The ship loaded with deck sections for the new Tacoma Narrows bridge hit the old bridge Friday for the simplest imaginable reason: Engineers at Tacoma Narrows Constructors got the height of the underside of the old bridge wrong.
“It was an error in a calculation,” Linea Laird, the state’s project manager, said Tuesday. “An elevation was wrong. It was based on an erroneous assumption, and it just got carried on through the process.”
Fixing the problem will not be as easy as simply adding more ballast to the ship and trying again, said Laird, who oversees the $849 million Tacoma Narrows project for the state Department of Transportation.
“The impact is significant,” she said.
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Because the elevation error was used in all calculations for the complex 41/2-month deck-lifting process, everything has to be rethought, from the timing of lifts to the length of anchor chains used to keep the ship in position.
“A lot of things have to be reconsidered and re-evaluated,” Laird said. “There’s no sense in being short-sighted at this point. The important thing is to get it right.”
The mistake is an excruciating public embarrassment for TNC, the Bechtel-Kiewit partnership building the bridge.
Throughout the 31/2 years of construction so far, the company has prided itself on its precise calculations and safety record, measuring tolerances in fractions of inches even with components weighing hundreds of tons. It has gone to great lengths to avoid negative publicity.
So far, the bridge builder has offered no public explanation for the incident. TNC spokeswoman Erin Hunter responded to inquiries Tuesday with a short, e-mailed response: “The evaluation of Friday’s incident is still ongoing and we have no final conclusions to share with you at this time.”
The mistake happened early in the engineering process when someone came up with the wrong figure for the amount of clearance between the water and the underside of the bridge, Laird said.
The bridge builders thought the ship – with 16 deck sections stacked 135 feet above the water level – would glide under the sidespan of the old bridge with 36 feet to spare, even at a 12.4-foot tide, one of the year’s highest.
Instead, temporary scaffolding on top of the deck sections hit the underside of the bridge and toppled over with a resounding boom.
No one was hurt, and, according to Laird, there was no damage to the existing bridge.
Laird said she does not expect the setback to further delay the bridge project, which already is three months behind schedule.
The loaded ship was to have been anchored in the Narrows for up to two weeks before the lifting process began, she said, so in all likelihood, the only impact will be that the ship will not have to sit idle for as long.
“What’s critical right now is getting the gantry cranes ready to go,” she said. “That’s what on the critical path.”
The gantry cranes, which ride on the new bridge’s main suspension cables, lift the deck sections, some of which weigh more than 500 tons, off the ship.
TNC has so far not changed its estimate that the first deck section will be in place by mid-July. No date has yet been set for the second attempt to move the ship into the Narrows.
Laird said the mishap has not shaken her confidence in TNC.
“They have years of experience at this, and this hasn’t changed my opinion a bit,” she said. “It was a very simple mistake and they’ll do what’s necessary to fix it.”
Rob Carson: 253-597-8693