A composite beam is a construction element typically consisting of a reinforced concreted slab attached to and supported by profiled steel beams. Composite beams are stronger than the sum of their constituent parts and exhibit a favorable combination of the strength characteristics of both materials. This means a steel and concrete composite beam will posses both the compressive strength of concrete and the tensile strength of steel. There are several other types of composite beams used in the construction industry which combine various grades of concrete with plastic composites and timber. The steel and reinforced concrete composite beam is, however, the most commonly used.
Joining two dissimilar materials to form a composite does not only combine the collective strengths of the two materials. Forming a union between relevant materials actually enhances their physical characteristics and makes the composite stronger than the sum of their strengths. In large scale construction, steel and concrete are most frequently used combinations for composite beams. The concrete lends the composite mass, stiffness, and compressive strength and reduces deflection and vibration in the slab. The steel members give the beam its tensile strength with excellent strength to weight ratios and rapid construction times.
One of the most important parts of a composite beam are the fixing points or shear connectors between the two materials. The correct connection of the two parts of the composite allows the materials to act as a unit and gives the composite beam its inherent strength. These shear connectors are typically studs welded to the steel beams and set into the concrete slab. The number and size of these shear connectors are carefully calculated as they represent a critical part of the composites mechanical performance.
A concrete and steel composite beam may use previously cast concrete slabs or be cast on site. Pre-cast slabs are constructed with slots or pockets in them which accept the fixing studs. These pockets are then filled up with concrete when the slab is correctly positioned. Fabricating slabs on site is a little more complex and requires a profiled deck sheet to be laid prior to pouring the concrete. This deck is typically light gauge sheet steel featuring different profiles depending on the application.
The sheet is laid on top of the beams, and then shear connectors are fixed to the beams using either a “through-deck welding” process or by shot firing. Once the deck is in place, the concrete is poured on top and reinforcing bars added. The deck serves to shutter or contain the wet concrete and also lends strength to the composite once the slab has cured.
Several other composite beam types are used in smaller scale construction. These include timber beams which support lightweight concrete strips used as floor slabs in many timber homes. Concrete filled resin composite boxes are often used in the construction of bridges. They are strong, quick to fabricate, and relatively light weight requiring a minimum of heavy machinery to put in place. Although very different in the materials used, these structural elements all make use of the same principle of shared strengths as the more traditional steel and concrete beams.