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Composite beams with partial shear connection are widely used for buildings. The type and the number of shear connectors between the concrete slab and the steel beam decisively influence the load-bearing behaviour and the economic performance of the composite beam. For reasons of economy, composite beams should if possible be designed using the plastic method. To be able to carry out plastic design of a composite beam the chords must exhibit sufficient rotational capacity, and the shear connectors greater ductility and deformation capacity than required by the composite beam. Otherwise, the bending resistance of a composite beam is less than that determined with the plastic design method. With shear connectors of insufficient ductility, the shear forces cannot be fully redistributed among all shear connectors. Therefore, the maximum axial force in the chords is smaller than predicted. With shear connectors of insufficient deformation capacity, the maximum axial force in the chords is reached before the chords exhibit sufficient rotation to develop their full plastic bending moment. The ductility and the deformation capacity required by a composite beam strongly depend on the resistance and the stiffness of the shear connection. Nevertheless, the shear connectors used in steel-concrete composite construction are usually rather stiff. Therefore, the influence of the shear connection‘s stiffness on the required ductility and deformation capacity in a composite beam is usually neglected and rigid behaviour is assumed. The effects of the shear connection‘s resistance on the required ductility and deformation capacity in a composite beam are considered by defining a minimum degree of partial shear connection. In composite beams with a degree of partial shear connection lower than the minimum degree of partial shear connection, the required ductility and/or deformation capacity is greater than the ductility or deformation capacity, respectively, provided by the shear connectors used. The composite beam would thus fail at lower loads than predicted by the plastic design method. Therefore, the plastic design method must not be used with composite beams with degrees of partial shear connection lower than the minimum one. The values for the minimum degree of partial shear connection given in today’s relevant standards are based on the load-bearing behaviour of the most commonly used welded headed studs. However, with alternative connector systems exhibiting load-bearing behaviours differing greatly from that of a headed stud, the minimum degree of partial shear connection of a composite beam can differ greatly from that of a composite beam with headed studs. The present publication reports on comprehensive experimental, numerical and analytical investigations performed in recent years at the Institute of Structural Engineering (IBK) of the Swiss Federal Institute of Technology (ETH) in Zurich in order to develop a novel design method for composite beams which deals consistently with a wide variety of shear connectors. Apart from a novel, deformation-based analytical design method for the minimum degree of partial shear connection, a comprehensive numerical model suitable for the advanced analysis of composite beams is presented.

Load-bearing behaviour of composite beams in low degrees of partial shear connection