Formulation of Duration-Dependent Inelastic Seismic Design Spectrum

In this paper, a procedure for developing the duration-dependent inelastic seismic design spectrum is proposed. The approach assumes that the lateral strength of the structure can be increased to compensate for the increased number of inelastic cycles that may be imposed on the structure under a long duration ground motion. Seismic input energy is considered as a part of the demand on the structure, and an empirical prediction of the seismic input energy in terms of the ground motion duration is made. Seismic design is established by requiring the structure to have a cyclic plastic strain energy capacity that is larger or equal to the portion of the seismic input energy contributing to cumulative damage. The plastic strain energy capacity of the structure is calculated using an energy-based cumulative damage model. Results from this study indicated that an increase in the lateral strength demand can be expected for an increase in the duration of the ground motion, and the increase in lateral strength is largest near the characteristic period of the ground motion. The strength deterioration parameter that characterizes the plastic strain energy capacity of the structure has been found to have a significant influence on the duration-dependent inelastic seismic design spectrum.