Setting the standard: Load factors expected to extend some Illinois bridges' life cycles

We drive over them without a second thought. Yet, last January the American Road & Transportation Builders released its 2017 report labeling 2,303 Illinois bridges structurally deficient using figures from the Federal Highway Administration. 

Replacing and repairing bridges is extremely costly for the Illinois Department of Transportation. The agency continuously seeks ways to maintain its infrastructure to ensure motorist safety while minimizing cost.

A bridge’s ability to withstand wear and tear often depends on its load rating ― the safe capacity for live load (the weight of goods in transit) over a highway bridge without harming it.

Bridges are often equipped to handle everyday vehicle traffic, but when it comes to overweight vehicles, such as heavy commercial freight, it’s a different story. Yet, strengthening every bridge to an overly conservative high requirement translates to unnecessarily high cost to taxpayers.

That is something Illinois Center for Transportation and IDOT want to address. The two were up to the task of refining bridges’ load and resistance factor rating (LRFR) by teaming up in a project, R27-171 Refinement of Load Factors for Illinois-specific LRFR Bridge Load Rating Using WIM Data, to analyze commercial freights’ weigh-in-motion data ― the measurement of static vehicles’ tire loads ― from 20 Illinois WIM sites, which are located near weigh stations, and thereby develop corresponding live-load factors for LRFR in Illinois.

The team’s Principal Investigator Gongkang Fu, an Illinois Institute of Technology Department of Civil, Architectural, and Environmental Engineering professor, is no stranger to this research topic. Fu previously worked on state DOT-specific load factors for commercial vehicles in California, Michigan, and New York.

ICT and IDOT examined each WIM facility’s traffic volume in the driving and passing lanes, observing vehicles’ gross weights, configurations, and the probability for two lanes of vehicles loading the same bridge span having various lengths. These cases of trucks clustered on the same span constitute possibly the worst load cases to bridges.

ICT and IDOT researchers observe weigh-in-motion readings of truck axle weights.

The two agencies scrubbed the data and evaluated each of the 20 facilities’ WIM data quality for the purpose of calibrating Illinois LRFR live-load factors. This helped the agencies focus on maximum loading cases, such as truck clusters and extremely heavy single-unit vehicles.

The four cases of load rating addressed in this research effort include the design load rating, legal load rating, routine-permit load rating, and special-permit load rating.

“(The research) will also allow the bridge owners in the state to more precisely load-rate their bridges with respect to the truck traffic,” Fu said. “When implemented, the needs for bridges will be adequately addressed without over- or under-conservative decisions in terms of the truck loads they are required to carry. This will maximize the effect of available and constrained funding for bridge repair, rehabilitation, and replacement.”

In other words, ICT and IDOT’s work has paved the path to LRFR implementation in Illinois based on the state’s WIM data.

The two agencies not only streamlined the process, but their recalibration determining live-load factors will help IDOT paint a more precise picture of the load-carrying capacity for Illinois’ highway bridges. Their efforts will also help focus infrastructure improvements where they are most needed with the limited funding available.

“If the WIM data shows that IDOT can lower the load factors to reflect actual vehicular traffic, fewer structures would need to be posted and more overweight vehicles would be permitted to move across structures, allowing traffic to move more freely while continuing to ensure the safety of the traveling public,” said Tim Armbrecht, engineer of structural services for IDOT’s Bureau of Bridges and Structures and Technical Review Panel chair for the project.

Going forward, IDOT plans to implement the state-of-the-practice load factors into its policy of bridge evaluation, which will extend the life cycle of some Illinois highway bridges in the state’s extensive infrastructure.