Composite floors using conventional shallow decking

In composite floors, steel decking acts as reinforcement to the concrete and supports it during construction when it is wet.

Composite steel deck floors are one-way spanning slabs in which the compressive strength is provided by insitu concrete and the tensile strength derives from profiled steel sheeting. This also serves as the shuttering for the wet concrete during construction.

Shallow deck composite slabs are typically about 125mm thick with a maximum span of 3.5m.

The structural behaviour is similar in principle to that of a reinforced concrete slab, with the sheeting acting as the bottom tension reinforcement. Profile heights are usually in the range 38mm to 75mm and slab thicknesses above the profile typically vary between 65mm and 120mm providing a maximum span of about 3.5m.

A number of deck profiles are available, based on thin, galvanised steel sheeting.

The sheeting is of thin gauge (typically 0.9mm or 1.2mm thick) and is therefore galvanised to provide corrosion protection. A number of companies manufacture steel sheeting for this purpose and a range of profiles is therefore available.

The design of shallow deck composite floors must ensure adequate strength and stiffness both during construction and in service.

In designing composite floors it is necessary to ensure that strength and deflections both during construction (when the concrete is wet) and in service (when composite action has been achieved) are satisfactory. The following must therefore be checked:

• Bending strength of the steel sheeting to ensure that the weight of wet concrete can be supported during construction, before composite action is developed;
• Flexural stiffness of the sheeting to prevent excessive deflections during construction;
• Tensile strength of the sheeting to provide the necessary reinforcement to the slab in its final composite form;
• Strength of the concrete in compression for which the floor is considered as an equivalent reinforced concrete slab;
• Stiffness of the composite slab to prevent excessive deflection under normal working loads.
• Bond between the concrete and the steel in order to achieve composite action;

The first two are concerned with the cross-sectional details of the sheeting, the strength and stiffness being derived from the profiling. The appropriate loading condition is the weight of wet concrete and an allowance for construction loads.

For the last four items normal loading conditions (dead and imposed) apply. The strength and stiffness of the slab are dependent on the overall depth of the composite floor, whilst the provision of ribs within the steel sheeting in the longitudinal direction ensures a good key with the concrete.

In practice design is normally simplified and based on safe load tables.

Because of the need to satisfy the various conditions outlined above and the complexity of certain aspects of the behaviour of this structural form, the design of composite floors is normally based upon manufacturers' data. The critical parameters in this are:

• Span
• Thickness of concrete
• Imposed load intensity
• Whether or not temporary props are used.

The tabulated information takes account of all possible failure modes. Typically the critical conditions are the bending strength of the steel during construction and deflections of the composite floor in service. There is thus a contradiction in the design - a thick topping of concrete provides an increase in stiffness of the composite slab but also adds more load to the non-composite section during construction. The use of design tables however enables a simple selection to be made.

A typical design table is shown below.