AISC LRFD 1.pdf

Comm. I3.] FLEXURAL MEMBERS 221whereA r = area of properly developed slab reinforcement parallel to the steel beamand within the effective width of the slab, in. 2 (mm 2 )F yr = specified yield stress of the slab reinforcement, ksi (MPa)Q n = sum of the nominal strengths of shear connectors between the point ofmaximum negative moment and the point of zero moment to either side,kips (N)A third theoretical limit on T is the product of the area and yield stress of the steelsection. However, this limit is redundant in view of practical limitations on slabreinforcement.The nominal plastic moment resistance of a composite section in negative bendingis given by the following equation:( ) ( )M = T d + d + P d -dn1 2 yc 3 2(C-I3-11)whereP yc = the compressive strength of the steel section; for a non-hybrid section P yc =A s F y , kips (N)d 1 = distance from the centroid of the longitudinal slab reinforcement to the topof the steel section, in. (mm)d 2 = distance from the centroid of the tension force in the steel section to the topof the steel section, in. (mm)d 3 = distance from P yc to the top of the steel section, in. (mm)Transverse Reinforcement for the Slab. Where experience has shown that longitudinalcracking detrimental to serviceability is likely to occur, the slab should be reinforcedin the direction transverse to the supporting steel section. It is recommendedthat the area of such reinforcement should be at least 0.002 times the concrete areain the longitudinal direction of the beam and should be uniformly distributed.3. Design Strength of Concrete-Encased BeamsTests of concrete-encased beams demonstrated that (1) the encasement drasticallyreduces the possibility of lateral-torsional instability and prevents local buckling ofthe encased steel, (2) the restrictions imposed on the encasement practically preventbond failure prior to first yielding of the steel section, and (3) bond failure doesnot necessarily limit the moment capacity of an encased steel beam (ASCE, 1979).Fig. C-I3.2. Plastic stress distribution for negative moment.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION