Composite Action Without Shear Connectors

In composite construction of building floor slabs, the common method for transferring shear stresses from the concrete slab to the steel beam is to use stud shear connectors. An alternate method, which is seldom used, is to completely encase the steel beam in concrete. When the steel beam is completely encased, the 1970 AISC Specification assumes the beam to be interconnected to the concrete by natural bond without additional anchorage. This assumption may be used when at least 2 in. of concrete cover is provided over the sides and bottom of the beam and at least 1½ in. of cover is provided at the top. An additional requirement is that adequate mesh or reinforcing steel be used in the encasement to prevent spalling of the concrete. In most composite construction the neutral axis of the composite section is located either in the floor slab or at a very small distance below the bottom of the slab. Since the specifications do not permit the concrete on the tension side of the neutral axis to be used as a structural element, the concrete below the neutral axis on a fully encased beam does not theoretically increase the stiffness of the beam. The tension concrete does provide more bond area between the steel and concrete; however, the advantage of this additional bond may be offset by the additional dead weight of the concrete used to encase the beam.