Fire resistance of compartments within a high-rise steel frame: New sub-frame and isolated member models

To study the structural response of a fire compartment within a steel frame, it is expensive and time-consuming to conduct full-frame finite element (FE) analyses, although they can better represent the actual behaviour. For design purposes, engineers have recourse to an isolated member or sub-frame model. Although such an approach is relatively quick, it does not simulate the effects of boundary restraints exerted on heated members. Thus, based on a simple design approach, this paper proposes a new sub-frame model and isolated member model to ascertain the fire resistance of beams and columns subjected to compartment fires. The boundary restraints are represented as a combination of linear and rotational springs, where the spring stiffnesses are derived based on the assumption of semi-rigid beam-to-column connections. The proposed models are verified using a two-dimensional (2D) full-frame analysis. The comparison shows that the member internal forces and displacements predicted by both sub-frame and isolated-member models agree well with the 2D full-frame predictions. The limitations of the proposed methods are also discussed.