According to the 2004 National Bridge Inventory, nearly 28% of Iowa's 25,000 bridges are rated as structurally deficient or functionally obsolete. Many of these structures are rated so by traditional visual bridge inspection and codified rating procedures. With many of these structures needing replaced or repaired, the problem is compounded by increasing bridge project costs and decreasing funding for such projects. A diagnostic tool is often necessary to accurately quantify the actual behavior and load carrying capacity of these deficient structures.;Iowa State University recently completed an evaluation of a commercially available diagnostic load testing system that combines the field testing equipment along with a software package for data interpretation, finite element modeling, and correlation of models to field data. The entire system produces a calibrated model of the load tested bridge for purposes of load rating. The system evaluated as part of this study was the Bridge Diagnostics, Inc. (BDI) Structural Testing System (STS). This paper documents a number of the efforts involved with the evaluation of the BDI system.