AISC LRFD 1.pdf

Comm. I3.] FLEXURAL MEMBERS 217may be needed to avoid overloading the component (steel section or concreteencasement) to which the load is applied directly.Although it is recognized that force transfer also occurs by bond between the steeland concrete, this is disregarded for encased sections (Griffis, 1992). Force transferby bond is commonly used in concrete-filled hollow structural sections (API, 1993)as long as the connections are detailed to limit local deformations, but no guidelinesare available for structures other than fixed offshore platforms.When a supporting concrete area is wider on all sides than the loaded area, the nominalbearing strength for concrete can be taken as085 . φ B f c ′ A2 / A1where A 1 is the loaded area and A 2 is the base of a frustum extending 45 in plan andat a 50 percent slope in elevation from the loaded area (ACI, 1995, 1995a). Thevalue of A2 / A1must be less than or equal to 2. In most practical cases, this limitwill be reached and thus the Specification uses a nominal bearing strength of17 . φ B f c ′ A B . The resistance factor for bearing, B , is 0.65 in accordance with AppendixC in ACI 318 and ACI 318M.I3. FLEXURAL MEMBERS1. Effective WidthLRFD provisions for effective width omit any limit based on slab thickness, inaccordance with both theoretical and experimental studies, as well as compositebeam codes in other countries (ASCE, 1979). The same effective width rules applyto composite beams with a slab on either one side or both sides of the beam. To simplifydesign, effective width is based on the full span, center-to-center of supports,for both simple and continuous beams.2. Design Strength of Beams with Shear ConnectorsThis section applies to simple and continuous composite beams with shear connectors,constructed with or without temporary shores.Positive Flexural Design Strength. Flexural strength of a composite beam in thepositive moment region may be limited by the plastic strength of the steel section,the concrete slab, or shear connectors. In addition, web buckling may limit flexuralstrength if the web is slender and a significantly large portion of the web is in compression.According to Table B5.1, local web buckling does not reduce the plastic strength ofa bare steel beam if the beam depth-to-web thickness ratio is not larger than376 . E/F y . In the absence of web buckling research on composite beams, thesame ratio is conservatively applied to composite beams. Furthermore, for moreslender webs, the LRFD Specification conservatively adopts first yield as the flexuralstrength limit. In this case, stresses on the steel section from permanent loadsapplied to unshored beams before the concrete has hardened must be superimposedon stresses on the composite section from loads applied to the beams after hardeningof concrete. In this superposition, all permanent loads should be multiplied bythe dead load factor and all live loads should be multiplied by the live load factor.For shored beams, all loads may be assumed as resisted by the composite section.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION