Modelling of unpropped semi-continuous composite beams

For ease of construction steel–concrete composite beams are usually unpropped during construction, with the steel section alone supporting the weight of the concrete slab during casting. The subsequent applied loading is resisted by the composite action of the steel and concrete. This mode of construction results in significant stresses built into the steel section. A numerical method is presented for modelling the load–deformation behaviour of composite beams unpropped during construction and incorporating composite connections at the supports. The method accounts for the effect of dead load stresses built into unpropped beams. A 3D moment–curvature–dead load stress relationship is defined, utilising the non-linear stress–strain properties of the steel and concrete. This relationship is used subsequently for calculating the distribution of curvature along the length of a beam. The curvature may be double integrated, using the support displacements as boundary conditions, to determine the deflected shape. The accuracy of the technique has been assessed experimentally using a 24 m long test frame incorporating unpropped composite beams and composite connections. The model predictions of end rotations and displacements are shown to be in good agreement with observed values. Standardised composite connection details are currently only available for use with propped composite beams. This work has been carried out to assess the conditions in which these details can be safely used with beams, which are unpropped during construction. The work is of particular interest to those attempting to model accurately the load–deformation response of unpropped composite beams.