Seattle transportation managers are raising doubts about a long-term repair option for the cracked West Seattle Bridge, while raising hopes the city can build a new high-rise span within three years.

Their role model is the Lake Champlain Bridge between New York state and Vermont. It was completed in late 2011 using “accelerated bridge construction” tactics, such as skipping a full environmental-impact statement and building a giant arch off-site.

The Seattle Department of Transportation (SDOT) plans to release information next week, along with cost estimates for various options. Previously, the agency speculated a new bridge would take until 2026, which itself would be faster than typical Seattle-area projects.

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“That might change the dynamic when we talk about replacement,” SDOT director Sam Zimbabwe said Wednesday of a three-year timeline. “Five to six years is a very challenging time frame — to think the bridge could be closed for that amount of time.”

Mayor Jenny Durkan will take longer than her self-imposed Oct. 21 deadline to decide whether to pursue repairs or replacement, her staff confirmed Wednesday.

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SDOT closed the high-rise bridge March 23, when shear cracks accelerated 2 feet in two weeks. Cracks had been worsening for months and the city was on the brink of reducing lanes and weight limits anyway, internal document show.

A coalition called West Seattle Bridge Now insists the 590-foot concrete mainspan should be repaired by next year. That would provide the soonest relief for drivers who typically lose 15 to 30 minutes per trip taking the detour route, via the First Avenue South drawbridge.

But city project manager Heather Marx raised doubts about repairs last week: “SDOT perceives there is a significant risk the structure would not fulfill the remaining design service life … There is a significant amount of uncertainty about the ability of the repaired bridge to last 40, 15, or even five years,” she told the task force.

Her remarks differ from a city-recruited panel of independent project engineers, who find it’s reasonable to assume the span can last another 15 to 40 years with repairs.

That could be accomplished by adding high-tension steel cables inside the hollow girders, to compress the concrete from end to end — along with carbon fiber wrap. Both are common techniques.

Replacement

Seattle residents often ask, “Why can’t we build a replacement in two years like Genoa, Italy?” That city’s new bridge, made of simple steel trusses on viaduct-style columns, opened Aug. 3 to replace a ruined cable-stayed bridge.

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A closer example is Lake Champlain, where officials closed a 1929-vintage steel bridge whose concrete columns had dangerously eroded.

Seattle DOT recently hired HNTB — the same company that designed the new Lake Champlain crossing — for a potential $50 million to $150 million engineering contract in West Seattle.

HNTB’s team on the Lake Champlain project, led by Theodore Zoli, managed to win a “categorical exclusion” that cut permitting from five years to just weeks. Project leaders demonstrated how its new arch would rise over old bridge alignment, and therefore not require full environmental study, says a retrospective by the New York State DOT.

However, Seattle City Councilmember Lisa Herbold said SDOT hasn’t specified an arch or any specific design type, in bridge briefings with her.

Recapturing the Lake Champlain magic wouldn’t be easy in Seattle.

Demolition might take longer. The Genoa bridge had already collapsed and killed 43 people — a fate SDOT structures director Matt Donahue may have averted this year with his emergency closure. The Lake Champlain bridge was brought down by explosives, then retrieved.

The Duwamish Waterway, by contrast, sustains a salmon run and crucial supply barges that serve Alaskan towns. Northwest projects typically must avoid negatively affecting waterways.

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And because the West Seattle bridge contains high-tension steel, and three connected spans, they must be carefully cut away, unlike how the Alaskan Way Viaduct succumbed to jackhammers and crunching jaws.

The existing 1984 bridge, itself fast-tracked after a ship ruined a drawbridge, took two years to engineer and four to construct.

On the other hand, assembly of the Lake Champlain arch was done off-site, so the structure could be barged and hoisted into place. If similar barging is possible in Seattle, that could shave months off the timeline.

Meanwhile, SDOT mentioned that a new bridge may be better positioned to win grants from a federal infrastructure package, as an emergency project.

Some residents suggest a “hybrid” of immediate repairs, then a more deliberate new-bridge plan years later. Yazmin Mehdi, an aide to U.S. Rep. Pramila Jayapal, mentioned Wednesday it may be hard to get federal money for both.

Repair

This spring, SDOT wrote an evacuation map for a potential collapse, but those fears have nearly vanished now that contractors have begun to stabilize the high-rise bridge.

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Radar tests, concrete samples and calculations found no hidden threats of either corrosion or broken internal steel that might snap. Therefore the girders are repairable, according to a city-chosen panel of engineers.

SDOT said it hasn’t identified specific problems that would reduce the longevity to less than 15 years. “As any bridge structure approaches its design life, we would expect some issues to emerge,” a spokesperson replied to Seattle Times questions.

Donahue told the task force he can’t be sure about the bridge’s durability until after more stabilization work this fall, such as repair of a stuck bearing.

Former Mayor Greg Nickels, co-chair of the task force, said it’s understandable SDOT, as the owner of the bridge, would be extremely cautious about how long a restored 36-year-old structure might last.

SDOT said last week long-term maintenance would make repairs almost as costly as a new structure, but didn’t supply figures, just four dollar signs: $$$$.

Herbold questions that point, saying afterward that “where the greatest risk lies is the condition of the high-tension steel cables. They are in good shape.”

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John Persak, a task force member and port advocate from Georgetown, agreed with SDOT’s argument. “We are talking about putting a substantial amount of money in a failing asset. Replacement is a matter of ‘pay me now or pay me later.'”

Kevin Broveleit, co-founder of the pro-repair West Seattle Bridge Now coalition, said that even 2 1/2 years without the corridor is unreasonable, given that repairs can be done in months.

Meanwhile, community advocates said the city should make struggling businesses and travelers a top priority in cost-benefit decisions.

“I think two years is hard, three years is harder, and six years is really awful,” said task force member Anne Higuera, co-owner of Ventana Construction in West Seattle.