Seismic behavior of composite steel/concrete MRFs: deformation assessment and behavior factors

The seismic inelastic behavior of regular planar composite steel/concrete MRFs consisting of I steel beams and concrete filled steel tube (CFT) columns is investigated. For this purpose, a family of 96 regular plane CFT-MRFs are subject to an ensemble of 100 ordinary (far-field type) ground motions scaled to different intensities in order to accommodate different performance levels and their response to these motions is recorded to form a response databank. On the basis of this databank nonlinear regression analysis is employed in order to derive simple formulae which offer a direct estimation of seismic displacements, drift and ductility demands, and the strength reduction (or behavior) factor q, which describes the seismic strength requirements in order to restrict maximum roof ductility demand to a predefined value. The influence of specific parameters on the maximum structural response, such as the number of stories, the beam-to-column stiffness ratio, the column-to-beam strength ratio, the level of inelastic deformation induced by the seismic excitation and the material strengths, is studied in detail. Furthermore, emphasis is given to the ability of the proposed formulae to be employed in the framework of seismic design methods which utilize response spectrum analysis. This essential aspect makes possible both the simple and direct seismic assessment of existing structures and the straightforward deformation-controlled seismic design of new ones because the q factor is a function of deformation.