AISC LRFD 1.pdf

40Chap. ICHAPTER ICOMPOSITE MEMBERSThis chapter applies to composite columns composed of rolled or built-up structural steelshapes, pipe or HSS, and structural concrete acting together and to steel beams supportinga reinforced concrete slab so interconnected that the beams and the slab act together toresist bending. Simple and continuous composite beams with shear connectors andconcrete-encased beams, constructed with or without temporary shores, are included.I1. DESIGN ASSUMPTIONS AND DEFINITIONSForce Determination. In determining forces in members and connections of a structurethat includes composite beams, consideration shall be given to the effectivesections at the time each increment of load is applied.Elastic Analysis. For an elastic analysis of continuous composite beams withouthaunched ends, it is permissible to assume that the stiffness of a beam is uniformthroughout the beam length. The stiffness is permitted to be computed using theweighted average of the moments of inertia in the positive moment region and thenegative moment region.Plastic Analysis. When plastic analysis is used, as limited in Section A5.1, thestrength of flexural composite members shall be determined from plastic stressdistributions.Plastic Stress Distribution for Positive Moment. If the slab in the positive momentregion is connected to the steel beam with shear connectors, a concrete stress of0.85f c is permitted to be assumed uniformly distributed throughout the effectivecompression zone, where f c is the specified compressive strength of the concrete.Concrete tensile strength shall be neglected. A uniformly distributed steel stress ofF y shall be assumed throughout the tension zone and throughout the compressionzone in the structural steel section. The net tensile force in the steel section shall beequal to the compressive force in the concrete slab.Plastic Stress Distribution for Negative Moment. If the slab in the negative momentregion is connected to the steel beam with shear connectors, a tensile stress of F yrshall be assumed in all adequately developed longitudinal reinforcing bars withinthe effective width of the concrete slab. Concrete tensile strength shall be neglected.A uniformly distributed steel stress of F y shall be assumed throughout the tensionzone and throughout the compression zone in the structural steel section. The netcompressive force in the steel section shall be equal to the total tensile force in thereinforcing steel.Elastic Stress Distribution. When a determination of elastic stress distribution isrequired, strains in steel and concrete shall be assumed directly proportional to thedistance from the neutral axis. The stress shall equal strain times modulus of elas-LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION