The Aurora Bridge will lose a southbound lane for several days, until the state severs and replaces a segment of corroded beam supporting the road deck.

The pavement sagged a half inch near the middle of the bridge, in the right lane that carries 50,000-pound buses every few minutes, along with general car and truck traffic.

The weakened beam triggered the Washington State Department of Transportation (WSDOT) to close the outer lane after a weekend bridge inspection. Engineers hope to devise a work plan within a day or two, followed by an emergency repair order.

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“It’s unlikely we’ll have the work done this week,” Mark Gaines, state bridge and structures engineer, said Monday afternoon. The sidewalks remain open.

A second lane may close during the repairs, for the safety of crews and machinery on the deck.

WSDOT released pictures Monday showing where the beam end flaked away from rust, and where positioning plates were bent. A pair of four-inch-long cracks formed nearby, Gaines said.

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State workers discovered the rust in September during a regular full-bridge inspection. The bridge had been recently sandblasted and painted, during a two-year, $35 million project.

Rust stains had seeped through the new paint, a telltale sign of advanced corrosion, Gaines said. That prompted inspectors to take a closer look.

“The bridge, for the most part, is in pretty good shape,” state traffic spokesman Bart Treece said.

WSDOT says full inspections of the 2,955-foot-long crossing were performed in 2015, 2017 and 2019, in accordance with national standards.

The corroded beam is called a stringer and runs lengthwise immediately beneath the concrete bridge deck. Stringers are fastened to and rest upon the bridge’s major crossbeams. Any given cross-section of roadway sits atop 13 stringers, approximately two per lane.

Though severe, the rust affects only one of 52 similar connections, Gaines said.

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The beams aren’t primary pieces of the steel-arch truss structure, which remains stable. However, continued traffic over the damaged beam would lead to metal fatigue in the adjacent stringers — requiring more extensive closures and repairs down the road, Gaines said.

To rebuild this connection, workers must jack up the deck to relieve pressure, cut away the corroded area and then fasten a short replacement-beam segment with supporting plates.

Most lanes can stay open, because the deck will flex sideways just enough to create a tiny separation, where workers can insert the new parts, Gaines said. The new parts will be delivered from the side, using an under-bridge maintenance truck parked on the outer lane.

The rusty area sits beneath an old-fashioned “finger joint” style expansion joint, where water, de-icing fluids and grit constantly flow between hundreds of interlaced phalanges.

“It’s been out there for 90 years, and we design bridges for 75 years,” Gaines said. “Especially when it’s structural steel, it’s out there in the weather, carrying heavier loads and volumes than it was expected to. Just over time, we start to see deterioration out there on steel connections.”

If needed, WSDOT can bypass bidding procedures to expedite repairs. After the Interstate 5 Skagit River Bridge collapse May 23, 2013, the agency’s contractors erected a military-style bridge of modular steel pieces by June 19, and rolled a permanent concrete bridge into position in September.

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Asked if state teams should have spotted rust sooner, Gaines said: “I think we found this at an appropriate sort of time frame.” Washington state meets federal goals of fewer than 10 percent of bridges in poor condition.

From the deck Monday morning, snarled traffic appeared back up to Woodland Park. But once motorists passed the weak spot, the bridge reopened to three southbound lanes, where vehicles cruised at 30 to 40 mph.

King County Metro Transit said its buses were delayed four minutes at 7:30 a.m. and eight minutes by 8 a.m.

The Seattle Department of Transportation has studied four- and five-lane configurations on the bridge but suspended that effort after a 2015 crash involving a Ride the Ducks vehicle and a charter bus that killed five people, injured 71 others, and spawned lawsuits and financial worries for the city.

Carrying 62,000 vehicles per day, Aurora Bridge is the city’s fourth-busiest arterial after the West Seattle Bridge, East Marginal Way South and the Montlake Bridge.

Some vulnerable bridges in Washington state are checked annually. WSDOT hasn’t determined yet whether the Aurora Bridge, built in 1932, should now be inspected every year.