uneven displacement, and localized overstressing.

Vertical irregularities occur due to discontinuities in mass, stiffness, geometry, or strength along the building height. Common types include soft storeys (stiffness irregularity), mass irregularity, vertical geometric setbacks, in-plane discontinuities of vertical resisting elements, and weak storeys. Such irregularities disrupt the vertical load path, increase inertial forces, and have been frequently observed as major contributors to structural failures in past earthquakes.

Overall, the text emphasizes that many real-world buildings inherently contain irregularities, and their combined effects significantly escalate seismic vulnerability. Understanding, identifying, and appropriately designing for these irregularities are essential for achieving earthquake-resistant structures and ensuring safe performance under seismic loading.

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 B.V.S Chandra Harsha, 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.