AISC LRFD 1.pdf

18 SECOND ORDER EFFECTS [Sect. C1.where M 1 /M 2 is the ratio of the smaller to larger moments at the endsof that portion of the member unbraced in the plane of bending underconsideration. M 1 /M 2 is positive when the member is bent in reversecurvature, negative when bent in single curvature.(b) For compression members subjected to transverse loading betweentheir supports, the value of C m shall be determined either by rationalanalysis or by the use of the following values:For members whose ends are restrained. . . . . . . . . C m = 0.85For members whose ends are unrestrained . . . . . . . C m = 1.001B 2 =(C1-4)1−⎛ ∆ohΣP⎞u ⎜ ⎟⎝ ΣHL⎠or1B 2 =1− ⎛ (C1-5)⎝ ⎜ ΣPu⎞⎟ΣPe2 ⎠P u = required axial strength of all columns in a story, kips (N) oh = lateral inter-story deflection, in. (mm)H = sum of all story horizontal forces producing oh , kips (N)L = story height, in. (mm)π 2 EIP e2 = , kips (N)( )2KLwhere I is the moment of inertia in the plane of bending and K is the effectivelength factor in the plane of bending determined in accordance withSection C2.2, for the unbraced frame.C2. FRAME STABILITY1. Braced FramesIn trusses and frames where lateral stability is provided by diagonal bracing,shear walls, or equivalent means, the effective length factor K for compressionmembers shall be taken as unity, unless structural analysis shows that a smallervalue may be used.The vertical bracing system for a braced multistory frame shall be determined bystructural analysis to be adequate to prevent buckling of the structure and to maintainthe lateral stability of the structure, including the overturning effects of drift,under the factored load combinations stipulated in Section A4.The vertical bracing system for a braced multistory frame may be considered tofunction together with in-plane shear-resisting exterior and interior walls, floorslabs, and roof decks, which are properly secured to the structural frames. The columns,girders, beams, and diagonal members, when used as the vertical bracingsystem, may be considered to comprise a vertically cantilevered simply connectedtruss in the analyses for frame buckling and lateral stability. Axial deformation ofLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION