Soil-Structure Interaction (SSI) analysis is the study of the dynamic response of a structure as influenced by the interaction with the surrounding soil. The SSI response is sensitive to the characteristics of the soil, structures, and ground motion, as well as the depth of embedment. Availability of soil dynamic properties is, therefore, of paramount importance for performing such SSI analysis. However, detailed soil information and associated engineering properties may not always be available at the beginning of a project. Therefore, the analyst may rely on simplified yet conservative methodologies to estimate the dynamic response of the coupled soilstructure system to generate preliminary or interim seismic responses. This paper examines a particular case of nuclear power structures founded on competent rock material, in which the diminished SSI effects allows for a fixed-base treatment of the various safety related buildings. To evaluate the adequacy of this simplified approach for interim type of analysis, two structures are considered in this study. The first structure has a large footprint and shallow embedment and is mostly subject to rocking responses. The second structure has a small footprint and relatively large embedment. The two structures are studied with varying backfill conditions and modeling approaches. SSI analysis is completed using SASSI2010 [2011] and the following outputs are considered for evaluation purposes: transfer functions, zero-period accelerations, and acceleration response spectra. Results are presented in the paper to demonstrate the validity of the approach as well as the limitations when considering embedment effects
