When composite slabs are used in conjunction with conventional composite beam construction, the number of shear connectors used along the length of the beam may be limited, resulting in incomplete interaction. This is known to affect the performance of the beam at ambient temperature, but nothing is known about its influence in fire. This thesis is primarily concerned with the development of a two-dimensional non-linear finite element approach to investigate the structural behaviour of unprotected composite beams at elevated temperatures by considering the influence of slip at the interface. The shear connection is modelled as a linking medium characterised by an assumed force-slip relationship. Semi-rigid connection characteristics are also incorporated as zero length spring elements connected to the steel beam and reinforced concrete slab. A semi-rigid connection model optimising the connection characteristics for a composite beam with partial interaction is proposed, using a pair of rotational springs and a linear spring. The computer model has been validated against experimental data both at ambient temperature and at high temperatures. A parametric study investigating the influence of shear connection, semi-rigid joint characteristics, reinforcement ratio and different temperature profiles across the cross-section at elevated temperatures has been carried out. It is shown that the failure temperature is little affected by the force-slip characteristics or their spacing. A noticeable improvement in fire resistance is seen when the rigidity of the connection or reinforcement ratio in the concrete slab is increased. It is also found that the failure temperature of composite beams can be significantly higher than that of bare steel beams.