Bridge Ratings Based on Field Measurements

BDI uses the following general procedures to develop accurate load ratings for highway bridges:

1. Obtain strain responses due to a known load.
   Continuous response histories are obtained by recording strains at many locations (usually a minimum of 32) as a loaded truck is driven slowly across the bridge. A key aspect of this test is that the position of the vehicle is tracked and recorded as well.
2. Preliminary investigation of data.
   Information concerning the structural behavior can be determined directly from raw strain data such as the verification of linear behavior, the neutral axis locations for flexural members, and indications of possible moment resistance at beam supports.
3. Develop representative model.
   A realistic simulation of a bridge can be developed with simple finite element techniques. The actual geometry of the structure is represented, including span lengths, girder spacing, skew, transverse members, and deck characteristics.
4. Simulate load test on computer model.
   Use a two-dimensional model of the test vehicle and apply it to structure model at the same locations as those recorded during the field tests. Perform analysis and compute strains at each gage location for each truck position.
5. Compare measured and computed strain values.
   Systematically compare the results at all gage locations and truck positions. Various local and global error values are computed directly from the analysis program and visual comparisons are also made.
6. Improve the model based on data comparisons.
   Engineering judgment and experience is required to determine which variables are to be modified. General rules have been defined to simplify this operation. An automated process built into the analysis program is used to evaluate the adjustable parameters so as to obtain the best correlation.
7. Perform load rating on calibrated model.
   Use standard design, rating and permit loads. Rating factors are determined by an elastic structural analysis and the same rating equation specified by the AASHTO - Manual for the Condition Evaluation of Bridges is applied:
   
   
   Stress envelopes are generated for several truck paths, and envelopes for paths separated by normal lane widths are combined to determine multiple lane loading effects. The only difference between this rating technique and standard beam rating programs is that a more realistic model is used to determine the dead- and live-load effects. Two-dimensional loading is applied because wheel load distribution factors are not applicable to a planar model. The detection and evaluation of "unreliable stiffening effects" during the model identification process does not require that these factors be implemented in the rating process. This means that the final rating factors are still a function of the engineer’s judgment.