Seismic failure of major concrete dams can be disastrous due to sudden release of reservoir water. At the present study, 203？m DEZ arch dam was selected as case study, and two types of nonlinearity were incorporated in seismic analysis of dam, joint nonlinearity and material nonlinearity. The finite element model of the dam, soil, and water was excited using multicomponent maximum design earthquake record which was extracted from seismic hazard analysis of the dam site. Also seismic performance of the dam was evaluated based on linear analysis. The extension of overstressed areas, demand-capacity ratio, and cumulative inelastic duration were used to identify the necessity of nonlinear analysis. It was found that when contraction joints between dam blocks are modeled, the direction of the principal stresses and their distribution patterns are changed meaningfully. In addition, overstress surfaces on the dam body change in comparison with the model without contraction joints. 1. Introduction Generally, analyses of arch dams and preliminary study of responses are based on linear elastic (LE) model of dam-soil-water coupled system. If observed responses of the dam under seismic loads and estimated damage by engineering judgment satisfy predefined criteria, so using only LE model for interpretation of results is enough. On the other hand, if severe damages, high stresses, and abnormal drifts are observed during seismic load, it is impossible to ignore the necessity for further investigations considering nonlinear behavior of concrete and/or joints. There are several approaches for modeling the stress-strain relationship of the mass concrete. In this regard, several researches have been conducted to study the dynamic behavior of concrete arch dams. Hall proposed a simple smeared crack model for modeling contraction and construction joints and diagonal cracking that is neglected in his work [1]. Espandar and Lotfi applied nonorthogonal smeared crack approach and elasto-plastic models on Shaheed Rajaee arch dam in Iran [2]. Mirzabozorg et al. developed a model based on damage mechanics approach in 3D space [3, 4]. Also, they developed a model based on smeared crack approach in 3D space. In their work, they analyzed 3D models including dam-reservoir interaction effects and considered nonlinear behavior of structure [5]. Calayir and Karaton analyzed gravity dam-reservoir system using continuum damage model of concrete [6]. Their model was a second-order tensor and includes the strain softening behavior. Akk？se et al. investigated elastoplastic response of arch
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