Design principles for composite beams

The slab thickness is determined by its function as a floor, and the composite beam design is therefore concerned with sizing the steel section.

The philosophy of designing composite beams is to utilise the implicit strength of the concrete slab which is to be used for the floor. The form and thickness of this will have been determined by its functional requirements as a slab, taking account of the span between supporting beams, load and fire resistance requirements. The sizing of the composite beam therefore starts with the thickness of the slab predetermined, and the process is to select a suitable steel cross-section (normally a Universal Beam) which, in conjunction with the slab, will satisfy the requirements of the beam in terms of strength and stiffness.

The beam must be designed to have sufficient bending strength and stiffness, and secure connection to the slab.

The principal aspects of the behaviour of the composite beam which need to be considered in this respect are bending strength, the adequacy of the connection between the slab and beam, and its deflection performance. The detailed design procedures for checking these are embodied in BS 5950 and are discussed further in the following sections.