AISC LRFD 1.pdf

Sect. C3.] STABILITY BRACING 19all members in the vertical bracing system shall be included in the lateral stabilityanalysis.1a. Design by Plastic AnalysisIn braced frames designed on the basis of plastic analysis, as limited in SectionA5.1, the axial force in these members caused by factored gravity plus factored horizontalloads shall not exceed 0.85 c times A g F y .2. Unbraced FramesIn frames where lateral stability depends upon the bending stiffness of rigidly connectedbeams and columns, the effective length factor K of compression membersshall be determined by structural analysis. The destabilizing effects of gravityloaded columns whose simple connections to the frame do not provide resistance tolateral loads shall be included in the design of the moment-frame columns. Stiffnessreduction adjustment due to column inelasticity is permitted.Analysis of the required strength of unbraced multistory frames shall include theeffects of frame instability and column axial deformation under the factored loadcombinations stipulated in Section A4.2a. Design by Plastic AnalysisIn unbraced frames designed on the basis of plastic analysis, as limited in SectionA5.1, the axial force in the columns caused by factored gravity plus factored horizontalloads shall not exceed 0.75 c times A g F y .C3. STABILITY BRACING1. ScopeThese requirements address the minimum brace strength and stiffness necessary toensure member design strengths based on the unbraced length between braces withan effective length factor K equal to unity. Bracing is assumed to be perpendicularto the member(s) to be braced; for inclined or diagonal bracing, the brace strength(force or moment) and stiffness (force per unit displacement or moment per unitrotation) must be adjusted for the angle of inclination. The evaluation of the stiffnessfurnished by a brace shall include its member and geometric properties, as wellas the effects of connections and anchoring details.Two general types of bracing systems are considered, relative and nodal. A relativebrace controls the movement of the brace point with respect to adjacent bracedpoints. A nodal brace controls the movement at the braced point without directinteraction with adjacent braced points. The strength and stiffness furnished by thestability bracing shall not be less than the required limits. A second order analysisthat includes an initial out-of-plumbness of the structure or out-of-straightness ofthe member to obtain brace strength and stiffness can be used in lieu of the requirementsof this section.2. FramesIn braced frames where lateral stability is provided by diagonal bracing, shearwalls, or other equivalent means, the required story or panel bracing shear force is:P br = 0.004P u(C3-1)LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION