The installation of giant girders over Interstate 5 where it meets Highways 18 and 161 near Federal Way will close southbound lanes, some northbound lanes and some ramps this weekend, causing delays of up to 5 miles.

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Drivers heading south on Interstate 5 could face backups of five miles Friday night and Saturday night approaching Federal Way, when workers install overhead girders for the state’s massive “triangle” interchange project.

Southbound lanes will gradually close Friday starting at 9 p.m., with full shutdown by 11 p.m. Then they will reopen at 7 a.m. Saturday. The southbound lanes will again close at the same time Saturday night until Sunday morning.

Drivers are warned to expect 30-minute delays. Traffic will exit at South 348th Street, then follow Highway 99 to re-enter southbound I-5 at Fife, a nearly six-mile detour, said spokesman Mike Murphy.

The northbound side will also be affected — two left lanes will close Friday and Saturday nights from 9 p.m. until 6 a.m.

Ramps will also close overnight from westbound Highway 18 to southbound I-5, and from southbound I-5 to eastbound Highway 18.

There will also be some weeknight lane and ramp closures next week.

The $113 million project is meant to reduce bottlenecks at the confluence of I-5, Highway 18 to Auburn and Snoqualmie, and Highway 161 to Puyallup.

This weekend, crews will set girders for an aerial curve linking Highway 18 to southbound I-5.

This is the first of three weekends when a side of the freeway will be closed overnight. A full northbound closure is planned the nights of Feb. 17 and 18, followed by a southbound closure the night of Feb. 24 and northbound closure the night of Feb. 25.

New ramps will reduce bottlenecks caused when cars merge, according to the state Department of Transportation.

Among other changes, the new southbound ramps will deposit cars directly onto I-5 and Highway 161, more than a half-mile beyond the crowded cloverleaf interchange that now exists.

Cloverleaves were state-of-the-art in the 1960s, but since then traffic has quadrupled at Federal Way, says DOT.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.