A Comparison of AASHTO Bridge Load Rating Methods

Abstract

Visual bridge inspections provide engineers with information to quantify the degradation in structural integrity of a bridge (i.e., the reduction in C). The trends in RF over time can be employed by bridge owners to make decisions regarding bridge maintenance and replacement. For example, when a bridge is first constructed, RF=1.3 means that a bridge can safely carry 1.3 times the weight of its design live load (i.e., that C-D, the existing capacity after accounting for dead load, is 1.3 times the design live load L). If the RF decreases to 0.8 after 20 years of service, deterioration of the primary structural components has most likely occurred and rehabilitation or replacement should be considered. Equation (1) is a simple idea, but C, D, and L can be highly variable and difficult to characterize depending upon the bridge location, bridge type, daily traffic flow, structural system (e.g., simple or continuous span) and choice of construction materials (e.g. steel, reinforced or prestressed concrete, composite construction). The American Association of State Highway and Transportation Officials (AASHTO) Manual for Condition Evaluation of Bridges (MCEB) provides a formal load rating procedure to assist engineers in the evaluation of existing bridges [AASHTO 1994 with interims through 2003]. The MCEB provides two load rating methods, one based on an allowable stress approach (ASR) and another based on a load factor approach (LFR). Both the ASR and LFR methods are consistent with the design loading and capacity calculations outlined in the AASHTO Standard Specification for the Design of Highway Bridges [AASHTO 2002]. Recently momentum has shifted towards a probabilistic-based bridge design approach with the publication of the AASHTO LRFD Bridge Design Specifications [AASHTO 2007]. Bridges designed with this code have a uniform probability of failure (i.e., a uniform reliability). The AASHTO Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges [AASHTO 2003] extends this idea of uniform reliability from LRFD to the load rating of existing bridges and is currently the recommended load rating method (over the ASR and LFR methods) by the Federal Highway Administration (FHWA). The transition from ASR and LFR to LRFR bridge load rating methodology represents a positive shift towards a more accurate and rational bridge evaluation strategy. Bridge owners are optimistic that the LRFR load rating methodology will improve bridge safety and economy, but they are also currently dealing with the tough questions related to its implementation. Why do