ABSTRACT
Bridges without design plans are an existing problem throughout the United States and other countries. Conventional load rating techniques cannot be utilized when evaluating planless prestressed concrete bridges, as no design documentation exists to indicate the amount and location of reinforcement. This paper presents an implementation plan for load rating prestressed concrete bridges without plans using advanced structural analyses in conjunction with nondestructive experimental methods.
A multi-step procedure was developed that includes the following. First, the material properties of the bridge, amount of prestressing steel, and strand eccentricity are estimated based on historical code requirements and Magnel diagrams. Second, the estimate of the number of prestressing strands and locations is validated, and the shear reinforcement layout in the bridge beams is determined using non-destructive testing equipment (e.g., rebar scanner). In addition, a Windsor Probe penetration test is conducted to verify the concrete strength assumption. Third, the available strain capacity in the existing bridge beams is calculated based on the structure dimensions, material properties, physical condition, and estimated cracking moment. Also computed is the minimum target proof load needed to reach the available strain capacity based on the axle weights and spacings of the test vehicles. Fourth, the bridge is instrumented to measure the strains induced by physical testing. Fifth, a diagnostic load test is conducted under a truck load of roughly 60% of the target proof load to determine the critical load paths and sixth, a proof test is performed until the available strain capacity (based on concrete cracking) or the target proof load is reached. Seventh, rating factors for AASHTO and New Mexico legal loads are computed for the serviceability limit state (i.e., concrete cracking) based on the proof test results in accordance with the AASHTO Manual for Bridge Evaluation (2011). Eighth, using a load rating software program, the rating factors for the strength limit state (i.e., shear or flexural capacity) are determined based on the measured bridge dimensions and estimated material properties. Finally, the serviceability ratings from proof testing and strength ratings from the load rating software program are compared to determine the final load ratings and need for posting the bridges. This procedure was organized into a flowchart, shown in Figure 1. The initial condition of the bridge and the results from non-destructive experimental testing are used in the flow chart to guide the evaluation process for each bridge by selecting the most efficient load rating procedure. Load rating flowchart. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig199_1.tif"/>
Finally, three case studies involving double T-beam, box beam, and I-girder bridges are presented to show the inspection and evaluation process. Overall, the implementation plan is shown to be effective in load rating prestressed concrete bridges, regardless of superstructure type.
