A structural model for panel zones in non linear seismic analysis of steel moment-resisting frames

With the aim of providing a tool for the comprehensive seismic analysis of steel moment resisting frames (MRFs), the paper presents the development of a structural model which is able to describe the deformation modes of the panel zone in beam-to-columns joints, without resorting to a computationally heavy FE discretization. The model is able to take into account the finite dimensions of the joint, differently from the “scissor model” currently adopted, and can incorporate the most recently proposed constitutive models for monotonic and cyclic loading. The proposed approach, based on a simple structural representation of the joint, does not modify the centre-to-centre line scheme and the nodal degrees of freedom usually adopted in the analysis of plane frames, but adds up to five more degrees of freedom to each node. These describe the deformation modes of the panel zone, namely extensional, flexural and due to shear. A non-linear behaviour can be associated to each of these modes; based on experimental results, a simple hysteretic model is proposed for the shear deformation. The panel zone model has been implemented in an ad hoc developed computer code for the non linear seismic analysis of steel MRFs (named STEFAN, STEel Frame ANalysis), including a non-linear fracturing beam model as well. As an example of application, the dynamic behaviour of a 5-bay 9-story frame designed according to Eurocodes 3 (EC3) and 8 (EC8) has been investigated, for different panel zone stiffness and with different models, both in the linear and non linear range, emphasizing the need for a correct panel zone modelling.