A finite element code has been developed at the University of Sheffield to simulate the structural response of steel and composite framed buildings subjected to fire. The steel skeleton is represented using two-noded line elements, the steel-to-steel connections using spring elements and the flooring system by isotropic flat shell elements. Structures are therefore considered as a complete entity, allowing a more realistic prediction of structural behaviour at elevated temperature. A series of numerical simulations of fire tests carried out on the full-scale, eight-storey composite frame at the BRE laboratory at Cardington in 1995 and 1996 have been conducted. These tests have been subject to a number of significant parametric studies including slab thickness and secondary beam connection strength and stiffness. The concrete floor slab element has also been extended to a layered flat shell element allowing the inclusion of material non-linearities, thermal bowing, thermal degradation, anisotropic properties and a more advanced cracking model. Using the new concrete floor slab element the Cardington fire tests have been simulated in detail, to further understanding of the structural reaction in fire. Another series of parametric studies have been conducted considering again the thickness of the floor slab, the effect of the slab temperature gradient, the compressive strength, tensile strength and load ratios. These have all been compared to results from the Cardington fire tests. Current design methods based on isolated element design are considered by comparing the results of analyses in which the concrete floor is either included as a continuous slab in an extensive subframe, or is treated simply as forming the flanges of composite beams in a three-dimensional skeleton. These examples show clearly the effects of membrane and bridging actions of the continuous floor slab. The implications for future design developments are discussed with particular reference to the parametric studies conducted.