Applications of composite beams.

In buildings and bridges, steel beams often support concrete slabs.Under load each component acts independently with relative movement or slip occurring at the interface. If the components are connected so that slip is eliminated, or considerably reduced, then the slab and steel beam act together as a composite unit (Fig. 21.1).

There is a consequent increase in the strength and stiffness of the composite beam relative to the sum of the components.

Behaviour of composite and non-composite beams
Fig. 21.1 Behaviour of composite and non-composite beams

The slab may be solid in situ concrete or the composite deck slab considered later. It may also comprise precast concrete units with an  in situ concrete topping. In buildings, steel beams are usually of standard UB scction, but UC and asymmetric beam sections are sometimes used where there is need to minimize the beam depth.A typical building under construction is shown in Fig. 21.2.Welded fabricated sections are often used for long-span beams in buildings and bridges.


Composite building under construction showing decking and shear-connectors
Fig. 21.2 Composite building under construction showing decking and shear-connectors

Design of composite beams in buildings is now covered by BS 5950: Part 3, although guidance was formerly available in an SCI publication.

The design of composite beams incorporating composite slabs is affected by the shape and orientation of the decking, as indicated in Fig. 21.3.


Composite beams incorporating composite deck slabs: (a) deck perpendicular to beam, (b) deck parallel to beam
Fig. 21.3 Composite beams incorporating composite deck slabs: (a) deck perpendicular to
beam, (b) deck parallel to beam

One of the advantages of composite construction is smaller construction depths.
Services can usually be passed beneath, but there are circumstances where the beam depth is such that services can be passed through the structure, either by forming large openings, or by special design of the structural system.A good example of this is the stub-girder.

The bottom chord is a steel section and the upper chord is the concrete slab. Short steel sections or ‘stubs’ are introduced to transfer the forces between the chords.

Openings through the beam webs can be provided for services. Typically, these can be up to 70% of the beam depth and can be rectangular or circular in shape.

Examples of the above methods of introducing services within the structure are shown in Fig. 21.4.

Different methods of incorporating services within the structural depth
Fig. 21.4 Different methods of incorporating services within the structural depth

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