Performance of Mass Irregular RC Framed Structures Subjected Wind Loads

Abstract

Structural irregularities are a major cause of poor performance in reinforced concrete (RC) framed buildings under wind loading. Among these, mass irregularities resulting from significant variations in mass distribution between consecutive storeys can critically affect wind-induced dynamic response. Such discontinuities alter the distribution of inertia forces caused by wind gusts, amplify oscillatory effects, and increase the likelihood of localized damage compared with regular frames having uniform mass distribution. The present study investigates the wind response of RC framed buildings with mass irregularities and compares them with regular buildings. A six-storey (G+5) RC frame is modelled and analysed using Equivalent Static Wind Load Analysis as per IS 875 (Part 3):2015 provisions. Key wind response parameters such as lateral displacement, inter-storey drift, and base shear are evaluated under design wind speeds corresponding to different terrain categories and importance factors. The results reveal that mass irregularities significantly influence the overall wind response, with irregular models exhibiting higher inter-storey drifts, increased lateral displacements, and uneven base shear distribution. These findings emphasize the need for careful consideration of mass irregularities during the design stage and highlight potential mitigation measures to enhance the wind resistance and overall safety of irregular RC structures.

Introduction

Structural behaviour is strongly affected by irregularities, especially under wind loads, which are dynamic and depend on height, shape, stiffness, and mass distribution. Irregular buildings—with discontinuities in geometry or lateral force-resisting systems—experience amplified stresses, inter-storey drifts, torsion, vibrations, and possible failures. Wind-resistant design, in line with codes like IS 875 Part 3–2015, is crucial for resilience in tall or complex structures.

Types of Structural Irregularities:

1. Vertical Irregularities: Abrupt changes in stiffness, strength, mass, or geometry across storeys, causing uneven force distribution. Examples include soft storeys, setbacks, and vertical geometric irregularities.

   • Stiffness irregularity (Soft storey): Lateral stiffness <70% of the storey above or <80% of the average of three upper storeys. Extreme soft storeys are <60% or <70% of average.

   • Mass irregularity: A storey’s mass >150% of the adjacent lower storey.

   • Vertical geometric irregularity: Horizontal dimension of lateral force-resisting system >125% of adjacent storey.

2. Horizontal (Plan) Irregularities: Discontinuities or asymmetries in horizontal resisting systems, such as re-entrant corners, torsional irregularity, diaphragm discontinuities, out-of-plane offsets, and non-parallel systems. These features can amplify torsion, stress concentrations, diaphragm deformation, and lateral displacement.

   • Torsional irregularity: Maximum floor displacement >1.5× minimum displacement in lateral force direction.

   • Re-entrant corners: Inside corners causing force concentration, typical in L, T, H, or + shaped plans.

   • Diaphragm discontinuity: Openings or stiffness changes >50% affecting horizontal force transfer.

   • Out-of-plane offsets & non-parallel systems: Misalignment of vertical elements causing discontinuous load paths and increased torsion.

Conclusion

The wind response of Regular Frame (RF) and Mass Irregular Frame (MIF) buildings was studied across all wind zones of India. The major conclusions are: 1) The Mass Irregular Frame (MIF) consistently exhibits higher lateral displacement compared to the Regular Frame (RF), indicating that the concentration of mass at upper levels significantly increases the flexibility and overall deformation of the structure under wind loads. 2) For both RF and MIF, lateral displacement grows almost linearly with an increase in basic wind speed. However, the rate of increase is steeper for the MIF, highlighting its greater vulnerability at higher wind intensities. 3) The maximum storey drift is observed in the lower to mid-storey region for both frames, with the MIF showing drift values 20–45% higher than the RF. This concentration of drift at lower levels may lead to serviceability and non-structural damage concerns. 4) Due to the increased inertia effect from concentrated roof mass, the MIF demonstrates a sharper increase in storey drift with wind speed, suggesting higher inter storey deformation demands and potential instability in extreme conditions. 5) The storey shear distribution shows that MIF experiences 30–50% higher base shear than RF, primarily due to its larger effective mass. This indicates that heavier upper storeys significantly influence the overall lateral load transfer mechanism. 6) The overall response trends indicate that mass irregular buildings require enhanced stiffness and strength provisions in their lower storeys to control excessive displacement, drift, and base shear, ensuring safety and serviceability under high wind speeds.

