WSDOT: Eroding steel beam blamed for SR-99 Aurora Bridge lane closure

SEATTLE – The state Department of Transportation is telling commuters who drive into Seattle to be prepared for delays in the foreseeable future.

A steel beam that holds a portion of the southbound SR-99 Aurora Bridge deck is failing after years of corrosion and heavy traffic.

And while WSDOT engineers are busy formulating a plan to fix the problem, getting all the southbound lanes back open may take longer than a week.

The problem was first spotted during a routine two-year inspection last August, according to a WSDOT spokesperson. But the severity of the problem was only discovered this past weekend during a more close-up inspection.

The agency insists the bridge is safe, and closing the far-right lane to traffic is being done out of an abundance of caution.

The SR-99 Aurora Bridge is about 90 years old. While it’s in no danger of collapsing because of this past weekend’s discovery, the old structure is starting to show its age.

“The bridge is still safe, I want to stress that, the bridge is still safe for people to use,” said WSDOT spokesperson Bart Treece. “This lane closure is a precautionary measure until we can get that repair underway.”

This weekend a follow-up inspection discovered that one of the steel beams that supports a section of road decking had begun to fail.

WSDOT says after years of erosion and traffic, a portion of the right hand, the southbound lane had sunk by about half an inch under the pressure.

The supporting structure - called a stringer - is partially eroding, says WSDOT engineers. There are more than 50 stringers along the bridge’s deck. The follow-up inspection showed not only has one of the stringers begun to corrode from exposure and traffic pressure, so are some of the stringer’s supports.

“If we had noticed this earlier and had been aware it was a problem, we certainly would have addressed it earlier on,” said WSDOT bridge and structure engineer Mark Gaines.

Now the challenge remains how to fix the problem from above and on one of Seattle’s busiest highways.

WSDOT says engineers are working out how to get materials and construction crews into position while minimizing the impact to traffic – but completing that job may last beyond this week and could include even more lane closures.

Also, considering the bridge’s age, WSDOT says similar repairs could become more common as the materials that make up the structure have likely outlived their intended lifespan.

“Obviously, the cost to replace this bridge would be substantial, much less the traffic impacts to do that,” said Gaines. “So, I think we’re in a position to do the best we can.”

The lane closure on the right-hand southbound lane will last at least throughout the week as WSDOT says it doesn’t know yet exactly how long repairs could take.

Plus, completing the job may take even longer and an exact timeline is still being calculated.