Seismic time history analyses are generally required for dams with high seismic risk in case of failure. Several calculation approaches with various levels of complexity can be applied. Considering a massless foundation with the Westergaard’s added masses for the dam/reservoir interaction is one the most common but rather simplified approach used for concrete dams. The very few back-analyses of existing dams show that this simplified approach generally leads to significant overestimation of the dam response, especially for gravity dams. A more advanced approach, taking into account the radiative boundary conditions for both foundation and reservoir exists and is more and more used. The current paper shows that such approach is able to simulate the recorded seismic behavior of a large Japanese gravity dam with a very satisfactory accuracy. Owing to this approach, the engineering consulting firms are now able to provide the dam Owners with optimized seismic numerical simulations. The main target is to avoid unnecessary strengthening works for some old dams but also to achieve significant saving on concrete quantity for new projects.