AISC LRFD 1.pdf

226 COMBINED COMPRESSION AND FLEXURE [Comm. I4.The nominal axial strength of a beam-column is obtained from Section I2.2, whilethe nominal flexural strength is determined from the plastic stress distribution onthe composite section. An approximate formula for this plastic moment resistanceof a composite column is given in Galambos and Chapuis (1980).( ) w yh2 - 2c ær h AF ö2Mn = Mp = ZFy + AFr yr+ -AwFy(C-I4-1)3 çè2 1.7 f ¢ch ÷1 øwhereA w = web area of encased steel shape; for concrete-filled HSS, A w = 0, in. 2 (mm 2 )Z = plastic section modulus of the steel section, in. 3 (mm 3 )c r = average of distance from compression face to longitudinal reinforcement inthat face and distance from tension face to longitudinal reinforcement inthat face, in. (mm)h 1 = width of composite cross section perpendicular to the plane of bending, in.(mm)h 2 = width of composite cross section parallel to the plane of bending, in. (mm)The supporting comparisons with beam-column tests included 48 concrete filledpipes or tubing and 44 concrete-encased steel shapes (Galambos and Chapuis,1980). The overall mean test-to-prediction ratio was 1.23 and the coefficient ofvariation 0.21.The last paragraph in Section I4 provides a transition from beam-columns tobeams. It involves bond between the steel section and concrete. Section I3 forbeams requires either shear connectors or full, properly reinforced encasement ofthe steel section. Furthermore, even with full encasement, it is assumed that bond iscapable of developing only the moment at first yielding in the steel of the compositesection. No test data are available on the loss of bond in composite beam-columns.However, consideration of tensile cracking of concrete suggests P u / c P n = 0.3 as aconservative limit. It is assumed that when P u / c P n is less than 0.3, the nominalflexural strength is reduced below that indicated by plastic stress distribution on thecomposite cross section unless the transfer of shear from the concrete to the steel isprovided for by shear connectors.I5. SHEAR CONNECTORS1. MaterialsTests (Ollgaard, Slutter, and Fisher, 1971) have shown that fully composite beamswith concrete meeting the requirements of Part 3, Chapter 4, “Concrete Quality,” ofACI (1999), made with ASTM C33 or rotary-kiln produced C330 aggregates,develop full flexural capacity.2. Horizontal Shear ForceComposite beams in which the longitudinal spacing of shear connectors was variedaccording to the intensity of the static shear, and duplicate beams in which the connectorswere uniformly spaced, exhibited the same ultimate strength and the sameamount of deflection at normal working loads. Only a slight deformation in the concreteand the more heavily stressed connectors is needed to redistribute the horizon-LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION