Composite Beams

Continuous steel-concrete composite beams are largely used in building and bridge construction but are characterized by a very complex behaviour even for low-stress levels. In fact, composite action depends on the interaction between three main components: the reinforced concrete slab, the steel profile, and the shear connection ( Mechanical properties and arrangement of common composite cross sections ensure a good response to positive bending, but a reduced bearing capacity when negative bending is considered. Therefore, redistribution of the internal forces at the ultimate limit state is a key factor in the design process, as it allows a reduction of bending moments at the internal support and the exploitation of the positive bending resistance. A reliable assessment of available rotation capacity is required in order to define design criteria and simplified code provisions. Experimental and theoretical analyses have focused mainly on the steel component affected by the buckling phenomena, which reduce the rotation capacity. Nevertheless, many experimental results on joints and semi-continuous beams shows that the collapse is often due to fracture of reinforcement placed in the slab and pointed out that properties and arrangement of reinforcement can influence the structural response of such beams.
Furthermore relative displacements between slab and profile due to mechanical connecting devices that are not completely rigid increase the deformability of the system and affect the global behaviour of members. The solution of a continuous composite beam can be performed using a unified approach to the modelling of the cross-section. In fact, each slip and the related interaction phenomenon are described by a static parameter that can change between an upper limit and a lower limit depending on the properties of the materials. Furthermore, when the cross section is subjected to negative bending, the moment-curvature relationship is defined assuming a given value of Tct and changing the interaction force F. In particular, if the results of tensile stresses on the effective area is zero, tensile stresses cannot arise in the slab, and the cross section is cracked. As a result, from a static point of view, the equilibrium conditions for the concrete slab are not strictly related to the assumed kinematic model. Thus the moment-curvature relationship for composite sections under positive bending belongs to the family of curves generated in compliance with the assumptions for a composite section under negative bending. This remark allows a unified approach to modelling of composite beams since the proposed generalized moment-curvature relationship is able to fully describe the flexural response of the section and can be used as a powerful tool to perform refined structural analyses.