City staff lean toward replacing, not repairing, West Seattle Bridge; could be done in 3 years

Seattle transportation managers are raising doubts about a long-term repair option for the cracked West Seattle Bridge, and embracing the hope of building 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 so-called “accelerated bridge construction” tactics, such as skipping a full environmental-impact statement and building an arch off-site.

The Seattle Department of Transportation (SDOT) plans to release information next week, along with cost estimates. Previously, the agency speculated it would take until 2026 to build an all-new bridge, which itself would be faster than normal Seattle-area projects.

“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, who is facing an Oct. 21 deadline to choose between repair or replacement, will take a while longer, Zimbabwe told the project’s advisory task force.

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

Many residents on the peninsula insist 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 last week.

Her remarks differ from those of 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 added steel and carbon-fiber reinforcement.

Meanwhile, the city has mentioned arguments for replacement — for instance, better odds to gain federal funding as an emergency project.

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.

A closer example is Lake Champlain, where a 1929-vintage steel bridge closed in 2009 when inspectors found severe erosion within its concrete columns.

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 environmental permitting from five years to just weeks, while negotiating the new ship channel with the Coast Guard in two months. Engineers designed the new bridge at the same time.

An overhead arch like the Lake Champlain bridge has would keep support structures out of the Duwamish River waterway, where all parties insist on keeping the 140-foot-high navigation channel. It might be possible to go a bit higher, which could help SDOT in talks with the Coast Guard.

It’s not clear yet whether the old 1984 columns would support an arch, or if new columns are needed.

Recapturing that magic won’t be easy in Seattle.

Demolition may take longer. The Genoa bridge had already collapsed and killed 43 people — a fate SDOT structures director Matt Donahue averted this year — while the Lake Champlain bridges were quickly 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 water impacts.

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

The existing 1984 bridge, which itself was fast-tracked in response to a damaged drawbridge below, 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 several months off the timeline here.

Repair

This spring, the city and consulting firm WSP prepared for a potential collapse, but those fears have nearly vanished now that the team has begun to stabilize the high-rise bridge.

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

Pressed about the repair prospects, Donahue said SDOT can’t be sure how durable the bridge could be 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 that SDOT, as the owner of the bridge, would be more cautious than the outside experts about how long a restored 36-year-old structure could last.

Meanwhile, some community advocates expressed skepticism about how fast a new bridge could be delivered, and said reducing stress on travelers and businesses should be a top priority.

“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.