BACKGROUND

According to the National Bridge Inventory, approximately 6200 bridges in the United States have span lengths greater than 200 feet. Although the current American Association of State Highway and Transportation Officials (AASHTO) specifications for load and resistance factor rating (LRFR) and load factor rating (LFR) provide sufficient direction for evaluating bridges with span lengths ≤ 200 feet, they are not fully developed for evaluating structures with span lengths > 200 feet. Long-span bridges get special engineering analysis in the design phase, but methods for evaluating them in service are limited. To ensure public safety while sustaining commerce and preserving vital infrastructure, methods for rational load rating, permitting, and posting are essential. Research is needed to help state departments of transportation evaluate their long-span bridges.

OBJECTIVE

The objective of this research is to develop procedures for load rating, posting, and permitting of highway bridges with spans greater than 200 feet in length. It shall consider all applicable load effects.

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 the state of the practice on the design, analysis, and load rating of 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 The review shall include published and unpublished documentation, past surveys, and research conducted through the NCHRP; the Federal Highway Administration; other national, state, and local agencies; and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 the research plan to be executed in Phase II to achieve the research objective. At a minimum, the research plan shall include:

• A plan to identify and engage key subject-matter experts (e.g., long-span bridge owners, consulting engineers, etc.) to seek input on the state of the practice.
• Identification and collection of existing traffic data.
• The basis for determining which bridge span configurations (e.g., continuous and cantilever) and which bridge elements are covered by this research.
• Development of a calibrated procedure based on available traffic data for load rating, posting, and permitting. The procedure shall include determination of live load factors, multiple presence factors, and reliability index.
• Evaluation and comparison of force effects for various live load types and placements, such as 
  • Vehicle type (e.g., design, legal, and/or permit vehicles).
  • Transverse placement (e.g., same vehicle in all lanes of traffic and/or mixed vehicles).
  • Longitudinal placement (e.g., truck train or uniform with concentrated load model).
  • Combinations of transverse and longitudinal placement (e.g., truck train in study lane with specified vehicle in adjacent lanes or uniform load in all lanes).
  • Any other configurations deemed appropriate by the research team.
• Development of definitions for (1) span length and/or influence length, (2) truck train, (3) study lane, (4) adjacent lanes, and (5) other terms relevant to the research.
• Development of procedures to account for site-specific effects such as thermal, wind, dynamic, and pedestrian loads. Include consideration for use of design lanes vs. striped lanes.
• Development of procedures to determine posting restrictions. Include the case when lane loading governs.
• Development of procedures for incorporating state-specific legal vehicles and routine permit vehicles.
• Recommendations for modifications to the existing requirements for load rating using LRFR and LFR. Identify any potential changes to existing bridge design requirements. 
• A list of rating examples that covers a range of geometry, materials, and types to demonstrate the use of the proposed procedures. The use of existing bridges is preferred.

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 through IV.

PHASE II—Execution

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

Task 6. Develop the procedures for load rating highway bridges as described in Task 3.

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 Phases III and IV.

PHASE III—Draft AASHTO Deliverable

Task 8. Revise the proposed procedures after consideration of the panel’s review comments.

Task 9. Develop draft language that may be considered by the AASHTO Committee on Bridges and Structures for inclusion in its next update of the AASHTO Manual for Bridge Evaluation (MBE) for both LRFR and LFR. The draft language hereafter will be called the AASHTO Deliverable.

Task 10. Prepare load rating examples according to the approved Interim Report No. 2. The examples shall follow the current format of the AASHTO MBE.

Task 11. Prepare Interim Report No. 3 that documents Tasks 8 through 10 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 IV.

PHASE IV—Final Products

Task 12. Revise the AASHTO Deliverable and load rating examples after consideration of the panel’s review comments.

Task 13. Submit the draft final deliverables, including (1) the AASHTO Deliverable, (2) a final report that documents the entire research effort, (3) load rating examples, and (4) 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.