Estimation of seismic inelastic deformation demands in plane steel MRF with vertical mass irregularities.

An extensive parametric study on the inelastic seismic response of plane steel moment-resisting frames with vertical mass irregularity is presented. A family of 135 such frames, designed according to the European seismic and structural codes, are subjected to an ensemble of 30 ordinarily (i.e., without near-fault effects) earthquake ground motions scaled to different intensities in order to drive the structures to different limit states. The statistical analysis of the created response databank indicates that the number of storeys, beam-to-column strength ratio and the location (top, midheight and bottom) of the heavier mass influence the heightwise distribution and amplitude of inelastic deformation demands, while the response does not seem to be influenced by the mass ratio. Nonlinear regression analysis is employed in order to derive simple formulae which reflect the aforementioned influences and offer, for a given strength reduction (or behavior) factor, three important response quantities, i.e., the maximum roof displacement, the maximum interstorey drift ratio and the maximum rotation ductility along the height of the structure