When the new Tolt Hill Bridge was built in 2008, it was built exactly according to the plans, King County Roads officials say, only the plans were wrong.
Now, the nine-year-old, $17 million bridge is closed indefinitely — and has been since Friday evening, June 16 — while engineers determine how to make the needed repairs.
And many of the residents who rely on that bridge for their commutes filled the Tolt Middle School gym Wednesday evening at a town hall meeting on the issue, to ask how this could even have happened on a bridge that shows no signs of stress.
“It’s hard to explain,” admitted Road Engineer Rick Brater, because of the many elements of bridge maintenance and management involved but he did so. The bridge should have been built to 2005 standards but wasn’t because of a problem transferring the required specifications for steel plating and bolts from the design documents to the actual construction plans. The bridge designer, Lin and Associates of Seattle, also created the construction plans.
In a nutshell, Brater said, the 36,000 bolts connecting the horizontal bridge plating to the support structure, “are smaller in diameter, too few and not of the right strength.”
Seven-eighths-inch thick steel plating in the design was reduced to three-fourths-inch in the construction plans; one-inch bolts in the design were seven-eighths-inch, and a lower-strength steel in the plans.
The problem wasn’t caught when the bridge was completed and inspected before opening, because the bridge inspector checked the actual bridge built against the plans for it, not the original designs, Brater said at the town hall meeting.
Five years later, in 2014, the Federal Highway Administration issued updated regulations for bridge load ratings. The changes, motivated by the 2007 collapse of a steel truss bridge in Minneapolis, as well as by the heavier trucks being allowed on more roads, added a new requirement to bridge inspections, an analysis of the bolted connections of bridge plates to the supports.
“It was the connection that failed in Minneapolis, that caused the bridge to collapse,” Brater noted.
That connection was also never previously part of any bridge inspections.
“Up until this new standard we didn’t have to do any load rating that looked at those connections,” he said. “With this new standard, we do the analysis and that tells us we have no safety factor left in the bridge.”
Thus, although the bridge was inspected every two years as required by law, and has never shown any signs of damage or stress, the Tolt Hill Bridge is now closed indefinitely.
Carnation Mayor Jim Berger, attending the town hall meeting, asked Brater if the complete closure of the bridge was really necessary, saying “A lot of people are concerned that it’s an overabundance of caution.”
Thursday, Berger said he felt that the engineers made it clear the closure was necessary.
“I think the overall analysis shows that it needs to be done,” he said in a phone conversation.
“When you’ve been asked by the federal governement to verify the safety of your bridges and your initial calculations don’t even come close….”
Berger also shared the top question on most residents’ minds: Could the bridge be opened on a limited basis, to lighter-weight vehicles?
“That’s one of our first efforts,” roads engineer Larry Jaramillo told a group of residents Wednesday, although he estimated it would take a couple of weeks to figure that out.
The Tolt Hill Bridge handled about 3,000 vehicle trips a day before its closure last week.
Berger said that the city has no real say on the county owned bridge, so “the only thing we can do is honor the detour and keep their feet to the fire and make sure they do everything they can do.”
Asked how he would do that, he said, “We’ll have to stay in pretty close communication with Executive Constantine’s office, most likely through our Representative (Kathy) Lambert, she does a great job of protecting the Valley.”
The county is more than a fourth of the way through its bridge re-evaluation process to meet the 2014 federal highway standards by 2022, with about 50 of its 178 bridges inspected to date. The Tolt Hill Bridge is the only one so far to have a significant problem with design, Brater said, but some of the bridges inspected would likely have their load ratings changed.
Because of its similarity to the Mount Si bridge in North Bend, the Tolt Hill Bridge was actually inspected earlier than required, Jaramillo told his listeners. The bridge, Brater explained, had similar parameters to the one on Mount Si Road, so making the changes to the testing model after completing the Mount Si Bridge — which was fine — were easy to do at the same time.
Those results triggered the investigation into the discrepancies in the bridge construction that led to the current closure, Brater said. He estimated it would be at least two weeks before they could determine either if the bridge could be opened to lighter traffic, or the extent of the repairs needed.
“We’re really at the beginning stages of delving into what the repairs are going to be,” he said.
The detours from Carnation for drivers who regularly use the Tolt Hill Bridge are either north on S.R. 203 to NE Carnation Farm Road/NE Ames Lake Road to S.R. 202, south on S.R. 203 to Fall City and S.R. 202, or north on S.R. 203 to NE 124th Street, to NE Novelty Hill road.
For the latest information on the project, visit http://www.kingcounty.gov/depts/transportation/roads/tolt-bridge.aspx.