Loss of interaction in short-span composite beams and plates

Abstract The shear connection in composite beam or plate cannot function unless there is slip at the interface between the concrete slab and the steel member; yet slip is neglected in design, and this assumption has been thoroughly validated for spans in the range 5 m to 15 m. In geometrically-similar beams of different span, one expects to find similar strain distributions; so slip should be proportional to span. But stud shear connectors are always about 20 mm in diameter, irrespective of span. This suggests that in spans below about 5 m slips may be so low that connectors are ineffective, and that in long spans slips may be so large that some connectors fail before much redistribution of longitudinal shear can occur. This paper gives a theoretical study of the first of these two problems, in relation to the composite plates of span about 3 m that occur in some box-girder bridges. It is shown that loss of interaction does indeed occur, but that the resulting increases in stress are acceptable. The problem of slip capacity in long spans is thought to be important only when partial shear connection is used. This method is therefore recommended only for beams in building, of span less than 20 m.