Study on damping reduction factor for buildings under earthquake ground motions

This paper discusses the rationality of the design force and damping reduction factors adopted by a few seismic design provisions for buildings with and without added passive energy dissipation systems. The issue will first be pointed out that the damping reduction factors adopted by those provisions are derived from the effects of viscous damping on displacement responses, but are used to reduce the design force of buildings. Statistical results from 1053 ground motions recorded in the U.S. show that it may lead to unconservative results, especially for systems with damping ratios greater than 10% and periods longer than 0.15 s. Furthermore, although there is no doubt that the additions of extra damping to a structure will always reduce the displacement responses, many documents argue the effect of added damping to reduce the force responses of the buildings. Therefore, this paper also addresses the effects of viscous damping on the inertial force and elastic restoring force in order to use the damping reduction factors correctly. Results of this study suggest that if the damping of structures comes from the hysteretic response of the building, the design force of the structures should be the inertial force and the damping reduction factors should be derived from the acceleration responses. Otherwise, if the additional damping of structures comes from the added energy dissipation devices, the design force should be the restoring force and the damping reduction factors should be derived from the displacement responses.