Page B2.2 . 17 October 2007                     
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    Tacoma Narrows Number Three

    continued

    And that's just for starters. As a "semi-submersible carrier," the Swan and her sister ships can submerge to load and discharge cargo by a float-on/float-off method, as well as roll-on/roll-off, skid-on/skid-off and lift-on/lift-off procedures or any combination of these methods. One of Swan's brethren in the Dockwise fleet, the Black Marlin, was famously used to ferry home the wounded destroyer USS Cole halfway around the planet, after the Cole had been seriously damaged by a bomb attack while in harbor in Yemen.

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    During the Tacoma Narrows work, the Swan was aided by a sophisticated positioning tug, in addition to her own main prop, powered by a 9,642-kilowatt Frederikstad engine and an additional 735-kilowatt bow thruster. In her partial-submersion work, she uses four electrically driven ballast pumps with a combined capacity of 2,800 cubic meters per hour (98,900 cubic feet per hour), plus three liquid cargo pumps with a combined capacity of 3,000 cubic meters per hour (105,900 cubic feet per hour). Working together, their pumping capacity is enough to drain an area about the size of a football field, filled two meters (6.6 feet) deep, every hour.

    New Span on the Tacoma Narrows

    The new 2007 bridge that the Dockwise Swan helped to build is the third major suspension bridge to be built across the Tacoma Narrows. But because the original suspension span failed generations ago in one of the longest, most spectacular, and best documented bridge collapses ever the new 2007 bridge is only the second span standing, neatly alongside the very similar second bridge built, which opened in 1950 and has given sturdy service ever since.

    The new bridge is the primary element of a $849 million construction project. When it opened to traffic on July 16, 2007, the newest Narrows bridge took over the eastbound traffic on Washington Route 16 heading toward the city of Tacoma, with four 11-foot- (3.4-meter-) wide lanes. At the same time, the 1950 span was converted from two-way to westbound-only traffic.

    The new bridge incorporates an up-to-date toll plaza, with discount crossing rates for electronic payers, both as a financing mechanism and for planned demand management, with the State of Washington Department of Transportation projecting slower growth of traffic volumes in the corridor due to the eastbound tariff. The last previous highway tolls in Washington State expired at the Hood Canal Bridge in 1985. The narrows tolls are projected to continue until about 2030, when the construction debt is expected to be paid off.

    Building the new bridge alongside the operating 1950 span followed the typical sequence for a suspension bridge: towers and main cable anchorages, then the main cables, suspender cables, deck frame, deck, and then finishing, which will still continue for several months, with the eventual bicycle/ pedestrian path providing construction staging until the end of work. The bridge deck frame itself is made up of 46 modular sections, averaging about 120 feet long by 78 feet wide by 30 feet high (36.6 by 23.8 by 9.1 meters), including hand railing, with a total suspended roadway weight of 26,500 U.S. tons (24,040 metric tons).

    Our photographer was in Tacoma to capture an interesting stage of the construction process, where the deck frame appears relatively quickly as the large truss modules are hoisted up and attached to the suspender cables and to each other. This primary assembly work has to be done in a balanced sequence across the main span and the two side spans so as to balance the incremental stresses throughout the process, especially on the towers and main cable connections. And because of the inherent flexibility of the suspension structure, during the construction process major elements experience very visible curvature, seeming to sag and uplift dramatically compared to their final configuration.

    Storied Crossing

    Back in 1940, the first span to make the 5,979-foot (1822-meter) overall crossing developed a wind-induced harmonic oscillation and collapsed catastrophically in undulating drama, captured in historic film, to earn a permanent station in the civil engineering textbooks. That span, created with Leon Moisseiff of New York as lead engineer, became known as Galloping Gertie, and still lies in part at the bottom of the Tacoma Narrows channel.

    Moisseiff was a leading bridge engineer, with an important role in the Manhattan Bridge in New York City, and a consulting role on the Golden Gate Bridge (behind Charles Ellis and Joseph Strauss). His theory of the capacity of long structures to accommodate flexibility meshed conveniently if fatally with administrative desires to reduce the projected construction budget for the original span across the Tacoma Narrows, and the 24-foot- (7.3-meter-) deep girders of early designs were reduced to a skinny 8 feet (2.4 meters).

    When the original bridge opened, the extreme flexibility of its span under normal traffic loads promptly became an issue of serious concern, and analysis and model testing at that point by engineering Professor F. Bert Farquharson at the University of Washington suggested the additional potential of serious problems in high winds. Minor damping modifications had been put in place, and further wind-fairing modifications were in planning when, on November 7, 1940, only four months after the bridge opened, in approximately 42-mile-per-hour (68-kilometer-per-hour) winds, the extra degrees of resonant torsional flutter tore the bridge apart.   >>>

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    The completed 2007 Tacoma Narrows Bridge, in front of its 1950 counterpart.
    Photo: Patrick S. O'Donnell Extra Large Image

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    Arriving with deck truss modules from South Korea, the Dockwise Swan and her sister ship the Teal waited briefly in the Tacoma harbor en-route to the 2007 Tacoma Narrows Bridge construction site.
    Kevin Matthews /Artifice Images

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    The Dockwise Swan was positioned below the 2007 Tacoma Narrows Bridge during construction.
    Kevin Matthews /Artifice Images Extra Large Image

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    Gantry cranes on the main suspension cables lift a prefabricated deck truss module into place, where it will be connected to the vertical suspender cables, at the 2007 Tacoma Narrows Bridge.
    Kevin Matthews /Artifice Images Extra Large Image

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    HABS/HAER elevation drawings and detailed comparative analysis of the 1940 and 1950 Tacoma Narrows Bridges.
    Image: HABS/HAER Extra Large Image

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    Detailed drawings and diagrams showing the structural collapse of the 1940 Tacoma Narrows Bridge, and subsequent design modifications to prevent the same in its 1950 replacement.
    Image: HABS/HAER Extra Large Image

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    The Dockwise Swan and its sister ship the Teal transported prefabricated deck truss segments from South Korea.
    Kevin Matthews /Artifice Images Extra Large Image

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    Early in the assembly of deck modules for the 2007 Tacoma Narrows Bridge span, uneven construction loading pulls the main suspension cables into a vee shape, noticably different from the finished catenary curve of the 1950 span behind.
    Kevin Matthews /Artifice Images Extra Large Image

     

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