MOUNT VERNON, Wash. - Experts said the Skagit River Bridge was vulnerable to big blows.
Although the 1955 bridge is not considered structurally deficient, it is rated as fracture critical, which means there's no redundancy built in if a piece of the bridge fails.
"Once one element is damaged or destroyed, the bridge essentially collapses because it cannot redistribute those loads," said Jeffrey Berman, University of Washington civil engineering professor.
Berman said 40 percent of the nation's bridges are considered “fracture critical,” and they rarely fall.
Berman said engineering standards changed in the 1970s to include more structural redundancy, so it is much less likely a newer bridge would fall if hit by a truck.
Stewart Gloyd, a retired chief bridge engineer for Washington State Department o Transportation, said he used to see bridges get hit by overloaded log trucks.
"The state bridges were constantly being repaired for this kind of damage, but I don't recall a time it caused a bridge to totally collapse," Gloyd said.
The opposite side of the Skagit River Bridge was struck by a vehicle in 2012 and was repaired.