Elastic behaviour of panel zone in steel moment resisting frames at elevated temperatures

In a steel framed building, the deformations that occur in the panel zones of rigid and semi-rigid joints have a pronounced effect on the structural stiffness and strength of the frame and on the distribution of inelastic deformations, especially for connections that are subjected to unbalanced moments. This effect can be significantly more pronounced if the joints are subjected additionally to elevated temperatures, as would occur in a compartment fire, and for which restraint against thermal expansion of the beams can produce large axial forces. This paper presents an elastic analysis of a panel zone in a rigid or semi-rigid joint in a steel frame at elevated temperatures. An analytical structural model based on simple equilibrium considerations that takes into account the shear and flexural deformations of the panel zone is developed firstly. A comprehensive parametric study is then presented, that includes variations of the elastic stiffnesses and the yield moment and for which the corresponding displacements have been computed. This shows that the main characteristics of the panel zone depend significantly on the thermal regime in a compartment, and on the large compressive axial load induced in the joint in the initial stages of the fire loading that develops because of the restraint to thermal expansion. The formulation is useful insofar as it has a prescriptive nature that allows for potential codification using the methodology of the component method of joint design.