Comparison of Load Factor Rating (LFR) to Load and Resistance Factor Rating (LRFR) of Prestressed Concrete I-Beam Bridges

Abstract

Bridge load rating involves performing a series of calculations synonymous with bridge design calculations in order to determine if a bridge is safe for public traffic loads. The AASHTO Load Factor Rating (LFR) is the currently used method in bridge load rating. The Load and Resistance Factor Fating (LRFR) is a new guide manual adopted by the American Association of State Highway and Transportation Officials (AASHTO) in 2001 for condition evaluation of highway bridges. The guide manual reflects the most current technologies and builds on the structural reliability approach inherent in specifications for Load and Resistance Factor Design (LRFD). In this paper, seven prestressed concrete bridges of moderate span length (50 - 117 ft) are analyzed and rated using the AASHTO LFR and AASHTO LRFR methods. The selected bridges include one straight simple span Bulb Tee-girder, three skewed simple span I-girder, and three skewed continuous multi-span I-girder bridges. The comparison study for the prestressed concrete I-section bridges reveals some differences between the rating results using LFR and new LRFR methodologies. The majority of load ratings achieved using the LRFR approach for the prestressed concrete bridges are governed by shear rather than flexure, which is a substantial difference from the LFR method where flexural ratings typically govern for all bridge types.