ACI 423.10R-16 Guide to Estimating Prestress Loss

ACI 423.10R-16 Guide to Estimating Prestress Loss. 3.3—Industry practice The Post-Tensioning Institute’s manual (PTI 2006) provides a discussion of long-term losses and a detailed discussion of friction losses, which are addressed else- where in this guide, including 4.4.2. While PTI refers to Zia et al. (1979), it is noted that precise determination of losses in post-tensioned members is not critical. The discus- sion is closed with the statement, “A 100% variance in the estimate for total long-term losses generally results in less than a 10% difference in stress in the prestressing steel at nominal strength.” Kelley (2000) indicates that for approx- imating prestress losses, tendons are typically assumed to be stressed to 0.8f pu ; for ASTM A416/A416M Grade 270 (1860 MPa), this equates to a tendon stress of 216,000 psi (1490 MPa). In addition, it is typically assumed that the average tendon stress along a tendon after seating is 0.7f pu . Based on an assumed fnal effective prestress of 175,000 psi (1210 MPa), the total prestress loss is 41,000 psi (282 MPa), or 19 percent of the initial jacking stress. This results in a force of 26,800 lb (119,000 N), often rounded to 27,000 lb (120,100 N), for a single 0.5 in. (12.7 mm) diameter strand. Before low-relaxation strand gained popularity in the 1970s, use of stress-relieved strand resulted in higher losses due to relaxation of the prestressing strand. At that time, a total loss of 52,000 to 57,000 psi (359 to 393 MPa) was typi- cally assumed, resulting in a fnal effective prestress force of approximately 24,000 to 25,000 lb (106,800 to 111,200 N) per strand. The PCI Design Handbook (PCI 2010) indicates that losses have no effect on the ultimate strength of a fexural component unless the tendon is unbonded or has an effec- tive prestress of less than 0.5f pu . In addition, the handbook indicates that overestimated or underestimated losses may impact service limit states such as camber, defection, or cracking. PCI (2010) also provides an estimate of total losses for typical concrete components: (a) Normalweight concrete: 25,000 to 50,000 psi (172 to 345 MPa) (12 percent to 25 percent) (b) Sand lightweight concrete: 30,000 to 55,000 psi (207 to 379 MPa) (15 percent to 27 percent) These values are presented as total losses for typical components. Losses in terms of the percentage of an assumed initial jacking force of 0.75f pu are also...