For structures with underground stories, defining the location of the seismic base is crucial for determining the seismic forces induced in the structure during an earthquake. While various building codes define the seismic base differently, most commonly, it is placed at the ground level when the structure and retaining walls are connected, such as in the top-down construction method. In this study, the seismic response of a superstructure adjacent to soft soils is examined in three different states to investigate the effects of connecting the superstructure and retaining walls. The results show a significant increase in the structural response, while current seismic codes typically predict a slight decrease. Additionally, determining the seismic base at the foundation level, which is commonly considered the worst-case scenario, does not account for the observed significant increase in seismic response. Therefore, the recommendations in current seismic codes may be inadequate or misleading in certain situations. The analysis, based on numerical simulations using FLAC2D and the Capped Hardening model for non-linear soil behavior, showed significant increases in structural responses: Shear force = 21%, Acceleration = 14%, Drift = 25%, and Displacement = 19%. These findings indicate that incorporating more detailed simulations may improve the ability of seismic codes to capture the complex behavior observed in this study, particularly for buildings on soft soils. This research contributes to bridging the knowledge gaps in seismic design practices for high-rise buildings on soft soils.

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