AISC LRFD 1.pdf

203Comm. ECHAPTER ECOLUMNS AND OTHER COMPRESSION MEMBERSE1. EFFECTIVE LENGTH AND SLENDERNESS LIMITATIONS1. Effective LengthThe Commentary on Section C2 regarding frame stability and effective length factorsapplies here. Further analytic methods, formulas, charts, and references for thedetermination of effective length are provided in Chapter 15 of the SSRC Guide(Galambos, 1998).2. Design by Plastic AnalysisThe limitation on c is essentially the same as that for l/rin Chapter N of the 1989AISC Specification—Allowable Stress Design and Plastic Design.E2. DESIGN COMPRESSIVE STRENGTHFOR FLEXURAL BUCKLING*Equations E2-2 and E2-3 are based on a reasonable conversion of research data intodesign equations. Conversion of the allowable stress design (ASD) equationswhich was based on the CRC—Column Research Council—curve (Galambos,1998) was found to be cumbersome for two reasons. The first was the nature of theASD variable safety factor. Secondly, the difference in philosophical origins of thetwo design procedures requires an assumption of a live load-to-dead load ratio(L / D).Since all L/Dratios could not be considered, a value of approximately 1.1 at equal to 1.0 was used to calibrate the exponential equation for columns with thelower range of against the appropriate ASD provision. The coefficient with theEuler equation was obtained by equating the ASD and LRFD expressions at of 1.5.Equations E2-2 and E2-3 are essentially the same curve as column-strength curve2P of the Structural Stability Research Council which is based on an initialout-of-straightness curve of l/1500 (Bjorhovde, 1972 and 1988; Galambos, 1998;Tide, 1985).It should be noted that this set of column equations has a range of reliability ()values.At low- and high-column slenderness, values exceeding 3.0 and 3.3 respectivelyare obtained compared to of 2.60 at L/Dof 1.1. This is considered satisfactory,since the limits of out-of-straightness combined with residual stress have notbeen clearly established. Furthermore, there has been no history of unacceptable*For tapered members see Commentary Appendix F3.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION