Figure 4.3-2 Conceptual Profile Alternative Massachusetts Department of Transportation 4-6
Figure 4.3-3 Profile Figure 4.3-4 Typical Cross Section Massachusetts Department of Transportation 4-7
Figure 4.3-5 Bascule Bridge Elevation 4.3.2.1 Bascule Bridge A bascule bridge consists of cantilevered leafs that rotate on a horizontal axis at a trunnion located on a pier (see Figure 4.3-5). The leafs are balanced by counterweights that swing down as the bridge opens. The proposed bascule bridge would have a distance of at least 315 feet from trunnion-to-trunnion to clear a channel width of 225 feet. The structure at Leaf the trunnion would be nearly 30 feet deep, and the movable span would weigh approximately sixteen million pounds. Opening the bridge would require an estimated 800 horsepower to achieve a two minute and 30 second opening time for a total cycle time of 13 minutes. These are near record-setting dimensions and properties. If built, the Fore River bascule bridge would be one of the largest in the U.S. Trunnion The bascule span would have a four-leaf arrangement. The movable span would consist of two parallel double-leaf bascules in a deck truss configuration. The approximate angle of opening would be 70 degrees. The length of the truss itself from toe to tail would be approximately 215 feet for each leaf. The trusses would range from an eight-foot depth at the toe to a 30-foot depth at the trunnion centerline. The total width of the bascule span would be 73 feet, 9 inches. Due to the size of the movable span and the necessary counterweight arm, the bascule pier would need to be constructed in a closed pit configuration, as the counterweight would swing down well below the mean high water line but could not be submerged. This closed pit pier would support the control house, electrical equipment, and generator room. In addition, the pier would support live load bearing on the front wall and the live load anchorage in the back wall. Also supported on the back wall of the bascule pier would be the approach span superstructure. Each massive pier structure would occupy an area of approximately 9,000 square feet of the river. The proposed bascule alternative spanning 225-foot navigation channel the absolute minimum that the USCG will accept also presents a potential for vessel allisions with the bridge superstructure itself when the bridge is in the open position. If a Panamax vessel were to strike the fender system at an angle of less than 15 degrees, the bow may arch over the fender system and strike the bridge superstructure when the vessel is unloaded. Increasing the distance between the centerlines of trunnions would alleviate this potential for allisions, but it would require a substantial and unacceptable increase in span length. Massachusetts Department of Transportation 4-8
Figure 4.3-6 Bascule Bridge Proposed Roadway Surface Construction of the bascule bridge span would most likely be performed onsite. Even if large sections were constructed off-site and barged into place to accelerate the construction process, the deck construction would have to occur after the leafs were installed. Deck construction must occur with the leafs in a horizontal position, which leaves the movable span closed to navigation traffic. As an example, a five-month restriction of navigation in the Taunton River began in April 2010 when the channel was closed to vessels requiring more than 60 feet of air draft because the Brightman Street bascule bridge deck had to be constructed in the horizontal position. 2 Construction of a bascule bridge at the Fore River site would require a similar extended channel closure that would not be acceptable to the USCG or the users of the navigation channel. Figure 4.3-6 depicts the roadway surface elevation for the bascule bridge evaluated in this (EA) compared to the roadway surface elevation for the 1936 bascule bridge. Figures 4.3-7 to 4.3-10 provide a rendering of the bascule bridge s appearance at the site in both the open and closed positions. 2 Local Notice to Mariners, District 1, Week 28/10, U.S. Department of Homeland Security, United States Coast Guard, http://www.navcen.uscg.gov/pdf/lnms/lnm01282010.pdf. Massachusetts Department of Transportation 4-9
Fore River Bridge Replacement Project Figure 4.3-7 Figure 4.3-8 Bascule Bridge Closed Position, Looking North Bascule Bridge Open Position, Looking North Massachusetts Department of Transportation Figure 4.3-9 Figure 4.3-10 Bascule Bridge Closed Position, Looking East Bascule Bridge Open Position, Looking East 4-10