Mayor Jenny Durkan announced Thursday the city will repair the cracked West Seattle Bridge, instead of rebuilding its center spans, because repairs are the surest and quickest option to restore traffic to 125,000 daily travelers.

She hopes to reopen the bridge in 2022, then collaborate with Sound Transit to design and build a multimodal crossing by the early 2030s that serves light rail, personal vehicles, freight and bicycles.

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West Seattle motorists have detoured since March 23, when quickly growing cracks in the concrete girders caused city structures director Matt Donahue to call for an emergency shutdown.

Detours for drivers coming from and going to West Seattle via the First Avenue South Bridge typically require an extra 30 minutes — and that’s while thousands stay home due to the COVID-19 epidemic.

Durkan said the Port of Seattle will reopen Terminal 5 in West Seattle next year, another reason to unclog the area.

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Repairs would prolong the 36-year-old bridge’s life an estimated 15 to 40 years. The primary method is additional “post-tensioning,” in which strands of steel rope are strung through the hollow girders, then tightened at extreme pressure to strengthen the concrete.

The Seattle Department of Transportation (SDOT) will go ahead with a “type, size and location” study for a replacement structure. That study will likely include not only bridge concepts but a shallow immersed-tube tunnel, said Heather Marx, city mobility director, in a briefing Wednesday to reporters.

Repairs are estimated at $47 million, while a new steel-arch bridge ranged from $390 million to $522 million, SDOT’s technical updates say.

Either option would only affect the cracked central mainspan and two sidespans, whose girders are under severe stress. The approach decks are in good condition.

Durkan said she was predisposed to favor the so-called rapid replacement option, suggested in October by the city-hired HNTB company, which sought to assemble twin steel bridges off-site, then fasten them above the Duwamish River by spring 2023.

But it didn’t seem so rapid after Durkan studied local projects, such as the Elliott Bay Seawall, which needed a year or more to win environmental permits. Also, briefings by members of Congress convinced her the federal government wouldn’t act on an infrastructure bill, which could make bridge grants available, until mid- to late next year, she said.

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Her hopes for a federal stimulus plan this fall that would include infrastructure dollars were dashed by a negotiating stalemate. Durkan also told an online town hall Monday that project decisions could rest with Senate Majority Leader Mitch McConnell, R-Kentucky, who’s got a damaged bridge in his home state.

“I could not tell the public we would have that [steel bridge] operable and be able to restore mobility in three years,” Durkan told reporters. “I think it would be longer than that. Maybe five years.”

It’s unclear what a future mixed-use crossing would be. Another steep structure at least 140 feet high, a level drawbridge, or something else? Would it be designed to serve freight when the lower swing bridge built in 1992 ages out?

Durkan said she’s posing similar questions internally, while SDOT Director Sam Zimbabwe said it’s too soon to predict the outcome.

“Ten, fifteen or twenty years from now we should be a city that is not reliant on vehicles and cars. We should be a city that relies on more climate-friendly options,” she added Thursday morning, when asked why Seattle won’t pursue a complete replacement of the road bridge for more than $1.5 billion.

SDOT will conduct a traffic and revenue study for a future bridge. That’s a prelude to tolls, which in turn may be required for certain federal loans. For now, there’s no proposal to charge travelers in 2022.

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And in the short-run, SDOT isn’t estimating when it could complete repairs. The recent cost-benefit study by WSP forecast August 2022, while SDOT this week described a possible early-2022 return of some traffic and full use in mid-2022.

Some repair proponents and industry sources say bridge strengthening, which entails more post-tensioning steel cables to compress and tighten the concrete, ought to go faster.

Barbara Moffat, co-chair of the city’s panel of outside technical experts, called the mid-2022 target “realistically conservative.” It could be shortened through streamlined contracting methods, she said, such as hiring a single team to both engineer and manage the fixes.

“Would I like to see it done quicker, yes. Do I believe it’s possible, yes,” she said. “But there are a lot of factors to consider.”

Engineering alone was projected to need six months to design lasting repairs. Seattle officials have modeling data, but say they can’t complete that stage until after they observe how the bridge — newly unstuck at a damaged bearing — behaves during winter’s freeze-and-thaw cycles.

On the other hand, Zimbabwe said the city has hired WSP, its bridge consultant, to design the repair project, which follows emergency stabilization this year. The firm produced more than 400 pages of technical findings about the seismic qualities and internal girder forces of a repaired bridge.

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SDOT staff has said that over many decades, a repaired bridge costs nearly as much as new span in part because of high maintenance costs, followed by replacement in 2060. In presentations, officials cited Seattle’s history of underfunding bridge maintenance as a good argument to build anew.

“That’s a challenge to council members to find maintenance money. I’m taking up a challenge,” Seattle City Councilmember Lisa Herbold, who strongly backs the repair option, said in an interview Monday.

She said that argument inspired her to co-sponsor a proposed $20 car-tab fee to double SDOT’s thin $7 million yearly bridge-maintenance budget. An audit requested by Councilmember Alex Pedersen said the maintenance amount should be at least $35 million.

If the bridge is repaired, SDOT has pledged to conduct full inspections twice a year with specialized worker-lift trucks; to spend extra time dealing with the tremendous volume of new tensioning steel; and to watchdog a full electronic network of monitoring devices.

“We have substantial risks before we are back to restoring traffic, and may not even know, until we are back to the point of restoring live load on our repaired bridge, exactly now the bridge responds,” Zimbabwe said at the online town hall Monday.

Durkan was asked if she fears future mayor and councils will neglect maintenance.

Her answer: given the havoc that would result from losing the West Seattle high-rise connection, the city will feel more than enough pressure to take care of the bridge.