Development of Improved Analytical Load Rating Procedures for Flat-Slab Concrete Bridges - A Thesis and Guidelines

Current AASHTO provisions for the conventional load rating of flat slab bridges rely on the equivalent strip method of analysis for determining live load effects. This is generally regarded as overly conservative by many professional engineers. As a result there are a significant number of slab bridges in Maine that are (or will be) posted for reduced truck weights, when in reality such postings may not be necessary. The objective of this study is to verify a program called SlabRate which was created using MATLAB, a numerical computing tool, and to explore the potential benefits of using it over the conventional strip width method. SlabRate computes the rating factors for simply-supported, continuous flat slab bridges using finite element analysis (FEA). The verification includes creating parallel models of identical bridges in both SlabRate and the commercial software, and comparing the maximum moments and locations of those moments due to a variety of live and dead loads. These models were used to assess SlabRate’s finite-element implementation and verify the assumptions that are used in SlabRate. In addition to using commercial software to verify SlabRate, live load testing of a reinforced flat-slab concrete bridge was done. These results were then compared with the predictions of SlabRate. To explore the potential benefits of using SlabRate over the conventional strip width, twenty existing bridges were load rated using both. Twenty-one different truck configurations were analyzed, these include AASHTO’s design and legal trucks, along with AASHTO’s specialized hauling vehicles and the Maine Department of Transportation’s rating trucks. The results from comparing SlabRate to commercial models and the live load test showed that its finite element implementation is correct for evaluating simply-supported and continuous flat slab bridges. It was also concluded that SlabRate can be reliably used to load rate flat slab bridges having skew angles of 20° or less. Only fourteen bridges of the original twenty bridges met this criteria, of which seven would have rating factors above one using SlabRate while below one using the conventional strip width method.