Analytical and Testing Methods for Rating Longitudinal Laminated Timber Slab Bridges

Over the past 10 years, WisDOT, Bureau of Structures (BOS) bridge rating engineers have been working on a program to update the load ratings of all bridges in Wisconsin. This effort was also driven by the mandate from the Federal Highway Administration (FHWA) to evaluate the State and Local inventory for Specialized Hauling Vehicles (SHVs). BOS developed and executed a data-driven analytical process that concluded with updated load ratings and load postings added, as required. BOS used existing plan information, field measurements, and standard structural analysis computer files to accomplish this task efficiently and effectively. As part of this process, BOS examined the 500 locally owned timber bridges. The analysis determined that these timber bridges are unable to carry contemporary AASHTO rating loads, based on standard structural analysis practices. This has resulted in the posting of these bridges, restricting traffic and impacting commerce. Current AASHTO guidance on wheel load distribution widths for timber slab bridges load rated using Allowable Stress may be conservative and results in unnecessary reduction in capacity and resulting load postings. While significantly less conservative, wheel load distribution widths for timber slab bridges (load rated using Load Resistance Factor Design) are not prescribed for older timber slab bridges that do not meet modern detailing requirements required by LRFD. More accurate/reflective wheel load distribution widths for laminated slab timber bridges of the type used in Wisconsin would help reduce the number of unnecessary load restrictions and impacts to commerce. Efforts to increase load capacity of timber slab bridges have typically been based on increasing the effective wheel load distribution. Methods include: tightening existing spreader beams, adding spreader beams, using transverse post-tensioning and using 4-inch thick laminated spreader deck panels placed transversely on top of the existing slab. These methods are meant to increase the effective wheel width; however, further research is required to more reliably quantify the effective wheel distribution width of these retrofit conditions. WisDOT is interested in the development of refined wheel load distribution widths for longitudinal laminated deck slab timber bridges and the development of methods for standardized proof load testing for this type of superstructure. We are also interested in the effectiveness of retrofit techniques to increase live load capacity. The focus of this study is related to evaluation of super-structures only. The objective of this research is to improve tools used to assess the condition, load distribution factor and load rating of Wisconsin timber slab structures by: A. Using a combination of an analytical program and a field testing program to develop refined wheel load distribution widths for laminated timber deck slab bridges, based on the actual measured load responses of Wisconsin timber bridges. B. Developing a process to easily load test and proof rate legal load capacity of timber slab bridges, with common loads (e.g., County dump trucks). To achieve these objectives, it is expected that the research team would establish load response relationships for the longitudinal laminated timber structures. C. Evaluating the effectiveness of retrofit techniques to increase live load capacity: tightening existing spreader beams, adding spreader beams, using transverse post- tensioning and using 4-inch thick laminated spreader deck panels placed transversely on top of the existing slab. D. Making in-situ moisture content and temperature (above and below freezing) measurements of timber bridges used in this study and comparing these measurements with current WisDOT assumptions and practices. E. Completing the study with the following expected implementable results: removal of load postings of good-condition timber slab bridges and allowance for more capacities than current standard analytic methods quantify.

Language

  • English

Project

  • Status: Active
  • Funding: $220000
  • Contract Numbers:

    0092-20-01

  • Sponsor Organizations:

    Wisconsin Department of Transportation

    4802 Sheboygan Avenue
    Madison, WI  United States  53707
  • Project Managers:

    Pence, Alex

  • Performing Organizations:

    Iowa State University, Ames

    Center for Transportation Research and Education
    2711 South Loop Drive, Suite 4700
    Ames, IA  United States  50010-8664
  • Principal Investigators:

    Phares, Brent

  • Start Date: 20191001
  • Expected Completion Date: 20211001
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01722026
  • Record Type: Research project
  • Source Agency: Wisconsin Department of Transportation
  • Contract Numbers: 0092-20-01
  • Files: RiP, STATEDOT
  • Created Date: Nov 11 2019 4:42PM