Composite Flexural Members

Reinforced- and prestressed-concrete, composite flexural members are constructed from such components as precast members with cast-in-place flanges, box sections, and folded plates.
Composite structural-steel-concrete members are usually constructed of cast-inplace slabs and structural-steel beams. Interaction between the steel beam and concrete slab is obtained by natural bond if the steel beam is fully encased with a minimum of 2 in of concrete on the sides or soffit. If the beam is not encased, the interaction may be accomplished with mechanical anchors (shear connectors). Requirements for composite structural-steel-concrete members are given in the AISC Specification for Structural Steel for Buildings Allowable Stress Design and Plastic Design, and AISC Load and Resistance Factor Design Specification for Structural Steel Buildings, American Institute of Steel Construction.

The design strength of composite flexural members is the same for both shored and unshored construction. Shoring should not be removed, however, until the supported elements have the design properties required to support all loads and limit deflections and cracking. Individual elements should be designed to support all loads prior to the full development of the design strength of the composite member.
Premature loading of individual precast elements can cause excessive deflections as the result of creep and shrinkage.
According to the ACI 318 Building Code, the factored horizontal shear force for a composite member may be transferred between individual concrete elements by contact stresses or anchored ties, or both. The factored shear force Vu at the section considered must be equal to or less than the nominal horizontal shear strength Vnh multiplied by  @= 0.85.

 1.0 for normal-weight concrete
 0.85 for sand-lightweight concrete
 0.75 for all-lightweight concrete

Precast-concrete members are assembled and fastened together on the jobsite. They may be unreinforced, reinforced, or prestressed. Precasting is especially advantageous when it permits mass production of concrete units. But precasting is also beneficial because it facilitates quality control and use of higher-strength concrete.
Form costs may be greatly reduced, because reusable forms can be located on a casting-plant floor or on the ground at a construction site in protected locations and convenient positions, where workmen can move about freely. Many complex thinshell structures are economical when precast, but would be uneconomical if cast in place.

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