Problem #____: Bottom Flange b/t Limits for Steel Box Girders

Submitted by:

AASHTO Subcommittee on Bridges and Structures

Chair – Gregg Fredrick, Wyoming DOT

Technical Committee on Structural Steel Design (T-14)

Technical Chair: Norman L. McDonald, Iowa DOT

PROBLEM STATEMENT:

Designers can be tempted to specify very thin/slender bottom flanges for steel box girders in positive moment regions (where the bottom flange is in tension). However, specifying too thin or too slender of a bottom flange could result in problems during fabrication, transportation, erection, or future inspection/maintenance, as well as potential strength and fatigue issues.

Currently there is no consensus on minimum thickness and maximum slenderness limits for steel box girder bottom flanges. Industry recommendations on minimum bottom flange thicknesses vary, but are generally in the range of ¾” to 1”. Industry recommendations for bottom flange width to thickness (b/t) slenderness ratios also vary, generally over a range from 80 to 135. However, during a recent design project, an informal survey was conducted of previously completed box girder bridge plans; bottom flanges as thin as ½” and width to thickness (b/t) slenderness ratios as high as 172 were noted. And because there are currently no limits in the AASHTO specifications, designs with even higher slenderness ratios are not precluded. Clearly, clarification of rational limits is required to ensure adequate strength, service, and fatigue limit state performance and to improve constructability.

This problem statement proposes a brief synthesis to review pertinent previously published literature and establish rational minimum thickness and maximum slenderness limits for steel box girder bottom flanges, and to recommend additional research if needed to provide an analytical basis for such limits.

The need for this guidance is urgent and immediate. Steel box girders are being used on a more and more widespread basis as they represent an alternative to steel I-girder bridges (in appropriate applications) with many advantages, including:

  • Pleasing aesthetic characteristics
  • Structural efficiencies in severely curved bridges
  • Long span capacity
  • Reduced number of girders to erect
  • Durability and maintainability

However, if improperly designed and detailed, steel box girders can experience significant problems. Specifically, specifying a bottom flange which is too thin or too slender could result in:

  • Bottom flange distortion during fabrication
  • Bottom flange or web buckling during handling, transportation, or erection
  • Insufficient strength and/or stiffness to resist permanent loads plus live loads occurring during construction or future inspection/maintenance
  • Incompatibility with standard preferred box girder details
  • Invalidation of standard practice methodology for estimating distortional stresses

Publication of rational minimum thickness and maximum slenderness limits for steel box girder bottom flanges would minimize the risk of problems during fabrication, handling, erection, and long-term performance of steel box girders.

Following is a partial list of related previous research and publications:

  1. American Association of State Highway Transportation Officials (AASHTO), LRFD Bridge Design Specifications, 7th Edition, 2014, with Interim Revisions through 2016.
  2. Coletti, D.A., Fan, Z., Gatti, W., Holt, J., Vogel, J., Practical Steel Tub Girder Design, April 2005, National Steel Bridge Alliance.
  3. Heins, C.P., and Hall, D.H., Designer's Guide to Steel Box Girder Bridges, Booklet No. 3500, February, 1981, Bethlehem Steel.
  4. Helwig, T., Yura, J., Herman, R., Williamson, E., Li, D., “Design Guidelines for Steel Trapezoidal Box Girder Systems,” Report No. FHWA/TX-07/0-4307-1, April 2007.
  5. Mattock, A.H., “Development of Design Criteria for Composite Box Girder Bridges,” Development in Bridge Design and Construction, Crosby Lockwood & Sons, London, England, 1971.
  6. Texas Steel Quality Council, Preferred Practices for Steel Bridge Design, Fabrication, and Erection, 2015.
  7. Wolchuk, R., and Mayrbaurl, R.M., “Proposed Design Specifications for Steel Box Girder Bridges,” Report No. FHWA-TS-80-205, Federal Highway Administration, January 1980.
  8. United States Steel, Steel / Composite Box-Girder Bridges – A Construction Manual, ADUSS 88-7493- 01, December 1978. This guide has long been out of print.
  9. Fan, Z., and Helwig, T.A., "Distortional Loads and Brace Forces in Steel Box Girders," ASCE Journal of Structural Engineering, Vol. 128, No. 6, June, 2002, pp 710-718.

OBJECTIVES:

The primary objective of this project is to develop rational minimum thickness and maximum slenderness limits for steel box girder bottom flanges which can be published in the AASHTO LRFD Bridge Design Specifications or in an appropriate guide specification or industry document (such as an AASHTO/NSBA Steel Bridge Collaboration Guideline document). The following issues should be investigated as well as any other issues that the investigator may discover to be important during the conduct of this study:

  1. The relationship between bottom flange thickness / slenderness and the potential for flange or web distortion during girder fabrication
  2. The relationship between bottom flange thickness / slenderness and the potential for flange or web buckling during handling, transportation, or erection
  3. The relationship between bottom flange thickness / slenderness and the strength and stiffness required to resist permanent loads plus live loads occurring during construction or future inspection/maintenance
  4. The relationship between bottom flange thickness / slenderness and compatibility with standard preferred box girder details
  5. The relationship between bottom flange thickness / slenderness and the validity of standard practice methodology for calculating distortional stresses

The secondary objective is to recommend additional research if needed to provide an analytical basis for such limits.

FUNDING REQUESTED: $50,000

EXPECTED TIME TO COMPLETE THE WORK:

Task 1: Literature Survey / 2 months
Task 2: Synthesis of Recommendations / 2 months
Task 3: Draft Final Report / 1 month
Task 4: Final Report / 1 month

Total duration would be 6 months.

CONTACT PERSON:

Mr. Norman L. McDonald

Chair, Technical Committee T-14

State Bridge Engineer

Iowa of Transportation

800 Lincoln Way

Ames, IA 50010

515-239-1206 voice

515-239-1978 fax

Endorsements:

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