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West Seattle Swing Bridge

The West Seattle Swing Bridge carries the Spokane Street traffic across the Duwamish River. Its two 7500 ton concrete spans are the heaviest swing bridge spans in the world. While at Hamilton Engineering, Gil Lund was Principal Engineer, responsible for the design of the unique hydraulic operating system for this bridge.

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During the early bridge concept design phase it was discovered that using concrete instead of a conventional steel structure for the bridge spans could provide significant savings in the construction cost. To accomplish this, it was necessary to design a swing bearing that could handle the heavy loads of the concrete span without at the same time producing a large friction torque from the swing bearings. Antifriction rolling element bearings were considered, however the load of the concrete spans was too heavy for the capacity of any available designs. Using conventional plain bearings would have been possible. However, the high friction torque and resulting large power requirements would have increased the cost of the swing drive system and more than offset the savings possible by replacing the steel structure with a concrete design.

The selected bearing design uses conventional plain bearings to support the spans in a static position. In order to swing the spans, they are lifted off the plain bearings by 103" diameter hydraulic lift cylinders located under the bridge pivot shafts. They are then rotated around the centers of the cylinders on a cushion of pressurized oil. This arrangement significantly reduces the frictional torque and power required to open the bridge. The additional cost of the hydraulic lift cylinders compared very favorably with the savings derived from using concrete in place of steel for the span structure.

This bridge design has received many awards including the "Outstanding Civil Engineering Achievement Award of 1991".