Comparative study of LRFR and LFR methods of rating for Missouri concrete bridges

Abstract

Bridge load rating is a critical component of evaluating and managing bridge infrastructure, offering crucial insights into a bridge's capacity to safely handle traffic loads. The process is fundamental for both new and existing bridges, establishing weight restrictions on vehicles to prevent structural damage and potential failure of the bridge. Communicating these rated bridge loads to road users is achieved through visible signage indicating truck weight limits, commonly known as load posting. Currently, the Missouri Department of Transportation (MoDOT) employs the Load Factor Rating (LFR) methodology to determine load postings for bridges, with detailed procedures outlined in MoDOT's Bridge Inspection Rating Manual (MoDOT, 2022). This research aims to contribute to the establishment of new Load and Resistance Factor Rating (LRFR) posting policies and methodologies, aligning with the existing LFR methodology in Missouri. Three key studies form the foundation of this work. The first involves a comparative analysis of rating load and rating factors between LFR and LRFR methodologies for five vehicles: H20 Legal Load, MO3S2, CZSU, CZRT, and SU5. The second study focuses on establishing thresholds and posting factors for MoDOT legal load vehicles, encompassing Special Hauling Vehicles (SHVs) and AASHTO vehicles. Lastly, the third establishes the relationship between the LRFR rating factor and the reliability index, obtained from the reliability analysis of the bridge. Utilizing AASHTOWare BrR v7.1 for bridge rating analyses across a sample of concrete bridges in Missouri, this research determined that the rating load and the number of bridges requiring posting are similar between LFR and LRFR methodologies. The second study suggests specific thresholds and posting factors for MoDOT vehicles, aiding in the envelope of SHVs and AASHTO vehicles. The third study established a directly proportional relation between the rating factor and the reliability index. The study of determining threshold values and posting factors for Missouri Department of Transportation (MoDOT) vehicles in relation to Special Hauling Vehicles (SHVs) and AASHTO vehicles indicates the following findings: H20L has a threshold set at 31 tons with a posting factor of 0.97; MO3S2 has a threshold established at 40 tons and a posting factor of 0.93; CZSU's threshold is equivalent to its own gross weight (40.8 tons) with a posting factor of 0.92; CZRT has a threshold matching its gross weight (51 tons) and a posting factor of 1. We conducted a reliability analysis to establish the relationship between the reliability index and the rating factor obtained from the LRFR methodology. As a result, we could see the rating factor and reliability index were directly related. Bridges with high rating factors to certain vehicles produced a high value of reliability index. The reliability index and rating factor also produced a similar curve. However, for a small change in the rating factor, the reliability index had a severe change in the slope of the curve. Apart from this, the rating factor of 1 was nearly equal to the -1.6 of reliability index. So, any reliability index values that are greater than -1.6 is said to be a safe structure with a very low probability of failure and is equal to rating factor 1 or greater than 1.