Engineer Says Truck Strike Could Bring Down Bridge

May 24, 2013
Originally published on May 23, 2013 11:09 pm

Just prior to the I-5 bridge collapse Thursday night north of Seattle, eyewitnesses report an oversized load struck a portion of the bridge’s steel superstructure. That’s the frame that’s key to holding the bridge up.

The I-5 bridge over the Skagit River was built in 1955. It’s a truss style structure. Bridge engineer Stanley Ryter says all that steel above.

"Think of it as a 16 ft tall beam and if any part of that breaks then you lose the ability to carry the load," Ryter says.

Reports are that a large load struck the steel portion of the bridge. Ryter says bridges are built to withstand all sorts of things: wind, earthquakes, heavy loads.

“But you don’t design necessarily for a truck actually hitting the bridge itself while driving on it. You design the bridge so that the bridge is tall enough so that the truck doesn’t hit any part of it,” Ryter says. 

Washington Department of Transportation records show the I-5 Skagit bridge had a weight restriction of 20,000 pounds per axle. But it was not on the state’s deficient bridge list.

“Just shocked.” Those are the words House Transportation Committee Chair Judy Clibborn is using to describe the collapse.

“I mean I-5 is our major corridor through the state so it will be something that will have repercussions," Clibborn said. "You know that people will be looking at all the bridges and it will call us to look at them quickly and make sure that we’re doing the right prioritization with our dollars.”

Clibborn, a Democrat, says the state doesn’t have enough money for preservation of existing roads and bridges. She’s been pushing for a ten-cents-a-gallon gas tax increase to fund ongoing maintenance along with new road projects. That funding package is a priority for Governor Jay Inslee in the current overtime session of the legislature. But some lawmakers have said in a bad economy this is not the year to impose a higher gas tax.

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