AISC LRFD 1.pdf

Comm. F1.] DESIGN FOR FLEXURE 2092b. Doubly Symmetric Shapes and Channels with L b > L rThe equation given in the Specification assumes that the loading is applied alongthe beam centroidal axis. If the load is placed on the top flange and the flange is notbraced, there is a tipping effect that reduces the critical moment; conversely, if theload is suspended from the bottom flange and is not braced, there is a stabilizingeffect which increases the critical moment (Galambos, 1998). For unbraced topflange loading, the reduced critical moment may be conservatively approximatedby setting the warping buckling factor X 2 to zero.An effective length factor of unity is implied in these critical moment equations torepresent a worst case pinned-pinned unbraced segment. Including considerationof any end restraint of the adjacent segments on the critical segment can increase itsbuckling capacity. The effects of beam continuity on lateral-torsional bucklinghave been studied and a simple and conservative design method, based on the analogyof end-restrained nonsway columns with an effective length factor less thanone, has been proposed (Galambos, 1998).2c. Tees and Double-AnglesThe lateral-torsional buckling strength (LTB) of singly symmetric tee beams isgiven by a fairly complex formula (Galambos, 1998). Equation F1-15 is a simplifiedformulation based on Kitipornchai and Trahair (1980). See also Ellifritt, Wine,Sputo, and Samuel (1992).The C b used for I-shaped beams is unconservative for tee beams with the stem incompression. For such cases C b =1.0 is appropriate. When beams are bent in reversecurvature, the portion with the stem in compression may control the LTB resistanceeven though the moments may be small relative to other portions of the unbracedlength with C b 1.0. This is because the LTB strength of a tee with the stem in compressionmay be only about one-fourth of the capacity for the stem in tension. Sincethe buckling strength is sensitive to the moment diagram, C b has been conservativelytaken as 1.0. In cases where the stem is in tension, connection details shouldbe designed to minimize any end restraining moments which might cause the stemto be in compression.3. Design by Plastic AnalysisEquation F1-17 sets a limit on unbraced length adjacent to a plastic hinge for plasticanalysis. There is a substantial increase in unbraced length for positive momentratios (reverse curvature) because the yielding is confined to zones close to thebrace points (Yura et al., 1978).Equation F1-18 is an equation in similar form for solid rectangular bars and symmetricbox beams. Equations F1-17 and F1-18 assume that the moment diagramwithin the unbraced length next to plastic hinge locations is reasonably linear. Fornonlinear diagrams between braces, judgment should be used in choosing a representativeratio.Equations F1-17 and F1-18 were developed to provide rotation capacities of at least3.0, which are sufficient for most applications (Yura et al., 1978). When inelasticrotations of 7 to 9 are deemed appropriate in areas of high seismicity, as discussed inCommentary Section B5, Equation F1-17 would become:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION