AISC LRFD 1.pdf

44 FLEXURAL MEMBERS [Sect. I3. b = 0.85; M n shall be determined from the plastic stress distribution on thecomposite section.(b) For h / t w > 376 . E/F yf : b = 0.90; M n shall be determined from the superposition of elastic stresses,considering the effects of shoring.The negative design flexural strength b M n shall be determined for the steel sectionalone, in accordance with the requirements of Chapter F.Alternatively, the negative design flexural strength b M n shall be computed with: b= 0.85 and M n determined from the plastic stress distribution on the composite section,provided that:(1) Steel beam is an adequately braced compact section, as defined in Section B5.(2) Shear connectors connect the slab to the steel beam in the negative momentregion.(3) Slab reinforcement parallel to the steel beam, within the effective width of theslab, is properly developed.3. Design Strength of Concrete-Encased BeamsThe design flexural strength b M n shall be computed with b = 0.90 and M n determinedfrom the superposition of elastic stresses, considering the effects of shoring.Alternatively, the design flexural strength b M n shall be computed with b = 0.90and M n determined from the plastic stress distribution on the steel section alone.If shear connectors are provided and the concrete meets the requirements of SectionI2.1(2), the design flexural strength b M n shall be computed based upon the plasticstress distribution on the composite section with b = 0.85.4. Strength During ConstructionWhen temporary shores are not used during construction, the steel section aloneshall have adequate strength to support all loads applied prior to the concrete attaining75 percent of its specified strength f c . The design flexural strength of the steelsection shall be determined in accordance with the requirements of Section F1.5. Formed Steel Deck5a. GeneralThe design flexural strength, b M n , of composite construction consisting of concreteslabs on formed steel deck connected to steel beams shall be determined bythe applicable portions of Section I3.2, with the following modifications:(1) This section is applicable to decks with nominal rib height not greater thanthree in. (75 mm). The average width of concrete rib or haunch w r shall be notless than two in. (50 mm), but shall not be taken in calculations as more than theminimum clear width near the top of the steel deck. See Section I3.5c for additionalrestrictions.(2) The concrete slab shall be connected to the steel beam with welded stud shearconnectors 3 4-in. (19 mm) or less in diameter (AWS D1.1). Studs shall beLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION