The seismic behaviour of moment resisting steel frames (MRFs) is strongly dependent on the plastic response of beams and columns. In this typology, it is common practice to foster the hysteresis in the beams ends and at the base of the columns of the first storey, thus relying on the resources of local ductility and energy dissipation capacity of the profiles. Therefore, in order to perform seismic non-linear analyses (pushover or time-history) mathematical models able to predict the inelastic response of the steel profile in terms of resistance and rotation capacity are usually needed. Currently, in technical literature, also due to the higher simplicity of the testing rigs, the available studies mainly regard the behavior of beams subjected to non-uniform bending, while the case of columns, which are subjected simultaneously to axial and bending loads, is frequently neglected.
Within this framework, in this paper an experimental and numerical work devoted to the monotonic behavior of square hollow sections (SHSs) (which are shapes frequently employed to realize columns) subjected to combined bending and axial loads, is presented. Results of experimental tests on steel profiles subjected to bending only or to simultaneous axial and bending loads are reported and, subsequently, with the aid of FE, a numerical analysis regarding about two thousand different cases is conducted to cope for variation of the influent parameters. The results of the experimental and FE studies are summarized in regression models expressing the ultimate resistance and rotational capacity of SHSs as a function of the main geometrical and mechanical parameters. The presented models represent a basic tool to run non-linear analysis of steel MRFs. Indeed, the maximum resistance of the member influences the redistribution of the actions in the structure, while the rotation capacity governs the local and global ductility of the structure. Both parameters are estimated properly accounting for the influence of non-dimensional axial load.
