A constant ductility response spectrum (CDRS) readily provides the yield strength of a single-degree-of-freedom (SDF) system necessary to limit the ductility imposed by a ground motion to a specified value. The allowance of inelastic deformation of an SDF system leads to reduction of the required yield strength. In this study, we discussed inelastic behavior of SDF systems located on floors by investigating CDRS for floor motions (CDFRS) amplified by the structures. We indicate that the reduction of the required yield strength depends on three effects; strength increasing, deformation increasing and hysteresis damping with increasing value of the ductility factor, and the required yield strength becomes less sensitive to the natural period of the system. We also indicate that the second stiffness of a system doesn't strongly affect the required yield strength. Based on these results, we propose a simplified method to obtain the yield strength of an SDF component necessary to limit the ductility imposed by a floor motion to a specified value by using the CDFRS.

For seismic design of components, elastic response spectrum analysis, which is based on linear response using elastic response spectrum, is generally used as a dynamic analysis of multi-degree-of-freedom(MDOF) systems not only elastic response but also elastoplastic response. Elastoplastic response is different from elastic response due to nonlinearity of the system by yielding of components. Several researchers have proposed response spectrum analysis methods of nonlinear MDOF systems using equivalent linearized methods, which replace a nonlinear system to the linear system equivalent to the nonlinear system on the response. We developed a response spectrum analysis method using elastoplastic response spectrum and not replacing a nonlinear system to the linear system as a dynamic analysis method of nonlinear MDOF systems. This method is obtained from exact equations of motions by just two assumptions; no interaction between natural moods and modal combination rules. We verified this method was effective in the calculated 2-DOF systems. This method is also considered of value for interpretation on the response of nonlinear MDOF systems by spectrum modal approaches.

The ductility factors of nonlinear SDOF systems at Service Limits Ds "using elastic analysis design" in JEAC4601 are investigated, and it was confirmed that the ductility factors depend on the natural frequencies of systems, seismic motions and constant loads. Based on the above results, an acceptance criterion of components to prevent ductile failure and plastic collapse is proposed. The criterion is given as a limit of ductility factor for Service Limits Ds "using elastoplastic analysis design". The proposed limit of ductility factor allows single state for nonlinear systems, and doesn't depend on the natural frequencies of systems, seismic motions and constant loads. JEAC4601-2015JEAC4601- ( 2015 Ds Ds