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 world’s 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. "There’s a lot of history
and baggage," says Linea Laird, project manager for the
Washington State Dept. of Transportation. "But we’re
on schedule and on budget."
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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 river’s
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 bridge’s
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 Japan’s Nippon Steel
and Kawada Bridge will begin fabricating the deck’s 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 bridge’s orthotropic steel
deck and framing act as one with the truss structure, with
no expansion joints, says Spoth.
