A simply supported pretensioned beam has a span of 75 ft (22.9 m) and the cross...

Transcribed Image Text:

A simply supported pretensioned beam has a span of 75 ft (22.9 m) and the cross section shown in Figure P3.1. It is subjected to a uniform gravitational live-load intensity W =1,200 plf (17.5 kN/m) in addition to its self-weight and is prestressed with 20 stress-relieved in. dia (12.7 mm dia) 7-wire strands. Compute the total prestress losses by the step-by-step method, and compare them with the values obtained by the lump-sum method. Take the following values as given: f = 6,000 psi (41.4 MPa), normal-weight concrete fa = 4,500 psi (31 MPa) fpu = 270,000 psi (1,862 MPa) fpi = 0.70fpu Relaxation time t = 5 years e, = 19 in. (483 mm) Relative humidity RH = 75% v/S = 3.0 in. (7.62 cm) Assume SD load = 30% LL. 30 (76 cm) w. - 1200 plf (17.5 kN/m) 8" 19 in. cge 50" (127 cm) 50 cgs (127 cm) 75'-0 (22.9 m) (a) 30 (b) A simply supported pretensioned beam has a span of 75 ft (22.9 m) and the cross section shown in Figure P3.1. It is subjected to a uniform gravitational live-load intensity W =1,200 plf (17.5 kN/m) in addition to its self-weight and is prestressed with 20 stress-relieved in. dia (12.7 mm dia) 7-wire strands. Compute the total prestress losses by the step-by-step method, and compare them with the values obtained by the lump-sum method. Take the following values as given: f = 6,000 psi (41.4 MPa), normal-weight concrete fa = 4,500 psi (31 MPa) fpu = 270,000 psi (1,862 MPa) fpi = 0.70fpu Relaxation time t = 5 years e, = 19 in. (483 mm) Relative humidity RH = 75% v/S = 3.0 in. (7.62 cm) Assume SD load = 30% LL. 30 (76 cm) w. - 1200 plf (17.5 kN/m) 8" 19 in. cge 50" (127 cm) 50 cgs (127 cm) 75'-0 (22.9 m) (a) 30 (b)