In composite construction, steel beams and a concrete slab are connected so that they act together to resist the load on the beam. The slab, in effect, serves as a cover plate. As a result, a lighter steel section may be used.


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There are two basic methods of composite construction.

Method 1. The steel beam is entirely encased in the concrete. Composite action in this case depends on the steel-concrete bond alone. Because the beam is completely braced laterally, the allowable stress in the flanges is 0.66F y , where F yis the yield strength, ksi (MPa), of the steel. Assuming the steel to carry the full dead load and the composite section to carry the live load, the maximum unit stress, ksi (MPa), in the steel is

F s = ( M D / S S ) + ( M L / S t r ) ? 0.66F y 
where M D = dead-load moment, in-kip (kN-mm)
M L = live-load moment, in-kip (kN-mm)
S s = section modulus of steel beam, in 3 (mm 3 )
S t r = section modulus of transformed composite section, in 3 (mm 3 )
An alternative, shortcut method is permitted by the AISC specification. It assumes that the steel beam carries both live and dead loads and compensates for this by permitting a higher stress in the steel:


f s = M D + M L / S s ? 0.76 F y 

Method 2. The steel beam is connected to the concrete slab by shear connectors. Design is based on ultimate load and is independent of the use of temporary shores to support the steel until the concrete hardens. The maximum stress in the bottom flange is

 F s = M D + M L / S t r £ 0.66 F y 

To obtain the transformed composite section, treat the concrete above the neutral axis as an equivalent steel area by dividing the concrete area by n, the ratio of modulus of elasticity of steel to that of the concrete. In determination of the transformed section, only a portion of the concrete slab over the beam may be considered effective in resisting compressive flexural stresses (positive-moment regions). The width of slab on either side of the beam centerline that may be considered effective should not exceed any of the following:

1. One-eighth of the beam span between centers of sup- ports
2. Half the distance to the centerline of the adjacent beam
3. The distance from beam centerline to edge of slab