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Field Studies of Steel Girder Fit-up


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. With respect to vertical deflection, girders experience three conditions from inception to fabrication to completion of the bridge in place in the field: No-Load (NL), Steel Dead Load (SD), and Total Dead Load (TDL). As such, 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 AASHTO LRFD Bridge Design Specifications (Article 6.7.2) 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.

Past studies of steel girder fit-up have been only analytical. The project will include field measurements of existing steel girder systems to determine the condition of the girders relative to the design. Researchers will compare the field results to the fit condition that the girders were fabricated to. If necessary, current guidelines (Ref. 1) will be revised based on these field studies, and/or new guidelines on the best detailing practices impacting girder fit-up can also be established.


The objective of this research project is to investigate the condition of steel girders in the field that were originally detailed for different fit conditions and compare analytically.

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

  1. Extend Bridge Service Life: N/A2. Assess Bridge Condition: N/A3. Maintain and Enhance a Knowledgeable Workforce: The research findings would increase the overall knowledge of steel I-girder fit condition. It would also enhance the already analytical studies by conducting field measurements of existing bridges.4. Maintain and Enhance AASHTO Specifications: The research findings could be implemented into the AASHTO LRFD Bridge Design Specifications.5. Accelerate Bridge Delivery and Construction: With a streamlined fit condition detailing guide, designers, fabricators, and contractors will work more efficiently and consistently.6. Optimize Structural Systems: The research could potentially benefit steel I-girder bridges by enhancing constructability and long term performance.7. Model and Manage Information Intelligently: N/A8. Contribute to National Policy: N/A


This project will provide a foundation to base an engineer’s decision on girder-fit up and potentially have a positive impact on steel bridge constructability. It will also provide an assessment of long-term behavior of the girders as a function of the as-built condition.

Related Research:

The authors of this RNS document are not aware of any other similar research underway.

  • Task 1 – Literature Review
  • Task 2 – Finite Element Analysis* Task 3 –Existing Bridge Selection – The research team would prepare a list of potential bridge candidates to perform the field testing on. The research team would obtain fabrication shop drawings for each bridge selected.
  • Task 4 – Field Measurements of selected bridges
  • Task 5 – FE Model Calibration and Parametric Studies – The research team would calibrate the FE Model based on the field results from Task 4. The research team would perform parametric studies, using finite element analysis, to investigate the different results of girder fit condition. The team will include different girder sizes and spans, horizontal curvature, skew angle, and other design variables in their study.
  • Task 6 – Review Existing Guidelines and Develop New Guidelines – The research team will review existing guidelines (Ref 1) regarding steel bridge fit and offer revisions if necessary, or if required, develop new guidelines for choosing the best detailing method for steel girders.
  • Task 7 – Develop Specification Language – The research team would review all results and propose revisions/additions to the AASHTO LRFD Design Specification if their recommendations differed from current practice.
  • Task 8 – Final Report – The research team would submit all research findings, guidelines, and specification language for final review.

This research project would not need an implementation project. As soon as the research is completed, the detailing guidelines could be used by bridge engineers, fabricators, and contractors.


The potential users of this research would be the bridge engineering and construction community.

Sponsoring Committee:AKB20, Steel Bridges
Research Period:24 - 36 months
Research Priority:Medium
RNS Developer:Jamie Farris, PE
Source Info:Chavel, B., et al (2016). “Skewed and Curved Steel I-Girder Bridge Fit.” National Steel Bridge Alliance, August 2016.
Date Posted:04/09/2020
Date Modified:05/06/2020
Index Terms:Field studies, Geometry, Girder bridges, Steel bridges, Girders, Bridge design,
Cosponsoring Committees: 
Bridges and other structures

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