For decades, the Herbert C. Bonner Bridge served as a transportation lifeline for crossing Oregon Inlet until it was dismantled over the last several years to make way for the Marc Basnight Bridge.
Now, pieces from the six-decades-old span are serving another purpose – to help engineers build better bridges in the future.
Girders from the now-demolished Bonner Bridge have been sent to N.C. State University in Raleigh to undergo stress testing at the college’s Constructed Facilities Lab.
The goal – to use lessons learned about how an aged bridge exposed to a half-century of extreme weather conditions can inform better bridge designs in the future.
The N.C. Department of Transportation is funding the project and facilitated the work to salvage the girders and test them in Raleigh.
“This is a very unique opportunity,” said Neil Mastin, who manages NCDOT’s Research and Development unit. “It’s not often you get a bridge in that extreme environment for nearly 60 years that was intact enough that we can actually use it for testing.”
The test consisted of two parts. First, the girder was subjected to low-level cyclic loading in which force is applied, removed and re-applied repeatedly in a manner similar to the kinds of stresses a bridge takes on from passing vehicles.
Then, it was subjected to monotonic loading until reaching peak load levels – essentially pressing on it continuously until it breaks.
How much force did it take to break the bridge girder?
More than 200,000 pounds.
Put another way, stack 2.5 tractor trailers on top of each other on just one wheel and then lower all that weight onto the girder.
Sensors with cameras were used to observe how the beam responds to loads, how it moves and where cracks form when pressure is added to the girder.
“What we’re trying to really investigate is the amount of pre-stressing in this structure,” said N.C. State University Assistant Professor Giorgio T. Proestos, who is leading the project at the university. “Is it enough? Should there be more? Should there be less? And how does that pre-stressing change in 60 years? Based on the results of the experiment, there might be changes in the way things are done moving forward.”
NCDOT will publish the results after the project concludes. Mastin said the results of this project will likely inform bridge design and building decisions nationally, not just in North Carolina.