Engineering and Construction

Composite floor is the general term used to denote the composite action of steel beams and concrete or composite slabs that form a structural floor. Composite deck slabs, in this context, comprise profiled steel decking (or sheeting) as the permanent formwork to the underside of a concrete slab spanning between support beams. The decking acts compositely with the concrete under service loading.

It also supports the loads applied to it before the concrete has gained adequate strength. A light mesh reinforcement is placed in the concrete. A cross-section through a typical composite slab is shown in Fig. 20.1. Shear-connectors are used to develop composite action between the concrete slab and steel beams.

Fig. 20.1 Section through a typical composite deck slab

The decking has a number of roles. It

(1) supports the loads during construction
(2) acts as a working platform
(3) develops adequate composite action with the concrete
(4) transfers in-plane loads by diaphragm action to vertical bracing or walls (in conjunction with the concrete topping)
(5) stabilizes the beams against lateral buckling provided that the ribs run perpendicular to the beam or are at an angle of at least 45° to the beam
(6) acts as transverse reinforcement to the composite beams
(7) distributes shrinkage strains preventing serious cracking.

In addition it has a number of advantages over precast or in situ concrete alternatives:

(1) construction periods are reduced
(2) decking is easily handled
(3) attachments (e.g. ceiling hangers) can be made easily
(4) openings can be formed
(5) shear-connectors can be welded through the decking
(6) decking can be cut to length and is less prone to tolerance problems.

The main economy sought in buildings is speed of construction and for this reason slabs and beams are generally designed to be unpropped during the construction stage. Spans of the order of 3m to 3.6m between support beams are most common, but can be increased to over 4m if propping is used. Design of composite slabs is covered by BS 5950: Part 4.