Comparative Study of LRFR and LFR Methods of Rating for Missouri Prestressed Bridges

Abstract

Bridge load rating is a crucial aspect of bridge evaluation and management, providing essential information regarding a bridge's capacity to safely accommodate traffic loads. For public safety, it is a crucial process for both new and existing bridges by establishing the weight restriction on heavy vehicles crossing the bridges to prevent structure damage and potential failure of the bridges. Rated bridge loads can be effectively communicated to road users through the installation of visible signage of truck weight limit (i.e., load posting). The Missouri Department of Transportation (MoDOT) currently utilizes the Load Factor Rating (LFR) methodology to determine load postings for bridges. The procedures and policies for this load rating process are specified in MoDOT’s Bridge Inspection Rating Manual (MoDOT, 2022). The primary goal of this research project is to help in establishing new LRFR posting policies and methodology which should have outcomes similarly to the current LFR methodology used in Missouri. Three studies are conducted in this work. The first is to compare rating load and rating factor between LFR and LRFR methodologies for five vehicles: H20 Legal Load, MO3S2, CZSU, CZRT, and SU5. The second study is to establish threshold and posting factor for MoDOT legal load vehicles to envelope SHVs and AASHTO vehicles. The last study is to answer the question to MoDOT whether serviceability limit state should be included in the legal load rating. Analyses of bridge rating was performed using the load rating software AASHTOWare BrR v7.1. With comparative rating results in LFR and LRFR methodology over hundred sample prestressed bridges across Missouri state, this study found that outcomes are similar in terms of rating load and number of bridges required posting. Using LFR method, there is one bridge that is required to post for vehicles CZSU and CZRT, while none of bridges require posting using the LRFR method. The ratio of rating load between LRFR and LFR have sample mean closed to 1.00 with Standard Deviation around 0.1. The second study of establishing threshold and posting factor for MoDOT vehicles to envelope Special Hauling Vehicles (SHVs) and AASHTO vehicles have suggested that; H20L has threshold of 31 tons and posting factor of 1; MO3S2 has a threshold of 40 tons and posting factor of 0.97; CZSU has a threshold equal to its own gross weight (40.8 tons) and posting factor of 1; CZRT has threshold equal to its gross weight (50 tons) and posting factor of 1. The serviceability limit state investigation has found that only six over one hundred sample bridges would have LRFR service III controls rating. The service III rating load of those bridges are about 64% to 67% compared to the strength – flexure limit state rating loads. While bridges are not showing distress over the course of the service, serviceability limit state might not need to consider for prestressed bridges in Missouri state.