Comparative Load Rating Study Under LRFR and LFR Methodologies for Alabama Highway Bridges

Abstract

Currently, the Alabama Department of Transportation (ALDOT) uses the load factor rating (LFR) methodology of the American Association of State Highway and Transportation Officials (AASHTO) Manual for Condition Evaluation (MCE) of Bridges (1994) in load rating of highway bridges across the state. With the introduction of the new AASHTO MCE and Load and Resistance Factor Rating (LRFR) of Highway Bridges (2003), the need arose to assess the impact of implementing the new manual on ALDOT’s current bridge rating practices. To this end, a comparative study was performed between ALDOT’s current rating practices utilizing the older LFR methodology, according to the AASHTO MCE (1994), and the new LRFR methodology. This comparative study was performed on a representative sample of 95 bridges from Alabama’s state and county owned bridge inventory at all three primary levels of vi LRFR rating: Design, Legal and Permit rating levels. The load models that were utilized in the rating analysis were the AASHTO design load models, AASHTO standard legal loads, ALDOT state legal loads, and a sample of ALDOT overweight loads. The bridges were modeled in AASHTO BridgeWare’s Virtis version 5.6 (2007) and analyzed in BRASS-GIRDER LRFR and LFR analysis engines (2007). Rating results were generated for interior and exterior girders of each bridge analyzed as well as for moment and shear load effects. The rating data at all three primary levels of rating indicated that the LRFR methodology produces lower rating factors than the LFR. It was therefore concluded that adopting the AASHTO MCE LRFR (2003) can have a significant impact on the rating practices of ALDOT. Comparisons were additionally made between the LRFR and LFR rating data, at the Design rating level, in the context of estimated probability of failure for a bridge based on the Monte Carlo simulation technique. This comparison showed that rating factors produced under the LRFR methodology have strong correlation to a bridge’s estimated probability of failure, whereas rating factors under the LFR methodology showed only sporadic correlation to a bridge’s estimated probabilities of failure.