Composite construction

Composite construction makes steel and concrete work as one member. The commonest case is a steel beam supporting a concrete slab, with shear connectors, usually welded headed studs, joining the two. Without connectors, the slab and the beam slip past each other and bend separately. With them, the slab becomes a wide compression flange and the steel works mostly in tension, so the combined section is much stronger and stiffer than the two parts acting alone.

For bending at the ultimate limit state, the usual approach is plastic: the slab, over an effective width, is assumed to carry a uniform compressive stress block, and the steel to yield. If the steel's full yield force can be balanced by concrete within the slab depth, the plastic neutral axis lies in the slab and the moment capacity is the steel force times the lever arm between the steel centroid and the centre of the concrete block. If not, the neutral axis drops into the steel.

The shear connectors must transfer the longitudinal force between the point of zero moment and the point of maximum moment. Full interaction needs enough connectors to carry the smaller of the steel yield force and the slab crushing force; partial interaction uses fewer and accepts a lower capacity. Connector spacing, edge distance and slab reinforcement across the connectors are detailed so that the slab does not split.

During construction, before the concrete hardens, the steel beam alone carries the wet concrete unless it is propped. Composite columns, steel sections encased in or filled with concrete, and composite bridge decks use the same principles. Indian guidance is in IS 11384; international practice in EN 1994 and AISC 360.