AISC LRFD 1.pdf

Comm. B5.] LOCAL BUCKLING 181load. A design procedure for plate girders including tension field action is given inAppendix G.The values of the limiting ratios p and r specified in Table B5.1 are similar to thosein AISC (1989) and Table 2.3.3.3 of Galambos (1976), except that: (1) p =038 . E/F y , limited in Galambos (1976) to indeterminate beams when momentsare determined by elastic analysis and to determinate beams, was adopted for allconditions on the basis of Yura et al. (1978); and (2) p = 0.045E/F y for plasticdesign of circular hollow sections was obtained from Sherman (1976).The high shape factor for circular hollow sections makes it impractical to use thesame slenderness limits to define the regions of behavior for different types of loading.In Table B5.1, the values of p for a compact shape that can achieve the plasticmoment, and r for bending, are based on an analysis of test data from several projectsinvolving the bending of pipes in a region of constant moment (Sherman andTanavde, 1984, and Galambos, 1998). The same analysis produced the equation forthe inelastic moment capacity in Table A-F1.1 in Appendix F1. However, a morerestrictive value of p is required to prevent inelastic local buckling from limitingthe plastic hinge rotation capacity needed to develop a mechanism in a circular hollowbeam section (Sherman, 1976).The values of r for axial compression and for bending are both based on test data.The former value has been used in building specifications since 1968 (Winter,1970). Appendices B5 and F1 also limit the diameter-to-thickness ratio for any circularsection to 0.45E/F y . Beyond this, the local buckling strength decreases rapidly,making it impractical to use these sections in building construction.Following the SSRC recommendations (Galambos, 1998) and the approach usedfor other shapes with slender compression elements, a Q factor is used for circularsections to account for interaction between local and column buckling. The Q factoris the ratio between the local buckling stress and the yield stress. The local bucklingstress for the circular section is taken from the inelastic AISI criteria (Winter,1970) and is based on tests conducted on fabricated and manufactured cylinders.Subsequent tests on fabricated cylinders (Galambos, 1998) confirm that this equationis conservative.The definitions of the width and thickness of compression elements agree with the1978 AISC ASD Specification with minor modifications. Their applicabilityextends to sections formed by bending and to unsymmetrical and hybrid sections.For built-up I-shaped sections under axial compression, modifications have beenmade to the flange local buckling criterion to include web-flange interaction. The k cin the r limit, in Equations A-B5-7 and A-B5-8 and the elastic buckling EquationA-B5-8 are the same that are used for flexural members. Theory indicates that theweb-flange interaction in axial compression is at least as severe as in flexure.Rolled shapes are excluded from this criterion because there are no standard sectionswith proportions where the interaction would occur. In built-up sectionswhere the interaction causes a reduction in the flange local buckling strength, it islikely that the web is also a thin stiffened element.The k c factor accounts for the interaction of flange and web local buckling demonstratedin experiments conducted by Johnson (1985). The maximum limit of 0.763corresponds to F cr = 0.69E / 2 which was used as the local buckling strength inLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION