This paper introduces the concept of a wide fl ange steel beam with the bottom fl ange encased in pre-cast concrete. These composite beams utilize the merits of both steel and concrete materials. The effective interaction between the two materials can reduce the size of the steel beams. The reinforcement and the concrete are pre-integrated with the bottom fl ange of the steel beam at a manufacturing plant. In this paper, the analytical investigation of the fl exural moment strength of the composite beams at both the yield limit state and the maximum load limit state is performed and compared with the experimental results. The depth of the equivalent rectangular stress block of the beams is obtained using an equilibrium equation when both the compression steel reinforcements are present. The post-yield behaviour of the composite beams is also investigated based on the normalized effec-The stiffness of the composite beams degrades gradually with suffi cient ductility and dissipating energy capability. A six-step procedure provides a fast, effective and accurate way of investigating the post-yield behaviour of the composite beams. Macorini et al. (2006) presented a fi nite element model suitable for the long-term analysis of steel-concrete composite beams. The structure was modelled using one-dimensional beam elements for the steel profi le and two-dimensional shell elements for the concrete slab. The two types of elements are interconnected with special-purpose link elements. Nie et al. (2005) investigated the stiffness and the capacity of steel-concrete composite beams with profi led sheeting by considering shear slip effects. In addition, Nie et al. (2006) also tested nine specimens of steel-concrete composite beams with openings in the concrete fl ange. The main parameters they considered were the size and position of the fl ange openings. Two different failure modes were observed during testing, depending on the loading arrangement and the position of the openings. The rigidity of the composite beams was also investigated. Thevendran et al. (2000) carried out an experiment on curved steel-concrete composite beams. Beams with varying radii of curvature were tested to failure. Weng et al. (2002) presented an experimental study that focused on the shear splitting failure of composite concrete-encased steel beams. Nie et al. (2008) presented a new defi nition for the effective width used in the analysis of a composite beam under sagging moments. Through an experimental study and fi nite element modelling, the distribution of longitudinal strain and stress across a concrete slab were examined and expressed using simplifi ed formulae. Lamont et al. (2007) compared the behaviour of composite steel-framed structures with applied fi re protection to external beams without fi re protection. Jurkiewiez et al. (2005) studied a new connection device based on horizontal shear connectors, eliminating the need for welding. The resulting steel-concrete composite beam was subjected to a static bending test. An experimental analy...