Structural Safety Evaluation of Residential Apartment Located at Indore City Having Plan Irregularity and Considering Different Importance Factors

Abstract

In the domain of structural engineering, significant focus has been placed on the issue of plan irregularities in multistoried buildings. Plan irregularities have been defined as the misalignment or displacement of structural components in relation to vertical load resisting members. To investigate this, a series of simulations were conducted considering importance factors of 1.0, 1.2, and 1.5 as per Indian standards (SS-A1 to G1, SS-A2 to G2, SS-A3 to G3). A total of 21 distinct cases involving plan irregularities were examined to develop a comprehensive understanding. By conducting the drift analysis as per special limitations for plan irregularities, the performance of each case has carefully documented. Among all variations, several cases like SS-A1, SS-B1, and SS-C2 remained well within the codal drift limits, even for Z-direction displacements. On the other hand, configurations like SS-G3 approached the upper bound of permissible drift, underscoring the need for careful placement of irregularities and appropriate factor selection. Through this approach, optimal structural resilience in buildings with plan irregularities has effectively identified.

Introduction

In multistoried building construction, columns and beams are ideally aligned vertically and horizontally, but deviations occur due to foundation settlement, fabrication errors, or construction tolerances. Minor misalignments usually have little impact, but larger offsets can reduce structural capacity by inducing extra bending stresses, threaten stability through buckling or tilting, and cause aesthetic damage. To avoid these issues, strict monitoring, quality checks, proper reinforcement, and accurate installation are essential.

The Indian seismic code IS 1893:2016 introduces an importance factor to address building occupancy and function during earthquakes. Ordinary buildings have a factor of 1.0, while critical structures like hospitals and emergency centers have higher factors (1.2 or 1.5), which increase the design base shear and strengthen the structure accordingly, aligning Indian seismic practices with global standards.

The study involves seismic analysis of a ten-story residential apartment with plan irregularities introduced at various floor levels. Different cases consider three importance factors (1.0, 1.2, and 1.5). The analysis uses real soil data, seismic zone III parameters, and response spectrum methods per IS 1893 standards.

Research objectives include:

1. Creating models with plan irregularities at different floors and importance factors.

2. Using actual soil conditions for accurate behavior prediction.

3. Applying response spectrum dynamic analysis via software.

4. Evaluating and comparing story drift in two directions to assess irregularity effects.

5. Validating structural responses against IS 1893 drift limits, especially stricter criteria for irregular buildings.

The goal is to identify the optimal plan irregularity case under different importance factors that meets Indian code requirements and can be recommended for safe residential construction.

Conclusion

Conclusion pointed out as per objectives selected are as follows:- 1) The creation of various cases of residential apartment with plan irregularities introduced at various floor levels, under varying seismic importance factors (I = 1.0, 1.2, and 1.5) has completed, considered 7 each importance factor related cases, modelled total of 21 cases. 2) The actual soil condition has introduced in all simulated cases by taking a soil investigation report and determined the horizontal spring stiffness (in this research, the value comes to be 7544444 kN/m respectively) 3) The Response Spectrum Method under dynamic analysis has used with zone factor Z = 0.16 using analysis software. 4) The evaluation and comparison of story drift responses in X and Z directions across different structural models (SS-A1/A2/A3 to SS-G1/G2/G3) has conducted. The results obtained for story drifts in both X and Z directions indicate a consistent trend as the importance factor increased, the corresponding story drifts also increased. For all structural series (SS-A through SS-G), it was observed that: • Models with higher importance factors consistently recorded greater drift values, especially at the mid and upper storeys. • Among all levels of plan irregularity, irregularities placed at higher storeys (e.g., SS-G series at 9th floor) showed more significant drift amplification, such that upper level irregularities are more critical in seismic response. • The Z-direction drift was generally higher than the X-direction, suggesting directional stiffness variation in the structural layout or irregularity impact. • The increase in drift was not linear across importance factors, the jump from I = 1.2 to I = 1.5 induced relatively larger increments compared to I = 1 to I = 1.2, highlighting the more seismic sensitivity. • This analytical investigation thus confirms that both the placement of plan irregularities and the selection of importance factor critically influence the seismic behaviour of multistory residential buildings. The validation of drift responses against codal storey drift limitations as per IS 1893 (Part 1): 2016, with special attention to more severe drift criteria (L/500) applicable to irregular buildings obtained as: Storey Drift Limitation as per IS Code (Clause 7.1, Table 5, sub section iv) According to IS 1893 (Part 1): 2016, the maximum allowable storey drift under seismic loading for general buildings shall not exceed 0.004 times the storey height (L/250). However, for buildings with discontinuities such as plan irregularities, vertical offsets, or setbacks, the permissible limit is more stringent i.e. 0.002 times the storey height (L/500) to ensure structural integrity under seismic excitation. In the present study, the evaluated models with different importance factors and irregularity levels were assessed against these codal limits. It was observed that in most configurations, models with higher importance factors (I = 1.5) exhibited higher story drift values, especially in cases where the plan irregularity was located at upper storeys (e.g., SS-G3). However, in several cases like SS-A1, SS-B1, and SS-C2 remained well within the codal drift limits, even for Z-direction displacements. On the other hand, configurations like SS-G3 approached the upper bound of permissible drift, underscoring the need for careful placement of irregularities and appropriate factor selection. Hence, ensuring compliance with these drift limitations is has recommended that the codal checks confirm the analytical results and highlight the importance of combining importance factor selection with architectural regularity to prevent drift-induced damage or instability during earthquakes.

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Copyright © 2025 Salim Shaikh, Chetan Gurjar. 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.