## 2 Modelling and Analysis of Buildings  
This study focuses on investigating a series of reinforced concrete (RC) structures with identical floor plans. Figure 1 illustrates the 3D structural models of the buildings under consideration, specifically a five- and a ten-story building. Without a soft story, the story height of each building (reference model) has been kept constant at 3 m. The chosen structures represent the dynamic behavior of medium- and high-rise buildings, respectively [16]. The structural models are considered to be RC moment-resisting frames (MRFs). In the case of soft-story buildings, it is anticipated that the soft-story level would possess one of two distinct heights, namely 5 m or 7 m. For all building models, a bay width of 3 m has been maintained. The ten-story and five-story reference models are defined in the current study as M**10 ref** and M**5 ref**, respectively. In the building models under consideration, it is presumable that the soft story will be provided at the top, middle, and bottom levels. Soft stories with 5 m height at the top, middle, and bottom levels of a five-story model are labelled as M**5**5**, M**5**ms5**, and M**5**bs5**, respectively, as shown in Figure 1b-d. Similarly, soft stories with a 7 m height at the top, middle, and bottom levels of a five-story model are labelled as M**5**5**, M**5**ms7**, and M**5**bs7**, respectively. The M**10**ss5**, M**10**ss5**, and M**10**bs5** configurations, which correspond to a 5 m soft story on the top, sixth, and bottom floor levels, respectively, are shown in Figure 2. Moreover, the soft-story height for the M**10**bs7**, M**10**bs7**, and M**10**bs7** types is 7 m. The soft-story buildings meet the IS 1893(Part 1) 2016 [57]irregularity requirements. According to the IS 1893 standard, the stiffness irregularity checks, \(K_{i}/K_{i+1}\) (ratio of lateral stiffness of \(i^{th}\) to \((i+1)^{th}\) story), are performed in both plan directions, and the relevant values are provided in Table 1. This particular study explored the effect of a soft story and its location on the floor response spectra. Previous researchers did not study this effect. Hence, the authors were limited to a linear analysis of simple regular buildings and NSCs under strong ground motions. This study serves as a preliminary investigation for future studies to obtain more generalized research outcomes by considering more complex structures. For the purposes of modelling reinforced concrete, the concrete and steel grades M 30 and HYSD 415 are used. In accordance with IS 875-Part 2 [58], the floor finishes and live load have been set at 1.5 kN/m\({}^{2}\) and 3 kN/m\({}^{2}\), respectively. According to IS 1893-2016, seismic zone V is supposed to represent the location of the buildings. The minimum dimensions specified in IS 13920: 2016 [59] were used to determine the column and beam's primary dimensions. For the frames, the dimensions of the beams and columns (230 mm \(\times\) 450 mm and 300 mm \(\times\) 450 mm) have remained constant. Stiffness irregularity can also be achieved by altering the sizes of columns. By maintaining uniform column sizes from the ground floor to the top floor in the software analysis, the authors aimed to simplify the input and reduce errors. In this research, the authors addressed vertical stiffness irregularities in the structure by adjusting the story heights to meet the criteria outlined in IS 1893-2016. The thickness of the RC slab is fixed at 150 mm for all the models. To evaluate the behavior of the models, the software SeismoStruct (2023 version) [60] was utilized to conduct a linear time-history analysis. This analysis focused on examining the bare frames' elastic responses. The elastic model of the structure that is employed as a reference case aims to imitate the theoretical behavior of the structures while ignoring nonlinear effects during the dynamic response. The columns and beams are defined using the elastic frame element. [PERSON] et al.