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It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

The lateral torsional buckling (LTB) resistance equations for beams in design specifications require the calculation of an effective length. In lieu of using rigorous analyses, design specifications allow the calculation of an elastic effective length factor (K) for beams. The most commonly used method in calculating K for LTB resistance of a critical beam span was proposed by Nethercot & Trahair, which makes use of alignment charts for braced columns, while accounting for the restraint provided by its adjoining segments. The authors find that this approach results in values of K that are larger or smaller than the true K for certain bracing and loading conditions. A K estimate that is larger than the true solution adversely effects the formulation of the design LTB curve, wherein a larger beam capacity is ascribed to a given effective length. A K estimate that is smaller than the true solution may result in unconservative design solutions. This paper discusses some conditions which are commonly encountered in design and experimental test setups, where unfavourable K estimates are made. The examples presented include cases such as multiple equal laterally unbraced lengths under a uniform moment, beams where the critical segment is an end segment, and cases where the critical segment is bounded by unsymmetric restraining conditions at its two braced ends. A modified approach to estimate K is presented for such cases, while also considering singly-symmetric cross-sections. The paper is limited to elastic buckling of compact (Class I) I-sections. © (2019) by the Structural Stability Research Council (SSRC) All rights reserved.

Department Of Civil Engineering

Structural Stability Research Council Annual Stability Conference 2019, SSRC 2019

A modified approach towards estimating the lateral torsional buckling effective length

The current AASHTO and AISC Specification equations characterizing the lateral-torsional buckling (LTB) resistance of steel I-section members are the same, with minor exceptions, and are based in large part on unified provisions calibrated to experimental data. This paper takes a fresh look at the correlation of the flexural strength predictions from these equations with a large experimental data set compiled from research worldwide. To account fully for the moment gradient and end restraint effects present in the physical tests, the study employs practical buckling calculations using inelastic stiffness reduction factors (SRFs) based on the design resistance equations. The study focuses on uniform bending tests as well as moment gradient tests in which the transverse loads are applied at braced locations. Reliability indices are estimated in the context of building design. It is shown that a proposed modified form of the current resistance equations provides a more uniform level of reliability, as a function of the LTB slenderness, consistent with the target intended in the AISC Specification. The paper also calls attention to the limited experimental data pertaining to the inelastic LTB resistances in certain cases. The paper concludes by providing additional recommendations for LTB strength calculations in routine design, including illustrative plots conveying the impact of the proposed changes. © 2018, American Institute of Steel Construction Inc. All rights reserved.

Engineering Journal

Assessment of I-section member LTB resistances considering experimental test data and practical inelastic buckling design calculations

The current AASHTO specification requirements for the web bend-buckling strength-reduction factor (Rb) do not consider the influence of longitudinal stiffeners on the flexural resistance of steel I-girders subsequent to the theoretical bend-buckling of a slender web. This is a deficiency that can have significant impact, especially in regions of continuous-span girders subjected to negative moment. This paper evaluates the contribution of single-web longitudinal stiffeners to the yield limit state flexural resistance of slender-web steel I-girders, often referred to as the plateau resistance for the lateral torsional and flange local buckling limit states. Based on test simulation studies, the authors recommend a cross-section model that captures the response of the postbuckled web for both homogeneous and hybrid girders subjected to uniform moment. The proposed model offers up to 60% improvement relative to the current AASHTO specifications. © 2016 American Society of Civil Engineers.

World University root node

Journal of Bridge Engineering

Flexural resistance of longitudinally stiffened I-girders. I: Yield limit state

The current AASHTO specifications impose a penalty on the strengths corresponding to the compression flange in slender-web cross sections due to bend-buckling of the web in flexural compression. This web bend-buckling factor (Rb ) does not account for any potential benefits from the influence of web longitudinal stiffening. In a companion paper, the authors developed a cross-section model that can be used to estimate the flexural capacity of I-girders at the yield limit state. In this paper, an improved handling of combined web bend-buckling and lateral torsional buckling (LTB) of longitudinally stiffened I-girders is proposed based on finite-element test simulations. Results from the proposed prediction model are compared to available experimental tests. In addition, the Rb calculated from the proposed model, used in conjunction with the current specification flange local buckling (FLB) equations, is shown to provide an improved characterization of the FLB capacity of these types of members. © 2016 American Society of Civil Engineers.

Flexural resistance of longitudinally stiffened I-girders. II: LTB and FLB limit states

The lateral torsional buckling (LTB) resistance equations of the current AISC and AASHTO Specifications are based on unified design provisions. These provisions are a fit to a large body of experimental test data for light to medium weight rolled and welded I-section members. The resulting nominal LTB resistances differ considerably from those of the Eurocode provisions, which tend to provide a lower-bound fit to experimental and simulation data. It is observed that finite element test simulations conducted with defacto standard practices commonly predict lower capacities than indicated by experimental tests. This disconnect is shown to be partly due to the use of simple deterministic residual stress patterns and geometric imperfections in test simulations that are conservative approximations of the true values occurring in physical tests. This paper aims to resolve this disconnect by employing an “inverse solution” approach, wherein appropriate simplified residual stresses and geometric imperfections to model in test simulations are determined based on correlation with experimental data. In addition, the paper presents the results of extensive sensitivity studies on I-section members subjected to uniform moment. These studies justify the use of as low as one-half the magnitudes of common nominal residual stresses and geometric imperfections in test simulations. In addition, these comparisons justify reductions in the “plateau length,” Lp, used in the AISC and AASHTO flexural resistance calculations, as well as in the stress limit FL in AISC and Fyr in AASHTO corresponding to the applicability of the theoretical elastic LTB equations.

