|Two caissons (above) will support towers for new Tacoma Narrows bridge (below) parallel to existing ones. (This photo and below courtesy of Washington State Department of Transportation)|
Amid the treacherously powerful swirls of Gig Harbor currents, the massive foundations for the third incarnation of one of the worlds most recognizably titled bridges are taking shape. Steeped in history, the new $849-million Tacoma Narrows Bridge represents global efforts as construction hits critical milestones.
Tacoma Narrows Constructors, a Tacoma, Wash., based joint venture team led by Kiewit Pacific and Bechtel Infrastructure, holds a $615-million design-build contract to complete the 5,400-ft suspension bridge by 2008. It will stand 185 ft center to center from the second Tacoma Narrows Bridge, which is built on the footprint of the infamous "Galloping Gertie" that collapsed in 1940. The current bridge carries 90,000 vehicles daily, far beyond the goal of 60,000.
Since 1994, the efforts to build the new bridge have survived various legislative amendments and rulings to allow public-private financing, local input and bidding and tolls. "Theres a lot of history and baggage," says Linea Laird, project manager for the Washington State Dept. of Transportation. "But were on schedule and on budget."
Two 130-ft x 80-ft caissons anchored by cables sit within feet of the existing bridge foundations. This August, four tugboats hauled the second of the 14,000-lb caissons from the nearby Port of Tacoma to the rivers western edge.
Crews are placing concrete in 10-ft lifts at about 90 cu yd/hour to sink each caisson into about 150 ft of water and 70 ft of riverbed, says Timothy Moore, WashDOT senior structural bridge engineer. Each caisson is held in place by anchor cables attached to 32 steel plates 5 ft x 8 ft x 3 in. buried about 47 ft deep in the riverbed, which are environmentally more sound than the second bridges concrete blocks, he says.
The box-like, steel-framed caissons contain 15 dredge wells, each 22 ft x 22 ft, domes of air for flotation control and hydraulic jacks that keep pressure on the walls as the caisson sinks to prevent the walls from deflecting. A 20-ft-deep rock trench with 5-ft-thick layers of stone protects the caissons from a potential 100 ft worth of scouring. Workers used a site-scanning sonar to map out where the remnants of the old "Galloping Gertie" bridge lies on the sea floor.
The caissons will support 510-ft concrete towers for a 2,800-ft suspension main span and two side trusses. A joint venture of Japans Nippon Steel and Kawada Bridge will begin fabricating the decks 18,000 tons of steel at a Samsung shipyard in Korea, says David Climie, TNC superstructure manager. The 46 bridge "slices" will begin arriving in 2006. Each slice, about 120 ft long and 450 tons, will be placed by gantry cranes.
The new bridge is designed with lower lateral bracing that can be removed to make room for a lower roadway or rail system, says Tom Spoth, design manager for the joint venture design team of Parsons Transportation Group, Pasadena, Calif., and HNTB Corp., Kansas City, Mo. The designers hired RWDI, Ottawa, Ontario, to do extensive wind tests on a model of the bridge. Rather than a concrete deck atop a truss, the new bridges orthotropic steel deck and framing act as one with the truss structure, with no expansion joints, says Spoth.