Problems with Rayleigh Damping in Base-Isolated Buildings

In dynamic analysis, the energy dissipation in a base-isolated building is typically accounted for by allocating damping properties independently to the superstructure and to the isolation system. Rayleigh damping applied to the superstructure component alone is often used to represent the superstructure energy dissipation. At least one study has indicated that when used improperly, superstructure Rayleigh damping leads to excessive damping of the response of a base-isolated structure. As shown here, even when used as recommended for combining subsystems with disparate damping properties, Rayleigh damping results in undesirable suppression of the first mode response. To correct this behavior, stiffness-proportional damping can be used in lieu of Rayleigh damping. Stiffness-proportional damping is demonstrated to negligibly affect the first mode response, yet provide the expected energy dissipation in higher modes.