Toward the Recognition of Unaccounted for Flange Local Buckling and Tension Flange Yielding Resistances in the ANSI/AISC 360 Specification

The Flange Local Buckling (FLB) and Tension Flange Yielding (TFY) limit states in Chapter F of the ANSI/AISC 360 Specification tend to underestimate I-section member flexural resistances as the web and/or the compression flange become increasingly slender. The characterization of FLB only considers the compression flange buckling strength without accounting for its reserve postbuckling capacity. For the TFY limit state, the current equations limit the maximum moment of singly-symmetric slender-web I-section members to the first yield of the tension flange. However, the actual cross-section typically is able to develop extensive yielding of the tension flange and the tension region of the web. The FLB limit state check can be improved by implementing a form of the unified effective width approach, which recognizes the postbuckling resistance of slender flange elements. For the TFY limit state, simple calculations can be configured from mechanics of materials concepts, recognizing the ability of the region in flexural tension to develop extensive spread of yielding. These calculations eliminate the conservatism of the TFY equations while accurately characterizing the associated member structural stability. This paper proposes updated provisions recognizing the above FLB and TFY limit states behavior, and compares the performance of these provisions to the current limit state equations and to the results from full nonlinear FEA parametric studies. Recommendations are provided for further research to evaluate the impact and implications of these improvements more fully.

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