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Design Guidelines for Lateral-Torsional Buckling of Nonprismatic Steel I-Girders

Description:

The flexural behavior and design of steel I-girders in negative moment regions at interior piers of continuous-span bridges is complex. The girders are often nonprismatic in these regions due to the introduction of flange transitions at welded shop splices or at field splices, and/or variations in the depth of the members (i.e., use of haunched girders at the piers). Historically, design specifications have been silent regarding the influence of changes in the cross-section along the unbraced length on the girder lateral-torsional buckling (LTB) resistance. The 8th Edition of the AASHTO LRFD Specifications includes a few highly simplified recommendations addressing LTB of nonprismatic (stepped) flanges as an effort to accommodate economical designs. However, studies have shown that these provisions are highly conservative in certain cases, and that they produce anomalous results in some situations (Grubb and Schmidt, 2015; Sause et al. 2016).

An effort is underway by an Ad Hoc Task Group, sponsored by the AASHTO T-14 Committee, to scrutinize LTB concepts and calculation procedures for these girder types. This effort has led to a draft agenda item for updating of the AASHTO LRFD Specifications. Additional efforts are needed to support this initiative and to facilitate the application of some form of these highly improved design methods. Specifically, further analytical studies and experimental testing are highly desirable to provide a comprehensive assessment of recommended procedures. In addition, a design guidelines document should be developed, focused on steel I-girders in composite continuous-span bridges. These developments will provide for the greatest potential success of the new AASHTO LRFD provisions.

Objective:

The first objective of this research is to conduct analytical and experimental studies targeted at further evaluation of the LTB behavior of nonprismatic I-girders in negative bending. This work should be targeted specifically at evaluation and potential improvement of the draft updated AASHTO LRFD Specifications being developed by the AASHTO T-14 Ad Hoc Task Group. The second objective is to develop a formal design guidelines document, similar to the AISC/MBMA Design Guide 25 (White and Jeong, 2020), providing a clear explanation of the concepts as well as detailed practical examples to assist designers with understanding and applying the new procedures. It is anticipated that these examples can be developed based largely on case studies being performed by the Ad Hoc Task Group.

COBS Prioritized Objective(s)(“2018 SCOBS Strategic Plan”) Addressed by this Problem Statement:

  1. Extend Bridge Service Life: The research findings will extend bridge service life by reducing the need for load posting, retrofit and rehabilitation.2. Assess Bridge Condition: This research will lead to improved load rating.3. Maintain and Enhance a Knowledgeable Workforce: The research findings would increase the overall knowledge regarding the consideration of cross-section transitions and variable web depth in steel bridge I-girder design.4. Maintain and Enhance AASHTO Specifications: The research findings should resolve current anomalies and shortcomings in the AASHTO Specifications.5. Accelerate Bridge Delivery and Construction: With clearer and more streamlined calculations, the design process can be accelerated as well as enhanced.6. Optimize Structural Systems: This research statement aligns with this objective because the potential benefit would be more efficient and accurate design of steel I-girders having nonprismatic geometry.7. Model and Manage Information Intelligently: N/A8. Contribute to National Policy: N/A

Benefits:

This project will allow engineers to more easily recognize additional structural capacity in nonprismatic steel I-girders in negative moment regions over interior piers, thereby leading to more economical designs and increased load rating factors. As a result, owners will benefit from lower initial construction costs and avoidance of unnecessary load posting, retrofit, or replacement of existing bridges.

Related Research:

A number of experiments have been conducted over the years (Climenhaga and Johnson, 1972; Carskaddan, 1980; Vasseghi and Frank, 1987; Tansil, 1991; Kemp, 1996; Baskar and Shanmugam, 2003) contributing to the present understanding of the behavior of composite steel I-girders subjected to negative bending. However, to the knowledge of the authors, no tests have been conducted on nonprismatic I-girders. In addition, many of the prior studies have been conducted on relatively small members representative of building construction or shorter-span bridge construction. Vasseghi and Frank (1987) are the only investigators who have addressed simulation of unshored composite construction in experimental tests. A number of practical nonprismatic girder design cases have been collected by the AASHTO T-14 Ad Hoc Task Group, and LTB design calculations have been prepared for these cases. These practical cases can serve as one starting point for design of experimental tests aimed at evaluating the performance of the new AASHTO LRFD provisions. It is clear that the LTB capacities of nonprismatic girders can be limited in particular when initial yielding occurs near the middle of the unbraced lengths, due to the loading and nonprismatic geometry effects, and that enhancement of the capacity via torsional restraint from the bridge deck is often limited due to the slenderness of the girder webs.

