The Skagit River Bridge wasn’t particularly worrisome to state engineers. Structural inspections showed its condition to be average. But bridges of its generation, circa 1955, often were designed in a manner described today as fracture-critical — meaning a failure in a key location can ruin an entire span.
Which is apparently what happened Thursday night.
Officials believe an oversized truck traveling south on Interstate 5 hit the bridge and triggered the collapse, said Bart Treece, state Department of Transportation (DOT) spokesman. One of the bridge’s four spans fell into the water.
Steel-truss bridges like this one are designed with a complicated web of forces distributed through the interdependent parts.
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It’s a trait shared with a bridge in Minneapolis, which fell Aug. 1, 2007. Rush-hour traffic stalled on that bridge went into free-fall, and dozens of vehicles plummeted into the Mississippi River. Thirteen people died, and 145 were hurt. The Minneapolis bridge, supported from beneath, was known as a decaying structure, unlike the Skagit bridge.
The I-5 bridge’s superstructure — including steel overhead trusses — was rated as “fair” while the substructure, or columns and foundations, was “satisfactory” in the National Bridge Inventory.
The bridge is on a 24-month inspection cycle, and also must undergo an underwater inspection every 60 months. It was inspected in August and November last year and repairs were made, said DOT Secretary Lynn Peterson.
There are 362 fracture-critical bridges in Washington state, says the Federal Highway Administration.
“It doesn’t imply anything bad about the bridge. It just means that if a certain component fails, it can lead to the complete collapse of the bridge,” said Jugesh Kapur, former head of bridges and structures for the DOT.
Kapur said a truck hitting a vertical or diagonal part of the truss certainly is capable of causing a failure. Guardrails or similar barriers are supposed to prevent strikes.
Three I-5 bridges appear as “structurally deficient” in the national database: at the Stillaguamish River near Arlington; the Samish River south of Bellingham; and the East Fork Lewis River, at Woodland in the southern part of the state.
The Stillaguamish and East Fork bridges also are fracture-critical, steel-truss bridges. The I-5 Ship Canal Bridge in Seattle, built in 1962, has “satisfactory” or “good” ratings structurally, but is functionally obsolete because of traffic safety or capacity problems.
The Skagit River Bridge has a federal sufficiency rating of 57.4, which is not particularly bad compared to, for instance, a rating of 9 for the Alaskan Way Viaduct in Seattle. These numbers are a combination of structural soundness and traffic-carrying ability.
The Skagit bridge was rated “structurally deficient” in 1992, needing $8.3 million in fixes, according to a report by uglybridges.com, which cites the National Bridge Inventory data for that year. There was erosion on the stream banks below, and serious road-deck damage or wear. By 1994, the federal database listed the bridge as “functionally obsolete,” an improved rating but an indication it still wasn’t adequate for modern traffic.
These days, it’s more common to build steel-girder bridges whose main support comes from beams under the deck, Kapur said.
Kapur, 52, worked in the bridge division 22 years. He was fired in April because of flaws in the new Highway 520 bridge pontoons. He is fighting his termination.
Background on I-35W Minneapolis collapse from the book “Too Big to Fall,” by Barry LePatner.