For more than 25 years, the sound of rattling chains has pierced the air whenever Lyle Peterson crossed one of the bridges spanning the Missouri River at one the U.S. Army Corps of Engineers - Omaha District’s six mainstem dams.
As a civil engineer, one of Peterson’s priorities is to conduct a federally mandated biennial inspection of the bridges on Corps’ property. Part of the inspection process is a technique called chaining.
“We walk back and forth dragging a set of chains across the concrete deck of the bridge. If there are delaminated spots in the concrete, they make a hollow sound when the chain passes over them,” Peterson explained. “We mark those areas and write them down on the drawing of the bridge so that we can see how the delaminations are progressing; if it's getting worse.”
Delamination occurs when the layer of concrete above the reinforcing bars becomes separated (debonded). This generally occurs when water penetrates through cracks in the concrete trickling down causing the rebar to corrode. That corrosion then expands and begins to break loose some of that upper layer of concrete, causing potholes.
“Once it begins, the process slowly accelerates,” Peterson said. “If we can catch it early, we can either do some patching or get some sealant and put on the deck to extend the life of that deck.”
According to the U.S. Department of Transportation there are more 600,000 bridges in the United States, with 54% of them 40 years or older; including all of the bridges at the Omaha District’s mainstem dams.
Keeping the bridges open is vital for those who live in the area, as well as ensuring commerce continues to flow smoothly. Many of the Corps’ bridges are the only way to cross the Missouri River for miles on end.
Jeffrey Forster, a civil engineer at the Corps’ Big Bend Dam near Fort Thompson, South Dakota, estimates as many as 500 vehicles a day cross the two bridges that span the dam.
“These bridges are very important to local communities because they provide transportation between the two tribal areas,” he said. “They also provide transportation for the goods and stuff that needs to be brought back and forth across the Missouri River because the two closest bridges, Chamberlain (25 miles downstream) and Pierre (60 miles upstream), are so far away. So it's very important to public.”
To ensure those bridges remain safe to cross, Peterson and his team inspect all components of the bridge to include approach guard rails, rails on the bridge, signage, the superstructure and the substructure.
“All of those checks are to assure the safety of the bridge, and also to see if there are any recommendations for maintenance that might preclude more expensive repairs in the future,” Peterson said.
Inspecting every inch of a bridge takes nerves of steel. In order to inspect the underside of the structure Peterson has to utilize an under bridge inspection unit, commonly called a snooper. The snooper allows inspectors to stand in a bucket suspended above the river.
“I guess we're not thinking about it, we're paying attention to the work we're doing. We're wearing fall protection harnesses tied off to the basket of the snooper as an extra degree of safety,” Peterson explained. “But we're so focused on the work of looking at the bridge, looking at the steel beams and girders, looking at the condition of the paint. We’re also looking to see if there's corrosion on the steel, checking for missing bolts, and inspecting the welds.”
All bridges open to the public are required to be inspected by a certified bridge inspector who has successfully completed the 80- hour Federal Highway Administration bridge inspection training course followed with refresher training every five years in order for a bridge inspector to keep their certification.
Bridge inspection results can be viewed on the USDOT’s website at https://www.fhwa.dot.gov/bridge/inspection/.