Collapsed I-5 bridge characterized as functionally obsolete, fracture critical
After the collapse of the I-5 bridge over the Skagit River, it’s come to the public’s attention that the design of this particular bridge is described as fracture-critical, which basically means that each part of the bridge is critical to its integrity.
“This is what they call a fracture-critical bridge, which means if any part of the bridge fails, there is no redundancy built into it so it will all collapse,” CBS News Transportation Safety Analyst and former NTSB Chair Mark Rosenker tells KIRO Radio Seattle’s Morning News.
He says the bridge was rated “functionally obsolete,” but that doesn’t mean it is unsafe, that just means it’s old. According to Rosenker, bridges with this type of characterization are not entirely uncommon. “There are about 18,000 bridges in our nation today that have that type of characterization.”
“Certainly it would be wonderful to be able to replace every single bridge that is both fracture-critical and also functionally obsolete. But where are we going to find the money to do that,” says Rosenker.
Dr. Fari Barzegar, the founding principal engineer at Habitat Engineering & Forensics, tells Seattle’s Morning News bridges that are characterized as “functionally obsolete” are bridges that have passed their life span.
“This particular bridge is now around 57 years old and with corrosion taking its toll, as well as what we call fatigue of the steel structure itself, so they are past their design life.”
Greg DiLorento, president of the American Society of Civil Engineers, tells KIRO Radio’s Ross and Burbank Show that the average age of bridges around the nation is 42 years old, and those that are viewed as structurally deficient aren’t necessarily in fear of failing but rather they just need to be inspected annually and maintained.
“They need to be inspected every year so that we make sure that any maintenance needs are accommodated or if the loadings need to be reduced on the bridge.”
“This is just bad luck of where it hit and how it hit,” said Lynn Peterson, State Secretary of Transportation, on the truck that struck the metal structure before the bridge’s collapse. “It’s basically not a structural deficiency within the bridge itself.”
The Interstate 5 bridge that collapsed into the Skagit River was built in 1955 and was inspected two times last year in August and November.
“I want to reiterate over and over, that we do meet the national standards for inspection for all our bridges,” said Peterson.
Transportation Department spokesman Noel Brady in Seattle says it had a sufficiency rating of 47 out of 100. The state average is 80, according to an Associated Press analysis.
The I-5 bridge that collapsed was a steel truss bridge, meaning it has a boxy steel frame. The vertical clearance from the roadway to the overhead supports is 14.6 feet.
Witness reports of an oversize truck hitting a part of the overhead portion of the bridge may have been involved in the collapse. Rosenker says standards today would require a 16 foot overhead clearance.
“It would be nice to be able to upgrade all of these bridges, but it’s billions and billions and billions of dollars of expenditures and it’s not easy to find,” says Rosenker. “We’ve just got to continue to maintain these bridges to the best possible way they can.”
Rosenker says all bridges in use in the U.S. are regularly inspected and should generally be safe for the public’s use.
“There are about 607,000 bridges in the United States. If a bridge is nearly falling down, clearly the state knows that by virtue of its required inspections that occur at least every two years. There are maintenance protocols that have to be done but I would suggest that the virtually all the bridges that we’re operating under today are certainly adequate enough to be able to safely transverse them.”