A comparison of Load Factor Rating (LFR) and Load and Resistance Factor Rating (LRFR) in reinforced concrete box and slab culverts

Load rating of new and existing bridges have been used in state departments of transportation for decades as a method to determine if a bridge is adequate for the expected vehicle traffic on the bridge. Currently, there are several methods available for rating existing bridges; each method is similar to its corresponding design philosophy. The two primary methods used in state departments of transportation are Load Factor Rating (LFR) and Load and Resistance Factor Rating (LRFR). Previously, the reinforced concrete culverts in Tennessee, which is approximately 40% of the bridge inventory, were not load rated. This was because they were not considered critical structures. However, a Federal Highway Administration (FHWA) mandate requires that all existing culverts be load rated. Also, FHWA recommends that LRFR be used on existing culverts in lieu of LFR. The objective of this research was to provide an accurate comparison of LFR to LRFR for reinforced concrete box and slab culverts. It was also necessary to check if the FHWA recommendation is valid or not. To accomplish the comparison, a database of existing culverts from 1980 to 1990 was assembled. The rating factors for LFR and LRFR were compared via graphical analysis for each member of the culvert for positive flexure, negative flexure, and shear. The graphs plotted the rating factors from LFR on the vertical axis and the rating factors from LRFR on the horizontal axis. The graphs were also separated into box culverts and slab culverts for each of the different levels of load rating performed. From this, it was easy to obtain which method had lower, more severe rating factors. A comparison was also made using the average ratio of LFR to LRFR for all of the results for each load for each member. The controlling live load model for LRFR, live load surcharge, live load distribution factors, and horizontal soil pressure application all affect the rating factors for LFR and LRFR. The results of the study indicate that LRFR is not a viable option for existing reinforced concrete culverts. Furthermore, the recommendation to load rate existing bridges with LRFR is not valid