REDUCED FREQUENCY-INDEPENDENT MODELS FOR SEISMIC ANALYSIS OF CONCRETE GRAVITY DAMS

A methodology is presented to allow a consistent and rational transition from linear frequency-domain model of concrete gravity dam-foundation-reservoir systems to time-domain models suitable for nonlinear seismic analysis. Two types of reservoir models are considered. These are an added mass approach based on the Westergaard method, and a finite element formulation using displacements as nodal variables to represent the reservoir. Coordinate reduction techniques are used to reduce the number of degrees-of-freedom needed to represent properly the reservoir and the foundation. A simplified time-domain model of energy dissipation in the reservoir and the foundation is developed by providing boundary dampers at the interface of the different media along with added stiffness and mass matrices. The added system matrices are calibrated to provide low level (elastic) time-domain response compatible to that obtained by using frequency-dependent system properties. The application to a typical gravity dam shows that the seismic response in time-domain can be properly predicted using the proposed methodology to develop frequency-independent system properties.