A bascule bridge rotates in a vertical plane (up and down) around a horizontal axis, much like a seesaw. Although the bascule had been in use in Europe for more than five centuries, it was not until the 1890s that engineers successfully developed innovative bascule designs that produced a balanced system, making large bascule bridges easy to raise and lower. In comparison to swing span bridges, bascules operate quickly. They also do not require a clear turning radius that would prohibit the construction of docks adjacent to the bridge site, a particular concern at crowded waterfronts.
North Carolina's oldest historic bascule is the South Mills Bridge that carries US 17 Business over the Dismal Swamp Canal (Camden County Bridge 14). Built in 1934, the bridge is a type of bascule known as a rolling lift, which has a moveable leaf that rolls back on segmental girders. Its bascule "walks back" just a few feet which, though barely perceptible, provides just that much additional clear opening for navigation. The bridge is owned by the US Army Corps of Engineers, which built it as part of a mid-1930s project to widen and deepen the canal.
The rolling lift design was eventually eclipsed m popularity by the simple trunnion bascule, a design first advocated by the City Engineering Department of Chicago in 1902 that then spread to other parts of the country. A horizontal steel pivot, called a trunnion, supports the entire weight of the bridge when it is in operation or in the open position. A massive counterweight, which serves to balance the long end with the shorter end, is attached at the heel. Significant simple trunnion bascule highway bridges in North Carolina include the C. Heide Trask Memorial Bridge on US 74/US 76 over the Intracoastal Waterway between Wrightsville Beach and Wilmington (New Hanover County Bridge 12), built in 1956, and the Grayden Paul Bridge carrying US 70 over Beaufort Channel in Beaufort (Carteret County Bridge 29), built in 1957. Both bridges employ a Hopkins Drive system, an ingenious refinement that simplified the erection and alignment of high-speed operating machinery.