ABSTRACT
Load rating of concrete bridges with no design plans is currently an issue in the United States and other countries. Missing or incomplete design documentation creates uncertainties in establishing the safe load limits of bridges to carry legal vehicles. Guidance for evaluating planless concrete bridges, in particular prestressed structures, is limited in the AASHTO Manual for Bridge Evaluation and very few state departments of transportation have rating procedures for such bridges. In this chapter, a multi-step load rating procedure is described for planless prestressed bridges that includes (1) estimating the material properties from past specifications and amount of prestressing steel using Magnel diagrams; (2) verifying the steel estimate by rebar scanning; (3) field testing at diagnostic and/or proof load levels based on strain measurements; and (4) rating the bridges using the proof test results. Three prestressed concrete bridges (including a double T-beam, box beam, and I-girder bridge) are evaluated using nondestructive load testing and material evaluation techniques to illustrate the procedures. Rating factors are determined for AASHTO and New Mexico legal loads using the proof test results for the serviceability limit state (i.e. concrete cracking). Using a load rating software program, rating factors are also computed for the strength limit state (i.e. shear or flexural capacity) based on the measured bridge dimensions and estimated material properties. Finally, load ratings for serviceability and strength are compared to establish the bridge capacities.
