A composite beam is a structural element combining two or more materials — typically a steel section acting together with a concrete slab — to form a single load-bearing member.
How does a composite beam work?
In a non-composite floor system, the steel beam and concrete slab carry loads independently. In a composite arrangement, shear connectors — usually headed studs welded to the top flange of the steel beam — mechanically bond the two materials together. This connection forces the concrete slab to work as the compression flange of the beam while the steel section carries the tension.
The result is a member with a much larger effective moment of inertia than either component alone. Because concrete is strong in compression and steel is strong in tension, each material is used to its greatest advantage. Composite action can increase the load capacity of a steel beam by 30 to 50 percent compared to its non-composite equivalent, or alternatively allow a lighter steel section to be used for the same span.
Applications
Composite beams are standard practice in modern multi-storey construction:
- Office buildings and commercial floors — achieving long, column-free spans with shallow structural depth.
- Car parks — minimising floor-to-floor height to reduce overall building height.
- Bridge girders — combining steel plate girders with a reinforced concrete deck slab.
- Industrial mezzanines — providing high load capacity with economical sections.
Proper design must account for the construction stage (when the beam supports wet concrete alone) as well as the composite stage, and must ensure adequate shear connection throughout the beam length.
Related terms
- Steel
- Concrete
- Reinforcement
- Column
- Construction
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