Journal of Constructional Steel Research

Resolving the disconnects between lateral torsional buckling experimental tests, test simulations and design strength equations

The noncompact web-slenderness limit in current design specifications is based on an assumed value of the plate buckling coefficient for a web subjected to flexure that is between the values for fixed and simply-supported edge conditions. While the limit provided in current specifications is a good representation in many situations, there is evidence that the limit overestimates the magnitude of the restraint provided by smaller flanges. This paper shows that the ratio of the area of the compression flange to the area of the web in compression is a good indicator of the restraint provided to the bend-buckling of the web plate. The overestimation of the plate buckling coefficient can result in the classification of cross sections having relatively small flanges as noncompact web sections when their behavior is more akin to that of slender web sections. Slender web sections have a maximum possible flexural resistance, referred to as the plateau strength, less than the yield moment Myc, because of load shedding from the web. Furthermore, the St. Venant torsional stiffness contribution to the lateral torsional buckling resistance is neglected for these sections due to web distortional flexibility. This paper evaluates the appropriate noncompact web-slenderness limit via test simulation of a targeted set of short girder specimens in which the calculated resistance is governed by the plateau strength and proposes a modified equation to rectify the problem. The impact of this change on the prediction of a broad range of member limit states is illustrated via comparison to other test simulations and to experimental results. © 2016 American Society of Civil Engineers.

Department Of Biotechnology

Journal of Structural Engineering (United States)

Improved Noncompact Web-Slenderness Limit for Steel I-Girders

The I-section member lateral torsional buckling (LTB) resistance equations of the unified provisions underlying the current AISC and AASHTO specifications are a fit to a large body of experimental test data. It has been observed that finite-element (FE) test simulations commonly predict smaller capacities than the AISC/AASHTO LTB equations, especially in the inelastic LTB region. One reason for this disconnect is the fact that the residual stresses and geometric imperfections are often approximated by conservative nominal values in test simulations. Another reason for the discrepancy is the common lack of consideration of inelastic effective length effects within calibrations to experimental test results. This paper recommends improvements to the current LTB resistance equations to address these shortcomings. The impact of inelastic end restraint in representative beam LTB experimental tests is illustrated through FE test simulations. In addition, extensive test simulation results are presented, based on reduced residual stresses and geometric imperfections determined in separate research, which illustrate the quality of the proposed prediction equations for I-section members of various cross section types subjected to uniform moment. © 2016 American Society of Civil Engineers.

Reassessment of the lateral torsional buckling resistance of I-section members: Uniform-moment studies

Longitudinal stiffeners are welded to the webs of slender-web I-girders to restrict the web lateral deformations at service and construction load levels. The AASHTO 2015 equations recognize an increase in the bend-buckling resistance of girder webs reinforced by longitudinal stiffeners. However, in girders where web bend-buckling occurs prior to reaching the girder ultimate flexural resistance, a portion of the web becomes ineffective and flexural stresses are redistributed largely to the compression flange. The current Specification equations impose a penalty on the strength of the compression flange by a load shedding factor, Rb. However, this load shedding factor neglects the contribution of the longitudinal stiffener to the web post- buckling resistance. The authors have previously developed a cross-section model that can be used to estimate the flexural capacity of I-girders for both homogenous and hybrid girders at the yield limit state. In this paper, an improved handling of combined web buckling and lateral torsional buckling of longitudinally stiffened plate girders is proposed based on finite element test simulations. In addition, the Rb calculated from the proposed model, used in conjunction with the current Specification flange local buckling equations is shown to provide a better characterization of the flange local buckling capacity of longitudinally stiffened I-girders.

Structural Stability Research Council Annual Stability Conference 2016, SSRC 2016

Improved stability design of longitudinally stiffened plate girders

The lateral torsional buckling (LTB) resistance equations for rolled and welded I-sections differ considerably across the AASHTO/AISC, Eurocode, and other international design codes. These differences stem largely from residual stress and geometric imperfection sensitivity in the inelastic and elastic LTB regions of the curves. AISC/AASHTO employ a resistance calculation method by which the LTB resistance curve is clearly segmented into three distinct parts: the yield or plateau region, the inelastic buckling region and the elastic buckling region. This curve was established on the basis of experimental data, but tends to give larger strengths than common FEA test simulations in many situations. This paper evaluates the sensitivity of LTB resistances to imperfections and residual stresses on compact and on non-compact web sections. The paper then recommends nominal residual stresses and geometric imperfections to be used when conducting FEA simulations to study LTB, as well as modified LTB resistance equations based on evidence from experimental tests and test simulations. Copyright © 2015 by the Structural Stability Research Council.

Structural Stability Research Council Annual Stability Conference 2015, SSRC 2015

Evaluation of lateral torsional buckling resistance equations in AISC and AASHTO

The current AASHTO specification requirements for the web bend buckling factor Rb do not consider the effect of longitudinal stiffeners in developing plate girder web post buckling flexural resistance. This is a deficiency that can have significant impact especially in regions of negative flexural resistance. This paper evaluates the impact of longitudinal stiffeners on the flexural resistance as well as the requirements on longitudinal stiffeners to develop post buckling capacity. The paper discusses test simulations on straight girders with single longitudinal stiffeners. It is found that there can be up to 50% of savings relative to the current conservative estimates in AASHTO in the negative flexural regions under certain conditions by including the effects of longitudinal stiffeners on the girder flexural capacity. The authors study the impact of various parameters on the contribution of longitudinal stiffeners to the plate girder strength.