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Steel Designers' Manual - 6th Edition (2003)
Chapter 21
Composite beams
by MARK LAWSON and PETER WICKENS
21.1 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.
The slab may be solid in situ concrete or the composite deck slab considered in
Chapter 20. 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.
Design of composite beams in buildings is now covered by BS 5950: Part 3, 1
although guidance was formerly available in an SCI publication. 2 The design of composite
beams incorporating composite slabs is affected by the shape and orientation
of the decking, as indicated in Fig. 21.3.
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. 3 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.
Guidance on the design of composite beams with web openings is given in Reference
4. Examples of the above methods of introducing services within the structure
are shown in Fig. 21.4.
21.2 Economy
Composite beam construction has a number of advantages over non-composite
construction:
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