Concrete Slab Stresses in Partial Composite Beams and Girders

Although calculation of compressive flexural stresses in the concrete slab of composite steel beams and girders is a routine procedure, engineers are discovering certain ambiguities in such an analysis when the partial composite concept is used. The problem presents itself when applying the rules in Section of 1.11.2.2 of the AISC Specification. This section states, in part, "The actual section modulus of the transformed composite section shall be used in calculating the concrete flexural stress...." The intent of the Specification is to determine the actual stress distribution corresponding to the actual number of shear connectors in the partial composite design approach. This paper describes a suggested procedure to determine concrete slab stresses based on assumed elastic behavior. The basic justification for the partial composite theory is the fact that shear connectors are inadequate to maintain strain compatibility at the interface of the concrete slab and the top of the steel beam. This results in a limited amount of horizontal slip, which would reduce the concrete compressive strains, and their corresponding stresses