Loading up: Evaluating bridge load capacity in the field

In 2021, the average age of existing U.S. bridges was 42 years, a number that’s rising according to the National Bridge Inventory.

Accurately determining the condition of aging bridges is key to effectively managing bridge inventories.

Illinois Center for Transportation and Illinois Department of Transportation sought to better evaluate bridges’ load capacity in a joint project, “R27-205: Load Rating of Reinforced Concrete Slab Bridges Using Field Testing.”

Bassem Andrawes, University of Illinois CEE Excellence Faculty Fellow, led the project with Ruben Boehler, IDOT’s Engineer of Structural Services.

All bridges must undergo regular inspection and load rating to assess the weight they can safely support. The load rating factor, calculated using analytical design specifications, indicates whether the bridge should be closed to traffic or restricted to lighter loads.

In many cases, however, this load rating factor is overly conservative, leading to unnecessary bridge postings and load restrictions.

“Design codes give us basic fundamental equations in a very perfect world that rarely exists,” Andrawes said. “Maybe for new bridges they exist. But after 20, 30, 50 years, that bridge will behave differently.”

“It’s like our bodies when they age,” he added. “Bridges age differently depending on the climate, environment and loads.”

Andrawes and Boehler aimed to develop a method for IDOT to assess the structural condition of bridges under real-world conditions.

“The field test in this project aimed to put the bridge to an actual test, like when we put our bodies in an MRI or an X-ray or a device that can actually get some vital, important information for us,” Andrawes said. “It will tell us how the bridge is actually feeling when we poke it, when we push it this way, when we put the loads right there.”

Andrawes and Boehler conducted two types of field tests — diagnostic and proof load — on six concrete slab bridges in Illinois with varying characteristics and ages.

“So as far as load testing the bridges, what we end up doing is taking a bridge that’s in the field and apply heavier and heavier loads to it while we’re measuring the response of the bridge,” Boehler said. “Then we can extrapolate what the actual, true load capacity of that bridge is outside of the design specifications.”

To conduct diagnostic testing, they put a precalculated load on a truck that is less than what the bridge typically services, measuring the response of the sensor-equipped bridge to identify any potential issues.

After diagnostic testing, they conducted proof load testing, where the researchers loaded the bridge to the designed amount and even beyond.

Researchers loaded the bridge incrementally until the bridge either hit the target load — the designed amount — or hit the maximum load, at which point the bridge would start to show signs of distress.

Andrawes’ team conducted field tests on the first four bridges while training Boehler and IDOT staff, allowing the agency to conduct the tests for the remaining two.

“It’s very, very helpful. Not only the theoretical side from the research, but the practical side of being able to apply it,” Boehler said. “Ensuring that we know how to do it ourselves for future use is very useful.”

Through the load tests, Andrawes and Boehler found that either the load is being distributed differently than predicted throughout the bridge or it has more capacity than predicted using analytical methods.

The developed method will allow IDOT to evaluate the capacity of Illinois bridges more accurately and reduce existing load postings that are proven to be overly conservative.

“In the future if we come across a bridge that is showing that it would need to be load posted, we have the option of going and doing a load test and verifying,” Boehler said.