As if the West Seattle Bridge closure isn’t bad enough, the city is writing contingency plans to close the lower swing bridge too, in case growing cracks drive the overhead span to the brink of collapse.

A low-bridge closure would divert the remaining 8,000 to 15,000 daily vehicles that still cross the Duwamish Waterway there. The low bridge is restricted to buses, freight, emergency vehicles and longshore workers, but is also used by other drivers when police aren’t watching.

Closing the bridge would also dash citizens’ hopes that health-care staff, private buses or people who work night shifts might cross there. And it would exacerbate a traffic nightmare expected when stay-at-home restrictions to slow the coronavirus are lifted.

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The Seattle Department of Transportation (SDOT) revealed its worries about what it calls the “worst-case scenario” of collapse in a blog post Friday and a City Council briefing Monday.

“We’re prepared to remove traffic from the low bridge and surrounding areas underneath the bridge for some period of time in the event that the High-Rise Bridge becomes unstable,” the agency said in the blog post.

The port, businesses, marinas, fire department and Coast Guard are being asked to help plan for a potential low bridge closure, the city’s West Seattle Bridge Safety Project team said. The SDOT is working on methods to send emergency alerts if needed.

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Several fast-moving cracks in concrete girders on the high bridge have continued to grow since the March 23 emergency closure, but at a slower pace, SDOT director Sam Zimbabwe told the Seattle City Council on Monday.

“It’s still our assessment that the bridge is not at imminent risk of failure,” he said.

However, the SDOT doesn’t know yet whether the bridge can be repaired or even be stabilized soon enough to make repairs possible, he said.

Additional sensors to measure changes in the cracks will be installed by May, Zimbabwe said.

“The information that we’re going to get from the real-time monitoring systems will help us understand and then have as much lead time as possible, if things start to go towards failure,” he said.

Related coverage on the West Seattle Bridge

The span is Seattle’s busiest bridge, carrying 100,000 vehicles and about 25,000 transit riders on normal weekdays, before the coronavirus outbreak.

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The low bridge, the walk-bike lane and a detour at West Marginal Way Southwest aren’t directly beneath the 590-foot-long central span, where the girders cracked. 

But the city is planning for a safety perimeter of 225 feet north and south of the high bridge, in case the high bridge fails or becomes “imminently expected to fail,” spokesman Ethan Bergerson said Tuesday morning. That area includes the low bridge. The 225-foot figure assumes a sideways tilt of the roughly 150-foot-high structure plus a 75-foot extra margin.

Besides a roadway plan, the city must prepare policies for maritime traffic, Bergerson said. When the lower-bridge decks pivot, to let ships and barges through, part of that bridge swings directly beneath the high bridge.

Consulting engineers and SDOT are still working to determine tolerances, such as what crack size or movement would prompt a warning of imminent collapse. New “bridge shape array instrumentation” should be in place by next week to monitor any changes in the geometry of the arch-shaped structure, Bergerson said.

Two decades ago in central California, the concrete Parrotts Ferry Bridge sagged 22 inches in the middle, but the state managed to save the structure, slightly misshapen, by fastening a steel arch support.

 SDOT’s engineering consultants are writing a report that models collapse scenarios, due to reach Mayor Jenny Durkan this month.

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“No matter what is decided about the future of the West Seattle high bridge, shoring and stabilization of the bridge have to happen to preserve the integrity of the bridge and to protect public safety,” Zimbabwe told the council.

“We need to address the continued growth of cracks — and those cracks have continued to grow even after we have removed vehicle live load from the bridge — and stabilize it to provide us the opportunity to make good decisions,” he said.

Councilmember Lisa Herbold said constituents have asked why the city would spend $33 million this year for interim shoring, engineering and traffic control rather than move ahead and build a new bridge.

“My understanding is, you have to spend the $33 million either way,” Herbold said. Demolition would require yet more money, Zimbabwe responded.

Traffic will not return until at least 2022, Zimbabwe said. Repairs would keep the bridge in use for another 10 years.