PSI - Issue 70

Aman Kumar et al. / Procedia Structural Integrity 70 (2025) 255–262

256

1. Introduction Past earthquakes in India have shown how buildings have experienced significant damage, highlighting the need for better safety measures. A building's seismic vulnerability during an earthquake depends on various factors such as ground shaking, structural dynamics, and local soil conditions beneath the structure. Ground motion amplification is a critical factor that influences the seismic vulnerability of buildings. Soil amplification is a phenomenon where local soil properties amplify ground motion intensity, which increases seismic forces that may surpass the design capacity of a structure, potentially leading to damage or complete failure. It can also induce liquefaction in loose, saturated soils, causing a loss of strength and resulting in foundation failure or the total collapse of the building (Seed and Idriss, 1982; Ozsarac et al.,2022). Furthermore, uneven site amplification across a building's footprint can lead to differential settlement, inducing structural stresses, cracking, and instability, further compromising the building's integrity. The seismic vulnerability of unsymmetrical reinforced concrete (RC) buildings is significantly affected by soil-induced seismic amplification (Kramer, 1996; Chopra, 2012). This effect is predominantly observed in irregular structures, where torsional effects increase the seismic demand. Soft soil profiles can amplify low-frequency ground motions, which resonate with the natural periods of mid- to high-rise unsymmetrical buildings, increasing inter-story drifts and plastic hinge formations (Karapetrou et al., 2015; Janous & Ghoulbzouri, 2023). The past study shows that ignoring soil amplification effects can underestimate seismic forces by up to 20 – 30% (Pitilakis & Petridis, 2021), increasing structural vulnerability to earthquake-induced damage or collapse (Tomeo et al., 2015). The performance-based study is an essential tool for developing precise fragility curves for seismic vulnerability assessment of buildings, as it offers a realistic and detailed understanding of how structures behave under seismic loads. Nonlinear analysis shows the true behavior of structures when elastic limits are exceeded. These approaches more precisely capture the progression of damage and the mechanisms of failure compared to linear methods. A fragility curve derived from the nonlinear analysis (pushover and time history analysis) allows for a more precise estimation of damage states and their corresponding probabilities (Kumar et al., 2024; Kumar & Ghosh,2023; 2024). While numerous studies have analyzed the seismic behavior of symmetrical reinforced concrete (RC) structures, research is scarce concerning the impact of soil amplification on unsymmetrical buildings. This study explores the impact of soil amplification on a 5-storey T-shaped RC frame building. It utilizes three compatible time history data with the Maximum Considered Earthquake (MCE) response spectrum. Soil amplification effects were assessed through borehole data, analyzing soil stratification, SPT-N values, shear wave velocity, and layer thickness. Pushover analysis and time history analysis evaluated the seismic response in terms of base shear capacity, roof displacement, and ductility demand of the building due to the amplified ground motions. The variation in damage probability of the building due to amplified ground motion has been estimated through the fragility analysis of the building. This study shows that ignoring the soil amplification effect in seismic design may cause an underestimation of seismic forces, which makes structures more susceptible to damage or collapse during an earthquake (Pitilakis & Petridis, 2022; Ozmen, 2023). By accurately considering soil amplification, engineers can design safer and more resilient structures. 2. Numerical Modeling and Structural Characteristics In present study considers a 5-storey T-shaped unsymmetrical building with a total height of 17.5 m (each story height 3.5 m). The building is fixed at the base. The numerical model of the building is presented in Figure 1 (a, b, and d), which represents the Plan, elevation, and 3d model of the building prepared in the SAP 2000. The building has a T-shaped plan with 4 bays along both the x-axis and y-axis, each bay having a width of 4 m. Dead and live load (Floor LL- 3 kN/m 3 and Roof LL -1.5 kN/m 3 ) are applied as per the IS 875:1975 Part 1& 2 and earthquake loading is considered as per IS 1893: 2016 part 1 (Zone Factor Z- 0.36 (V), Importance Factor I -1.5, Response reduction Factor R-5, and Medium stiff soil). Building has been analysed and designed using M25 grade of concrete (characteristic compressive strength f’ c of 25 MPa) and Fe 500 grade of steel (yield tensile strength f y of 500 MPa) as per the IS 456:2000. Slab thickness is 125mm and wall thickness is 230 mm. The beam and column sizes of the buildings are 230*350 mm and 350*350 mm, respectively. Section details and reinforcement details of the beam and column are presented in Figure 1c. The building’s demand capacity ratio is less than 1, and the inter -storey drift ratio is within the permissible limit as per Indian codes. To perform the nonlinear analysis, the plastic hinges are assigned to the beam