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Why did the ship hit the bridge? Math error

A ship carrying deck sections for the new Tacoma Narrows bridge hit the old bridge last week because engineers miscalculated the height of the underside of the old bridge, the state's project manager says.

The ship had 16 deck sections stacked 135 feet above the water level. The bridge builders thought it would glide under the sidespan of the old bridge with 36 feet to spare. But temporary scaffolding on top of the deck sections hit the underside of the bridge and toppled over on Friday.

No one was hurt and there was no damage to the existing bridge, said Linea Laird, project manager for the state Department of Transportation.

"It was an error in a calculation," she said Tuesday. "An elevation was wrong. It was based on an erroneous assumption, and it just got carried on through the process."

Laird said that because the elevation error was used in all calculations for the complex 4 1/2-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," she said. "There's no sense in being shortsighted at this point. The important thing is to get it right."

Tacoma Narrows Constructors, the bridge's builder, has not commented except to say that the accident was still being evaluated.

Laird said she does not expect the setback to further delay the $849 million bridge project, which already is three months behind schedule.

In May, the bridge builders told the state the opening would be delayed until at least July 2007.

The delay will cost the bridge builder significant fines. The contract calls for penalties of $12,500 a day for the first 90 days the bridge is late. If the bridge is not open by July 2, 2007, the penalty will jump to $125,000 a day, up to a maximum of $45 million.

When the new bridge opens, all traffic will temporarily be shifted to the new bridge so retrofitting work can begin on the existing 1950 span. There is 11 months of work to do on the existing bridge, said Claudia Cornish, a spokeswoman for the bridge project.

The company is contracted to complete the entire project by Feb. 26, 2008. Missing that deadline would trigger more penalties.

A mate of mine in the UK is a Quantity Surveyor and he mucked up some measurements on a bridge for a new motorway by 11mm....cost £6million to put right..!

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