## 6 Conclusions  
In structural engineering, accurately calculating the seismic loads on a structure is essential to ensuring its safety under dynamic loads. The most critical parameter for determining the seismic load imposed on a structure is the acceleration of earthquake ground motion. Since these accelerations cannot be predicted in advance, various approaches are adopted in structural design. One of the most important approaches is the creation of displacement and acceleration design spectra based on previously recorded earthquakes. Design spectra under different soil conditions are provided in building codes. Numerous studies have examined the dynamic interaction between structure, soil, and foundation. When a structure is placed on soft soil, displacements at the foundation level caused by inertial forces (resulting from rotation and translation) dampen seismic energy. Increased damping and decreased natural vibration frequencies significantly alter the structure's dynamic behavior. As damping and the period of the structure change due to the structure-soil interaction, the design force required for analysis becomes greater than that for fixed-base structures. This study investigates the dynamic interaction between the soil, superstructure, and rigid foundation block under seismic loading. Four types of soil are considered: strong rocks (ZB), weak rocks (ZC), solid clay (ZD), and soft clay (ZE). The goal of this study is to clarify the significance of SSI effects and to examine how key parameters, such as the dimensions of the foundation block (including its base area and thickness) and variations in the height of the superstructure, affect the displacement and acceleration response spectra of structures. The main findings of the study are as follows:  
* For superstructures with large foundation blocks, such as (20 m \(\times\) 20 m) and (30 m \(\times\) 30 m), response values increase regardless of soil type. This increase is primarily due to the larger foundation mass, which amplifies earthquake forces. Consequently,  
Figure 20: Displacement variation at the foundation level for a superstructure (\(H\) = 7 m, \(T\) = 0.5 s) with a (10 m \(\times\) 10 m \(\times\) 0.5 m) foundation. the effect of the soil is dampened, and the structure behaves more like a fixed-base structure (as shown in Figures 8 and 9). * When the foundation block size is reduced to (10 m \(\times\) 10 m), significant changes in spectral displacement (SD) and spectral acceleration (SA) values occur. The system's flexibility increases compared to a fixed-base structure, altering its natural vibration periods. This flexibility is more pronounced in buildings with smaller foundations situated on soft soils (as shown in Figure 7). * The flexibility of soil-interacting structures alters their natural vibration periods with softer soils increasing the spectrum characteristic periods and extending the plateau region in spectral acceleration (as shown in Figure 10). This shift highlights the importance of accounting for soil-structure interaction effects in design, as assuming a non-interacting system may underestimate the dynamic response. * Changes in foundation thickness (\(h\) = 0.5 m, 1.5 m, and 2.5 m) do not significantly affect the spectral displacement (SD) values for any soil type (as shown in Figures 11-14). However, structures on soft soils (ZD and ZE) exhibit a decrease in more than 15% in SD values compared to those on hard soils (ZB and ZC). This is due to the increased flexibility of soft soils, which reduces seismic loads on the structure. Superstructures on soft soils experience greater vertical settlements but less structural damage, while those on hard soils show higher relative displacement values. * The analysis of the effects of different building heights (\(H\) = 7, 14, and 21 m) on acceleration and displacement response spectra reveals significant findings.