This paper reports on investigations into seismic ductility reduction factors for vertically irregular multi-degree-of-freedom (MDOF) systems subjected to pulse-like ground motions. Ductility reduction factors of irregular MDOF systems are obtained by modifying those of reference regular MDOF systems through a modification factor. Specifically, the structural vertical irregularities are introduced by changing the combined-strength-and-stiffness at the first storey of a reference regular system. The five main parameters of irregularity ratio, ductility ratio, fundamental period, storey number and velocity pulse of ground motions, have been investigated to gain insight into the modification factors. It is demonstrated that ductility reduction factors for irregular MDOF systems are clearly smaller than those for regular MDOF systems. The modification factor is mainly affected by irregularity ratio, and ductility ratio, decreasing with decreasing irregularity ratio and increasing ductility ratio. However, the storey number and fundamental period are observed to have little influence on this modification. Finally, based on the statistical results of pulse-like and non-pulse-like cases, reference values of the modification factors are suggested, which can somehow accounts for the effects of vertical irregularity and velocity pulse on ductility reduction factor in engineering application.

ductility reduction factor, modification factor, vertical irregularity, pulse-like ground motion, sheartype, nonlinear dynamic analysis

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