The 26 utility poles that collapsed in Tukwila in April, crushing a car and sending two people to the hospital, toppled because several of the poles had rotten cores and were significantly weaker than when they were installed, an outside investigation concluded.

Six poles at the center of the collapse had “advanced internal decay,” the investigator hired by the Seattle City Attorney’s Office concluded, causing them to snap in high winds. Three of those poles were damaged internally by beetles, likely meaning they were not properly treated by the manufacturer, the report found.

The two poles that fell first, causing the rest to tumble like a 60-foot tall string of dominoes, had “a great deal of rot,” Debra Smith, CEO of Seattle City Light, said in an interview.

Seattle City Light, which owns the poles, said it will now revamp how it inspects and rates its 91,000 utility poles and will move as quickly as possible to replace 6,000 poles throughout the region that have the lowest strength ratings. Smith described a kind of “triage process” as it moves as to replace those 6,000 poles efficiently without unnecessary disruptions.

The utility still hasn’t determined how long it will take to replace the poles, but it will likely be years. Seattle City Light replaced about 11,300 poles from 2010 through 2018.

The Tukwila Police Department released video of power lines striking a car in Tukwila on Friday, April 5, 2019. A couple in the car suffered minor injuries.

The utility aims to inspect each of its poles every 10 years and poles should generally last 40 to 60 years. The two poles that first collapsed were around 25 years old, City Light said. Poles are rated on a scale of P1 to P5, with P1 meaning immediate replacement is needed and P5 meaning no maintenance is required.

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The six poles near the center of the string that collapsed along East Marginal Way in April were all rated P2 when they were last inspected in 2016, meaning they required replacement “within a practical time frame.” But that time frame is never defined, something that the outside investigator dinged the city for.

“‘Practical time frame’ is completely vague and provides no level of urgency,” Nelson Bingel, the investigator and the chairman of the National Electrical Safety Code, wrote in his report, which was completed last week. He also faulted the city for the broad category of poles that can all earn the same rating. A pole rated P2 could be anywhere from 75% as strong as when it was first installed down to 25%.

City Light, Bingel wrote, needs “tighter remaining strength ranges and more specific requirements for replacement or restoration.”

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The city has about 6,000 poles rated P3, or lower, meaning maintenance is required. Those poles are not concentrated in any one area, City Light said. The utility said it is starting an emergency contracting process to speed up the replacement of those poles. It won’t be cheap. Replacing an average utility pole costs around $13,000, City Light said, yielding a rough estimate of nearly $80 million to replace those 6,000 poles.

Smith said the utility will not seek rate increases to do the work, but will instead delay or put off other capital projects.

The string of power poles fell shortly before 4 p.m. on April 5 in front of the Museum of Flight. Right around that time, nearby Boeing Field measured gusts as high as 29 mph, according to Bingel’s report. But weather conditions resembled a “wet microburst” Bingel wrote, and he estimated an upper bound for gusts at the site of the collapse at 50 mph.

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The report found that all the poles exceeded design requirements when they were installed and, if they were in pristine condition, should have been able to withstand the winds. But they were not.

One of the poles crushed the car of Tom and Linda Cook, 64 and 71, respectively. The couple was trapped in their car for an hour, while first responders worked to shut off power to the live wires that surrounded them. Both suffered bruises and Tom Cook required five stitches.

The first two poles to collapse were each rated P2 and their remaining strengths were estimated at 33% and 57%. Their collapse put pressure on the poles next to them, on both sides, causing “sequential failures” including in poles that were well-rated and showed no signs of decay.

“The poles were varying ages, they were varying ratings,” Smith said. “But once it starts, the torque on the rest of the pole line is really crazy and pulled them down.”

The two poles were part of a string of six poles, all rated P2, that had been installed between 1991 and 1995 and all came from the same manufacturer, which has since gone out of business, City Light said. Three of those six also were damaged by Golden Buprestid beetles, which only survive if a utility pole isn’t properly sterilized during manufacturing, the report found.

City Light said it has identified 360 poles around the city, rated P3 or lower, that also likely come from the same manufacturer, and it will prioritize their replacement.

“We take this situation really seriously,” Smith said. “We have been working for months to understand what happened, which is where we are now, develop a plan to resolve the immediate situation and then a path forward that is more aggressive and results in more timely replacement of poles.”