Composite beam construction with alternative forms of floor slab

Composite action can be achieved with different slab forms.

The behaviour of composite beams described above relates to simply supported beams and insitu solid concrete slabs. Where other forms are used this may be slightly modified, although the principles and advantages of composite action remain.

Composite beam construction can be achieved with precast concrete slabs but the slabs should be detailed to allow proper shear connection.

The main additional point to be considered in achieving composite action with precast slabs is the need to provide adequate containment for the connectors within the slab. This is normally achieved by leaving pockets or a continuous gap for the connectors. These are infilled with insitu concrete, providing a bond between the connectors and the precast units. Careful detailing is needed to ensure that this bond is achieved and that there is adequate bearing for the precast slab.

The precast slab construction must be carefully detailed to ensure that it can make an effective contribution to the composite beam action.

It is clearly necessary that the precast units are capable of providing the compressive force along the length of the composite beam. Since the units are laid individually, continuity can only be ensured by the use of an insitu topping. Some types of unit, such as those with voids, may not be suitable since the longitudinal compression developed in the concrete slab cannot be transferred through the voids.

Composite beam construction with composite floor slabs is very common, and simply requires that the shear connectors are fixed on site.

Composite floors, which are described below, are commonly used in conjunction with composite beams. Because of the need to lay the profiled steel sheets over the steel beams, the stud connectors cannot be fitted during fabrication. Instead a system of 'through-deck welding' is normally adopted. This involves a semi-automatic welding procedure which effectively fixes the stud and decking to the flange of the beam. It is also possible to use a shot-fired connection, although the strength is less and a larger number of connectors is required.

Because of the deck profile, it may not be possible to locate the required number of shear connectors, and the full benefit of composite beam action will not be realised.

The design of the composite beam is largely unaffected, although it may be difficult to accommodate sufficient shear connectors. These can only be fitted where the profiled sheet is in contact with the steel beam, that is in the troughs of the profile.

Where this applies, the interaction between the slab and steel beam is partial. The bending strength of the composite beam must therefore be reduced in proportion to the number of connectors which have been omitted.