Bridge Ratings for Permit and Superload Vehicles

1 Million lb Load in Louisiana

Testing for permit and so-called “Superloads” is conducted in the same way as is done for posted bridges, however, if possible, a load test is arranged prior to the passage of the superload vehicle. The superstructure is instrumented with many strain sensors, and a heavy vehicle of known load crosses the bridge at crawl speed.  Typically this first set of tests is completed with a dump truck or similar readily available vehicle. The strains are continuously monitored during several of these passages, along with the position of the vehicle.

Next, a relatively simple finite element model of the structure is developed by using specialized software, and the field testing procedures are reproduced in the analysis. These analytical responses are then compared to the measured responses, and the modeling parameters are systematically refined until the calculated strains compare well with the measured strains.

This “calibrated” model that was developed with the smaller dump truck is then used for predicting stresses due to the permit or superload. The only assumption being made in this process is that the structure is behaving linearly.

Usually, the rating limits will increase since the actual load distribution (rather than that developed from the distribution factor approach) is being accounted for. However, we have tested structures where the recommended rating values were lower or approximately the same as those originally calculated.

800,000 lb Load in Toledo

Data Correlations for Overload Tests

Data Correlations for Overload Tests