A repaired West Seattle Bridge could safely reopen by 2022 for only $47 million, but its future costs might soar nearly as high as a partial replacement bridge, said a city cost-benefit analysis released Tuesday.

The partial-replacement could be finished by mid-2026 and last 75 years, for $383 million upfront construction costs, the report by a consulting team says.

In a response, the Seattle Department of Transportation (SDOT) wrote that the “superstructure replacement” alternative, to build three new spans the same size as the existing bridge, “was the best overall performer.”

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The 89-page document gives backers of either option ample ammunition for debate, while Mayor Jenny Durkan weighs whether to repair or replace Seattle’s busiest city-owned roadway, which closed March 23 because of runaway shear cracks.

To further thicken the plot, Ted Zoli, who led a two-year bridge replacement in Lake Champlain, New York, in 2011, is scheduled to speak to a West Seattle Bridge task force Wednesday. He’ll describe how his engineering company, HNTB, could design a similar steel-arch shape, barge the frame to the Duwamish Waterway and hoist it onto the existing bridge columns.

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Project officials haven’t explained yet how to hoist a structure for Seattle, which would be far bigger than the two-lane span built in New York.

The steel-arch concept emerged too late to be included in the cost report, but SDOT Director Sam Zimbabwe hopes to beat the report’s 2026 time frame estimate that assumes a concrete bridge.

Tuesday’s report, by the engineering firm WSP, doesn’t dwell on travel delays or business losses in West Seattle, costs that many residents say ought to cinch the argument for immediate repairs.

However, the report did call out the equity risk of prolonged traffic detours, which mainly harm neighborhoods with large communities of color, such as High Point, Highland Park and South Park.

Looking into the future, the precise-looking cost numbers become cloudier, because of unknown risk and operating needs.

Zimbabwe warned a volunteer task force last week to not take the figures literally, because the project is at “0% design.”

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And the report’s purpose is just to give rough order-of-magnitude estimates, to help the mayor make the basic repair or replace decision.

Only then would SDOT move to a more rigorous step called a Type, Size and Location Study, to compare different kinds of bridges or a shallow tunnel. That’s generally needed to compete for federal grants, city project manager Heather Marx has said.

Also, the WSP report escalates several cost figures in an attempt to “monetize” various risks and pile on future maintenance costs.

For instance, the simple $47 million repair estimate rises by $176 million, mainly because of “the risk of a reduced service life,” a chart shows.

A city-hired expert panel concluded most of the 1984 bridge is in good shape and is capable of lasting 40 more years after the cracks are sealed with high-tension steel cables and the girders are wrapped in carbon fiber.

WSP noted a slight chance, of only 5%, that a repaired bridge may fail to last 40 years, because of some hidden weakness designers fail to discover.

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City officials such as Structures Director Matt Donahue have more conservatively cited a 15- to 40-year range, and say maintenance would be expensive, including frequent inspections every six to 12 months.

The 36-year-old bridge has already behaved in ways models didn’t predict, all the way until 2020, SDOT spokesperson Michael Harold said. “Are we truly confident nothing will go wrong … ?” he said.

But its main shortcoming lies decades away, because even a well-repaired bridge must be replaced 35 years sooner than a new structure, the report says. That factor is priced at $1.3 billion minus the $648 million benefit of getting more life from the existing bridge, while maintenance and operations are $41 million.

All together, WSP assigned a $916 million “life-cycle cost” price over 75 years to the repair version, comparable to $1.01 billion for the three-span partial replacement.

Two more options — a completely new cable-stayed bridge or a shallow immersed-tube tunnel — were both far more expensive, the study found. On the other hand, either would be strong enough to carry future Sound Transit trains, a benefit not calculated in the report.

To the extent HNTB’s steel-arch concept can save construction time, that would only improve its scores for traffic and social equity, Harold said.

Port of Seattle Commissioner Peter Steinbrueck, a task-force member who is undecided about the best alternative, said, “I have tremendous skepticism about the ability to replace a bridge in three years. It would be a miracle in Seattle to do anything like that.”