Strength of Prestressed Concrete Beams after 12 Years of Live Load

摘要

Twelve post-tensioned prestressed concrete I-beams and three hollow box beams were loaded to failure. Four I-beams were tested at 21 days; all others were tested after 10 to 12 years of live load. Live load ranged from zero to 1.5 times the design live load. The I-beams were straight, 42 feet long, and 24 inches deep. Five I-beams were post-tensioned with ungrouted cables. Seven I-beams were post-tensioned with steel bars; four were grouted. The hollow box beams were gable-shaped with straight tendons. The box beams were 42 feet long, 33 inches deep, 18 inches wide, and post-tensioned with ungrouted, stranded-wire cables. In general, the flexural stiffness of all beams was less than the stiffness computed from the ACl code. The ultimate moment capacity of all beams was greater than values computed from the ACl code; the ultimate moment capacity was 13% greater for I-beams with bar tendons; 14% greater for I-beams with cable tendons, and 11% greater for the box beams. The difference was least 7% for beams with bonded tendons. The average ratio of clear span to collapse deflection for all I-beams was 64; the ratio was 52 for the box beams. The average factor of safety against collapse was 2.46 for the I-beams and 2.05 for the box beams. Prestress loss, type of tendon, time variation of concrete strength, and live-load history did not significantly affect the ultimate moment capacity of the beams. (Author)