The final and biggest phase of repairs began Monday at the highrise West Seattle Bridge, which remains on track to reopen mid-2022.

The bridge, built in 1984, should regain its full design life, providing another 30 years or more of vehicle traffic.

Completion is expected by June 30, in the agreement between the Seattle Department of Transportation and Kraemer North America. Then the bridge will undergo load testing for approximately two weeks before traffic returns.

Contractors will string 46 miles of steel cable within the hollow girders, to strengthen the bridge from within, city officials said. That technique, known as post-tensioning, will be applied not only to the severely cracked central main span, but also the two approach spans west and east, which show signs of fatigue.

Kraemer’s operation begins with high-pressure water jets, to blast temporary holes through the bridge deck. These openings provide anchor sites for suspended platforms, where workers stand beneath the bridge. They’ll fasten sheets of carbon-fiber wrap to the concrete, similar to what travelers saw in 2020, when the company performed emergency stabilization work.

Carbon wrapping is “similar to the way you put a cast on a broken bone, that will really last for years,” SDOT Director Sam Zimbabwe said.

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Pre-pandemic, the seven-lane bridge carried about 100,000 vehicles and 20,000 bus passengers on busy weekdays.

West Seattle motorists, frazzled by their congested 5-mile detour, often complain the city is taking too long.

Twelve months have passed since Mayor Jenny Durkan announced her decision to repair the bridge, which closed in March 2020, rather than replace it. The procurement process took months, despite the mayor’s emergency declaration, and ultimately SDOT decided to retain Kraemer, the same contractors used to stabilize the bridge in 2020.

Zimbabwe said Monday the city needed to secure federal money and follow federal contracting rules. SDOT also waited to observe how the stabilized bridge behaved last winter before soliciting bids by repair teams.

He also insisted, “We didn’t lose time in that process because we were still doing the design and assembling all the funding needed to get the project where we are today — starting the physical work.”

City Councilmember Lisa Herbold, who represents West Seattle, recalled Monday how shortly after the closure, SDOT estimated repairs might restore only 10 to 15 more years of bridge use. She supported repairs, and is glad the city chose not to demolish the bridge in favor of a new one, which would have triggered three additional years of detours and congestion.

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“I’m gratified to hear SDOT is now talking about it as an original-lifespan repair of 30 years,” Herbold said. To her, it proves the value of the city’s engineering cost-benefit analysis, performed in mid-2020.

Kraemer will be paid $45 million for final repairs. The full $175 million West Seattle corridor budget includes last year’s stabilization, swing-bridge retrofits next year, neighborhood safety, transit and signal projects, and engineering. About $38 million is federal money, plus $9 million from the Port of Seattle.

The seven-lane bridge closed March 23, 2020, when city roadway structures director Matt Donahue witnessed diagonal cracks moving 2 feet in two weeks. A main cause was the original bridge design, where internal steel cables didn’t tighten the bridge completely from end-to-end. Long-term concrete shrinkage, known as “creep,” also distorted the structural forces.

After SDOT discovered tiny hairline cracks in 2013, officials for years believed frequent inspections and epoxy sealants would suffice. But they reacted decisively in 2020 when cracks accelerated, in time to prevent a lethal collapse.

“Because of the work at SDOT, the end is in sight to reopen the bridge in the coming months. This is an important milestone for our residents, commuters, and businesses as we urgently work to reconnect West Seattle to the greater region,” Durkan said in a statement Monday.