The fit condition of a steel I-girder bridge refers to the deflected geometry associated with a specific load condition in which the cross-frames or diaphragms are detailed to connect to the girders. There are three fit conditions for steel girder bridges: no-load fit (NLF), steel dead load fit (SDLF), and total dead load fit (TDLF). Consideration of the fit condition is important because the appropriate fit decision can provide significant benefit to the constructability and the overall performance of the bridge system. The American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications specify that the fit condition of the cross-frames or diaphragms should be clearly stated in the contract documents for straight I-girder bridges with skewed supports, and horizontally curved I-girder bridges with or without skewed supports. If the appropriate fit condition is not selected, the bridge could be subjected to problems during erection and construction. It could also result in systems that have errors in final steel vertical profile and cross-slope or other long-term problems.
In 2016, the National Steel Bridge Alliance (NSBA) published a document titled “Skewed and Curved Steel I-Girder Bridge Fit.” The document provided guidelines for selecting the appropriate fit condition during design, however, it was based on analytical work and did not include field verifications. These guidelines have been accepted and are being used by bridge design engineers. Research is needed to validate these guidelines by conducting field measurements of existing steel girder systems to determine the condition of the girders relative to the design.
The objective of this research is to investigate the effects of the as-detailed cross-frame fit condition (SDLF and TDLF) on the as-built girder geometry in the field. It shall be limited to I-shape plate girders.
Accomplishment of the project objective will require at least the following tasks.
Task 1. Conduct a literature review of research on the effects of the fit condition on the final geometry of steel girder systems. The review shall include published and unpublished documentation and research conducted through the NCHRP; Federal Highway Administration; other national, state, pooled-fund sponsored research; and international research.
Task 2. Synthesize the results of the literature review.
Task 3. Propose the research plan to be executed in Phase II to achieve the research objective. At a minimum, the research plan shall:
- Identify existing I-girder bridges suitable for evaluation, including bridges that have the following parameters:
- Significant curvature, detailed for SDLF;
- Skew greater than 30 degrees, detailed for SDLF;
- Skew greater than 30 degrees, detailed for TDLF; and
- Skew index greater than 0.3 with skew greater than 30 degrees.
- From the identified bridges, propose bridges for field evaluation based on availability, accessibility, coordination with bridge owners, and budget. The NCHRP will review and determine acceptability.
- Include the collection of as-built and fabrication drawings, deck pouring sequence, and the fit condition detailed for each bridge selected.
- Include finite element analysis on the selected bridges.
- Include field measurements of out-of-plane girder rotation of the fascia girders on the selected bridges.
- Include a comparison of field measurements with analytical results.
- Include an assessment of the fit limits presented in the NSBA document titled “Skewed and Curved Steel I-Girder Bridge Fit.”
- Provide recommendations for detailing steel I-girder bridges in terms of fit condition.
Task 4. Prepare Interim Report No. 1 that documents Tasks 1 through 3 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.
Task 5. Execute the research plan according to the approved Interim Report No. 1.
Task 6. Based on the findings of the research, develop revisions of existing guidelines presented in the NSBA document titled “Skewed and Curved Steel I-Girder Bridge Fit” if necessary or, if required, develop new language for choosing the best detailing method for steel girders. The draft language hereafter will be called the AASHTO Deliverables.
Task 7. Prepare Interim Report No. 2 that documents Tasks 5 and 6 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 8. Update the AASHTO Deliverables developed in Task 6 after consideration of the panel’s review comments.
Task 9. Submit the final deliverables including (1) the AASHTO Deliverables, (2) a final report that documents the entire research effort, and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products.”
STATUS: A research agency has been selected for the project. The contracting process is underway.