Study on Response of Stiffness Irregular RC Framed Structures Subjected to Wind Loads

Abstract

Stiffness irregularities are among the most critical factors influencing the structural performance of reinforced concrete (RC) framed buildings under lateral loading. Abrupt reductions in stiffness—often resulting from weak storeys or sudden loss of lateral resistance—disrupt the uniform distribution of wind and seismic forces along the building height. This leads to stress concentration, localized deformations, and a significant increase in inter-storey drift, frequently culminating in soft-storey failures. In contrast, buildings with uniform stiffness exhibit more stable and predictable dynamic behaviour. The present study investigates the wind 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 for wind loads in accordance with IS 875 (Part 3): 2015, using ETABS. Key response parameters—including lateral displacement, storey drift, and base shear—are evaluated across different wind zones of India.

Introduction

The behavior of structures under wind and seismic loads is strongly influenced by structural irregularities, which can amplify lateral forces, stress concentrations, and deformations. Irregularities may be vertical (e.g., soft or weak stories, mass or stiffness discontinuities, vertical setbacks) or plan-based/horizontal (e.g., re-entrant corners, torsional asymmetry, diaphragm discontinuities, out-of-plane offsets, non-parallel systems). Such irregularities alter dynamic responses, increase inter-storey drift, and can compromise both serviceability and structural safety.

Plan irregularities—including torsional irregularity, re-entrant corners, diaphragm discontinuity, out-of-plane offsets, and non-parallel systems—affect lateral load distribution and generate stress concentrations, often exacerbating torsional effects. Vertical irregularities—including stiffness, mass, and geometric irregularities, as well as weak or soft stories—cause uneven load paths and increase susceptibility to localized failures. Codes like IS 1893 (Part 1)-2016 define thresholds for identifying such irregularities, such as drift limits, stiffness reduction, or excessive mass variations.

The study aims to evaluate the effect of stiffness irregularities on the wind performance of G+5 RC framed buildings using ETABS, comparing regular and irregular configurations under various Indian wind zones. Key response parameters include lateral displacement, storey drift, and base shear, with the objective of understanding how abrupt stiffness changes affect structural safety and stability.

Literature review findings highlight:

• Irregularities significantly amplify seismic demands, lateral displacements, storey drifts, torsional effects, and base shear compared to regular structures.

• Combined irregularities (plan + vertical + stiffness/mass) are the most critical, increasing the risk of soft-story or weak-story failures.

• Dynamic analyses (time history, response spectrum) provide more accurate predictions than linear static methods.

• Strengthening techniques, ductility-based design, base isolation, damping devices, and careful modeling of irregular configurations are essential for improving resilience.

• Soil–structure interaction (SSI) can worsen the performance of irregular buildings on soft soils.

Conclusion

The wind performance of Regular Frame (RF) and Stiffness Irregular Frame (SIF) buildings was evaluated. The primary conclusions drawn from the study are: 1) The SIF exhibits significantly higher roof displacement compared to RF, with top-storey displacements increasing by approximately 30–40% depending on the degree of irregularity. This indicates that stiffness discontinuity reduces global lateral stiffness, leading to larger deformations. 2) The lateral displacement profile of RF remains uniform and linear, whereas SIF shows pronounced deformation concentration at the irregular storey level. This soft-storey effect makes the structure more prone to localized damage. 3) The inter-storey drift in SIF is sharply peaked at the irregularity level, exceeding the corresponding drift values in RF by nearly 35–45%. Such concentrated drift demands enhanced ductile detailing and drift control strategies at the critical storey. 4) The storey drift distribution in RF is smooth and gradual, while SIF shows sudden drift spikes, indicating that irregularity disrupts energy dissipation and induces higher inelastic deformations in critical members. 5) The base shear demand in SIF is 20–25% higher than in RF, with abrupt shear variations above and below the discontinuity. This emphasizes the need for robust foundation systems and special detailing of columns and beams in the vicinity of the irregular storey. 6) Overall, SIF structures demonstrate greater wind vulnerability than RF. Stiffness discontinuity amplifies displacement, drift, and shear responses, particularly in higher wind zones. To ensure codal compliance and safety, design strategies must include stiffness balancing, drift limitation, and ductile detailing measures.

References

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Copyright

Copyright © 2025 Potti Pushapanjali, 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.