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Seismic Stability Analysis of Concrete Gravity Dams Bakenaz. A. Zeidan, Ayman Seleemah Faculty of Engineering, Tanta University, Egypt Email: b.zeidan@f-eng.tanta.edu.eg Abstract: The Analysis of dam-reservoir-foundation coupled system is a complicated problem due to mutual interaction between reservoir water, rock foundation and concrete dam. In order to design earthquake resistant dams, it is essential to have accurate and reliable analysis procedures to predict the dam response. In this paper, KOYNA concrete gravity dam subjected to four different normalized ground motion excitations was investigated as a typical case study. A 2-D Finite Element model of the dam was constructed using ANSYS computer code to simulate dam-reservoir- foundation coupled system. The reservoir water domain is represented as incompressible and inviscous fluid continuum having infinite length in the upstream direction. The foundation rock is represented as a viscoelastic half space considering both its mass and flexibility. Both the mass concrete and foundation rock are assumed to have homogenous, isotropic, linearly elastic properties. Seismic dam responses are expressed in terms of dam deformations, dam stresses, spectral displacements and accelerations, hydrodynamic pressure, natural frequency and response time history. Obtained results confirm that El-Centro earthquake caused the minimum displacement and stresses while Aqaba-EW earthquake resulted in maximum displacements and stresses. Spectral results confirm the significant effect of dam natural period on its seismic behavior. Keywords: Concrete Gravity Dams, FEM, Seismic Analysis, Dam-Reservoir-Foundation Interaction, Ground motion excitation, Spectral analysis, deformations, Stresses, ANSYS. Introduction Concrete dams are very important structures, regarding to requirements for continuous service during their life time, and catastrophic effects in cases of dam failure. Therefore, the safety of these structures should be investigated quite critically by logical and precise methods. The fluid-structure- foundation interaction is one of the main factors that affect dam behavior during earthquake excitations. The analysis of dam-reservoir-foundation coupled system is much more complicated than that of the structure alone. For a dam-reservoir-foundation system, the earthquake response is significantly influenced by the interaction of the dam with the impounded water and with the underlying foundation region, thus increasing the requirements for the analysis procedure to be used, and complicating what would otherwise have been considered a routine finite element analysis of a concrete cross-section. State of Art The seismic behavior of concrete gravity dams under strong ground motion was investigated by numerous researchers. Chopra (1967) conducted a study on the determination of the effects of foundation-structure interaction on the seismic response. Das Gupta and Chopra (1977) presented a procedure to produce a complex valued, frequency dependent stiffness matrix for the surface of a dam base which is supporting the structure. Chopra and Chakrabarti (1981) introduced a general procedure for analysis of the response of concrete gravity dams including the dynamic effects of 1474