Don’t say you haven’t been warned.

The state Department of Transportation says drivers should expect lengthy traffic delays this weekend as workers simultaneously shut down both the Highway 520 floating bridge and the northbound lanes of Interstate 405 through Bellevue.

“If you head out the door without a game plan this weekend, you could end up in some gridlock,’’ said Travis Phelps, state DOT spokesman. “We really need drivers’ help to keep traffic moving and avoid nasty backups.’’

On Friday, northbound I-405 will close at 10 p.m., and both directions of Highway 520 will close at 11 p.m. All lanes of both highways will reopen by 5 a.m. Monday.

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The DOT says Eastside and Seattle-area drivers should be prepared to spend an extra hour getting to their destinations, with the Interstate 90 bridge and Interstate 5 being used as alternate routes.

The closures will allow crews to replace 50-year-old concrete roadway sections on I-405, most of them between Main and Southeast Eighth streets, and conduct an annual inspection of the 520 bridge.

Crews also will install a fish culvert allowing Yarrow Creek to flow beneath a section of Highway 520 at 108th Street.

Without the full weekend closure, the work would have to be done over many weekends during nightly closures, said DOT spokeswoman Kris Olsen.

The DOT says work crews are taking advantage of summer weather and one of the few available weekends without major traffic events — such as the Seafair parade, the Bellevue Arts Fair or Bite of Seattle — to do the repairs and inspection.

However, the Mariners are playing at Safeco Field against the Los Angeles Angels Friday and Saturday night and Sunday afternoon, and the Seafair Milk Carton Derby is Saturday on Green Lake.

The DOT said it would do whatever it could to help keep traffic moving before and after the Mariners games.

The I-405 improvements are part of an $8.2 million concrete-pavement-rehabilitation project. This weekend, crews will replace broken concrete sections on the two left northbound lanes. Later this summer the DOT will replace sections in the two right northbound lanes.

Workers also will grind portions of the roadway. Once completed this fall, the surface will be smoother for driving.

There are no immediate plans to replace the concrete sections on the southbound lanes.

The 520 bridge will be closed once again later this summer for construction staging as part of the $306 million project to widen an 8.8-mile section of the highway east of the bridge, adding a transit lane.

Even though the highways may be a mess, “And we realize there are good reasons for this, the businesses are open as usual,” said Sara Garner, vice president of the Bellevue Chamber of Commerce.

The DOT recommends these alternate routes:

• To reach downtown Bellevue from I-405 headed north, take Southeast Eighth Street to 116th Avenue Northeast.

• Westbound Highway 520 drivers can use southbound I-405 or 148th Avenue Northeast to travel south.

• I-90 drivers wanting to head north on I-405 will be detoured onto Bellevue Way or 148th Avenue Southeast.

The DOT says drivers should plan for long delays on I-90 near the detour routes, on westbound Highway 520 approaching I-405, northbound I-405 where all traffic will exit to Southeast Eighth Street, and I-5 through downtown Seattle.

Surface streets in and around Bellevue will experience congestion as well.

Nancy Bartley: 206-464-8522 or