P1: GIG GRBT055-COM-E
258
E2.
AISC-Sample (LRFD)
June 17, 2005
17:57
Char Count= 0
SLENDERNESS LIMITATIONS AND EFFECTIVE LENGTH
[Comm. E2.
SLENDERNESS LIMITATIONS AND EFFECTIVE LENGTH The concept of a maximum limiting slenderness ratio has experienced an evolutionary change from a mandatory “. . . The slenderness ratio, KL/r, of compression members shall not exceed 200 . . . ” in the 1978 Specification to no restriction at all in this Specification. The 1978 ASD and the 1999 LRFD Specifications (AISC, 1978; AISC, 2000b) provide a transition from the rigid mandatory limit to no limit by the flexible provision that “. . . the slenderness ratio, KL/r , preferably should not exceed 200. . . .” This latter restriction is actually no limit at all, so the present Specification has disposed with the provision altogether. However, the designer should keep in mind that columns with a slenderness ratio of more than 200 will have a critical stress (Equation E3-4) less than 6.3 ksi (43.5 MPa). The traditional upper limit of 200 was based on professional judgment and practical construction economics, ease of handling, and care required to minimize inadvertent damage during fabrication, transport and erection. It is not recommended to exceed this limit for compression members except for cases where special care is exercised by the fabricator and erector.
E3.
COMPRESSIVE STRENGTH FOR FLEXURAL BUCKLING OF MEMBERS WITHOUT SLENDER ELEMENTS Section E3 applies to compression members with compact and noncompact sections, as defined in Section B4. The column strength equations in Section E3 are the same as those in the previous editions of the LRFD Specification, with the exception of the cosmetic replaceKL Fy ment of the nondimensional slenderness ratio λc = by the more familiar r E KL . For the convenience of those calculating the elastic buckling stress directly, r without first calculating K, the limits on use of Equations E3-2 and E3-3 are also provided in terms of Fe . Comparisons between the previous column design curves and the new one are shown in Figures C-E3.1 and C-E3.2 for the case of Fy = 50 ksi (345 MPa). The curves show the variation of the available column strength with the slenderness ratio for LRFD and ASD, respectively. The LRFD curves reflect the change of the resistance factor f from 0.85 to 0.90, as was explained in Commentary Section E1 above. For both LRFD and ASD, the new column equations give somewhat more economy than the previous editions of the Specification. KL E The limit between elastic and inelastic buckling is defined to be = 4.71 r Fy or Fe = 0.44Fy . For convenience, these limits are defined in Table C-E3.1 for the common values of Fy . One of the key parameters in the column strength equations is the elastic critical stress, Fe . Equation E3-4 presents the familiar Euler form for Fe . However, Fe can Specification for Structural Steel Buildings, March 9, 2005 AMERICAN INSTITUTE OF STEEL CONSTRUCTION, INC.