Missouri Bridge Rating by LRFR and LFR: MoDOT Comparative Study

During my graduate studies at the University of Missouri-Kansas City (UMKC), I conducted bridge load rating research under the guidance of Dr. Ganesh Thiagarajan alongside senior colleagues Bao Tran and Ronald Cheruiyot. This state research project evaluated highway bridges across Missouri, with each team member focusing on specific superstructure types: prestressed concrete bridges, steel girder bridges, and reinforced concrete bridges.

This article provides a summary of the methodology, bridge sample, vehicle live load models, and rating results comparing the Load Factor Rating (LFR) and Load and Resistance Factor Rating (LRFR) approaches for the Missouri Department of Transportation (MoDOT).

MoDOT Load Rating Research Background

Historically, the Missouri Department of Transportation (MoDOT) utilized the Load Factor Rating (LFR) methodology to determine live load capacities and establish posting limits for state bridges. With the nationwide adoption of the AASHTO Manual for Bridge Evaluation (MBE) and the Load and Resistance Factor Rating (LRFR) methodology, DOTs needed empirical comparisons to understand how LRFR rating factors compare to legacy LFR ratings across existing bridge inventories.

The primary objective of this research was to compare LFR and LRFR methodologies across representative Missouri bridges, quantify rating differences, and establish baseline correlation thresholds for MoDOT bridge management.

Load Rating Principles: Inventory vs. Operating Levels

Bridge load rating quantifies the safe live load carrying capacity of a structure. Expressed as a Rating Factor (RF), a value of 1.0 or greater indicates that the component can safely carry the evaluated vehicle configuration. Structural members are rated individually, with the lowest component rating controlling the overall bridge capacity.

Under both AASHTO LFR and LRFR, two core rating levels are evaluated:

• Inventory Level: Represents the routine, everyday live load capacity that the bridge can safely sustain for an indefinite design lifespan without damage.
• Operating Level: Evaluates the maximum permissible live load capacity for infrequent, controlled, or heavier commercial vehicle crossings.

Key Load Rating Terminology

• Rating Load: The maximum vehicle live load that a bridge can safely carry based on its structural condition, material properties, and traffic demands.
• Rating Factor (RF): The ratio of available structural capacity to the applied live load demand. An RF of 1.25 indicates that the bridge has 125% of required live load capacity for the rated truck.
• Posting Load: The maximum allowable gross vehicle weight posted on roadside signage when the rating factor falls below legal requirements.

Missouri State Legal Trucks & AASHTO Legal Vehicles

The comparative load rating analysis evaluated two major categories of vehicle configurations:

1. Missouri Department of Transportation (MoDOT) State Vehicles

• State Legal Vehicles: H20L (Single Unit Truck) and MO3S2 (Semi-Trailer Combination Truck).
• Commercial Zone Vehicles: CZSU (Commercial Zone Single Unit) and CZRT (Commercial Zone Combination).

These configurations represent standard hauling loads operating across the Missouri state highway network.

2. AASHTO Legal Loads & Specialized Hauling Vehicles (SHVs)

• AASHTO Legal Loads: Type 3 (3-axle single unit), Type 3-3 (6-axle combination), and Type 3S2 (5-axle semi-trailer).
• Specialized Hauling Vehicles (SHVs): SU4, SU5, SU6, and SU7 single-unit trucks with closely spaced multiple drop axles that induce high shear and negative moment demands on short-to-medium span bridges.

Analysis of 99 Missouri Concrete Bridges Using AASHTOWare BrR

The study evaluated a representative sample of 99 reinforced concrete bridges across Missouri. Each bridge model was built and analyzed using AASHTOWare Bridge Rating (BrR) software under both LFR and LRFR specification engines.

Key steps in the comparative analysis included:

1. Extracting moment and shear rating factors for inventory, operating, and legal load levels for all 99 structures.
2. Pairing vehicle analyses (Single Unit vs. Single Unit, Multi-Axle Combination vs. Combination) to observe trends across span ranges.
3. Plotting correlation scatter plots between LFR and LRFR rating loads to establish regression relationships and rating factor thresholds.
4. Determining the percentage of bridges requiring weight postings under LRFR compared to existing LFR postings.

Research Findings & Conclusions

The comparative study yielded key insights for Missouri bridge load rating:

• Enhanced Consistency: The LRFR methodology demonstrated more uniform reliability indices across varied span lengths and girder spacings due to its calibrated load and resistance factors.
• Rating Trends: For the majority of concrete bridges in sound structural condition, LRFR produced comparable or higher rating factors than LFR, particularly for standard legal vehicle types.
• SHV Sensitivity: Specialized Hauling Vehicles (SU4–SU7) controlled load ratings on short-span bridges under both methodologies, reinforcing the necessity of dedicated SHV checks in bridge management.

For more discussion on superstructure modeling and bridge layout standards, see the guide on Layout Length vs. Span Length in Bridge Design.