Seattle Swing

Robison, Rita; ,

Seattle Swing

by Rita Robison, Senior Editor; Civil Engineering, New York, NY,

Serial Information: Civil Engineering—ASCE, 1990, Vol. 60, Issue 9, Pg. 54-57

Document Type: Feature article

Abstract:

The swing bridge has long been out of favor in the U.S. because of the navigational hazard presented by mid-channel pivot piers. A consortium of engineers is updating the design to a state-of-the-art double-leaf concrete segmental structure that will move on equally state-of-the-art machinery to cross Seattle's Duwamish River. A replacement, it is placed on the existing bridge alignment to minimize right-of-way requirements and disruptions to the existing street network. The design engineers established the dimensions of each cantilever leaf according to its proximity to an adjacent high-level bridge. The bobtail spans are 173 ft and the span between the pivot piers is 480 ft from center to center. The control tower, adjacent to but separate from the west pivot pier, is 120 ft above the mean high-water mark. Because the concrete box girders have free ends rather than ends fixed to piers, control of long-term deformations was a major concern during design. There is about 30% more prestressing steel than necessary to satisfy service load stress conditions, the deck of the box girder is post-tensioned transversely and some vertical post-tensioning is included in the webs. The slewing machinery, which can rotate the leaves in 2 minutes, is located in both pier houses.

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