A pair of overnight repair closures this coming weekend, followed by five full days of lane reductions, will snarl traffic around the southbound First Avenue South Bridge — a key remaining route to West Seattle.

However, the job will wreak less havoc than the four-week project that the Washington State Department of Transportation (WSDOT) predicted back in October, before the winning construction bidder offered a tighter schedule.

The southbound Duwamish River crossing, part of state Highway 99, will fully close from 9 p.m. Friday until 6 a.m. Saturday, and then again from 9 p.m. Saturday to 6 a.m. Sunday. Two more similar closures are planned from 9 p.m. to 5 a.m. on Sunday, March 14, and Monday, March 15.

A greater traffic hassle arrives at noon Wednesday, March 10, when the southbound First Avenue South bridge will be reduced from four lanes to two lanes — 24 hours a day until noon Monday, March 15. The parallel northbound bridge will operate normally.

WSDOT warns travelers to expect delays, aggravated by the fact the southbound bridge was already a key detour route for drivers while the cracked West Seattle Bridge remains closed. Traffic has tripled on industrial roads near the West Seattle Bridge since the city closed that six-lane structure last March.

Contractors from Massana Construction, of Gig Harbor, will replace bearings under the First Avenue South bridge’s steel decks, under a $380,455 contract. The bearings consist of steel contact plates resting on grout. These enable the bridge to slide in response to normal temperature changes. But WSDOT says the 25-year-old grout is becoming worn out.

Damaged grout bearings beneath the southbound First Avenue South Bridge require replacement this winter. This photo shows a gap that was temporarily filled using steel shims this summer. A blue pen was placed in the gap for scale. (Photo from Washington State Department of Transportation)

“We have been watching the bearings that support the bridge for a couple of years,” said a statement by WSDOT project engineer Mike Askarian. “Last fall we found they had worn to the point where they need to be replaced. This is a challenging time for travelers with the West Seattle Bridge closed, but doing this work now will prevent future damage to the bridge.”

Detour routes include the South Park Bridge; Interstate 5; or the low-rise Tukwila International Boulevard South and East Marginal Way South bridges, both near Boeing Access Road.

The lower West Seattle swing bridge will remain limited to transit, freight, emergency vehicles and a limited number of workers with pre-registered city passes, except overnights. However, the usual 9 p.m. to 5 a.m. general traffic hours will be extended until 6 a.m. during the four overnight closures upstream, to match WSDOT’s work hours and give early travelers a break, city staff said Wednesday.