Influence of beam-to-column joints on the robustness of earthquake-resistant moment-resistant frames

Robustness of Moment Resistant Frames (MRFs) designed according to current seismic codes and subjected to a loss-of-column scenario is strictly linked to the deformation capacity of beam-to-column connections, which are usually subjected to combined axial and bending loads. In the seismic design of steel frames, once avoided the yielding of columns, a global dissipative collapse mechanism has to be ensured by imposing the formation of plastic hinges at the ends of the beams or in the connections and guarantying, provided that the elements involved in plastic range have an adequate energy dissipation capacity. Within this framework, the aim of this paper is to analyse the influence of the response of the connections designed for seismic application on the global behaviour of MRFs under a loss-of-column scenario providing an accurate modelling of connections subjected to combined axial and bending loads. To this scope four different beam-to-column joints designed for seismic actions, previously tested under cyclic load history at the STRENGTH laboratory of Salerno University, have been modelled and introduced in a case-study structure, whose response has been assessed by means of push-down analyses. The performances of the analysed frames have been analysed and is compared using the Residual Reserve Strength Ratio (RRSR) index combined with the energy balance method.