The Leonard P. Zakim Bunker Hill Memorial Bridge is a cable-stayed bridge across the Charles River in Boston, Massachusetts. It replaced the Charlestown High Bridge when it opened, and is the world's widest cable-stayed bridge. The main portion of the Zakim Bridge carries four lanes each northbound and southbound of the Interstate 93 and U.S. Two additional lanes are cantilevered outside the cables, which carry northbound traffic from the Sumner Tunnel and North End on-ramp. These lanes merge with the main highway north of the bridge. I-93 heads toward New Hampshire as the "Northern Expressway" and US 1 splits from the Interstate and travels northeast toward Massachusetts' north shore, crossing the Mystic River via the Tobin Bridge.
The bridge and connecting tunnel were built as part of the Big Dig, the largest highway construction project in the United States. Finished in 2003, the bridge's unique styling quickly became an icon for Boston, often featured in the backdrop of national news channels to establish location, and included on tourist souvenirs. The bridge is commonly referred to as the "Zakim Bridge" and as the "Bunker Hill Bridge" by residents of nearby Charlestown.
The Leverett Circle Connector Bridge was constructed in conjunction with the Zakim Bridge, allowing some traffic to bypass it. In a cable-stayed bridge, instead of hanging the roadbed from cables slung over towers, the cables run directly between the roadbed and the towers. Although cable-stayed bridges have been common in Europe since World War II, they are relatively new to North America.
The bridge concept was developed by Swiss civil engineer Christian Menn and its design was engineered by American civil engineer Ruchu Hsu with Parsons Brinckerhoff. Boston-based architect Miguel Rosales was the lead architect/urban designer and facilitated community participation during the design process. Neither Hsu nor Rosales served as the designer of record for the project. The engineer of record is HNTB/FIGG. The lead designer from HNTB was Theodore Zoli; and W. Denney Pate from FIGG. The bridge follows a new design in which two outer lanes is cantilevered outside of the wires another eight lanes run through the towers. It has a striking, graceful appearance that is meant to echo the tower of the Bunker Hill Monument, which is within view of the bridge, and the white cables evoke imagery of the rigging of the USS Constitution.
The bridge has an overall length of 436.5 m, a main span of 227.1 m, side spans of 81.4 m (downtown side) and 128.0 m (Charlestown side), and towers that are 82.3 m tall. While the highest wind gusts recorded in Boston were 110 MPH (175 km/h), the bridge is over engineered and is designed to withstand 400 MPH (640 km/h) winds.
The MBTA Orange Line tunnel lies beneath the bridge.
The roadway was not the only asymmetrical part of the bridge. The north and south towers, the spires of which are evocative of the nearby Bunker Hill Monument in Charlestown, are of differing heights (322 feet and 295 feet respectively) to account for the transition from elevated roadway to tunnel. The "pitch or transition grade between the new Central Artery ("Liberty") Tunnel and the bridge is as much as five percent. However, motorists do not notice the change in elevation: the tops of both towers are 266 feet above the roadway.
The hollow concrete towers were cast in place with walls only one to four inches thick. Steel boxes were drilled into the spires to fasten the cables into the towers. To prevent the additional weight of the cantilevered roadway section from throwing the entire bridge off-balance, the cables were shifted three inches off-center. To support the towers, workers drilled eight-foot-diameter steel shafts into bedrock. In order to avoid impact to the MBTA Orange Line and its ventilation building, the steel shafts were encased in tubing, and the legs of the towers were inverted at a 55-degree angle so as to straddle the MBTA tracks below.
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