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White,
Donald W.
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Donald W.
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ItemExperimental 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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ItemEnhanced 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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ItemResidual Stress Measurement of Built-Up I-Section Members via the Center-Hole Drilling Method(Georgia Institute of Technology, 2022-05) Phillips, Matthew L. ; Slein, Ryan ; Sherman, Ryan J. ; White, Donald W.The objective of this study was to measure the residual stresses of two built-up I-girders (one doubly symmetric and one singly symmetric) that are representative of all the experimental specimens tested in the lateral torsional buckling (LTB) investigations conducted by Slein et al. (2022) and Phillips et al. (2022). The measured residual stress profiles were compared to the one-half Best-fit Prawel residual stress profile used in a series of complementary shell finite element analysis (FEA) simulations conducted by Slein et al. (2022), Deshpande et al. (2021), and Phillips et al. (2022). Residual stresses are introduced into components during the manufacturing and fabrication processes and are independent of any externally applied forces or thermal loads. Resulting from mechanical, thermal, and/or metallurgical processes, residual stresses are introduced into built-up I-girders during the manufacturing of the individual plate elements and the welding of the plate components during fabrication. The residual stresses are self-equilibrating, meaning that the corresponding cross-section tensile and compressive forces must balance across the section. Applied stresses are additive to the residual stresses. For the stability limit state, compressive residual stresses at the flange tips have a detrimental impact on the overall buckling strength of the member due to the resulting premature yielding. Two primary classifications for residual stress measurement techniques exist: destructive and non-destructive. Destructive techniques involve the process of removing a small volume of material and then measuring the resulting strains, while non-destructive techniques involve using grain structure and material properties to measure the internal stresses of a member without any physical alterations. For the current study, the semi-destructive technique of center-hole drilling was selected. Center hole-drilling is semi-destructive as it involves the removal of a volume of material, but the holes are generally small enough that they could be repaired without hinderance to the performance of the specimen. The center-hole drilling technique is described in detail in ASTM E837 (2020). All the measurements performed during the current work were conducted in accordance with the governing standard. The measured residual stress profiles for the two specimens considered were small. All flange measurements resulted in a similar profile, where the outer thirds of the flange widths had littleto- no residual stress and the middle third had a spike in tensile residual stress apparently associated with the web-to-flange weld. Both the webs demonstrated an approximately uniform compressive residual stress throughout the web. The measured stresses suggest that the one-half Best-fit Prawel distribution provides a reasonable-to-conservative estimate of the residual stress profiles of the evaluated members; hence the use of the one-half Best-fit Prawel distribution is justified for the complementary shell FEA simulations.
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ItemBuilt-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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ItemFlexural Resistance of Longitudinally Stiffened Plate Girders(Georgia Institute of Technology, 2017-01) Subramanian, Lakshmi P. ; White, Donald W.The American Association of State Highway and Transportation Officials Load and Resistance Factor Design Specifications (AASHTO LRFD) require the use of longitudinal stiffeners in plate girder webs when the web slenderness D/tw is greater than 150. This practice is intended to limit the lateral flexing of the web plate during construction and at service conditions. AASHTO accounts for an increase in the web bend-buckling resistance due to a longitudinal stiffener in a plate girder. However, when the theoretical bendbuckling capacity of the stiffened web is exceeded under strength load conditions, the Specifications do not consider any contribution from the longitudinal stiffeners to the girder resistance. That is, the AASHTO LRFD web bend-buckling strength reduction factor Rb applied in these cases is based on an idealization of the web neglecting the longitudinal stiffener. This deficiency can have significant impact on girder resistance in regions of negative flexure. This research is aimed at evaluating the improvements that may be achieved by fully considering the contribution of the longitudinal stiffeners to the girder flexural resistance. Based on refined Finite Element (FE) test simulations, this research establishes that minimum size longitudinal stiffeners, per current AASHTO LRFD requirements, contribute significantly to the postbuckling flexural resistance of plate girders, and can bring about as much as 60% increase in the strength of the compression flange. A straincompatibility based cross-section Rb model is developed that can be used to calculate the girder flexural resistance at the yield limit state. This model is based on test simulations of straight homogenous girders subjected to uniform bending, and is tested extensively and validated for hybrid girders and other limit states. Hybrid girders use web plates of lower yield strengths than the compression flange plates, leading to early yielding in the web, and potential impact on girder strength. A simplified equation has also been provided that can be used to calculate Rb for both longitudinally stiffened I-girders that may be homogeneous or hybrid. In testing the Lateral Torsional Buckling (LTB) limit state, it is found that there is a substantial deviation between the AISC/AASHTO LTB resistance equations and FE test simulations. A comprehensive parametric study is conducted to determine the appropriate parameters to use in FE test simulations. The recommended parameters are identified as the ones that provide the best fit to the mean of experimental data. Based on FE simulations on unstiffened girders using these recommended parameters, a modified LTB resistance equation is proposed. This equation, used in conjunction with the proposed Rb model from the yield limit state also provides an improved handling of combined web buckling and lateral torsional buckling of longitudinally stiffened plate girders. In the course of evaluating the above limit states, it is observed that the noncompact web slenderness limit in the Specifications, which is an approximation based on nearly rigid edge conditions for the buckling of the web plate in flexure is optimistic for certain cross-sections with narrow flanges. This research shows that the degree of restraint at the edges of the web depend largely on the relative areas of the compression flange and the area of the web in compression. An improved equation for the noncompact web slenderness limit is proposed which leads to a better understanding and representation of the true behavior of these types of members. It is found that there is negligible interaction between the Flange Local buckling (FLB) limit state and the LTB limit states for noncompact flanges with the flange slenderness restricted as per the AASHTO 2014 Specifications. Also, the Rb calculated from the proposed model, used along with the current Specification FLB equations is shown to provide a better characterization of the flange local buckling capacity of longitudinally stiffened girders. Tests subjected to High-Shear High-Moment, and High-Moment High Shear are considered in order to characterize the girder shear resistances and potential moment-shear interaction for both homogenous and hybrid girders in the context of the above improvements. Preliminary studies on curved homogenous girders indicate that the proposed yield limit state model is valid for yield limit state of these types of members.
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ItemDevelopment of Design Equation for Curved Steel I Girder(Georgia Institute of Technology, 2001-10) White, Donald W.