The seismic performance of the Tsankov Kamak Dam is evaluated using a finite element approach in the time domain. The study investigates the impact of massless and massed foundation models, considering their influence on the dam's seismic response. The numerical model, developed in ANSYS, incorporates fluid-structure interaction, nonlinear interfaces, and the coupled damage-plasticity microplane model for mass concrete, and replaces the gated orifice spillways by interrupted upper arches. Effective earthquake forces are applied at the foundation boundaries to simulate free-field seismic wave propagation using a simplified approach. The results indicate that including foundation mass significantly reduces structural response compared to the massless foundation approach. Nonlinear interfaces properties affect stress redistribution and flexibility, while material nonlinearity reduces peak tensile stresses. Modeling continuous upper arches surrounding the gated orifice spillways, highlights the impact of local geometric discontinuities on stress distribution in the originallytruncated geometry model of the dam.