Like its taller neighbor, the low-rise West Seattle swing bridge has developed shear cracks in its concrete girder, which will need repairs.

But this time, Seattle Department of Transportation (SDOT) officials say they identified the risks soon enough to contain the damage and keep the swing bridge open for travelers.

The lower bridge is the only direct connection between West Seattle and downtown since the city abruptly closed the six-lane high-rise bridge in March due to accelerating cracking. The bridge is expected to remain closed for at least two years while the city decides whether to repair or replace it.

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SDOT will soon post flashing signs to encourage drivers to obey the 25 mph speed limit, a citywide safety rule that should also reduce stress on the span. Later the city will hire contractors to tighten the span with steel cables, and reinforce with carbon wrap by 2022, said city structures director Matt Donahue. He is seeking approval from the Federal Highway Administration (FHA), which oversees bridge maintenance nationally.

The city doesn’t know yet the cost of repairs or how many days the 28-year-old bridge might be closed to traffic during critical phases of the work, he said.

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The swing bridge is limited to trucks, buses, emergency vehicles and longshore workers, with general traffic currently allowed only from 9 p.m. to 5 a.m. Otherwise, SDOT fears it would become gridlocked if the 100,000 daily drivers formerly using the six-lane high bridge funneled through the two-lane lower crossing.

Small shear cracks have formed in four locations, 45 feet from the pivoting piers on either riverbank, according to bridge inspection reports obtained by The Seattle Times through a public records request.

SDOT leaders this spring said repairs are needed in the moving bridge pivots and underwater columns, and they stepped up their girder inspections. But the cracks and pending repairs were not publicly disclosed until now.

“This is a concrete structure that’s deteriorating, like concrete structures do,” Donahue said in an interview Monday. “First it’s a public safety issue. We are trying to be abundantly cautious, and still operate the bridge safely.”

The city began intensive crack mapping in April, and outfitted the swing bridge’s girder with gauges and meters by Colorado-based BDI — the same company that installed monitoring equipment on the neighboring highrise bridge. The gauges take readings every 20 seconds.

“Some of the cracks at this location are also apparent on the outside of the web,” says an April 29 inspection report, which included speculation they might penetrate the wall. But predominantly, they’re hairline cracks.

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“They’re only a few inches deep, they’re not growing,” Donahue said.

Donahue said cracks first appeared in a June 2019 inspection. The city probed further, in its already-scheduled program to revise bridge load ratings, a federal requirement. So SDOT would have caught the problem regardless of the crisis on the high-rise bridge, he said.

Concrete always cracks to some extent, and even shear cracks that can be seen from shore wouldn’t be unusual, or necessarily a hazard, said Marc Eberhard, a civil engineering professor at the University of Washington. That’s because modern bridges are crisscrossed by internal steel.

“As soon as a crack occurs, it engages the steel,” Eberhard said. “From a safety view, it’s all about how much steel you have to carry the forces.” But shear cracks should be fixed to assure long bridge life, he said.

While the swing bridge contains ample rebar, Donahue said the cracked spots lack vertical post-tensioning steel, which tightens the concrete like a rubber band.

On March 5 a report by Jacobs Engineering Group calculated that at the points where diagonal cracks appeared, the bridge was unfit to carry more than its own weight, so “the load ratings are shown as zero.” Jacobs called the finding “of high concern,” and said the city should conduct more thorough research, which SDOT did.

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The city ran a weighted truck over the bridge, and in a June 17 follow-up report, Jacobs recalculated its findings based on that “live load” test and support from the larger concrete section near the pier. That analysis found the actual strength satisfies the minimum for legal truckloads.

“We know the bridge has capacity, because it’s out there being operated right now,” Donahue said. He called the city’s research “a positive story about how we do our work.”

Still, the federal rating for the spans including girders, known as the superstructure, declined from “good” to “fair” condition, based on the National Bridge Inventory. As recently as 2018, the city reported a good superstructure rating, and an excellent score of 92 out of 100 for traffic and physical quality combined, the national database says.

Engineers in April started marking and mapping cracks inside the low West Seattle swing bridge. Here’s an image from the south wall of the hollow girder. (Seattle Department of Transportation photo.)
Engineers in April started marking and mapping cracks inside the low West Seattle swing bridge. Here’s an image from the south wall of the hollow girder. (Seattle Department of Transportation photo.)

The bridge has also suffered occasional clanging under heavy buses and concrete mixers. This could be seen from the walk-bike lane, when the deck bounced up about 2 inches sometimes. SDOT’s own April 2020 inspection report said that a center-pivot lock “bangs under heavy traffic” and needs fixing. Last weekend the spans appeared solid. Donahue says the city has done repairs and will increase pivot maintenance.

The low bridge is actually two cantilevered spans that swing in parallel to open the waterway for high loads, such as barges carrying food and building supplies headed to Alaska.

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Donahue told federal regulators SDOT intends to examine trucking records in the area and collaborate with the State Patrol to keep better track of any overweight loads.

Meanwhile 140 feet over the Duwamish Waterway, shoring at the closed high bridge starts this week, as workers stand on hanging platforms to fasten carbon sheets over the much-larger cracks.

If the six-lane bridge must be demolished and replaced, the area that calls itself West Seattle Island will depend on the swing bridge much longer than two more years.