In the aftermath of Hurricane Florence, transportation throughout North Carolina has become an issue. As the storm made landfall last weekend and slowly moved west, it dumped trillions of gallons of water onto North Carolina and South Carolina, creating a 1,000-year flood event. Highways throughout the state have become rivers. Wilmington and some other areas of the state are simply inaccessible. State officials and highway engineers are closely monitoring the safety of our state’s transportation infrastructure, including bridges.
Last month, as the death toll from last month’s Morandi Bridge collapse in Genoa, Italy, rose to 43, we also wondered about the safety of the structures that we cross every day on our commutes to work or school. “Can it happen here?” we asked.
Earlier this year, a pedestrian crossing at Florida International University in Miami fell onto a busy road, killing six. Even closer to home, a pedestrian bridge collapse on Wake Tech Community College’s northern campus killed one construction worker and injured four other people in 2014.
A bridge failure can have one of several causes, including natural disasters like an earthquake or flood, excessive loads, and wear and tear. Design flaws, construction flaws, or impacts from a heavy object such as a barge or truck may also be to blame. Alpha & Omega Group, as a bridge design and inspection firm, is dedicated to identifying potential issues and developing solutions that keep our bridges safe for the traveling public.
“There is inherent impact design built into a bridge, depending on the natural forces of a given region,” said A&O Vice President Glenn Zeblo, PE. For example, bridges at the coast are engineered with extra loading for heavy winds, to withstand hurricanes, he explained. Structures in the mountains of western North Carolina are designed to withstand loads of snow and ice. Similarly, bridge engineers in the western United States design for seismic activity to make bridges more resilient to an earthquake.
Another natural force that can destabilize a bridge is scour, or water erosion, of the supports. One high-profile example is the 1963 Herbert C. Bonner Bridge, connecting Bodie and Pea Islands in North Carolina’s Outer Banks. A&O performed a National Bridge Inspection Standards (NBIS) inspection on the Bonner Bridge in 2006. “The piles weren’t deep enough,” said Glenn. “The tide is strong there, and the current shifts the sand. As we were going out in the boat to inspect, the boat captain observed that in just one day, the navigational channel in the water shifted from one bridge span to the next. That’s 20 feet of sand moving overnight,” he added. To stabilize the bridge, A&O designed repairs to the structure, while NCDOT installed concrete “jacks” – giant star-shaped objects resembling the children’s toy – around affected bents and piles to hold sand in place. A replacement is currently under construction, expected to be complete by September 2019.
A&O has worked with the North Carolina Department of Transportation since 2000 to conduct NBIS element inspections on the state’s 18,000 bridges and culverts throughout North Carolina. Each bridge is inspected every two years by a certified company. To ensure the safety of all bridges for public use, we perform non-destructive “hands on,” visual inspections of each structure’s railings, decks, expansion joints, superstructure, and substructure. We cover the entire bridge from top to bottom, documenting and photographing cracks, impacts, and deteriorated concrete. We check for “spalls” and “delaminations,” or areas of concrete erosion caused by sodium chloride or pollutants in the air, by tapping with a small mallet or using an electro-sonic device, to detect hollow spots in the concrete where moisture has seeped in and corroded the steel reinforcement. The rust expands 12 times the original size to create the spalls and delaminations.
Other methods of inspecting may include destructive testing, taking core samples of concrete, to analyze the strength and quality of the concrete. Inspectors may also use ground-penetrating radar or LiDar (light detection and ranging), a method that uses light pulses to measure variable distances and generate three-dimensional images of the structure.
Each bridge inspector documents findings in NCDOT’s Wigins reporting software, where they are reviewed by inspection team leaders and project managers before sending to NCDOT. Once NCDOT engineers get the report, several sets of eyes will also review it and decide on what actions to take for anything needing attention, explained Glenn.
What if bridge inspectors see a problem on a bridge that needs immediate attention? According to Glenn, there are three levels of alerting NCDOT to urgent bridge maintenance issues: priority maintenance, in which NCDOT has a 30-day window to resolve an issue; critical find, which is an issue requiring remediation in just a few days; and imminent failure, reserved for issues that are so severe that the bridge should be closed immediately. Because of the two-year inspection cycle, said Glenn, bridges are rarely closed because of imminent failure – however, it occurs occasionally when a bridge is deemed unsafe.
A bridge in Iredell County, along State Road 1863 over Rocky Creek, was closed earlier this year, upon a recommendation by A&O inspector Michael Meyer, PE. Used by about 70 vehicles per day, the steel beams under the bridge were rusting through and beginning to “deflect,” or to bend and buckle – a visual indicator that there is a serious structural problem, explained Michael.
“NCDOT is already replacing two or three structures within a five-mile radius of this bridge,” said Michael, “so obviously there isn’t enough funding to go around and undertake another complete replacement. I believe they want to add a few more years or so to this bridge’s life by removing the timber deck and taking off all of the steel beams and use salvaged beams from another bridge that has already been torn down.”
A&O project manager Matthew Moyer, PE, talked about the importance of regular inspection and maintenance to extend the life of a bridge. “It’s like if you own a house or a car, and you do regular maintenance, that house or car lasts longer,” he said. “By understanding the inspection and what it tells you, you can find the appropriate fix,” he continued. So, for example, inspectors need to understand which crack or area of deterioration is important to the structure before recommending a repair. “Having a dent in the body of your car that doesn’t affect the way it runs is very different from having a hole in the radiator,” said Matt.
Glenn agreed. “The bridge inspection process is to determine the quality and condition of the material. Degradation is documented and noted. We take into consideration where it is today, where it will be in two years, or in 25 years, so we can prolong the life and prevent failure.”
No system is foolproof, Glenn points out. “The fact is, we are human. We all follow the checklist and guidelines, trying not to miss anything,” he said, but NCDOT’s system of having different companies and different people inspect each bridge on a two-year cycle greatly reduces the chance that a bridge will collapse simply from neglect. Applying knowledge and experience can help extend the life of our state’s bridges so that maintenance dollars help offset the escalating cost needed to replace our aging infrastructure.