AISC/MBMA Design Guide 25 (White and Jeong, 2020) provides one formal approach for the design of noncomposite nonprismatic I-section members. This procedure basically requires: 1) the calculation of an elastic buckling load factor, ge, which defines the theoretical elastic buckling load level and the corresponding elastic buckling member stresses, Fe, and 2) the calculation of the ratio of the flange stresses at the factored design load to the flange yield stress, fbu/Fyf for flexure. Once these values are obtained for a given unbraced length, the calculations proceed by mapping the theoretical elastic buckling resistance to the design resistance at a critical cross-section determined based on the ratio fbu/Fyf.

Tasks:
  • Task 1 – Conduct a literature synthesis starting with the known materials associated with the AASHTO T-14 Ad Hoc Task Group effort.
  • Task 2 – Design an experimental test program that considers both noncomposite girder actions, representative of construction conditions, and composite girder actions, representative of final constructed conditions (both involving moment gradient). As a minimum, these tests should include: 1) a constant depth I girder with a single flange transition, and 2) a variable web depth I-girder, also potentially containing flange transitions.
  • Task 3 – Perform full nonlinear finite element analysis simulations of the targeted experimental tests to evaluate the expected behavior in detail and to finalize the design of the tests.
  • Task 4 – Execute the experimental tests of the nonprismatic I-girders.
  • Task 5 – Synthesize the data from the experimental tests and recommend any adjustments or refinements to recommended AASHTO LRFD procedures based on the results.
  • Task 6 – Prepare a research report detailing the results from the above studies.
  • Task 7 – Prepare a guidelines document providing a clear illustration of recommended LTB concepts and design
Implementation:

The implementation prospects for this research are excellent given the AASHTO T-14 Ad Hoc Task Group effort, which has been underway for several years, and given the draft AASHTO LRFD Specifications agenda item developed within this effort.

Relevance:

It will be most efficient and most productive for the research to be executed as soon as possible, given the AASHTO T-14 efforts already underway. This research is highly relevant to the bridge engineering and construction community. Nonprismatic unbraced lengths are common in negative moment regions of continuous-span steel I-girder bridges, Corresponding design procedures should be well founded in experimental and analytical studies.

Sponsoring Committee:AKB20, Steel Bridges
Research Period:24 - 36 months
Research Priority:High
RNS Developer:White, Grubb, Sherman
Source Info:Baskar, K. and Shanmugam, N.E. (2003). “Steel-Concrete Composite Plate Girders Subject to Combined Shear and Bending,” Journal of Constructional Steel Research, 59, 531-557.
Carskaddan, P.S. (1980). “Autostress Design of Highway Bridges Phase 3: Interior-Support-Model Test,” AISI Project 188, Research Report 97-H-045(019-5), United States Steel Corporation.
Climenhaga, J.J. and Johnson, R.P. (1972). “Local Buckling in Continuous Composite Beams,” The Structural Engineer, 50(9), 367-374.
Grubb, M.A. and Schmidt, R.E. (2015). “Steel Bridge Design Handbook Design Example 1: Three- Span Continuous Straight Composite Steel I-Girder Bridge,” FHWA-HIF-16-002-Vol. 20.
Kemp, A.R. (1996). “Inelastic Local and Lateral Buckling in Design Codes,” Journal of Structural Engineering, ASCE, 122(4), 374-382.
Sause, R., Hodgeson, I. and Tahmesebi, E. (2016). “Lateral-Torsional Buckling Limit State Check for Haunched Girders,” Report to Pennsylvania DOT, ATLSS Engineering Research Center, Lehigh University, Bethlehem, PA.
Tansil, T.C. (1991). “Behavior of a Composite Plate Girder in Negative Bending,” M.S. Thesis, University of Texas, Austin, TX.
Vasseghi, A. and Frank, K.H. (1987). “Static Shear and Bending Strength of Composite Plate Girders,” PMFSEL Report No. 87-4, University of Texas, Austin, TX, June.
White, D.W. and Jeong. W.Y. (2020). Design of Frame Using Nonprismatic Members, AISC-MBMA Design Guide 25, 2nd Edition, American Institute of Steel Construction, Chicago, IL (in press).
Date Posted:04/09/2020
Date Modified:05/06/2020
Index Terms:Bridge design, Girder bridges, Steel bridges, Buckling, Torsional strength, Lateral strength, Girders, Flexural strength, Flexure,
Cosponsoring Committees: 
Subjects    
Highways
Design
Bridges and other structures

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