Traditionally, a bridge’s load-carrying capacity has been calculated based on conservative, non-site specific information. In some cases this results in overly conservative load ratings and load postings, which can restrict the transport of commercial goods through the State. One technique for more accurately evaluating a specific bridge’s safe load-carrying capacity involves field testing. Load tests often reveal that the load carrying capacity of the bridge is higher than estimated. Furthermore, due to recent technological advances in sensors and data acquisition, field testing is now economical and cost effective. Field testing, used in conjunction with traditional inspection and load rating procedures, can result in a more accurate load rating, can relax or eliminate load postings, can provide assurance of adequate capacity for superload crossings, and in some cases can eliminate the need for costly rehabilitation or replacement. While it may not be required of all bridges, field testing is now a cost effective tool for evaluating the condition and load carrying capacity of bridges.

     Researchers from the University of Delaware, Department of Civil and Environmental Engineering, for the past several years, have conducted 

research on the application of field testing for load rating and condition assessment of bridges. Through a grant from the Delaware Department of Transportation, a state-of-the-art bridge structural testing system has been purchased for this purpose. The system consists of quick mounting strain transducers, distributed data acquisition units and a main control unit. Using this equipment, investigators can deploy strain transducers over a wide area of the bridge deck and girders. In a controlled load test, loaded trucks of known weight are used as a load source. Normal traffic on the bridge is stopped and the loaded trucks traverse the bridge along a specified line. In some cases two trucks are run in tandem across the bridge. Strain data is continuously recorded while the truck crosses the bridge. The trucks are allowed to cross at a crawl speed, and also at the posted speed limit, to assess the effect of impact and dynamic response on the bridge behavior. A test is usually completed in a single day (this includes setup and take down of the instrumentation). The recorded strain data is then later analyzed to assess such things as load distribution, support fixity, composite action and impact.

     A number of bridges in Delaware have been field tested over the past four years. This includes two steel girder bridges and one reinforced concrete box culvert on Interstate 95, six reinforced concrete slab bridges on Route 13, a highly skewed concrete slab on steel girder bridge on SR1, and an advanced polymer composite bridge. Each of these tests has yielded valuable information about the current condition and behavior of the bridges. In some cases, the results of the field test have been used to determine a more realistic, yet still conservative load rating for the bridges. As a result, load postings have been relaxed or eliminated on some of these structures.
 

For more information on this topic contact Processor Michael J. Chajes (chajes@ce.udel.edu) or Professor Harry W. Shenton (shenton@ce.udel.edu) from the Department of Civil and Environmental Engineering at the University of Delaware.