Seismic Performance of Offset Irregular Building

Abstract

Here in this offset irregularity, the vertical members bearing horizontal force are located on other axes rather than its own axes. A 15-storey offset irregular building is modelled in SAP2000 software based on the codal provisions. Then pushover analysis is performed in which spectral displacements and base shears are examined in both X and Y directions. Fragility curves are developed to determine the failure probability of offset irregular building in slight, moderate, extensive and complete damage states. Using a thorough analysis using the pushover analysis approach, this study compares the seismic performance of a 15-storey offset irregular structure with a structurally regular building. For all building types, the study uses the SAP2000 software platform to assess spectral displacements and base shears in both the X and Y axis. The goal of this research is to measure and compare the complicated static response of the 15-storey building with that of a regular counterpart, given that the offset irregularity in the structure adds to its complexity. Accurate finite element models of both structures are developed, mode shapes and natural frequencies are extracted. After that, the pushover data is examined to determine the greatest displacements and base shear at crucial points in both the buildings. Understanding the impact of the offset irregularity on the static properties and seismic response of the 15-story building is given special attention. Moreover, the study goes so far as to use fragility curves to determine the likelihood of collapse for both buildings. Fragility curves provide a probabilistic evaluation of failure possibilities by offering important insights into a structure\'s susceptibility to seismic loads. The research looks at how the 15-story structure\'s abnormalities affect its fragility in comparison to the normal building, highlighting possible flaws and assisting in the improvement of seismic design. The results are intended to guide future design guidelines and construction rules, fostering increased safety and resilience in seismically vulnerable areas.

Introduction

I. INTRODUCTION

Irregular buildings, with their unconventional geometries, non-standard arrangements, and uneven mass distributions, pose a unique structural engineering problem. In contrast to its regular counterparts, which follow standard geometries and homogeneous layouts, irregular buildings contradict established structural design principles. These constructions may take on a wide range of forms, from asymmetrical floor plans and uneven facades to complicated geometries that defy traditional technical solutions. As a result, assessing and designing irregular structures need specialized knowledge, novel methodologies, and advanced computational tools to assure structural safety and performance, especially under seismic loading situations. An offset building is one having horizontally displaced floors or walls, resulting in a staggered or skewed look. It is used to improve aesthetics, break up homogeneous facades, and accommodate site restrictions. Offset structures can also give functional benefits such as more lighting, better vistas, and seclusion. However, they can cause structural engineering problems because to uneven load distributions and differential settling concerns.

Seismic analysis is an essential part of determining the earthquake resilience of buildings and other structures. It entails assessing how a structure responds to earthquake-induced ground motion in order to forecast future damage and ensure structural safety. Seismic analysis is very difficult in irregular buildings because to irregularities in geometry, mass distribution, and stiffness qualities. These variations can cause localized stresses, torsional effects, and uneven seismic force distribution, all of which might risk the building's structural integrity if not correctly accounted for throughout the study and design process.

II. METHODOLOGY

A. Pushover Analysis

Pushover analysis is an effective approach in structural engineering, notably for evaluating the seismic performance of buildings and other structures. It belongs to the domain of nonlinear static analysis methods and is commonly used in seismic design and retrofitting projects. The essential premise of pushover analysis is to mimic a structure's gradual collapse behavior under increasing lateral pressures, which are often seismic forces. To do a pushover analysis, engineers must first model the structure using finite element analysis or other numerical methods. This entails dividing the structure into smaller components, such as beams, columns, and braces, and describing their attributes and relationships. The structure is then exposed to lateral stresses in stages, beginning at a low level and gradually increasing to a predetermined maximum. During the study, the structure's reaction is tracked, including deformations, member forces, and displacements at crucial places. This data is used to create a force-displacement curve that depicts the connection between applied lateral forces and the resulting displacements. The force-displacement curve gives useful information on structural characteristics, such as strength, stiffness, and ductility.

B. Fragility Curves

Fragility curves are crucial for assessing structural seismic vulnerability, revealing the likelihood of different damage levels under different ground shaking levels. They depict the probability of exceeding certain damage states based on seismic intensity. The process involves defining damage states and analyzing structure’s response to seismic events using computational models and ground motion records. Damage states are estimated using simulated reactions at various degrees of ground shaking severity. Statistical approaches like as Bayes's inference and maximum likelihood estimation fit mathematical models, resulting in fragility functions. These probabilistic representations assist engineers in assessing seismic activity risk, making decisions about retrofitting structures, developing new buildings, and adopting mitigation measures to improve seismic resistance. Validating fragility curves against observed damage data from previous earthquakes is critical for ensuring their reliability and correctness. Engineers may also do sensitivity analysis to determine the influence of uncertainty in input parameters on fragility estimations.

