Retrofitting of Soft Storey Building Using Different Structural Systems: Software Based Study

Abstract

After the Bhuj earthquake took place, the IS 1893 code was revised in 2002, incorporating new design recommendations to address OGS framed buildings. According to this clause 7.10.3(a) of the same code states: “The columns and beams of the soft-storey are to be designed for the multiplication factor of 2.5 times the storey shears and moments calculated under seismic loads of bare frame”. The prescribed multiplication factor (MF) of 2.5, applicable for all OGS framed buildings, is proved to be fairly higher and suggests that all existing OGS framed buildings (those designed to earlier codes) are highly vulnerable under seismic loading. This MF value however does not account for number of storeys, number of bays, type and number of infill walls present, etc and hence it is independent of all of the above factors. Present study deals with various aspects related to the performance of OGS buildings. The values of magnification factor recommended in literatures vary from 1.0 to 4.8 (Kaushik, 2009). The main objective of present study is the study of comparative performance of OGS buildings designed according to various configuration using seismic analysis. As the more realistic performance of the OGS building requires the modelling the stiffness and strength of the infill walls, the stiffness and strength of the infill walls also considered. The variations in the type of the infill walls using in Indian constructions are significant. Depending on the modulus of elasticity and the strength, it can be classified as strong or weak. The two extreme cases of infill walls, strong and weak are considered in the study. The behavior of buildings depends on the type of foundations and soils also. Depending on the foundations resting on soft or hard soils, the displacement boundary conditions at the bottom of foundations can be considered as hinged or fixed. As the modeling of soils is not in the scope of the study, boundary conditions, fixed, that represent extreme condition is considered. It can be pointed out that the maximum storey drift is occurring in the case of open ground storey building. As the height of the building increases, the storey drift is considerably reducing, i.e. there is a larger drift at bottom storey as compared to other storey of the open ground storey building. It can be seen that the addition of shear wall greatly reduces the ground storey drift. As compared to the provision of diagonal steel bracing, the cross-steel bracing reduces the storey drift to a greater extent. The time period taken for first mode shape is 1.54 seconds for bare model. For OGS model, the time is 0.93. Thus, for OGS model, the time period is less by while compared to the normal model because of the OGS model has more stiffness.

Introduction

The text focuses on the seismic vulnerability of Open Ground Storey (OGS) reinforced concrete buildings, where the ground floor lacks infill walls while upper storeys are infilled. This configuration leads to soft-storey behavior due to a sudden reduction in lateral stiffness at the ground level, causing high stresses in ground-storey columns during earthquakes and often resulting in collapse. Conventional bare-frame analysis, which ignores the stiffness contribution of infill walls in upper storeys, underestimates seismic forces and is identified as a major reason for past failures.

Following the 2001 Bhuj earthquake, the Indian seismic code IS 1893 was revised to address OGS buildings by introducing a multiplication factor of 2.5 for designing ground-storey columns and beams. However, this factor does not account for variations in building height, bay configuration, or infill characteristics, suggesting that many existing OGS buildings remain highly vulnerable under seismic loading.

The study highlights the growing prevalence of OGS and high-rise buildings in urban India due to land scarcity and parking requirements. It emphasizes that seismic damage in multistorey buildings typically initiates at structural irregularities, such as stiffness and mass discontinuities, making irregular configurations less reliable during strong earthquakes. Recent earthquakes, including the 2015 Nepal event, further underscore the need to evaluate and strengthen existing buildings.

The objectives of the study include modeling a G+9 storey Ordinary Moment Resisting Frame with different structural configurations—bare frame, infilled frame, braced frame, and shear wall frame—to analyze soft-storey behavior in high seismic zones. ETABS software is used for analysis in accordance with IS 1893 standards. Structural responses such as storey displacement, drift, bending moments, and shear forces are compared across models.

Results show that models incorporating infill walls, bracings, or shear walls significantly reduce storey displacement and drift compared to bare-frame OGS buildings. The study concludes that strengthening measures like bracing and shear walls are effective in mitigating soft-storey effects and improving the seismic performance of high-rise RC buildings.

Conclusion

The shear wall reduced the Storey Displacement by more than half and storey drift by 99.86%. The Steel bracing reduced the Storey Displacement % and storeydrift . Finally, it has been found that the Shear wall reduces lateral displacement and storey drift, thus significantly contributing to greater structural stiffness. The analysis results recommended that the shear wall use reinforced concrete frames for the seismic hazard zones and the Steel bracing recommended for the high seismic zones. The storey shear for bare frame is lowest compared to infilled frame in X axis and in Y axis from the analysis. The storey shear was increased after providing shear wall at interior.

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Copyright

Copyright © 2025 Rahul Singh Bhadoria, Ms. Chaitali Gangwal , Dr. Chaitanya Mishra. 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.