BACKGROUND

Currently, in the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications (BDS), the nominal flexural resistance of I-girder bridges that are (1) kinked (chorded) and continuous, (2) horizontally curved, or (3) straight with supports skewed more than 20 degrees from normal is conservatively not allowed to exceed first yield at the strength limit state for the following reasons: 

• Such bridges typically experience significant differential vertical deflections at the cross-frames resulting in larger cross-frame forces in the elastic range. As interior-pier sections yield and begin to lose stiffness and shed their load, the forces in the adjacent cross-frames will increase. There is currently no established procedure to predict the resulting increase in the forces without performing a refined nonlinear analysis. 
• In horizontally curved bridges and severely skewed straight bridges with discontinuous cross-frames, significant lateral flange bending effects can occur. The resulting lateral bending moments and stresses are amplified in the bottom compression flange adjacent to the pier as the flange deflects laterally. There is currently no means to accurately predict these amplification effects as the flange is also yielding.
• Skewed supports also result in twisting of the girders, which is not recognized in plastic-design theory. The relative vertical deflections of the girders create eccentricities that are also not recognized in the theory.

Research is needed to address these load and resistance factor design (LRFD) issues to help state departments of transportation (DOTs) achieve more efficient designs and recognize improved accuracy in load ratings for existing bridges. 

OBJECTIVE

The objective of this project is to develop a methodology to determine the nominal flexural resistance beyond first yield at the strength limit state in I-girder bridges that are (1) kinked (chorded) and continuous, (2) horizontally curved, or (3) straight with supports skewed more than 20 degrees from normal. 

Accomplishment of the project objective will require at least the following tasks.

TASKS

PHASE I—Planning 

Task 1. Conduct a literature review of research and documents on the design and analysis of steel I-girder bridges relevant to this research. The review shall include published and unpublished documentation, past surveys, and research conducted through the NCHRP; the Federal Highway Administration; other national, state, and local agencies; international organizations; and industry groups.

Task 2. Identify and engage key stakeholders (e.g., AASHTO Technical Committee for Steel and Metal, state DOTs, consulting engineers, industry) to seek input on the state of the practice and to define practical and realistic I-girder bridge parameters.

Task 3. Based on the results of Tasks 1 and 2, identify knowledge gaps related to the project objective. These gaps should be addressed in the final product or the recommended future research, as budget permits.

Task 4. Propose the research plan to be executed in Phase II to achieve the project objective. At a minimum, the research plan shall include:

• An analytical investigative approach that considers the material and geometric non-linear responses of the structures.
• An examination of the AASHTO LRFD BDS requirements for the design, analysis, and load rating of I-girder bridges that are (1) kinked (chorded) and continuous, (2) horizontally curved, or (3) straight with supports skewed more than 20 degrees from normal.
• A study of the effects of exceeding the moment at first yield at the strength limit state in (1) composite sections in negative flexure, or non-composite sections, with compact or noncompact webs and (2) composite sections in positive flexure that would otherwise qualify as compact sections. Include consideration of cross-frame forces, flange lateral bending moments and stresses (and the amplification of those moments and stresses as applicable), and torsional displacements of the girders.
• Developing recommendations for modifications of the existing steel girder design requirements, if needed.
• A list of design examples to demonstrate the results of the research.

Task 5. Prepare Interim Report No. 1, which documents Tasks 1 through 4 and provides an updated work plan for the remainder of the research. The updated plan must describe the process and rationale for the work proposed for Phases II and III.

PHASE II—Execution

Task 6. Execute the research plan according to the approved Interim Report No. 1.

Task 7. Prepare draft language for consideration by AASHTO to incorporate the research results in the next update of the AASHTO LRFD BDS (herein called the AASHTO Deliverable).

Task 8. Prepare design examples according to the approved Interim Report No. 1.

Task 9. Prepare Interim Report No. 2, which documents Tasks 6 through 8 and provides an updated work plan for the remainder of the research. The updated work plan must describe the process and rationale for the work proposed for Phase III.

PHASE III—Final Products

Task 10. Revise the AASHTO Deliverable and the design examples developed in Phase II after consideration of the panel’s review comments.

Task 11. Prepare a stand-alone technical memorandum titled “Implementation of Research Findings and Products”.

Task 12. Submit the final deliverables including (1) the AASHTO Deliverable, (2) a final report that documents the entire research effort, (3) the design examples, and (4) the stand-alone technical memorandum, Implementation of Research Findings and Products.

STATUS: Proposals have been received in response to the RFP. The project panel will meet to select a contractor to perform the work.