The current research work investigates the behaviour of steel and composite beams as well as beam-to-column connections at elevated temperatures. Significant attention is needed to identify the main issues, as the behaviour is profoundly different from that at ambient temperature. Local buckling of the beam flange outstands has been found to be highly significant in accelerating the development of catenary action in fire, since this action is reliant on hinges forming which may result from local buckling. Local buckling of the beam web, which experiences a non-uniform temperature variation, is also important because the mechanical properties of the steel are degraded non-uniformly from their ambient values. Current formulations for web buckling at ambient temperature therefore need substantial revision at elevated temperatures. Very large compressive forces are developed within the beams initially and these induce large stresses in the column web; hitherto this has not been considered. Developing a formulation representing the mechanics of this potential failure zone in the column web in the compression region of the connection is a useful and needed addition to research in the topic. Predicting the critical temperature in a connection that causes failure of the bolts, end plate and column flange in the tension zone of the connection is considered in this research work. An elastic analysis of a panel zone in a rigid or semi-rigid joint in a steel frame, which is based on simple equilibrium considerations that takes into account the shear and flexural deformations of the panel zone, is developed at elevated temperatures. In order to model the structural response of a composite beam restrained by cooler members in a steel compartment fire in a frame structure at elevated temperature, recourse is needed to a geometric nonlinear formulation, since the beam transverse deflections are not negligible and the axial compressive force in the member is also substantial at the early stages of the fire. This thesis presents such a formulation, which incorporates partial interaction between the concrete slab and steel component, as well as the degradation of the stiffnesses of the components of the composite beam prior to yield at elevated temperature.