AISC LRFD 1.pdf

222 FLEXURAL MEMBERS [Comm. I3.Accordingly, the LRFD Specification permits three alternate design methods: onebased on the first yield in the tension flange of the composite section; one based onthe plastic moment capacity of the steel beam alone; and a third method based uponthe plastic moment capacity of the composite section applicable only when shearconnectors are provided along the steel section and reinforcement of the concreteencasement meets the specified detailing requirements. No limitations are placedon the slenderness of either the composite beam or the elements of the steel section,since the encasement effectively inhibits both local and lateral buckling.In the method based on first yield, stresses on the steel section from permanentloads applied to unshored beams before the concrete has hardened must be superimposedon stresses on the composite section from loads applied to the beams afterhardening of the concrete. In this superposition, all permanent loads should be multipliedby the dead load factor and all live loads should be multiplied by the live loadfactor. For shored beams, all loads may be assumed as resisted by the compositesection. Complete interaction (no slip) between the concrete and steel is assumed.The contribution of concrete to the strength of the composite section is ordinarilylarger in positive moment regions than in negative moment regions. Accordingly,design based on the composite section is more advantageous in the regions of positivemoments.4. Strength During ConstructionWhen temporary shores are not used during construction, the steel beam alone mustresist all loads applied before the concrete has hardened enough to provide compositeaction. Unshored beam deflection caused by wet concrete tends to increase slabthickness and dead load. For longer spans this may lead to instability analogous toroof ponding. An excessive increase of slab thickness may be avoided by beamcamber.When forms are not attached to the top flange, lateral bracing of the steel beam duringconstruction may not be continuous and the unbraced length may control flexuralstrength, as defined in Section F1.The LRFD Specification does not include special requirements for a margin againstyield during construction. According to Section F1, maximum factored momentduring construction is 0.90F y Z where F y Z is the plastic moment (0.90F y Z 0.90 1.1F y S). This is equivalent to approximately the yield moment, F y S. Hence,required flexural strength during construction prevents moment in excess of theyield moment.Load factors for construction loads should be determined for individual projectsaccording to local conditions, using the factors stipulated in ASCE 7 as a guide. Asa minimum it is suggested that 1.2 be the factor for the loading from steel framingplus concrete plus formed steel deck, and a factor of 1.6 be used for the live load ofworkmen plus equipment which should not be taken as less than 20 psf(unfactored).5. Formed Steel DeckFigure C-I3.3 is a graphic presentation of the terminology used in Section I3.5.When studs are used on beams with formed steel deck, they may be welded directlythrough the deck or through prepunched or cut-in-place holes in the deck. The usualLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION