Prestressed Concrete Stringer Bridges
Prestressed concrete girder leaving Durham plant on specially designed trailer truck for bridge under construction over Cape Fear River in Elizabethtown (Bladen County Bridge 17) in 1957 (source: North Carolina Roadways, March-April 1957)
After trial efforts with block slab and channel beam bridges, the State Highway Department was ready to expand its use of prestressed concrete for a variety of bridge types, including stringers. In 1957 the Department built its first major prestressed concrete bridge, which carried US 701/NC 41 over the Cape Fear River at Elizabethtown (Bladen County Bridge 17). The bridge has 98 prestressed concrete, I-shaped beams, each 110-feet-long and weighing11 tons, cast in the Durham plant of the North Carolina Concrete Products Corporation. Each concrete beam contains 26 steel cables with approximately five times the strength of ordinary reinforcing steel. Before the concrete was poured in the casting yards, the cables were stretched 34 inches by jacks exerting a force of 243 tons. Then the concrete was poured in steel forms around the cables while they were stretched. When the tension in the cables was released by cutting the cables at either end of the beam, the cables contracted, forcing the concrete together tightly and thus preventing the likelihood that the beam would crack or sag. The beams, although patterned after national standards, were still an impressive size for the state's first major effort. They were transported from Durham to Elizabethtown on a specially designed trailer truck.
The Department quickly adopted the prestressed concrete stringer technology, including it at bridges on the rapidly developing interstate highway system. By 1960 prestressed concrete stringer bridges were well on their way to becoming one of the dominant standard bridge types in the state. The Historic Bridge Inventory identified nearly 80 examples with dates of construction from 1957 to 1960. Chief state bridge engineer T. B. Gunter, Jr. reported that prestressed concrete stringer bridges had made the cast-in-place, tee-beam bridge practically obsolete. He noted that prestressed concrete allowed "standardization to be utilized to its maximum effect in girder design and details," both from the standpoint of the designing engineer and the casting company. The introduction of prestressed concrete marked one of the most significant technological changes in standardized bridge construction since the early years of the Department. The applications of prestressed concrete from the mid-1950s show important thinking about, and uses of, the new material.