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Aside from the usual complexities of bridge construction, site constraints made this job a special challenge.
The largest visible part of the "Big Dig" project in Boston, is the new cable-stayed bridge, crossing the Charles River. Named for Lenny Zakim, a local, beloved civil rights leader, and the nearby Bunker Hill Monument, which commemorates a Revolutionary War battle, the bridge incorporates elements of the monument in its design.
From the beginning, the public was involved in the bridge design. Their clear choice was a concept submitted by Swiss bridge engineer Christian Menn in 1994. The first step involved a preliminary design for the owner, the Massachusetts Highway Department (now the Massachusetts Turnpike Authority), and the Federal Highway Administration. Central Artery Management Consultants, Bechtel/Parsons Brinckerhoff, Boston, completed this phase of the design work in order to determine whether Menn's asymmetrical cable-stay bridge concept was feasible. In 1995, HNTB, Boston, in association with Figg Engineering Group, Tallahassee, Fia., won the contract to lead the final design effort. They realized that a hybrid main span, constructed with a relatively light steel frame, would be needed in order for the short south back span to properly counterbalance the main span.
According to Denney Pate, principal bridge engineer and senior vice president for Figg, the type of bridge construction used for a project is decided largely by the length of the main span. Box-girder construction works best for spans up to about 440 lineal feet. For spans between 500 and 2000 lineal feet, Figg prefers cable-stay construction. And when spans exceed 2000 lineal feet, a suspension bridge is the most economical. So, aside from aesthetic considerations, for this bridge with a main span of 745 feet, the cablestay method was the best choice.
Site constraints
Constructability was a primary concern from the very beginning because of the tight working space. An existing double-deck bridge interfered with both the design and construction on the southbound side of the bridge. Once the old bridge has been demolished, the new bridge will be completely open to traffic in January 2005. Currently it is carrying only northbound traffic. Pate explains that the back span on the south end of the bridge...