The state will cut, lift and straighten each of the 25 arched crossbeams on the Interstate 5 Skagit River bridge this fall, so they won’t be smacked by tall truckloads.

All four lanes of the bridge will provide clearance of 18 feet, compared to the current 15 feet, 6 inches above the right-side fog lines.

The job requires 40 overnight closures, most of which will crimp only one direction of traffic.

The goal is to avoid repeating the type of crash that caused the northernmost of four freeway spans to collapse May 23, when a tall load in the right lane hit several crossbeams, and the northernmost of four bridge spans fell into the river. To travel safely, the truck needed to be in the left lane.

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Since then, a temporary span has been installed, allowing I-5 to reopen June 19. The temporary span will be replaced by a $7 million permanent north span in September, supported by concrete girders below deck, instead of an overhead truss.

This fall’s $4.5 million truss retrofit, affecting the other three spans, will be federally funded. Construction closures will last from 8 p.m. to 8 a.m. Drivers will detour mainly across the nearby Riverside Drive bridge that connects Burlington to Mount Vernon.

The rationale is that shops along Burlington’s busy commercial strip don’t open until midmorning, so the freeway will be available in time to drain off nonshopping traffic. Merchants lost business after the collapse when traffic congestion discouraged customers.

“Essentially, I-5 will be open when the businesses of Burlington are open,” said Bryan Harrison, city administrator.

To change the crossbeams, workers will cut them in two places, so the level central part remains, and new steel will be fastened to square-off the areas that now slope downward.

The procedure shouldn’t weaken the trusses — and it won’t even require temporary bracing — according to Todd Harrison, state bridge engineer. Only six of the 25 support significant weight, while the rest serve merely to resist lateral forces, especially wind, he said. The entire freeway will close while those six are rebuilt.

The new straight crossbeams should work fine, he said, since the curvature of the 1955 beams was purely an architectural feature, Harrison said. “There’s no real structural significance to it.”

Jay Drye, assistant regional administrator for the state Department of Transportation, said he expects oversize loads to be more frequent in the future.

Though trucking firms are obligated to check the right-lane clearance, “We hope to reduce the chances of a similar bridge strike in the future,” he said.

The same bridge took a hit above the northbound lanes last year. Across the state, there were at least 55 bridge and overpass strikes recorded from 2010 to 2012.

Meanwhile, yellow warning signs have been recently affixed to low I-5 overpasses, including Northeast 45th Street and Northeast 50th Street crossings in Seattle, and near Joint Base Lewis-McChord south of Tacoma.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom