"This is not the sign of deteriorating infrastructure, this is a sign of vulnerable infrastructure," said Abolhassan Astaneh-Asl, a civil-engineering professor at the University of California, Berkeley.
Joel Millman, Zusha Elinson and Jim Carlton write that although the incident which caused two vehicles carrying three people to plunge off the four-lane interstate bridge into the Skagit River, resulting in "non-life-threatening injuries", "wasn't caused by structural flaws", according to state transportation officials, it raises "new questions about the nation's aged infrastructure".
The 1955 bridge is of a through-truss design, common for the era, that lacks the redundancy of more-modern spans—meaning that severing one truss can cause the entire bridge to collapse. (See companion piece below).
The bridge collapsed "after a flatbed truck with an oversize (but fully permitted) load hit the upper part of the bridge". In fact, the truck had a pilot car in front of it that took electronic measurements of the truck load's bridge clearance.
While a Washington state Department of Transportation spokesperson said "the bridge was structurally sound and entirely safe for travel", professor Astaneh-Asl's recommended that "today we should invest in getting these...out of the system."
In the companion piece, Kris Maher writes why these types of bridges, quite common of the era 50 years ago, are no longer built except for railroads.
The steel "through-truss" bridge has what is known as a "fracture-critical" design, which means that if any part fails the whole bridge could fail, experts said. That is what happened with the Skagit River bridge, which collapsed when a tractor-trailer took out an overhead girder, officials have determined.
"About 18,000 or 3% of the USA's 607,380 bridges are fracture-critical, lacking redundant supports". (USA Today).
As it turns out, "truck swipes" are not uncommon for bridge structures, "but in most cases it doesn't lead to a failure, said Timothy Galarnyk, who runs a St. Paul, Minn., company that conducts forensic investigations of collapses."
"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. (MYNorthwest.com).
However, 'bad luck' would 'likely' not have caused a collapse in a contemporary bridge asserts Galarnyk.
"With more current technology and engineering, bridges have more flexibility," he said. "This kind of collapse would likely not occur on a modern-day bridge."
The Skagit River Bridge was not considered "structurally deficient" - it was on the less serious and larger, "structurally obsolete" list. See the May 24 post here describing deficient and obsolete bridges, and Stateline article, "66,749 Structurally Deficient Bridges" on May 24 by Melissal Maynard.
Correspondent's note: Links to both Wall Street Journal articles may not provide complete access for non-subscribers after June 01.