AISC LRFD 1.pdf

Comm. I2.] COMPRESSION MEMBERS 215described in Commentary Section I3, and a discussion of the composite participationof slab reinforcement is presented.Plastic Stress Distribution for Negative Moment. Plastic stress distributions aredescribed in Commentary Section I3.Elastic Stress Distribution. The strain distribution at any cross section of a compositebeam is related to slip between the structural steel and concrete elements. Priorto slip, strain in both steel and concrete is proportional to the distance from the neutralaxis for the elastic transformed section. After slip, the strain distribution is discontinuous,with a jump at the top of the steel shape. The strains in steel and concreteare proportional to distances from separate neutral axes, one for steel and theother for concrete.Fully Composite Beam. Either the tensile yield strength of the steel section or thecompressive stress of the concrete slab governs the maximum flexural strength of afully composite beam subjected to a positive moment. The tensile yield strength ofthe longitudinal reinforcing bars in the slab governs the maximum flexural strengthof a fully composite beam subjected to a negative moment. When shear connectorsare provided in sufficient numbers to fully develop this maximum flexural strength,any slip that occurs prior to yielding is minor and has negligible influence both onstresses and stiffness.Partially Composite Beam. The effects of slip on elastic properties of a partiallycomposite beam can be significant and should be accounted for in calculations ofdeflections and stresses at service loads. Approximate elastic properties of partiallycomposite beams are given in Commentary Section I3. For simplified design methods,see Hansell, Galambos, Ravindra, and Viest (1978).Concrete-Encased Beam. When the dimensions of a concrete slab supported onsteel beams are such that the slab can effectively serve as the flange of a compositeT-beam, and the concrete and steel are adequately tied together so as to act as a unit,the beam can be proportioned on the assumption of composite action.Two cases are recognized: fully encased steel beams, which depend upon naturalbond for interaction with the concrete, and those with mechanical anchorage to theslab (shear connectors), which do not have to be encased.I2. COMPRESSION MEMBERS1. Limitations(1) The lower limit of four percent on the cross-sectional area of structural steeldifferentiates between composite and reinforced concrete columns. If the areais less than four percent, a column with a structural steel core should be designedas a reinforced concrete column.(2) The specified minimum quantity of transverse and longitudinal reinforcementin the encasement should be adequate to prevent severe spalling of the surfaceconcrete during fires.(3) Very little of the supporting test data involved concrete strengths in excess of 6ksi (41 MPa), even though the cylinder strength for one group of four columnswas 9.6 ksi (66 MPa). Normal weight concrete is believed to have been used inall tests. Thus, the upper limit of concrete strength is specified as 8 ksi (55LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION