1. School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798;
2. Key Laboratory of Disaster Prevention and Structural Safety of Ministry of Education, School of Civil Engineering & Architecture, Guangxi University, Nanning, 530004, China
Abstract：Traditional constant-strength ductility demand spectral is difficult to consider the influence of strength degradation, stiffness degradation, pinching effect and coupling effect of bi-directional restoring forces effectively. In this study, the fundamental equation of the inelastic seismic response spectral for the single mass with two-degree-of-freedom (SM-2DOF) system under bi-directional excitations was proposed based on the biaxial Bouc-Wen-Baber-Noori model. Then the influence of strength degradation, stiffness deterioration, pinching effect and coupling effect of the bi-directional restoring forces on the seismic ductility demand of the SM-2DOF system were investigated quantificationally. Furthermore, the mean and standard deviation prediction models for the constant-strength ductility demand spectral were developed. The results show that the strength degradation, stiffness deterioration and pinching effect could increase the seismic ductility demand of the SM-2DOF system significantly; and the influence of the coupling effect of bi-directional restoring forces on the seismic ductility demand is largely dependent on the competitive relationship between the hysteretic energy along the two principal direction of structures; the degradation, pinching and coupling effect could increase the standard deviation of the constant-strength ductility demand spectral, especially for the SM-2DOF system with high nonlinearity; Finally, the mean value and standard deviation of constant-strength ductility demand spectral decrease with the increasing of fundamental vibration period along the two principal direction of building structures.
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