Most mornings in the last decade, travelers on the West Seattle Bridge could see a menagerie of box trucks delivering food, 25-ton buses aligned nose to tail, flatbeds of steel rebar and hordes of cars, vans and pickups.

It turns out, we may have loved the concrete span to death.

The Seattle Department of Transportation (SDOT), which declared an emergency bridge closure Monday, is focusing on how overuse, including a bus-only seventh lane the city squeezed in last decade, contributed to midspan cracks that accelerated in length and risk — triggering a sudden safety threat last week.

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Weight and the forces it creates are the suspected culprits, according to SDOT Director Sam Zimbabwe. That includes uneven weight distribution, where the added eastbound lane shifted more vehicles toward the far right edge, he said Wednesday.

“I think it’s mostly a load issue. But a lot of the load is the bridge itself, so it’s a combination of things,” Zimbabwe said. With traffic, about 80% of the weight is the bridge itself, he said in a brief interview.

Cracks developed deep within the hollow twin girders that support the 590-foot main span, which opened in 1984. Inspectors noticed corrosion where air and water penetrated the steel-reinforced concrete, said Matt Donahue, chief structures engineer.

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The shutdown raises questions about whether SDOT should have realized the seriousness of the cracks sooner, and warned the public and the City Council earlier. City Council members, including Alex Pedersen, who chairs the Transportation and Utilities Committee, said they learned of the problem only Monday afternoon.

Repairs will likely take months. In normal times, 107,000 drivers and 25,000 transit riders a day rely on that corridor.

“I’m shocked,” said Phil Talmadge, a former state senator from West Seattle who worked alongside then-City Councilmember Jeanette Williams to get the bridge built.

“This bridge was meant to last 50 years,” Talmadge said. “I find it strange that abruptly, all of a sudden,  they discovered this problem that emerged out of the blue, as if this bridge was struck by a comet. If this was a wear-and-tear issue, it should have been monitored over time.”

As required by federal law, the bridge undergoes thorough inspections every two years. Other monitoring is performed by the city.

Zimbabwe said city engineers started noticing unusual crack patterns in mid-2019, and hired outside experts to help evaluate the bridge. What was previously thought to be superficial cracking within the girders turned out to be more serious, he said.

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Meanwhile, the Federal Highway Administration issued new, stricter load standards for bridges last year.

Another inspection last week found the cracking had gotten worse.

“The load analysis, coupled with the continued growth of cracking, became a major problem,” Zimbabwe said. “The whole idea of closing the bridge became very sudden for everyone.”

The Federal Highway Administration said Wednesday it’s monitoring the situation: “In accordance with federal regulations, Seattle DOT acted appropriately by immediately closing the bridge upon identifying the cracking during a routine inspection.”

The bridge merited only a 5, on a scale of 1 to 9, for both “superstructure condition” and “structural evaluation,” based on the city’s May 2017 inspection reported to the National Bridge Inventory. Such a score means: “Somewhat better than minimum adequacy to tolerate being left in place as is,” the inventory says.

Still, the bridge scored a so-called sufficiency rating of an acceptable 69 out of 100. Sufficiency rating is a federal metric that combines strength, traffic, environmental impact and navigation. By comparison, the Alaskan Way Viaduct rated only a 9 several years before it was demolished in 2019.

John Stanton, civil engineering professor at the University of Washington, said the bridge might be susceptible to concrete weakness that develops over time.

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The bridge was constructed with a network of steel tendons strung through the girders, then pulled tight like a rubber band, so the steel compresses and strengthens the concrete. That method is called post-tensioning. Concrete can gradually shrink, loosening the tendon pressure, which in turn weakens the concrete’s ability to withstand downward forces, Stanton said.

“Eventually, if you make the traffic heavy enough, you’ll have concrete cracking,” he said.

Leaky road decks pose another threat, as seen last fall when the state closed an Aurora Bridge lane to replace a rusty beam, where grit, runoff and de-icing salts trickled through.

But the road decks earned a 6 rating in the National Bridge Inventory, indicating they are generally kept in good shape.

The West Seattle Bridge’s twin girders, under tremendous forces, allow little margin for damage to structural steel, Stanton said.

Zimbabwe couldn’t say offhand Wednesday whether only the rebar within the concrete showed corrosion, or whether the post-tensioning cables are at risk. Travelers can see worm-shaped patches midspan, but those are just epoxy patches for the exterior, he said.

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SDOT didn’t make Donahue, the chief structures engineer, available Tuesday or Wednesday for a more detailed interview, nor did it answer Seattle Times requests for pictures and diagrams.

The crossing is unlike I-5 viaducts, where about a dozen stiff girders support each road span, or standard freeway overpasses that can often operate even if a truck batters one or two girders, Stanton said.

The West Seattle Bridge could be reinforced by carbon wrap, a common technique, he said.

A last resort is exterior steel bracing, essentially to tighten the bridge from outside, as post-tensioning does internally, he said.

“It would look as ugly as sin. If it was the only way to do it and get 20 or 30 years of life … they might do it.”