Performance Evaluation of Reinforced Concrete Box Culverts

Project summary:

The Minnesota Department of Transportation (MnDOT) has approximately 950 reinforced concrete bridge-class box culverts. These structures have an average age of 77 years and receive routine inspection as well as a load rating. Currently, the culverts are rated using assigned load ratings as a means of complying with the requirements of the National Bridge Inspection Standards (NBIS). Within this methodology, a load rating is assigned (not calculated) based on the load at the time of design and the condition rating, which is irrespective of design methodology. Approximately 99 percent of the concrete box culverts have an assigned inventory rating factor less than unity, which means that the structures require posting to restrict overweight vehicular loading for safety. However, the culverts have been carrying unrestricted traffic for an appreciable period with no significant signs of structural distress; historical visual inspection of the structures often indicates satisfactory performance.

The Federal Highway Administration (FHWA) provided five additional requirements for assigning inventory or operating level ratings to structures based on the design loading in 2011. One of these requirements states that the bridge must have been designed using either the American Association of State Highway and Transportation Officials (AASHTO) load factor design (LFD) or load and resistance factor design (LRFD) methods to at least the HL-93 or HS-20 live load level. This requirement made all assigned load ratings for MnDOT's reinforced concrete box culverts invalid because the structures were designed using the service load design (ASD) method. For culverts, MnDOT switched from the ASD design method to the LFD design method in the late 1970s. Therefore, approximately 950 reinforced concrete box culverts need load ratings determined via calculations. The main objectives of this project are to:

1. Collect data from field testing of up to 10 reinforced concrete box culverts to evaluate their performance under a known truck load.

2. Evaluate live load performance, culvert capacity, and load rating using refined methods of numerical analysis validated with field test data.

3. Calculate a refined load rating for approximately 20-30 additional culverts based on the field data and numerical analysis results to assess the impact of different culvert attributes using statistical analysis. This will allow for extrapolation of the performance trends to the larger inventory of in-service culverts.

4. Develop a load rating decision matrix based on culvert size, soil fill depth, and other pertinent attributes to guide implementation of load rating calculations for MnDOT's reinforced concrete box culvert inventory.