# Effect of Prestress Levels and Jacking Methods on Friction Losses in Curved Prestressed Tendons
## 4 Test Results and Discussion
### Normalized Friction Loss  
Figure 7 shows the measured friction losses normalized by those calculated using the model of Equation (4). The friction losses in the prestressing tendon continuously decreased as the prestress level was increased. Furthermore, the difference between the measured and theoretical friction losses tended to decrease from a certain prestress level regardless of the profile of the prestressing tendon. These prestress levels were 24% and 18% for tendon 3 in the GBE girder and tendon 1 in the GOE girder, respectively. However, the decrease in the prestress levels in tendon 3 was proportional to the decrease in the normalized friction losses when the prestress level was 24% or higher. The normalized friction losses in tendon 1 converged to one value, regardless of the prestress level, when the prestress was 18% or higher. These results were probably greatly influenced by the profiles of the prestressing tendon. Tendon 1 had a greater curvature compared with tendon 3. This greater curvature probably caused and maintained greater friction effects (curvature and wobble) between the prestressing tendon and sheath. On the other hand, the prestressing tendons in the GBE girder were arranged almost linearly, which resulted in very small friction effects. These friction effects probably disappear if a prestress force above a certain level is applied to the tendon.  
As shown in Figure 7 and Table 9, the lower the prestress level was, the greater the friction loss became, regardless of the prestressing method. At the time of jacking, the contact surface between the tendons is very rough. The lower the prestress level is, the less smooth this contact surface becomes and vice versa. Therefore, a greater friction loss must have occurred at a lower prestress level because the contact surface was less smooth.  
Figure 7: Effect of prestress levels on normalized friction losses.  
In PSC structures, the friction caused by the interaction between a tendon and a sheath during tension has a crucial effect on the prestress force and the elongation of the tendon. In particular, the underestimation or overestimation of the friction coefficient can cause unexpected structural behavior in relation to the camber, deflection, and stress distribution. Structural engineers obtain the friction coefficient from friction loss models specified in various design standards to evaluate the friction between a prestressing tendon and a sheath. Therefore, the accuracy of a friction loss model leads to an accurate decrease of prestress force caused by friction loss. A successful design requires accurate prediction of the prestress loss by friction. In this study, the friction losses were predicted using the theoretical model in Equation (4), and the predicted values were compared with the experiment results. The most important finding in this study is that the friction loss model provided a theoretical value that is very close to the experimental value at a high prestress level, but the theoretical values showed greater errors compared to the experimental values at the lower prestress levels. Therefore, the designer should take special care when applying a low prestress level to PSC structures.