References

[1] N. P. Modakwar, S. S. Meshram, and D. W. Gawatre, “Seismic Analysis of Structures with Irregularities,” IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), Int. Conf. on Advances in Engineering & Technology (ICAET-2014), pp. 63–66, 2014. [2] H. B. Khamkar, G. V. Tapkire, and S. M. Dumne, “Effects of Structural Irregularities on the Seismic Performance of Multi-Storey RC Buildings,” International Journal of Innovative Research in Science, Engineering and Technology (IJIRSET), vol. 5, no. 7, pp. 13615–13622, Jul. 2016. [3] M. T. Raagavi and S. Sidhardhan, “A Study on Seismic Performance of Various Irregular Structures,” International Journal of Research in Engineering and Science (IJRES), vol. 9, no. 5, pp. 12–19, 2021. [4] Tambare, O. Landge, P. Rakhunde, P. Raskar, and M. Deosarkar, “Study of Seismic Analysis of Plan Irregular Structures by Using ETABS Software,” Journal of Emerging Technologies and Innovative Research (JETIR), vol. 9, no. 5, pp. 111–115, May 2022. [5] S. Sabu and S. Raghavu, “Analysis of Irregular Structures Using ETABS Software,” International Journal of Engineering Research & Technology (IJERT), vol. 10, no. 6, pp. 272–276, 2022. [6] S. S. Girme and A. B. Pujari, “Review of Progressive Collapse Analysis of Reinforced Concrete Structures with Flat Slab Considering Effects of Geometrical (Horizontal and Vertical) Irregularities,” International Research Journal of Engineering and Technology (IRJET), vol. 9, no. 5, pp. 3526–3531, May 2022. [7] G. Kinagi and L. J. K., “Seismic Analysis of Structure with Structural Irregularities,” International Advanced Research Journal in Science, Engineering and Technology (IARJSET), vol. 9, no. 6, pp. 844–854, Jun. 2022, doi: 10.17148/IARJSET.2022.96134. [8] Dhalwar and S. P. Tak, “Seismic Analysis of Vertical Irregular Steel Structure with Seismic Resiliences,” International Journal of Creative Research Thoughts (IJCRT), vol. 10, no. 10, pp. 370–374, Oct. 2022. [9] Sam and M. M. Paul, “Performance Evaluation of Irregular Structures Under Seismic Response Considering Soil-Structure Interaction: A Review,” International Journal of Engineering Research & Technology (IJERT), vol. 12, no. 5, pp. 678–684, May 2023. [10] D. Bhagirath and O. Chirag, “Seismic Performance Analysis of Irregular Steel Building,” Journal of Emerging Technologies and Innovative Research (JETIR), vol. 11, no. 4, pp. 481–490, Apr. 2024. [11] A. Patil and R. Sutar, “Seismic Analysis of Multi Storey Irregular RCC Buildings with Bracing System,” IRE Journals, vol. 8, no. 3, pp. 206–213, Sep. 2024. [12] A. R. Babar and S. N. Patil, “Performance of Multi-Storied Irregular Steel Buildings: A Comprehensive Review of Dampers and Base Isolation Systems,” International Journal for Multidisciplinary Research (IJFMR), vol. 7, no. 3, pp. 1–12, May–Jun. 2025. [13] S. Nagar and M. Ahmed, “Investigating the Influence of Vertical Irregularities on Structural Integrity,” International Journal of Advance Scientific Research and Engineering Trends, vol. 9, no. 9, pp. 21–31, Sep. 2025. [14] Indian Standard Plain and Reinforced Concrete – Code of Practice (IS 456:2000), Bureau of Indian Standards, New Delhi, 2000. [15] Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures, Part 1: Dead Loads – Unit Weights of Building Materials and Stored Materials (IS 875 Part 1:1987), Bureau of Indian Standards, New Delhi, 1987. [16] Indian Standard Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures, Part 2: Imposed Loads (IS 875 Part 2:1987), Bureau of Indian Standards, New Delhi, 1987. [17] Criteria for Earthquake Resistant Design of Structures (IS 1893:2016, Part 1 – General Provisions and Buildings), Bureau of Indian Standards, New Delhi, 2016.

Copyright

Copyright © 2025 Pujari Raghu Varma, 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.