Ductility Demands Control Under Repeated Earthquakes Using Appropriate Force Reduction Factors

This paper develops a new procedure for the ductility demands control of single-degree-of freedom (SDOF) systems under multiple earthquakes. According to the proposed method, this objective can be achieved using appropriate force reduction (R) factors. In this way, the total ductility demands, and therefore the cumulative damage levels, are absolutely controlled. The influence of structural period, local site conditions, post-yield stiffness, and viscous damping is taken into account. In order to achieve a unique, simple, and effective empirical expression for the R factors, a comprehensive nonlinear regression analysis is applied. Thus, a complete statistical investigation of 3,360,000 inelastic time-history analyses are carried out to study 8400 SDOF models excited by 100 earthquake accelerogram records, under 4 cases of seismic sequence. It is found that multiple earthquakes require increased ductility demands and repeated frequent or smaller ground motions necessitate equal or smaller force reduction factors in comparison with the idealized or isolated "design earthquake." Therefore, the multiplicity of earthquakes leads to more conservative force reduction factors in comparison with the "design earthquake."