Although numerous techniques are available to evaluate the seismic performance of gravity dams, there is still a demand for practical approaches suitable for everyday engineering routine. Dynamic modal superposition and response spectrum analyses are popular choices among practicing engineers. However, they typically involve complex numerical formulations to couple the dynamic responses of the solid and fluid domains in structure-water systems like dams. Such advanced solutions generally require specialized software and a significant level of expertise. Traditional simplified techniques model dam-reservoir systems as equivalent single-degree-of-freedom systems to evaluate fundamental mode response. Higher vibration modes are accounted for approximately via static correction. This work presents a more accurate alternative to conduct time-history modal superposition and response spectrum analyses of dams while directly accounting for higher mode effects in lieu of static correction. The approach's main novelty consists of incorporating earthquake-induced hydrodynamic effects into modified versions of the input ground motion acceleration and its response spectrum. The resulting seismic demands can then be directly applied to the numerical model of the dam with empty reservoir, thus eliminating the need for specialized fluid-structure software. Numerical examples are provided to illustrate the application of the proposed technique and its performance in evaluating key seismic response indicators. Diverse results are discussed, notably that hydrodynamic effects generally amplify seismic demand relative to the dry dam case along the fundamental mode, while causing reductions in certain cases depending on the ground motion characteristics and the vibration mode under consideration.