Modeling viscous damping in nonlinear direct-integration and modal time-history analyses of base-isolated reinforced concrete buildings

The effects of modeling viscous damping on the response of base-isolated reinforced concrete buildings subjected to earthquake ground motions are investigated. The test results of a reduced-scale three-story building previously tested on a shaking table are compared with nonlinear direct-integration and modal time-history analyses. Nonlinear direct-integration time-history analysis results reveal that the damping ratio as well as the approach used to model damping has significant effects on the response, and quite importantly a damping ratio of 1% is more appropriate in simulating the response, than a damping ratio of 5%. It is shown that stiffness-proportional damping, where the coefficient multiplying the stiffness matrix is calculated from the frequency of the base-isolated building with the post-elastic stiffness of the isolation system, provides reasonable estimates of the peak response indicators, in addition to being able to capture the frequency content of the response very well. In contrast, for nonlinear modal time-history analysis, frequency-dependent damping, where zero damping is assigned to the frequencies below the fundamental frequency of the superstructure for a fixed-base condition and 5% damping is assigned to all other frequencies, is more appropriate, than 5% constant damping.