Abstract: Different earthquake input models, i.e. massless foundation and viscous-spring boundary models, are employed to analyze the seismic response of gravity dams. The effects of different spectral earthquakes, different ratios of modulus of foundation rock to dam concrete and different dam heights are taken into account. A concept of increased equivalent structural damping used in massless foundation model for approximately considering the effects of radiation damping is presented based on case studies. The results demonstrate that the crest displacements and the maximum tensile stresses in dams are reduced using the viscous-spring boundary model compared with those obtained by the massless foundation model. The reduction amplitude of the response ranges between 10% and 65%, and it decreases with the increasing modulus of foundation rock. It is concluded that the structural response using the massless foundation model with increased equivalent structural damping is close to that obtained from the viscous-spring boundary model with an error of 7%.