Partnership between NCDOT and NCSU on the Bonner Bridge research project
For decades, the Herbert C. Bonner Bridge served as a lifeline for people traveling to the Outer Banks until it was dismantled a few years ago to make way for a new bridge.
Now, the parts of the Bonner Bridge have another purpose: to help engineers build better bridges in the future.
The beams of the now demolished Bonner Bridge were sent to NC State University in Raleigh for stress testing at the college’s built facilities lab. The goal – to use lessons learned about how an aged bridge exposed to half a century of extreme weather conditions can inform better bridge designs in the future.
The North Carolina Department of Transportation is funding the project and facilitated work to salvage the beams and test them in Raleigh.
“This is a very unique opportunity,” said Neil Mastin, who heads the research and development unit at NCDOT. “It’s not often that you get a bridge in this extreme environment for almost 60 years that was intact enough that we could actually use it for testing.”
The test consisted of two parts. First, the beam has been subjected to a low level cyclic load in which the force is repeatedly applied, removed, and reapplied in a manner similar to the types of stresses a bridge experiences from passing vehicles. Then it was subjected to a monotonous load until it reached peak load levels – essentially pressing it continuously until it broke.
How much force did it take to break the beam of the bridge?
Over 200,000 pounds.
In other words, stack 2.5 semi-trailers on top of each other on a single wheel and then lower all that weight onto the beam. Sensors with cameras were used to observe how the beam reacts to loads, how it moves and where cracks form when pressure is added to the beam.
“What we’re really trying to study is the amount of prestressing in this structure,” said Giorgio T. Proestos, assistant professor at NC State University, who is leading the project at the university. “Is this enough? Should there be more? Should there be less? And how does this preload change in 60 years? Based on the results of the experiment, there could be changes in the way things are done in the future. ”
NCDOT will publish the results once the project is completed. Mastin said the results of this project will likely inform bridge design and construction decisions nationwide, not just in North Carolina.