The Seismic Behaviour of Plan Irregularity in Mid-Rise Reinforced Concrete Structures Using Response Spectrum and Pushover Analysis

The present study evaluated the seismic behaviour of mid-rise reinforced concrete (RC) buildings with different plan irregularities using Response Spectrum Analysis and Pushover Analysis in ETABS. A comparative assessment was carried out between a regular rectangular building and various irregular configurations, including L-shaped, U-shaped, C-shaped, and diaphragm discontinuity models, to understand the influence of plan irregularity on structural performance during earthquake loading. The analysis demonstrated that the geometry of a building plays a significant role in determining its seismic response, particularly with respect to base shear, storey displacement, torsional effects, and overall structural stability. The results revealed that the regular rectangular building exhibited the most balanced and predictable seismic behaviour because of its symmetrical distribution of mass and stiffness. In contrast, plan-irregular buildings experienced greater torsional effects, uneven force distribution, and increased lateral displacement. Among the analysed models, the diaphragm discontinuity configuration attracted the highest base shear, indicating greater seismic force demand, while the U-shaped building showed significant lateral displacement due to torsional irregularity. Although the L-shaped structure recorded comparatively lower displacement values, its asymmetric geometry still made it susceptible to torsional response during strong earthquake excitation. These observations confirm that plan irregularity significantly affects the seismic performance of reinforced concrete buildings and therefore requires careful consideration during structural design. The pushover analysis further demonstrated the importance of evaluating the nonlinear behaviour of structures beyond the elastic range. Capacity curves showed that different building configurations possess different levels of strength, stiffness, and ductility. The regular building displayed a stable nonlinear response with good deformation capacity, whereas irregular buildings exhibited variations in load-carrying capacity depending on their structural configuration. These findings highlight that relying only on conventional linear analysis may not provide a complete understanding of structural performance under severe seismic loading. Overall, the study concludes that advanced analytical methods such as Response Spectrum Analysis and Pushover Analysis are essential for accurately evaluating the seismic  behaviour  of  plan-irregular  RC structures. Strict compliance with IS 1893:2016 codal provisions, combined with appropriate structural planning and performance-based design approaches, can significantly improve earthquake resistance and ensure the safety, serviceability, and durability of buildings constructed in seismic regions. The outcomes of this research provide valuable guidance for structural engineers and researchers in designing safer and more resilient reinforced concrete buildings.

[1]  S. Sivakumar, R. Shobana, E. Aarthy, S. Thenmozhi, V. Gowri, and B. S. C. Kumar, "Seismic Analysis of Plan Irregular Building," Materials Today: Proceedings, vol. 72, pp. 1–8, 2023. [2]   P. K. Das, S. C. Dutta, and T. K. Datta, "Seismic Behavior of Plan and Vertically Irregular Structures: State of the Art and Future Challenges," Natural Hazards Review, vol. 21, no. 3, 2020. [3]    N. M. Naganur and Vijaykumar, "Effect of Seismicity on Irregular Shape Structure," International Journal of Civil Engineering and Technology (IJCIET), vol. 9, no. 13, pp. 2248–2264, 2018. [4]  S. A. Halkude, C. G. Konapure, and C. A. Madgundi, "Seismic Analysis of RC Building (G+9) by Response Spectrum Method," International Journal of Engineering Research & Technology (IJERT), vol. 3, no. 6, pp. 1–6, 2014. [5]   J. Browning, B. Warden, A. Matamoros, and A. Lepage, "Global and Local Seismic Drift Estimates for Reinforced Concrete Structures," Engineering Structures, vol. 30, no. 5, pp. 1248–1262, 2008. [6]  Bureau of Indian Standards, IS 1893 (Part 1): 2016, Criteria for Earthquake Resistant Design of Structures – General Provisions and Buildings, New Delhi, India, 2016. [7]  Bureau of Indian Standards, IS 456:2000, Plain and Reinforced Concrete – Code of Practice, New Delhi, India, 2000. [8]  Bureau of Indian Standards, IS 875 (Part 1):1987, Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures – Dead Loads, New Delhi, India. [9]  Bureau of Indian Standards, IS 875 (Part 2):1987, Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures – Imposed Loads, New Delhi, India. [10]   Computers and Structures, Inc., ETABS 21 – Integrated Building Design Software User Guide, Berkeley, California, USA. [11]  International Code Council, Uniform Building Code (UBC 1997/1998), Volume 2: Structural Engineering Design Provisions, California, USA. [12]  European Committee for Standardization, Eurocode 8: Design of Structures for Earthquake Resistance (EN 1998-1), Brussels, Belgium, 2004.