This study is focused on the nonlinear behavior of ground-supported open top circular concrete tanks under the effect of seismic loads. The tank support conditions are considered in this study where both flexible and nonflexible supports are investigated. A comparison between the behavior of reinforced concrete (RC) and prestressed concrete (PC) tanks is undertaken for flexible base condition. The finite element (FE) method is used to study the nonlinear response of circular tanks under dynamic time-history and push-over analysis. Furthermore, the response modification factors (R) included in current practice are evaluated based on the results of nonlinear dynamic and push-over analysis. Several tank configurations with different aspect ratios, construction method, and base conditions are used in this study to attain reliable results and to validate the Rvalues. The behavior of circular RC tanks under shrinkage effect is also investigated. Moreover, an innovative approach is presented in this study for flexile base tanks in order to further reduce the seismic response of these structures by using passive energy dissipation systems such as fluid viscose dampers (FVD). The results of this study show that higher R-values could be applied to fixed base tanks as compared to hinged base tanks. Also, shallower concrete tanks can be assigned higher R-values as compared to tall tanks. The results of this study show that the type of construction affects the tanks ductility. PC tanks show lower ductility as compared to RC tanks. Furthermore, this study shows that the flexible base tanks with seismic cables do not dissipate the seismic forces, as expected, due to the elastic behavior of the seismic cables. Based on the results of the FE analysis, it is shown that, using FVD reduces the tank response under seismic loads. The use of FVD improves the tank serviceability by reducing the concrete cracking. It is concluded that flexible based tanks equipped with FVD can be used as an economically feasible system in high seismic zones.