Composite beams are made of reinforced concrete slabs and profiled steel beams, creating a stronger combination of both materials. Steel and concrete are the most commonly used materials for composite beams, with shear connectors being a critical part of their mechanical performance. Pre-cast or on-site slab making methods can be used, and other materials such as wood and resin composites are also used in smaller scale construction.
A composite beam is a construction element typically consisting of a reinforced concrete slab fixed 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 that a steel-concrete composite beam has both the compressive strength of concrete and the tensile strength of steel. There are many other types of composite beams used in the construction industry that combine various types of concrete with plastic and wood composites. The mixed steel and reinforced concrete beam is instead the most used.
Joining two dissimilar materials to form a composite not only unites the collective forces of the two materials. Forming a bond between relevant materials enhances their physical characteristics and makes the composite stronger than the sum of their strengths. In large-scale construction, steel and concrete are the most frequently used combinations for composite beams. Concrete imparts composite mass, stiffness and compressive strength, and reduces deflection and vibration in the slab. The steel elements give the beam its tensile strength with excellent strength-to-weight ratios and fast construction times.
One of the most important parts of a composite beam are the fixing points or shear connectors between the two materials. Proper bonding 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 are a critical part of the mechanical performance of the composites.
A mixed concrete and steel beam can use pre-cast concrete slabs or be cast in place. Prefabricated slabs are constructed with slots or pockets that accommodate the fixing studs. These pockets are then filled with concrete when the slab is correctly positioned. On-site slab making is a little more complex and requires a profiled sheet to be laid before pouring the concrete. This platform is typically made of low gauge steel plate with different profiles depending on the application.
The sheet metal is laid over the beams, then the shear connectors are attached to the beams using a “through welding” process or by firing. Once the deck is in place, concrete is poured over it and reinforcing bars are added. The deck serves to clamp or contain the wet concrete and also imparts strength to the composite once the slab has cured.
Several other types of composite beams are used in small scale construction. These include wooden beams that support lightweight concrete strips used as floor slabs in many wooden houses. In bridge construction, cement-filled resin composite boxes are often used. They are robust, quick to fabricate and relatively light in weight requiring a minimum of heavy machinery to install. Although very different in the materials used, these structural elements all employ the same principle of shared force as the more traditional steel and concrete beams.
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