The West Seattle Bridge closed Monday night for several months, until the city can repair cracked support girders that render the span structurally unsound.

Built in 1984, the seven-lane crossing is the busiest city-owned thoroughfare, carrying 100,000 vehicles and about 25,000 transit riders in normal weekdays, before the coronavirus pandemic cut traffic in half last week.

During the prolonged and unexpected shutdown, the low-level swing bridge will be open only for transit, emergency vehicles and freight.

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“We want to make sure that West Seattle businesses and residents know we will be able to get needed goods to them, and we have plans in place,” Mayor Jenny Durkan said during a Monday afternoon teleconference.

She acknowledged that the current light traffic makes the high-bridge closure less disruptive, but said she’s hopeful the region will tamp down the viral outbreak so the economy and traffic recover.

Sam Zimbabwe, city transportation director, estimated the shutdown “will not be a matter of weeks, it will be a longer-term closure.”

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He said a combination of stricter federal bridge standards issued last year, along with continued deterioration seen during Seattle Department of Transportation (SDOT) inspections, led to another examination last week, followed by Monday’s decision to close off all traffic.

“We did not time this,” insisted Zimbabwe, who moved to West Seattle last year from Washington, D.C. “There would never be a time we would want to close the West Seattle high-pass bridge, because it is a critical corridor that carries more traffic than any other street in the city.”

City Council President M. Lorena González, also of West Seattle, said she supports the decision.

“We have seen too many times incidents where there are catastrophic results for not acting quickly to shut down public infrastructure, from Genoa to other cities,” González said. A highway bridge in the Italian city collapsed in August 2018, killing or injuring 41 people.

Officials described the cracks as a long-developing problem, rather than one triggered by a specific load or incident.

Matt Donahue, SDOT structures director, said the cracks in girders that support the bridge deck appeared in areas where concrete was poured, molded and compressed internally by steel bands, known as post-tensioning, during bridge construction.

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The agency had been inspecting, and trying to seal off, cracks that allowed air and water to penetrate  to the internal steel, Donahue said. Left alone, that would lead to corrosion and long-term weakness in the concrete. But despite city attempts to close the cracks, deterioration continued, officials said.

Over the years, truck loads on the bridge have grown heavier than when the bridge opened, Donahue said. Bus traffic has multiplied, too, carrying weights near 45,000 pounds each, though he didn’t cite those. Ever-increasing vehicle weights are affecting bridges across the state and the U.S.

Major repairs this year could include carbon wrap around the cracked girders, Zimbabwe said. The state used carbon extensively over the past decade in response to cracking in the Alaskan Way Viaduct, which was demolished in 2019.

“The columns themselves seem like they’re in fine condition,” Zimbabwe said of the huge vertical structures that hold the bridge 140 feet above the Duwamish Waterway.

Simply restricting load sizes or other traffic won’t be sufficient this spring, he said, because 80% of the bridge’s load is the bridge itself.

Barricades at the bridge entrances, and detour signs farther afield at Alaska Junction, Admiral Way Southwest and Delridge Way Southwest, were expected to appear Monday night.

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Drivers leaving West Seattle will need to use other routes, such as the First Avenue South Bridge and Michigan Street to reach Interstate 5.

King County Executive Dow Constantine said he’s instructed Metro Transit to immediately plan alternate bus routes, and to increase the frequency of and connections to the King County Water Taxi.

The Water Taxi has been operating at only a fraction of capacity, and as recently as last week the county planned to postpone the busier summer sailing season for weeks, in keeping with social distancing measures in place to combat the coronavirus.

For decades, the city of Seattle has underfunded, or at least struggled with, its aging structures, though at 36 years old the high bridge is only halfway through a normal design life. The Ballard Bridge, for instance, is 103 years old and the steel drawspan occasionally gets stuck, but no replacement funding is in sight.

Donahue himself said last year he could use $80 million a year to catch up with maintenance and major retrofits, compared to $17 million allotted by city budgets.

Voters have passed two property-tax levies for supplemental transportation funding since 2000, but those dollars are spread among many projects, including concrete repaving, safer crosswalk designs and the current replacement of a rotting timber-supported bridge at Fairview Avenue North.

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SDOT closed one northbound lane of the Argo Bridge near Georgetown in 2018, in part due to lack of repair funds. Trucks and buses were banned last year from several curbsides in Pioneer Square to protect 120-year-old sidewalks and underground passageways.

In the near future, the low West Seattle Bridge itself will need a retrofit of its underwater foundations, Donahue said last year.

A previous version of this story misspelled the last name of Matt Donahue, chief structures engineer at the Seattle Department of Transportation.