AISC LRFD 1.pdf

28 DESIGN COMPRESSIVE STRENGTH FOR FLEXURAL BUCKLING [Sect. E2.For members whose elements do not meet the requirements of Section B5.1, seeAppendix B5.3.E3. DESIGN COMPRESSIVE STRENGTH FORFLEXURAL-TORSIONAL BUCKLINGThe design strength for flexural-torsional buckling of double-angle and tee-shapedcompression members whose elements have width-thickness ratios less than rfrom Section B5.1 is c P n :where c = 0.85P n = A g F crft(E3-1)Fcrft⎛Fcry + Fcrz ⎞⎡4FcryFcrzH⎤= ⎜ ⎟⎢1− 1 −⎥2H ⎢ ( F + F )2⎥⎝ ⎠⎣ cry crz ⎦(E3-2)GJF crz =2Aror o = polar radius of gyration about shear center, in. (mm) (see EquationA-E3-8)2yoH = 1−2ro= distance between shear center and centroid, in. (mm)y oF cry is determined according to Section E2 for flexural buckling about the y-axis ofsymmetry for λ c = Kl Fy.ryπ EFor double-angle and tee-shaped members whose elements do not meet the requirementsof Section B5.1, see Appendix B5.3 to determine F cry for use in EquationE3-1.Other singly symmetric and unsymmetric columns, and doubly symmetric columns,such as cruciform or built-up columns, with very thin walls shall be designedfor the limit states of flexural-torsional and torsional buckling in accordance withAppendix E3.E4. BUILT-UP MEMBERS1. Design StrengthThe design strength of built-up members composed of two or more shapes shall bedetermined in accordance with Section E2 and Section E3 subject to the followingmodification. If the buckling mode involves relative deformations that produceshear forces in the connectors between individual shapes, Kl/r is replaced by(Kl/r) m determined as follows:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION