Seattle transportation managers are growing confident that repairs can squeeze at least 15 more years of use out of the cracked West Seattle Bridge.

The city would strive to restore six lanes of traffic in 2022, compared to an immediate demolition and replacement that would disrupt travel until 2026 or longer, at greater cost.

Although a repair-or-replace decision by Mayor Jenny Durkan isn’t due until at least October, the Seattle Department of Transportation (SDOT) has revealed some repair possibilities in forums and interviews.

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Contractors are working six days a week to stabilize the bridge by wrapping portions of the cracked concrete girders in carbon fiber. This month, they’ll strengthen it further by stringing groups of steel cables inside the hollow girders. Like a rubber band, those cables will compress the four weak zones where cracks emerged, a method known as post-tensioning.

To make the bridge resilient enough to handle 100,000 vehicles a day, SDOT must string a longer and more critical set of steel post-tensioning cables through the entire 590-foot central span from pier to pier.

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SDOT’s program manager, Greg Izzo, told an advisory task force in August the city might even get up to 40 more years from the 1984-vintage structure if it reinforces the foundations. The 40-year time frame isn’t a pinpointed forecast; instead, it suggests the bridge could be restored to its original estimated life span.

Managers have tried to temper their talk of a 40-year fix, and emphasize they can’t make a reliable prognosis until more emergency stabilization is done.

Nearly six months have passed since accelerating cracks forced SDOT to declare an emergency closure March 23. As people return from COVID-19 quarantines, traffic jams and delays are worsening on detour routes, which in turn builds support for repairs as the quickest remedy.

A West Seattle Bridge Now coalition of 4,000 members and followers has thrown its support behind prompt repairs.

Pro-repair sentiment is gaining in the West Seattle Chamber of Commerce, too, said Dan Austin, a pizzeria owner who is the chamber’s government-affairs chairperson. Austin also is on the 31-member task force of business, political and neighborhood representatives advising the city on the project.

“If we can get between 20 and 40 years out of the bridge, and it’s not close [in cost] to a brand new bridge, we should be aiming for that repair option within two years,” Austin said.

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This spring, SDOT Director Sam Zimbabwe worried that repairs might extend the span’s life for only 10 years — if it could be fixed at all. The city even wrote an evacuation plan, in case of a collapse.

Since then, radar examinations and test samples have found no corrosion within tons of existing steel strands, which run within the girder walls and hold the concrete together. If rust had run rampant, repairs would be hopeless.

And the removal of heavy traffic, along with the stabilization work, has halted the spread of dangerous shear cracks.

“It is estimated that traffic could be restored in 2022 and has the potential to add 15+ years of use to the bridge,” a city update said in July.

A technical advisory panel of seven outside engineers agreed in July that studies haven’t turned up “any information that indicates that a long-term repair is infeasible or economically unviable.”

What explains the gap between the 15-year and 40-year scenarios?

Matt Donahue, SDOT roadway structures director, said in an interview that if the city aims for decades of public use, it should strengthen the ground under the foundations to better withstand an earthquake.

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The Seattle Fault, which triggered a tsunami when it ruptured circa 930 A.D., is only a few blocks south, and the city is also vulnerable to regional Cascadia Subduction Zone quakes.

SDOT’s contemplated seismic improvements aren’t a legal code requirement, but a city policy, Donahue said.

In other words, he’s making a professional judgment that a 40-year time frame demands seismic upgrades, while a 15-year life span is tolerable with girder strengthening alone.

“This is one of the least risk bridges, from a seismic vulnerability standpoint,” because other bridges in Seattle are far older, he told the task force Wednesday.

The original foundations haven’t shifted or settled since 1984, Donahue said. They withstood the 2001 Nisqually earthquake, which moved the elevated span 3 inches. Column foundations rest on buried caissons as big as 54 feet wide, 99 feet long and 20 feet deep, which are braced by steel or concrete tubes that plunge 120 feet below sea level.

Seismic work could include injecting concrete grout into the ground, to solidify soil around the foundations, he said.

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That’s a common technique in Seattle-area soils. Harbor Island, where some of the foundations rest, sits on about 140 feet of weak fill, tideflats and alluvial Duwamish River sediments.

Current repairs to stabilize the bridge will continue through fall. A cost-benefit analysis to guide the city’s repair-or-replacement decisions is scheduled for October.

Seattle will spend $70 million through next spring on stabilization work, traffic control, major maintenance for the low swing bridge, and early design for a new high bridge, Councilmember Lisa Herbold of West Seattle reported.

There’s no budget yet for additional repairs next year, if the city chooses to do those.

Izzo said SDOT can’t be certain long-term repairs will succeed until after this winter’s freeze, when engineers double-check how the concrete expands and shrinks.

“It’s all dependent on the behavior of the bridge,” he told the task force.

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The team is already studying data on how solar radiation causes thermal expansion this summer, said consultant Greg Banks, of the firm WSP.

The additional repairs next year would add more post-tensioning steel to compress the entire 590-foot central span from pier to pier. Ducts and concrete diaphragms for this purpose were built into the bridge by original engineers and contractors.

There’s a “high probability” that after a 2022 traffic restart, the city can keep the bridge open to drivers while it reinforces the ground, except for sporadic bridge and marine channel closures, spokesperson Ethan Bergerson said.

Bob Ortblad, a Seattle megaprojects historian who is promoting an immersed-tube alternative, is not convinced bridge repair is possible.

“I doubt this will work. No project this big has ever been done,” Ortblad said. Carbon wrap is brittle and may break loose during thermal changes or traffic vibrations, he said. Even after post-tensioning, the cracked concrete will remain irretrievably weakened, he said.

But West Seattle Bridge Now member Adam Ludwig, who is a structural engineer for buildings, says his analysis of publicly available bridge reports leads him to believe SDOT’s expert advisers who say the bridge is repairable.

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“The kind of work they’re doing can be done in one year,” Ludwig said, if SDOT obtains the materials immediately.

Heather Marx, city mobility director, replied that people outside the technical team lack detailed knowledge to second-guess the 2022 reopening timeline.

“We know how important it is to the public to reopen the bridge as quickly as possible,” Bergerson said, “but we cannot rush into a literal life-and-death decision until we are confident that there is no possibility that repairs could fail while people are on it.”