BACKGROUND

The design and construction of steel girder bridges, most often, require the use of bolts to connect various components in the field. In some cases, bolt holes have to be located at critical locations such as the tension flange of a steel girder. The impact of these holes on the capacity and the performance of the steel girders are accounted for during design. The current American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications (BDS) Equation 6.10.1.8-1 provides a limit on the maximum major-axis bending stress permitted on the gross area of steel girder flange, neglecting the loss of area due to holes in the tension flange. This equation is used in lieu of the 15 percent rule that had existed in previous editions of AASHTO design specifications, which allowed holes with an area less than or equal to 15 percent of the gross area of the flange to be neglected. However, for modern steels with yield-to-ultimate tensile strength ratio (Y/T ratio) higher than Grade 36 steel, the 15 percent rule was revised.

The current provision is based on the yielding and fracture of axially loaded tension members as opposed to the yielding and fracture of tension flanges of flexural members. A refinement of the current provision may lead to improved design strength of steel I-girders, particularly for girders with longer spans where modern steels are employed. Research is needed to better understand the impact of tension flange holes on steel girder performance and possibly improve the current state-of-practice. 

OBJECTIVE

The objective of this research is to conduct an analytical study on the impact of tension flange holes on the strength and ductility of composite steel I-girders and recommend modifications to existing compact steel I-girder design requirements, if needed. At a minimum, the research shall focus on Grade 50 and hybrid Grade HPS70W steel girders.

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 impact of tension flange holes, including at bolted splices, on the strength and ductility of steel girders. The review shall include published and unpublished documentation and research conducted by the NCHRP; Federal Highway Administration; other national, state, and local agencies; and international organizations. 

Task 2. Synthesize the results of the literature review to identify knowledge gaps related to the research objective. These gaps should be addressed in the final product or the recommended future research, as budget permits.

Task 3. Propose a research plan to be executed in Phase II to achieve the research objective. At a minimum, the research plan shall include:

• An analytical investigative approach;
• The effect of various parameters such as the Y/T ratio and the dimensional characteristics of the cross-section;
• Composite girders in regions of positive flexure where the neutral axis is relatively high in the cross-section and the inelastic deformation demands on the tension flange are typically larger;
• Holes along the tension flange and at the net section of bolted splices;
• Evaluation of the applicability of AASHTO BDS Eq. 6.10.1.8-1;
• Recommendations for modifications of the existing steel girder design requirements, if needed, to ensure that adequate strength and ductility can be achieved when holes are present; and
• A list of design examples to demonstrate the results of the research.

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.

PHASE II—Execution

Task 5. Execute the research plan according to the approved Interim Report No. 1.

Task 6. Based on the findings of the research, develop draft language to modify existing steel I-girder design requirements if necessary. The draft language hereafter will be called the AASHTO Deliverable. 

Task 7. Prepare design examples according to the approved Interim Report No. 1.

Task 8. Prepare Interim Report No. 2 that documents Tasks 5 through 7 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 9. Revise the AASHTO Deliverable and the design examples developed in Phase II after consideration of the panel’s review comments.

Task 10. Submit the final deliverables including (1) the AASHTO Deliverable, (2) a final report that documents the entire research effort, (3) design examples, and (4) a stand-alone technical memorandum titled “Implementation of Research Findings and Products.”

STATUS: Research In Progress