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White, Donald W.

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    Built-Up I-Section Member Flexural Resistance: Inelastic Cb Effects from Web Shear Post-Buckling and Early Tension Yielding
    (Georgia Institute of Technology, 2021-03) Deshpande, Ajinkya M. ; Kamath, Ajit M. ; Slein, Ryan ; Sherman, Ryan J. ; White, Donald W.
    To address the influence of nonuniform bending on the lateral-torsional buckling (LTB) capacity of steel I-section members, the AISC 360 Specification directly scales the calculated uniform bending resistance by the moment gradient modification factor, Cb. Various Cb factors are recommended in the Specification and its Commentary. Most of these factors are derived from elastic LTB solutions using thin-walled open-section (TWOS) beam theory. When the LTB resistance is scaled to certain moment levels, additional flexural yielding occurs in the physical member. The corresponding reductions in member stiffness tend to limit the buckling strength. This behavior may be referred to as an “inelastic Cb effect.” The present AISC Cb calculations do not account for this effect. The resulting over-estimate of the strength tends to be relatively small in many situations; however, this effect can be significant in certain problems. For instance, significant reductions in flexural strength can occur due to web post-buckling distortion in thin-web members subjected to high shear demands. These reductions may be considered as a moment-shear interaction problem; however, they can be described more directly as an inelastic Cb effect. The more commonly recognized inelastic Cb effect is often influenced substantially by yielding induced by significant second-order compression flange lateral bending that occurs as the strength limit is approached; web shear post-buckling deformations exacerbate these effects. Several specific recent advances in I-section member design – advances that capture the substantial shear post-buckling strength of unstiffened webs, as well as improvements that recognize significant inelastic reserve strength in sections exhibiting early tension flange yielding – potentially can lead to larger inelastic Cb effects. This research aims to investigate the accuracy of recommended improvements to the AISC 360-16 Section F4 and F5 provisions for the design of general built-up I-section members, with a primary focus on addressing inelastic Cb effects in cases where they become important. The research evaluates the strength behavior and ultimate load capacity of a number of specific sets of I-section members having geometries particularly sensitive to these effects. Refined shell finite element analysis (FEA) test simulations are implemented to investigate the detailed influence of web shear post-buckling distortions as well as flexural yielding effects including early tension flange yielding. The results from the simulations are compared to “manual” calculations using the iv Specification Sections F4 and F5, as well as recently recommended improvements to these provisions. Refined TWOS inelastic buckling solutions using stiffness reduction factors based on the recommended equations are also considered. The research studies show that web transverse stiffening based on a rule of thumb originally recommended by Basler is effective to limit some of the largest reductions in strength due to web shear post-buckling distortion effects. In addition, it is found that the traditional application of Cb solely in the calculation of the elastic LTB stress, FeLTB, followed by the use of the ratio Fyc/FeLTB in a form of the recommended resistance equations, provides an accurate to conservative calculation of the flexural resistance in cases where simple scaling of the uniform bending resistance significantly over-estimates the capacity. These alternative calculations are the same as employed for general nonprismatic I-section members in the AISC/MBMA Design Guide 25, and are akin to the use of Fy /Fe in the AISC column strength equations. Guidelines are provided defining when these alternative calculations are needed, and when the more common scaling of the uniform bending resistance is sufficient.
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    Development of Design Equation for Curved Steel I Girder
    (Georgia Institute of Technology, 2001-10) White, Donald W.