ARLINGTON — Bridge work on I-5 over the Stillaguamish River will result in a major traffic disruption this summer.

The state Department of Transportation is replacing the concrete deck and part of the steel support frame of the bridge span that carries southbound traffic across the river.

Starting in mid-July, work crews will close the 607-foot-long span and redirect southbound vehicles across the median onto the bridge that currently carries northbound traffic.

The northbound bridge will be restriped to allow for two lanes each of northbound and southbound traffic, separated by a concrete barrier.

Each span of the bridge carries an average of 39,000 vehicles per day, but that can rise to 50,000 per day during summer. The heaviest traffic volume occurs between 3 to 6 p.m. Thursdays and Fridays and from 11 a.m. to 4 p.m. on the weekends northbound, and from 4 to 6 p.m. Sundays southbound.

The work is expected to take approximately four months, ending in late October or early November. An exact start date for the closure has not been set.

The bridge bearing the southbound lanes was built in 1933 to carry Highway 99 across the Stilly.

The bridge deck has been overlaid several times since then, but after a 2012 inspection it was put on the state’s “structurally deficient” list.

The northbound bridge was built in 1971 and is still rated as being in good condition.

“It’s come time that we need to replace the concrete deck,” said Todd Harrison, WSDOT’s regional assistant administrator.

The bridge deck has potholes and cracks, and some of the underlying steel beams and stringers — beams that run parallel to the direction of travel — that support the deck are corroding, Harrison said.

“Structurally deficient” does not imply the bridge is in danger of imminent collapse, but indicates that one or more components of the bridge need repair or replacement.

According to the transportation department’s website, there are 139 state-owned bridges in Washington with that rating.

The steel truss bridge comprises three spans over the river. The superstructure of the bridge is in good condition, Harrison said, and is not included in the project.

Last year, an oversize truck hit one of the overhead trusses on the Skagit River Bridge on I-5, causing a span to collapse. There is a significant difference between the two, however, in that the old Skagit River Bridge’s overhead trusses were arc-shaped, with just 15 feet, 3 inches of clearance at the outer edge of the travel lanes, which is where the truck hit the span, compared with 18 feet at the center of the roadway.

The Stillaguamish River Bridge’s trusses are horizontal, with uniform clearance of 16 feet, 5 inches all the way across, Harrison said. The new Skagit River span has horizontal trusses with 18 feet of clearance.

Once the work starts, speed will be reduced through the work area to 55 miles per hour, and the lanes will be reduced to 11 feet in width, from 12 feet.

Tow trucks will be in the area to quickly remove any disabled vehicles from the bridge.

“The goal is to keep traffic moving and keep it safe,” Harrison said.

The interchanges immediately north and south of the bridge, at 236th Street NE and Highway 530, will stay open.

During the work period, the transportation department is encouraging drivers to avoid traveling on the bridge during peak hours, to check the state’s website for updates (wsdot.wa.gov/projects/i5/stillaguamishbridgerehab), and to plan for delays of up to 35 minutes if you need to cross the bridge during those peak hours.

Alternate routes for local traffic include Highway 9 east of I-5 and Pioneer Highway west of the interstate.

Mowat Construction Co. was awarded the $8.7 million contract for the project. All but $350,000 is paid for by federal bridge preservation funds, with the state picking up the remainder.

The contract has a built-in incentive of $50,000 per day, up to a maximum of $500,000, if the work is finished in fewer than 120 days. It also has a disincentive built in if the work takes longer than expected.

Chris Winters: 425-374-4165; cwinters@heraldnet.com.