The principle of composite action

The principle of composite construction can be demonstrated by comparing the action of two joists placed one on top of the other. If these are physically connected the bending strength and stiffness are significantly improved.

The principle of composite beam behaviour can be illustrated with reference to a pair of timber joists. If these are simply placed one on top of the other and loaded as a beam there will be some relative movement between the two.

Both joists will contribute independently to the bending strength which will simply be the sum of the strengths of the two joists. If each joist has a breadth b and depth d the bending strength of each can be quantified as

• bd² / 6

and hence the combined strength is simply

• bd^2/3

If the joists are now connected together, say by spiking them at regular intervals or by gluing, the two will act together as a single unit with a depth of 2d.

The bending strength of the beam then becomes

• b(2d)^{2 / 6} = 2bd^{2 / 3}

Representing a doubling of the previous strength. The forces developed along the interface preventing slip are referred to as longitudinal shear forces.

Similar improvements in performance can be achieved by connecting the concrete floor slab to the steel beams which support it, using 'shear connectors'.

In traditional construction of steel framed buildings the steel beam and concrete slab which it supports are not positively connected. The contribution of the slab to the strength of the beam is generally small and can be ignored. The steel section alone is used to determine the beam strength and stiffness.

If the slab and steel beam are now connected, preventing any slip between the two, the strength will be increased as it was in the case of the timber joists.

The connectors accommodate the longitudinal shear force in the same way as the connection between the two timber joist sections described above. They are therefore referred to as shear connectors.

Significant savings are possible using composite beam construction, but details such as holes in the slabs need careful consideration.

Precisely how much better a composite beam is compared with the same beam used non-compositely depends upon both the beam size and the slab details. However it is likely that improvements of about 20% may typically be achieved. This means that composite beams are correspondingly lighter than non-composite beams for the same span and loading conditions. It should of course be recognised that the composite action is dependent on the integrity of the concrete slab. This means that great care must be exercised where openings occur in the slab, particularly if they are close to a beam and in such cases it may be that composite action cannot be used.