Hybrid Low-Viscous-Damping Inelastic Displacement Ratios for Seismic Design and Assessment of Structures

Until recently, the viscous damping ratio pervasively assumed has been ζ=5%. However, the dynamic identification of the experimental response recorded in recent large-scale shake-table tests shows that lower levels of viscous damping are more appropriate when material nonlinearities are explicitly accounted for. This paper investigates a hybrid constant-ductility inelastic displacement ratio, Cμ2, which identifies the inelastic demand for a viscous damping ratio ζ=2% when the elastic demand for ζ=5% is known. A comprehensive statistical analysis is developed, and analytical estimates are provided, for inelastic single-degree-of-freedom systems with a wide variation of the constitutive parameters and a large earthquake database.