Date:
Thursday, June 29, 2006
Contact:
Claudia Cornish, Tacoma Narrows Bridge Communications Manager, (253) 534-4646
TACOMA - Taking advantage of low tidal changes and currents, design-builder Tacoma Narrows Constructors relocated the SWAN from Commencement Bay to the site of the new Tacoma Narrows Bridge during pre-dawn hours this morning. The ship arrived at the bridge site around 3:30 a.m., where crews spent the next few hours mooring it under the bridge’s west side span. The SWAN will remain in its new temporary home up through August while its 16 bridge deck sections are hoisted into place.
Assembling the new bridge deck starts the final chapter of the 5½-year construction story of the new Tacoma Narrows Bridge. The entire deck-lifting process will take about four months to complete and includes three separate deliveries of deck sections to the Narrows. The deck sections will be lifted into place with gantry cranes that straddle the bridge’s two main suspension cables.
TNC plans to lift the first deck section exactly in the middle of the bridge during a slack tide period within the first two weeks of July. The remaining 45 deck sections will be lifted in an order that, to the casual observer, may appear almost random. It will be anything but. The non-linear order of lifting the deck sections is designed to maintain equal stresses on the bridge’s two towers, which will pull the towers back to a plumb vertical position. (People following the progress of the tower construction may recall that TNC used large cables to pull both tower tops back about two feet toward the shore. They did that because the weight of the new deck sections will pull the towers toward each other and back to a perfectly vertical alignment.)
While the SWAN is moored at the bridge site, the west side span will be closed to all marine traffic. Mariners are encouraged to stay informed of deck-lifting activities and the accompanying marine restrictions by visiting tacomanarrowsbridge.com. Marine restrictions will be in place not only around the SWAN itself, but also around the barge MARMAC and around gantry cranes and other equipment.
For more information on the new bridge, visit www.tacomanarrowsbridge.com.
Fun Facts - Deck Lifting and Assembly
- The average deck section weighs 450 tons and measures 120 feet long, 78 feet wide and 30 feet deep.
- The weight of the first deck section, along with the weight of the two gantry cranes lifting it into place, will cause the main suspension cables to deflect 12 feet vertically in length. This change will cause a rather dramatic change in the two suspension cables’ geometry. The weight of the first deck section will create a "v" shape at midspan that will be in place until sidespan deck sections are installed, which will act as counterweights to the middle deck sections.
- The weight of all 46 deck sections will pull the alignment of the main cables about 25 feet lower than their current, unweighted, profile. This stretching process will place the new bridge deck at the same level as the existing bridge deck.
- The new bridge deck will have only two expansion joints - one on the east side and one on the west side. Each will allow up to 30 inches of bridge deck movement. That movement can be caused by thermal (temperature) changes, seismic events, wind, and even forces generated by vehicles braking and accelerating on the bridge. That configuration is in contrast to the existing bridge deck, which has large expansion joints at each tower and smaller expansion joints every 120 feet along the length of the deck.
- The completed bridge deck is 5,400 feet long. The 46 deck sections that make up the bridge deck were preassembled in Korea before being shipped to the Narrows. The final deck sections fit within 1/8" of the initial engineering plans.
- Two different types of gantry cranes will be used to hoist the bridge deck sections into place. On the side spans, the gantries’ lifting mechanisms are winches located on the caissons. In the mid-span area, the gantry cranes lift the sections using strand jacks located on the gantries’ main girders.
- The tug and barge that will position deck sections for lifting are specially outfitted for the job. The tug has a global positioning satellite (GPS) unit to precisely navigate, and the barge has four barge thrusters to keep it exactly positioned throughout the deck lifting process.
< Back to News Home