AISC LRFD 1.pdf

17Chap. CCHAPTER CFRAMES AND OTHER STRUCTURESThis chapter contains general requirements for stability of the structure as a whole.C1. SECOND ORDER EFFECTSSecond order (P) effects shall be considered in the design of frames.1. Design by Plastic AnalysisIn structures designed on the basis of plastic analysis, as limited in Section A5.1,the required flexural strength M u shall be determined from a second-order plasticanalysis that satisfies the requirements of Section C2.2. Design by Elastic AnalysisIn structures designed on the basis of elastic analysis, M u for beam-columns, connections,and connected members shall be determined from a second-order elasticanalysis or from the following approximate second-order analysis procedure:M u = B 1 M nt + B 2 M lt(C1-1)whereM nt = required flexural strength in member assuming there is no lateral translationof the frame, kip-in. (N-mm)M lt = required flexural strength in member as a result of lateral translation of theframe only, kip-in. (N-mm)B 1C m= ≥1( 1−Pu/ Pe1)(C1-2)P e1π 2 EI=( )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 C2.1, for the braced frame.P u = required axial compressive strength for the member under consideration,kips (N)C m = a coefficient based on elastic first-order analysis assuming no lateral translationof the frame whose value shall be taken as follows:(a) For compression members not subject to transverse loading betweentheir supports in the plane of bending,C m = 0.6 0.4(M 1 / M 2 )LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION(C1-3)