Composite steel-deck floor

A composite floor in which a profiled steel deck acts as permanent formwork and, once the concrete has set, collaborates with the slab as bottom reinforcement. Laid on the beams and topped with a slab, it gives a light, very fast load-bearing deck, with no props over modest spans and no formwork. It is the typical solution in steel-framed buildings, where the deck is fixed to the beams and, with shear connectors, makes them act compositely with the floor.

The composite steel-deck floor unites two materials at their respective strengths: the steel of the deck, at the bottom, resists tension; the concrete of the slab, at the top, resists compression. The profiled deck, laid on the beams, first acts as permanent formwork — carrying the fresh pour — and then as tension reinforcement, collaborating with the hardened concrete.

During the pour the deck alone carries the weight of the fresh concrete and the workers, shedding it onto the beams: over modest spans no props are needed. Once cured it changes role and becomes a resisting part of the floor: the ribs with embossments or dovetails key into the concrete and prevent slip, so deck and slab work as one composite section.

For the steel beam too to collaborate with the floor, stud connectors («Nelson studs») are welded to its top flange, embedded in the pour. They transfer the shear at the beam-slab interface and make them a single composite beam, far stiffer and stronger than the bare beam: this is what allows wide spans with slim depths in steel buildings.

The system is light and fast, ideal for dry construction and for strengthening in refurbishment. Two points must be managed: the fire resistance, because the steel exposed at the soffit must be protected (intumescent paints, suspended ceilings, or added reinforcement) to ensure the REI class; and the routing of services, which the ribs themselves can house. Mesh or added reinforcement in the slab controls cracking and the negative moments over the supports.