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Author: Kamath, Ajit M. × End date: 2024 × Start date: 2020 × search.filters.applied.f.resourcesubtype: Technical Report ×

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Now showing 1 - 3 of 3
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    Experimental and Analytical Investigations of Doubly-Symmetric Built-Up I-Girders Subjected to Large Moment Gradient
    (Georgia Institute of Technology, 2022-08) Phillips, Matthew L. ; Slein, Ryan ; Kamath, Ajit M. ; Sherman, Ryan J. ; White, Donald W.
    Recent analytical studies have indicated that the current American Institute of Steel Construction (AISC) 360-16 Specification overpredicts the flexural resistance of certain built-up I-girders. The largest overpredictions are observed in I-girders subjected to a high moment gradient (i.e., high shear-to-moment ratios) having unstiffened webs with a large web slenderness ratio (i.e., height-to-depth ratio, h/tw). These recent analytical studies have consisted of elastic shell finite element analysis (FEA) buckling solutions and full-nonlinear shell FEA solutions. The elastic buckling solutions have targeted a wide range of web slenderness ratios and moment gradients, and the full-nonlinear solutions have targeted members with high web slenderness ratios and moment gradients. However, there is a lack of experimental data to confirm the analytical findings that these I-girders are susceptible to strength overpredictions by the AISC 360-16 Specification. Additionally, the analytical studies have focused exclusively on either purely elastic material idealizations or on high web slenderness ratios and moment gradients; therefore, a lack of analytical data exists to determine the combined influence of inelastic material effects (i.e., web postbuckling and/or the onset of yielding) and web slenderness for specimens subject to inelastic lateral torsional buckling limit states. The objectives of the current research are to: 1) Provide experimental validation of the strength overpredictions through large-scale experimental testing, 2) Validate the accuracy of full-nonlinear shell FEA solutions, and 3) Investigate the influence of web slenderness on the behavior of built-up I-girders subjected to large moment gradient. The current study is comprised of two main thrusts: a large-scale experimental effort and complementary FEA studies. The experimental effort consists of six large-scale tests targeting I-girders with an unbraced length near the intersection of the scaled AISC 360 inelastic/elastic LTB strengths with the strength plateau. Recent full-nonlinear analytical studies suggest that specimens at this unbraced length have the largest overpredictions by the AISC 360 Specification. The large-scale tests consist of three unique cross sections and include both single curvature (three point bending) and reverse curvature loading configurations. The single curvature configuration corresponds to a moment gradient factor, Cb, of 1.74, and the reverse curvature configuration corresponds to a Cb value of 2.31, calculated using common design equations. One suite of full nonlinear FEA simulations is conducted to evaluate the correlation with the experimental tests (using measured dimensions, material properties, geometric imperfections, and an assumed residual stress pattern), and a second suite of full nonlinear FEA simulations parametrically extends the experimental studies over a range of unbraced lengths (using nominal dimensions, material properties, geometric imperfections, and an assumed residual stress pattern). The results from these simulations are compared to predicted strengths from recommended AISC 360 Specification provisions and from the first-generation Eurocode 3 standard. A third suite of parametric FEA simulations further explores the influence of continuity across brace points, the effects of material nonlinearity on FEA solutions, and the use of common design approximations versus rigorous calculations for Cb. The results from the current study show: 1) The experimental specimens exhibited strengths significantly smaller than strengths predicted by recommended AISC 360 Ch. F provisions (up to 16 % and 32 % smaller for the single curvature and reverse curvature specimens, corresponding to professional factors of 0.84 and 0.68, respectively), providing validation of the analytical strength overpredictions. 2) The full-nonlinear shell FEA modeling approach implemented in the current research provides accurate simulations of the experimental test results. 3) Web slenderness directly influences the strength overprediction of the members (i.e., the members with smaller web slenderness values had larger normalized LTB resistances and smaller strength overpredictions by the AISC 360 procedures than the members with larger web slenderness values). 4) The strength overpredictions of the built-up I-girders by the AISC 360 design provisions is primarily a consequence of two factors: a. Web distortion effects that are exacerbated by web shear stresses and are not adequately accounted for, and b. The direct scaling of the AISC 360 uniform bending LTB strength curve by the elasticallyderived moment gradient factor, Cb, to levels of major-axis bending moment where significant yielding effects are encountered (other than the AISC LTB strength curves being capped by the plateau strength, additional yielding effects associated with the strength increases from the application of Cb are not accounted for when using the direct scaling methodology employed in AISC 360). 5) Built-up I-girders exhibiting early web shear buckling have significant web postbuckling strength. As such, full-nonlinear analyses must be used to accurately predict the strength of these members via FEA simulations. However, it was observed that the increase in strength from postbuckling action of the web is generally negligible when the theoretical web shear buckling strength is larger than the LTB strength. In addition, webs that exhibit early web local buckling in flexure exhibit substantial postbuckling strength, requiring full-nonlinear shell FEA to accurately predict the strength by FEA simulation.
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    Enhanced Characterization of the Flexural Resistance of Built-Up I-Section Members
    (Georgia Institute of Technology, 2022-07) Slein, Ryan ; Kamath, Ajit M. ; Phillips, Matthew L. ; Sherman, Ryan J. ; Scott, David W. ; White, Donald W.
    The AISC 360 Specification Chapter F I-section member flexural resistance provisions are a central part of structural steel design in the United States. The “unified” procedures of Sections F4 and F5 address general singly and doubly symmetric I-section members. Analytical studies and experimental tests subsequent to the implementation of these provisions within the 2005 AISC Specification suggest that the corresponding inelastic Lateral-Torsional Buckling (LTB) and Tension Flange Yielding (TFY) resistance calculations can be improved. Sixteen new large-scale experimental tests on thirteen specimens are targeted in this research to further investigate these predictions. In addition, extensive shell finite element analysis (FEA) test simulation studies are performed correlating with and parametrically extending the experimental results. The broad objective is to provide additional supporting data for improvements to the AISC Specification Section F3 to F5 provisions for general built-up I-section members. These improvements provide: (1) reductions as well as increases in calculated capacities via changes to the anchor points (Lp, Mmax) and (Lr, ML) in the LTB resistance equations, (2) increases in calculated capacities recognizing inelastic reserve strength in members experiencing early yielding in flexural tension, via a number of advancements, and (3) substantial shortening and streamlining of the Specification provisions by eliminating all TFY resistance checks and addressing the corresponding behavior in the primary limit states calculations. This report discusses the design and execution of the 16 experimental tests and hundreds of test simulations, including the details of how the test fixtures and bracing systems are configured to minimize incidental restraint in the physical tests, as well as the direct modeling of residual stresses and geometric imperfections in the test simulations. Updated professional factors, Mtest /Mn, obtained from the new tests and test simulations, considered in conjunction with recommended Chapter F provisions, show significant improvements relative to values obtained using the current Specification rules. The updated professional factors exhibit mean values close to 1.0, with relatively small dispersion, across the entire range of the design space. The results from the current research, combined with an updated assessment of historical test strengths versus predictions from the recommended procedures, shows close to a uniform reliability index, β, of 2.6 (for building design and a live-to-dead load ratio of 3.0) across the design space evaluated at the completion of this research.
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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.