Lynn Reeder/Tacoma Narrows Constructors
Spanning the Narrows.
Early this month, crews positioned a 592-ft vessel with the final 15 deck sections needed to complete the new 2,800-ft-long Tacoma Narrows suspension main span and two side spans. The design-build team hopes to make up lost time on the $849-million project, which was originally due for completion in April but is now slated for July.
The 5,400-ft-long crossing comprises 46 deck sections, each about 120 ft long, 78 ft wide, and 440 tons. Each section has a steel orthotropic road deck atop 26-ft-deep steel trusses. Tacoma Narrows Constructors, a local joint venture of Bechtel Infrastructure and Kiewit Pacific, in a $615-million design-build contract, will finish lifting the deck sections this winter. Bolting and welding will continue through spring 2007.
Then TNC must prepare to weatherproof the deck surface while still in the midst of the Pacific Northwest’s rainy season, due to almost three months of delays last winter. Delays included a lawsuit and work stoppage by the Korean deck section fabricator Samsung Heavy Industries, followed by the discovery of corrosion in one-third of the steel wire provided for the bridge cables. Liquidated damages start at $12,500 per day after April 2, 2007, but jump to $125,000 per day after July 1 with a $45-million cap.
Erin Hunter, TNC spokesperson, says weather conditions will be key to whether TNC meets its revised target completion date of July 1, 2007. The weatherproofing membrane can only be applied on dry days. “We’re praying for El Nino,” Hunter says.
For superstructural steel, including the deck sections and cables, TNC subcontracted to a Japanese joint venture of Nippon Steel and Kawada Industries. It also supplies construction engineering for the deck erection. The joint venture shipped the sections from South Korea on a cargo ship, now moored in a fixed location underneath the west side span. Deck sections are lifted one at a time off the ship and placed on a barge that moves them closer to their final locations on the bridge.
Lynn Reeder/Tacoma Narrows Constructors
Gantries transfer weight to temporary suspenders in an operation called "trapezing" to place some bridge segments.
Eight gantry cranes riding on the main cables lift the sections from the barge and hoist them to deck level. The gantries can move over the main cables like a train car on rails, up to about 300 ft per day, but never with an active load.
The forceful tides and high winds of the Tacoma Narrows make every lift an adventure, says Linea Laird, project manager for the Washington Dept. of Transportation. Lifts can only occur during four windows in a 24-hour period, in conjunction with slack tides and preferably in daylight. If winds reach 35 mph or currents above 3 knots, the contractor postpones lifting work.
When a section is lifted, the gantries clamp onto the main cables. The barge uses a computerized marine thruster system to compensate for the swift currents and keep it fixed at a single GPS coordinate. Each side span gantry uses a pair of caisson-mounted winches. Each winch is equipped with a 14-part block-and-tackle with four lines that pass through the tops of the towers. Each winch line is 1⁄14 the size needed to pick up the load, says Tim Moore, WashDOT senior bridge structural engineer. The gantries on the center span use a custom-designed lifting system. Pairs of gantries operate in tandem with a strand jack system, with 19 strands running through a 337-ton hydraulic jack.
Where a section’s final location is not accessible to the barge, such as near the anchorages and towers, the team must swing the section into place in a coordinated motion called “trapezing.” The gantries transfer weight to temporary suspenders, then progress along the main cable to take up the load again. The distance of each swing decreases as they move closer to the anchorages. Trapezing the first section to reach the west anchorage required 13 swings, Moore says, with the first traveling 150 ft and the last 12 ft.
After the new bridge is complete, TNC’s contract will continue for about a year to upgrade and seismically retrofit the older adjacent bridge, which has a concrete deck and steel towers.