Umi_Ryuzuki
Jun 28, 2006, 01:49 PM
Bridge crew had wrong height
Miscalculation on existing structure blamed for mishap in the Narrows
ROB CARSON; The News Tribune
Published: June 28th, 2006 01:00 AM
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.
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."
more and Picture...
http://www.komotv.com/news/story.asp?ID=44089
Miscalculation on existing structure blamed for mishap in the Narrows
ROB CARSON; The News Tribune
Published: June 28th, 2006 01:00 AM
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.
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."
more and Picture...
http://www.komotv.com/news/story.asp?ID=44089