Seattle Swings Again

by Rita Robison, Contributing Editor; Civil Engineering, 345 E. 47th St., New York, NY 10017,

Serial Information: Civil Engineering—ASCE, 1992, Vol. 62, Issue 7, Pg. 46-49

Document Type: Feature article

Abstract: The 1992 Outstanding Civil Engineering Achievement Award went to a bridge that the experts said couldn't be built—a concrete segmental box girder swing bridge with twin leaves that weigh 7,500 tons each. Constructed as an industrial workhorse to divert truck traffic from an adjacent graceful high level span, the $33.5 million West Seattle Low-Level Swing Bridge has leaves that pivot with a 173.5 ft tail span and a 240 ft channel span, clearing the channel by 55 ft. The span between the pivot piers is 480 ft. The unique design—this is the only hydraulically operated, double-leaf concrete swing bridge in the world—and unprecedented weight made normal mechanical components impractical. A 9 ft diameter hydraulic cylinder lifts the bridge off its stationary service bearings and provides an oil bearing for rotation. Hydraulic cylinders propel the leaves, drive and retract the center and tail locks. Operation is by computer, which the human operator can override in emergencies. To make the bridge earthquake resistant, the 36 in. foundation piles are surrounded by 48 in. diameter steel pipes that create a center of rigidity at the same location as the center of their mass. Provisions have also been made for safe access by inspectors and maintenance personnel, and replacement of all equipment.

Subject Headings: ASCE outstanding civil engineering | Awards | Concrete | Girder bridges | Hydraulics | Washington |

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