Seismic Reliability Assessment of Direct and Modal Methods: Application to Sarıyar Dam

Accurate seismic assessment of structures involving soil–structure interaction (SSI), water–structure interaction (WSI), and soil‐structure–water interaction (SSWI) is crucial for ensuring the safety of critical infrastructure, such as concrete gravity dams. This study presents a rigorous comparative analysis of the direct method (DM) and the modal analysis method (MAM) in evaluating the seismic responses of the Sarıyar Dam under three real earthquake records: 1967 Koyna, 1999 Kocaeli, and 1989 Loma Prieta. Finite element (FE) models developed in FORTRAN 90 were subjected to identical boundary conditions (BCs) to isolate the effect of the solution strategy. Whereas DM directly solves the coupled equations, MAM relies on modal superposition, and its accuracy depends on the selected modes. The modal sensitivity analysis reveals three key findings based on cumulative modal mass participation. Using only sloshing modes in SSWI captures less than 1% of the total mass, leading to errors approaching 99.9% in displacements and stresses. Including the first dominant structural mode increases participation to about 45% and reduces discrepancies to ≈14.2% for displacements and 34.4% for stresses. Using dominant structural modes alone achieves the closest agreement with the DM, with differences mostly about 6.2%, highlighting the governing role of structural modes in reliable seismic analysis.