Reinforced Concrete Tee Beam Bridges
Buncombe County Bridge 240 over the Swannanoa River in Asheville built by county in 1920: reinforced concrete tee beams retain marks of wooden formwork from when they were cast in place (source: NCDOT bridge inspection files)
Tee beam bridges have cast-in-place, reinforced concrete beams with integral deck sections to either side of the tops of the beams. In cross section the beams are deeper than their deck sections, which produces the T-shape that gives them their names. The primary reinforcing steel is placed longitudinally in the bottom of the beam to resist the tension on (the forces that would pull apart) the beam. The deck that forms the top part of the T-shaped beams is subject to compression (forces that squeeze or push it together). As concrete resists compression, it is concentrated in the deck along with less substantial reinforcing steel laid across the width of the bridge. The development of the tee beam bridge in the early 20th century reflected a better understanding by engineers of the forces of compression and tension within reinforced concrete bridges. This understanding allowed them to develop strong and economical tee beam bridges. The bridges were strong because the reinforcing steel and concrete were placed where they were most needed, and economical because material was not wasted.
Section of reinforced concrete tee beam with reinforcing steel shown as black circles (source: FHWA, Bridge Inspector's Reference Manual, 2012)
Tee beams were poured as a unit, regardless of how many parallel beams were required to form the bridge. They were generally used for spans 25- to 60-feet-long, but multiple spans allowed for the construction of long tee beam bridges. The no-longer-standing Tarboro Bridge (Edgecombe County Bridge 24), which was built to carry Tarrboro’s Main Street (NC 33) over the Tar River in 1931, had 10 equal-sized spans that extended a total of 490 feet.
Nationally, tee beams began appearing between 1905 and 1910 and spread rapidly in the 1910s. Tee beam bridges were first used in the North Carolina about 1910 by counties, cities, and railroads. Among the oldest surviving examples are three overpasses built by the Southern Railway from 1917 to 1919 in Bessemer City (Gaston County Bridge 165), Concord (Cabarrus County Bridge 266), and Kings Mountain (Cleveland County Bridge 426). The bridges were built as part of an improvement project important in the history of the Southern Railway—the realignment, re-grading, double-tracking, and grade-crossing improvement of the trunk line south from Washington to Atlanta.
The tee beam emerged as one of the most popular State Bridge Department designs, with standard plans first prepared in late 1919. Early prototypical examples are scattered throughout the state, many located on pristine sections of by-passed old state routes. The first standard designs consisted of three longitudinal beams, but in the late 1920s the standards were updated for wider roadways, and later examples usually consisted of four or more beams. The tee beam standards were used ubiquitously through the 1920s and 1930s and continued to be popular through the 1950s, although tee beams faced competition from steel stringer technology in the same range of span lengths. By the early 1960s the State Highway Department was phasing out the tee beam in favor of prestressed concrete beam bridges. The cast-in-place, tee beam bridges were labor intensive owing to the requisite form work and they had increasingly high labor costs. The state has 795 tee-beam highway bridges with dates of construction from 1916 to 1960.