The Tacoma Narrows’ wakes, winds and rapid currents presented bridge builders with a difficult challenge: how to keep the deck sections in one place on the water long enough for workers to hook up the cables dangling from gantry cranes.
Tacoma Narrows Constructors originally planned to pluck the sections off the delivery ships, jockeying the big transport vessels into position beneath the gantry cranes as needed.
But at 592 feet long and 106 feet wide, the ships are not easy to maneuver and secure, especially in conditions as tight and potentially treacherous as those in the Narrows.
That plan was discarded in favor of anchoring the ships near the shore and loading the deck sections onto a supercharged barge that would take them where they need to go.
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The barge has thrusters on all four corners and a computerized navigation system to keep it hovering within 3 feet of where it needs to be.
Seattle’s Foss Maritime outfitted the barge at its Ballard shipyard, acting on instructions from the Japanese bridge subcontractor, Nippon Steel and Kawada Bridge.
Together they came up with something like a flatbed truck modified to act like a sports car.
The barge is an oceangoing model named the Marmac 12, originally from New Orleans. It’s big, with an open deck 250 feet long and 72 feet wide, an area big enough to accommodate six tennis courts.
The barge is equipped with four 750-horsepower thrusters that extend 8 feet from the port and starboard sides, according to T.J. Paul, TNC’s liaison in the process.
“They’re kind of like big outboard motors,” he said.
A 30-foot mast on the barge’s deck picks up GPS signals and sends them into a computer in the barge’s onboard brain, in a control shack on the deck. The computer is loaded with an ideal GPS location for every lift.
It takes the constant stream of actual location data from the GPS signals and instructs the thrusters how to make corrections.
“We feed in raw data and it takes us to the coordinates and holds us there,” Paul said.
Operators will be able to override the automatic system and take over the controls if necessary, Paul said.
During lifts, the barge will have as many as a dozen people on board – the operator and oiler, plus a crew of NSKB technicians.
At least five ironworkers also will be on board. They will set up bearing blocks to make sure the deck sections are properly supported and strip off trusses that were attached to keep the sections stable during the trip from South Korea. They’ll also hook up the cables sent down from the gantries.
A Foss tug will be situated behind the barge at all times and will nudge it in any direction if necessary, Paul said.
TNC’s plan calls for the barge to remain in position after each deck section comes off and to wait there until the gantry crane raises the section enough to clear the 30 vertical feet of equipment on the barge.
Raising a section that high will take 20 to 30 minutes, Paul said.
TNC hopes to use the barge only in slack tides for the actual lifts, or “picks,” and only in daylight hours. That could change, Paul said, depending on how things go.
While the systems work on paper, exactly how the loaded barges will react in the Narrows is still unknown.
TNC spokeswoman Erin Hunter cautioned boaters to stay at least 500 feet away from the barge. As a precaution, she said, the gantries will be lighted with flashing strobes during lifts. The deck sections also will be lighted as they rise.