Ductility reduction factors for steel buildings considering different structural representations

The global ductility parameter (mu(G)), commonly used to represent the capacity of a structure to dissipate energy, and its effects, considered through the ductility reduction factor (R-mu), are studied for buildings with moment resisting steel frames (MRSF) which are modeled as complex multi degree of freedom systems. Results indicate that the mu(G) value of 4, commonly assumed, cannot be justified, a value between 2.5 and 3 is suggested. The ductility reduction factors associated to global response parameters may be quite different than those of local response parameters, showing the limitation of the commonly used equivalent lateral force procedure (ELFP). The ratio (Q) of R-mu to mu(G) is larger for the models with spatial MRSF than for the models with perimeter MRSF since their ductility demands are smaller and/or their ductility reduction factors larger. According to the simplified Newmark and Hall procedure, the Q ratio should be equal to unity for the structural models under consideration. Based on the results of this study, this ratio cannot be justified. The reason for this is that single degree of freedom systems were used to derive the mentioned simplified procedure, where higher mode and energy dissipation effects cannot be explicitly considered. A value of 0.5 is suggested for Q for steel buildings with perimeter MRSF in the intermediate and long period regions. The findings of this paper are for the particular structural systems and models used in the study. Much more research is needed to reach more general conclusions.