Steel box girder bridges are being used on a more and more widespread basis as they represent an alternative to steel I-girder bridges. 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, and 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. Since there are currently no limits in the AASHTO LRFD Bridge Design specifications, designs with even higher slenderness ratios are not precluded.
The objective of this project is to propose AASHTO guidelines for the minimum thickness and maximum slenderness limits for steel box girder bottom flanges to (1) minimize issues during fabrication, handling, and erection; and (2) enhance the long-term performance of steel box girders.
Task 1. Conduct a literature review of relevant domestic and international research, guidelines, and current practices to determine the current state of knowledge on (1) minimum thickness and maximum slenderness limits for steel box girder bottom flanges and (2) issues during fabrication, handling, erection, and in service. This information shall be assembled from published and unpublished reports. The results of this task should be summarized in a tabular form showing thickness ranges, slenderness limits ranges, and reported issues.
Task 2. Conduct a survey of bridge owners and other stakeholders to collect data related to the research objective. Follow-up with participants as needed. The survey plan, including questions and participants, shall be submitted to the NCHRP for review and approval prior to distribution.
Task 3. Conduct limited analytical studies as needed to quantify appropriate limits to flange thickness and/or slenderness. The studies should focus on secondary stress effects and/or distortion-induced stress effects not typically captured in routine design analysis. These analytical studies may include simplified hand analysis (classical solution) calculations and limited finite element modeling studies.
Task 4. Synthesize existing practice and issues. Prepare an outline of the proposed AASHTO guidelines, including discussion of the contents and intent.
Note: Submit a draft of the synthesis and detailed outline no later than 5 months after contract award. NCHRP approval will be required before proceeding with the remaining tasks.
Task 5. Develop the draft proposed AASHTO guidelines according to the approved outline. Submit the draft of the proposed guidelines no later than 1 months after the Task 4 approval.
Task 6. Revise the draft guidelines submitted in Task 4 according to the NCHRP project panel comments.
Task 7. Present the recommended guidelines to the AASHTO Committee on Bridges and Structures, Technical Committees: T-14 Structural Steel Design.
Task 8. Submit a final report describing the entire research effort with the proposed AASHTO guidelines. Following receipt of the draft final report, the remaining 2 months shall be for NCHRP review and comment and for research agency preparation of the revised final report.
STATUS: Research in progress.