AISC LRFD 1.pdf

34 DESIGN FOR FLEXURE [Sect. F1.2b. Doubly Symmetric Shapes and Channels with L b > L rThe nominal flexural strength is:M n = M cr M p(F1-12)where M cr is the critical elastic moment, determined as follows.(a) For doubly symmetric I-shaped members and channels:πEMcr= CbEI yGJ+ ⎛ I yCL b ⎝ ⎜ π ⎞⎟Lb⎠2w(F1-13)2CbSxX1 2 X1X2= 1 +Lb / ry 2 ( Lb / ry)(b) For solid rectangular bars and symmetric box sections:2Mcr57000Cb=L / rbyJA(F1-14)2c. Tees and Double AnglesFor tees and double-angle beams loaded in the plane of symmetry:π EIyGJ2Mn= Mcr= ⎡B+ 1+B ⎤L ⎣ ⎦b(F1-15)whereM n 1.5M y for stems in tensionM n 1.0M y for stems in compressionB = ±2.3(d/L b ) I Jy /(F1-16)The plus sign for B applies when the stem is in tension and the minus sign applieswhen the stem is in compression. If the tip of the stem is in compression anywherealong the unbraced length, use the negative value of B.3. Design by Plastic AnalysisDesign by plastic analysis, as limited in Section A5.1, is permitted for a compactsection member bent about the major axis when the laterally unbraced length L b ofthe compression flange adjacent to plastic hinge locations associated with the failuremechanism does not exceed L pd , determined as follows.(a) For doubly symmetric and singly symmetric I-shaped members with the compressionflange equal to or larger than the tension flange (including hybridmembers) loaded in the plane of the web:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION