AISC LRFD 1.pdf

Sect. C3.] STABILITY BRACING 214. BeamsBeam bracing must prevent the relative displacement of the top and bottom flanges,i.e. twist of the section. Lateral stability of beams shall be provided by lateral bracing,torsional bracing, or a combination of the two. In members subjected to doublecurvature bending, the inflection point shall not be considered a brace point.4a. Lateral BracingBracing shall be attached near the compression flange, except for a cantileveredmember, where an end brace shall be attached near the top (tension) flange. Lateralbracing shall be attached to both flanges at the brace point near the inflection pointfor beams subjected to double curvature bending along the length to be braced.(a) Relative BracingThe required brace strength is:P br = 0.008M u C d / h o(C3-7)The required brace stiffness is:4MCu dβ(C3-8)br= φ Lhb owhere = 0.75M u = required flexural strength, kip-in. (N-mm)h o = distance between flange centroids, in. (mm)C d = 1.0 for bending in single curvature; 2.0 for double curvature; C d = 2.0only applies to the brace closest to the inflection point.L b = distance between braces, in. (mm)(b) Nodal BracingThe required brace strength is:P br = 0.02M u C d / h o(C3-9)The required brace stiffness is:10MCu dβ (C3-10)br= φ Lhb owhere = 0.75When the actual spacing of braced points is less than L q , the maximum unbracedlength for M u , then L b in Equations C3-8 and C3-10 shall be permitted to be takenequal to L q .4b. Torsional BracingTorsional bracing can be nodal or continuous along the beam length. The bracingcan be attached at any cross-sectional location and need not be attached near theLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION