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Ironworkers weaving the suspension wires on the current phase of the Tacoma Narrows Bridge project will be working with more than steel cable.
They'll need to have nerves of steel, too.
Working on a 12-ft.-wide steel mesh catwalk 248 ft. high, over one of the most dangerous waterways in Puget Sound, has to be a daunting task by its-self.
Crews will be working 24 hours a day in all types of weather until the wind hits 45 mph. At that point, "it's not that the catwalks move or somebody could fall," said Reg Carlson, superstructure general superintendent with TNB Constructors, Gig Harbor Wash. "It's that it would be too hard to meet the tolerances required for placing the cable."
The bridge runs parallel to an existing span which now carries traffic in both directions. The $100 million project will ease traffic congestion through the area and take care of safety problems. The new span will also include a second deck that could carry either standard vehicles or light rail.
Suspension bridges are atypical; it's been two years since one was built anywhere in the United States. "The work is similar enough to other types of bridges that we don't really need any specialized skills," Carlson said. "The deck work is similar to building a cable-stay bridge."
The cable spinning is what makes this job unusual.
The process started at the end of August, when crews started hanging the catwalks from which they would work to place the cable. Crews pulled the catwalk with a winch attached to the bridge towers, across wires that start on the shore, "similar to a rope tow," Carlson said. After the cables - more than 19,000 mi. of steel wire - are placed, the catwalks will be removed.
The cable-spinning process involves carrying the steel wires between anchorages on shore and over the towers. When the actual spinning starts, massive spools of steel wire will be secured to each anchorage and a wheel will pull individual wires off the spools.
The pencil-thin wire is pulled across the water and back, with each wire being placed parallel to the other.
The wires will be placed in groups of four during the three-month job. When the first 464 wires have been placed, they will be twisted into a single strand. A total of 19 strands, each containing 464 wires, will be compacted and wrapped to complete the main suspensions cable. A total of 8,816 individual steel wires are needed to create a 20.5-in. cable.
Tensioning the wires also serves to keep the towers in a vertical position. The weight of cables causes them to tilt inward, and the tensioning pulls them back to the shore.
The new bridge deck is 5,400 ft. in length and 2,800 ft. long in the main span. When completed, it will carry two lanes of traffic and have two 10-ft. shoulders and a 10-ft. bike/pedestrian path.
Project completion is expected in 2007. Upgrades to the existing bridge will be completed in 2008.
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