The Zakim Bridge is part of the Central Artery/Tunnel Project and carries 10-lanes of traffic over the Charles River in Boston, MA. The bridge is unique among cable-stayed structures in several aspects. Its cable arrangement, slender inverted Y towers, and a two-lane roadway cantilevered outside of the eastern cable plane are among the bridge’s most notable features. The site constraints and the functional requirements drove the design of the bridge. The elegance and the economy of the structure are a classic testament to the effectiveness of the form-follows-function design philosophy. The 195-ft- wide, 745-ft long main span is of steel composite design with light-weight concrete used on the cantilevered ramp to reduce the eccentric dead loads. The 295-ft- and 425-ft-long asymmetric back spans are constructed of multi-cellular concrete box girders. This doubly asymmetric design was the result of the constraints of the heavily built-up site and the need to maintain the existing crossing for carrying traffic until the new bridge was operational. Sena Kumarasena, Ph.D., PE served as the Lead Engineer and the Deputy Project Manager for the final design phase and the Project Manager for the construction-phase engineering services for this landmark, award-winning, cable-stayed bridge. The cantilevered roadway produced 60% higher cable forces on the eastern cable plane than on the western. This, in turn, produced torsional moments on the tower spire that were effectively eliminated by adopting a compact, composite tower design and carefully manipulating the cable geometry. This and many other design and constructability challenges were overcome through the development of innovative design solutions. The design of this bridge also includes many first-time applications.

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