AISC LRFD 1.pdf

218 FLEXURAL MEMBERS [Comm. I3.When first yield is the flexural strength limit, the elastic transformed section is usedto calculate stresses on the composite section. The modular ratio n =E/E c used todetermine the transformed section depends on the specified unit weight andstrength of concrete. Note that this procedure for compact beams differs from therequirements of Section I2 of the 1989 AISC ASD Specification.Plastic Stress Distribution for Positive Moment. When flexural strength is determinedfrom the plastic stress distribution shown in Figure C-I3.1, the compressionforce C in the concrete slab is the smallest of:C = Asw Fyw +2 Asf Fyf(C-I3-1)C = 085 . f′A(C-I3-2)Ccc=Σ Q n(C-I3-3)For a non-hybrid steel section, Equation C-I3-1 becomes C =A s F ywheref c = specified compressive strength of concrete, ksi (MPa)A c = area of concrete slab within effective width, in. 2 (mm 2 )A s = area of steel cross section, in. 2 (mm 2 )A sw = area of steel web, in. 2 (mm 2 )A sf = area of steel flange, in. 2 (mm 2 )F y = minimum specified yield stress of steel, ksi (MPa)F yw = minimum specified yield stress of web steel, ksi (MPa)F yf = minimum specified yield stress of flange steel, ksi (MPa)Q n = sum of nominal strengths of shear connectors between the point of maximumpositive moment and the point of zero moment to either side, kips(N)Longitudinal slab reinforcement makes a negligible contribution to the compressionforce, except when Equation C-I3-2 governs. In this case, the area of longitudinalreinforcement within the effective width of the concrete slab times the yieldstress of the reinforcement may be added in determining C.The depth of the compression block isCa =0.85 f ¢ bc(C-I3-4)whereb = effective width of concrete slab, in. (mm)A fully composite beam corresponds to the case of C governed by the yield strengthof the steel beam or the compressive strength of the concrete slab, as in EquationC-I3-1 or C-I3-2. The number and strength of shear connectors govern C for a partiallycomposite beam as in Equation C-I3-3.The plastic stress distribution may have the plastic neutral axis (PNA) in the web, inthe top flange of the steel section or in the slab, depending on the value of C.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION