Swing Span Bridges

No-longer-extant Core Creek Bridge (1935), NC 101 over Intracoastal Waterway, Core Creek, Carteret County: center-bearing swing span bridge in process of turning on central pivot (Source: Library of Congress, Prints & Photographs Division, HAER, Reproduction number HAER NC-47-8) No-longer-extant Core Creek Bridge (1935), NC 101 over Intracoastal Waterway, Core Creek, Carteret County: center-bearing swing span bridge in process of turning on central pivot (Source: Library of Congress, Prints & Photographs Division, HAER, Reproduction number HAER NC-47-8)

A swing span bridge rotates in a horizontal plane around a vertical axis into a position parallel with the marine channel and shore, to allow boats and ships to pass along a waterway that would otherwise be obstructed by the bridge. When in operation, the movable span is supported by one of two methods: center-bearing on a vertical pin or pivot, or rim-bearing on a circular girder called a drum that moves on rollers. The most common swing span design is the lighter and more easily operated and maintained center-bearing design with the pivot set on a pier, known as the pivot pier. The rim-bearing design was less common and used for wider and heavier swing-span bridges. The superstructure of swing span bridges can be trusses, stringers, or girder-and-floorbeam. The choice of superstructure almost always reflects the prevailing practices of fixed bridge construction, with the specific type and design matched to the length and capacity needed at the crossing.

No-longer-extant Core Creek in open position (Source: HAER NC-47-9) No-longer-extant Core Creek in open position (Source: HAER NC-47-9)

Swing span bridges are rotated by a series of gear sets and a rack-and-pinion drive. Operator’s houses and equipment houses with the controls and machinery are located adjacent to or on the bridge, depending on its size. North Carolina's examples are electrically operated, although they often have gasoline or diesel motor, or even hand-cranked, back-up systems.

All of North Carolina's surviving swing-span bridges are later examples of the well-established technology. The state's oldest complete swing-span is the bridge carrying US 17 Business over the Perquimans River at Hertford (Perquimans County Bridge 8), built in 1929.

[June 2013]