Evaluating the Adequacy of Ductility Requirements of Seismic Design Codes

Seismic design philosophy is based on this principal that destructive earthquakes have a low probability of occurrence, and it is not economical to design structures to perform elastically during such rare events. Therefore structures are designed and constructed with much less strength than their elastic demand imposed by destructive earthquakes. This imposes nonlinear deformations and consequently damages to structures. However, preventing collapse necessitates that structures must be able to undergo such a large deformations without any significant reduction in strength and stiffness. This characteristic is referred as a ductility capacity of structures. Since ductility capacity, in design procedure proposed by seismic codes, is implicitly addressed by satisfying some prescriptive requirements, the fundamental question is: do these provisions ensure adequate ductility capacity to avoid collapse during a destructive earthquake? Answer to this question makes the cornerstone of our study. To this end, the present study examines three intermediate steel moment frames of 4, 5, and 7 stories designed according to Iran’s national building regulations. Next, the lateral yield strength and ductility capacity of the designed buildings are estimated by performing a nonlinear static analysis. By applying a set of destructive earthquakes and developing corresponding acceleration response spectrum, the maximum elastic strength demand is estimated. The force-reduction factor defined as a ratio of the maximum elastic strength demand to the actual lateral yield strength of the structures is then computed for each of the selected earthquakes. According to the computed force-reduction factors, ductility demands imposed by each of the selected destructive...