Assessment of seismic inelastic displacement profiles of steel MRFs by means of modal behavior factors

A new method for the estimation of inelastic displacements of steel moment resisting frames (MRFs) under strong seismic ground motions is presented. The method comes as an extension of previous work of the authors from reinforced concrete to steel MRFs. Key-elements of the present method are the modal behavior (strength reduction) factors, accounting for the first four modes of significance. These factors are expressed in terms of natural period, interstorey drift ratio and/or plastic rotation, soil type and performance level. These factors are multiplied by the corresponding elastic modal displacements to generate the inelastic modal displacement profile. Eventually, the inelastic displacements can be calculated after combining the modal responses through any combination rule (e.g. SRSS). The results of the presented method are compared with that of the well-known equal displacement rule and three others and attested through nonlinear time history analyses.