Seismic Response of Irregular Structures

Abstract

Structural irregularity is a common characteristic in modern buildings due to architectural and functional requirements. However, such irregularities significantly influence the seismic performance of structures, often making their behavior more complex and vulnerable compared to regular configurations. This study investigates the torsional response of reinforced concrete (RC) irregular buildings subjected to seismic loading. The analysis is carried out using the Equivalent Lateral Force (ELF) method as per IS 1893 (Part 1): 2002 provisions. Finite Element Modelling (FEM) is adopted to analyze four different 16-storey building models with varying irregularities, including mass irregularity, stiffness irregularity, and plan irregularity. These models are evaluated under different seismic zones of India. The primary objective of this study is to examine the influence of codal provisions on torsionally irregular structures and identify the most critical type of irregularity affecting seismic performance. Special emphasis is given to discontinuities in the lateral load-resisting system, such as offsets in vertical elements. The results highlight the importance of considering structural irregularities during seismic design to ensure safety and performance.

Introduction

The text discusses seismic behavior of irregular buildings and how structural irregularities affect their response during earthquakes. It explains that real-world buildings are rarely perfectly regular and often contain irregularities in mass distribution, stiffness, or plan geometry, which can significantly increase torsional effects during seismic events. Design codes such as IS 1893 (Part 1): 2002 provide guidelines to identify and analyze these irregularities, particularly torsional irregularity.

The study uses the Equivalent Lateral Force (ELF) method to evaluate seismic response within the elastic range, while noting that irregular structures may behave more complexly under nonlinear conditions. It aims to examine how different irregularities—such as uneven mass distribution, asymmetric stiffness, soft storeys, and re-entrant plan corners—affect seismic performance and torsional response, and compares these with a regular building model to identify the most critical case.

Four building models are analyzed: a regular structure, a structure with heavy mass at specific floors, a soft-storey model, and a plan-irregular structure with re-entrant corners. All models are 16-storey residential buildings designed using standard structural parameters and seismic zones as per IS 1893. The study assumes fixed base conditions and focuses on elastic analysis, while acknowledging that real seismic behavior may be more complex due to foundation effects and inelastic response.

Conclusion

The present investigation tries to assess the impact of firmness on seismic reaction of a vertical sporadic structure on seismic zones II, III, IV, and V on medium soil. The investigation likewise stretches out to discover the impact of Base shear, horizontal uprooting of structures and major normal time of the customary and unpredictable models. The examination prompts the accompanying expansive ends. The following are the key observations of this study- 1) The Base shear and sidelong removals are progressively expanded with increment in zone factors for all models. 2) The horizontal uprooting is less in normal model contrast with vertical unpredictable models. 3) The sidelong dislodging is most extreme in model kind 4 for example the Re-participant corner model. 4) The sidelong dislodging is least in model sort 1 for example the customary model.The base shear is maximum in the model type-2i.e. mass irregularity, heavy mass in 5th and 10th floor. 5) The base shear is minimum in model type-4 i.e. Re-entrant corner model. 6) From the modal analysis, it shows that the natural time period is gradually increases with the type of irregularity. 7) The timespan from the outset mode is high in model sort 3 for example solidness abnormality and low in the model sort 4 for example Re-contestant model.The base shear and lateral displacement are gradually increased with increase in zone factor for all types of models and maximum for severe zone-5. 8) The vertical irregular models i.e. model-3 and model-4 shows the less base shear compare to other type of models. 9) The base shear is almost same in regular model, model type-2 and model type-3. 10) The regular model shows less displacement compare to irregular model, but the displacement is almost same in regular model and irregular model i.e. model type-2. 11) At the point when sporadic structures are examined utilizing direct comparable static examination and Reaction range investigation thinking about various seismic zones as indicated by code arrangements, the outcomes got features the significance of mass, firmness and geometry of the structure. Following wide ends can be made in this regard: 12) This study measures the impact of vertical abnormalities in mass and firmness on seismic requests. 13) From the general examination and perception it tends to be inferring that, Base shear and sidelong dislodging will increments as the seismic force increments from zone-2 to zone-5 which shows progressively seismic interest the structure should meet. 14) Base shear for mass inconsistency is discovered more contrasted with every single other abnormality since base shear relies upon seismic load of the structure.

References

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Copyright

Copyright © 2026 Vithika Pole, Nupur Dewangan. 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.