C. Building Description

The buildings consist of 15 reinforced concrete floors, with each storey height of 3.0 m. The plan dimensions are 15.2 m x 12.0 m, with a 1.2m offset as shown in fig.3.1, fig.3.3 and fig.3.5. The plan dimension of regular building is 14 m x 12 m as shown in fig.3.2, fig.3.4 and fig.3.6. These buildings have four spans in each longitudinal and transverse orientations, with 3 m long spans in longitudinal and 3.5 m long spans in transverse. And offset building is having an extra span in longitudinal direction with 1.2 m span which is represented as offset. The lateral load resisting system consists of moment resistant frames in both directions.

Conclusion

The seismic performance and the failure probability of G+15 offset irregular RC building is compared with the G+15 regular building in which all the building parameters are considered to be same. The buildings were subjected to an elastic dynamic analysis using SAP2000 and applying loads according to codal provisions. Results from response spectrum analyses and fragility curves were used to estimate the seismic performance and failure probability of offset irregular building and comparison between offset irregular and regular is also done. The main conclusions from the findings are: 1) The maximum bending moment of offset irregular building is 45% greater than the regular building. 2) Pushover in the X direction, regular buildings have slightly higher base shears than offset irregular buildings for identical spectral displacements. 3) Pushover in the Y direction, offset irregular building exhibits high base shear at very less displacement. Regular buildings have higher base shears than offset buildings. 4) To improve the seismic performance of the offset irregular building dampers should be provided, it is seen that at 0.1% of damping ratios, the seismic performance of offset irregular building is similar to regular building. 5) The failure probability of offset irregular building is greater than the regular building in all damage states. It is concluded that for each damage condition, offset irregular building have larger spectral displacements, causes increasing in failure probability.

References

[1] Das, P. K., Dutta, S. C., & Datta, T. K. (2021). “Seismic behavior of plan and vertically irregular structures: state of art and future challenges”. Natural Hazards Review, 22(2), 04020062. [2] Haldar, P., & Singh, Y. (2009). “Seismic performance and vulnerability of Indian code designed RC frame buildings”. ISET journal of Earthquake Technology, 46(1), 29-45. [3] Soni, D. P., & Mistry, B. B. (2006). “Qualitative review of seismic response of vertically irregular building frames”. ISET Journal of Earthquake Technology, Technical Note, 43(4), 121-132. [4] Moehle, J. P. (2002). “Seismic response of vertically irregular structures. Journal of Structural Engineering”, 110(9), 2002-2014. [5] Moon, D. S., “Lee, Y. J., & Lee, S. (2018). Fragility analysis of space reinforced concrete frame structures with structural irregularity in plan”. Journal of Structural Engineering, 144(8), 04018096. [6] Phadnis, P. P., & Karjinni, V. V. (2019). “Fragility curves for steel–concrete composite shear wall building with torsional irregularity”. Asian Journal of Civil Engineering, 20, 1163-1178. [7] Saikrishna, J., & Aparna, K. (2021). “Response Reduction of Plan and Vertical Irregular Structures Using Dampers” [8] Zabihullah, P. S., & Aryan, M. Z. (2020). “Effect of (vertical & horizontal) geometric irregularities on the seismic response of RC structures”. International Journal on Emerging Technologies, 11(3), 965-974. [9] Phadnis, P. P., & Karjinni, V. V. (2019). “Fragility curves for steel–concrete composite shear wall building with torsional irregularity”. Asian Journal of Civil Engineering, 20, 1163-1178. [10] Moon, D. S., Lee, Y. J., & Lee, S. (2018). “Fragility analysis of space reinforced concrete frame structures with structural irregularity in plan”. Journal of Structural Engineering, 144(8), 04018096. [11] Salehi, I., & Nateriya, R. (2018). “Seismic Evaluation of Vertical Irregular Building with Setback”. [12] Huh, J., Tran, Q. H., Haldar, A., Park, I., & Ahn, J. H. (2017). “Seismic vulnerability assessment of a shallow two-story underground RC box structure”. Applied Sciences, 7(7), 735. [13] Shelke, R. N., & Ansari, U. (2017). “Seismic analysis of vertically irregular RC building frames”. Int. J. Civil Eng. Technol, 8(1), 155-69. [14] Sharma, A., & Bhadra, B. (2013). “Seismic analysis and design of vertically irregular RC building frames” (Doctoral dissertation).

Copyright

Copyright © 2024 Mohammed Khaja Muqeeth, Nagapurna Chandan Thammishetti. 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.