The jacking method will be determined based on several factors such as the number of strands, the shape of the post-tensioning duct and the profile shape of tendons. there are two methods used to tension prestressing steel.
Single (Mono) Strand Stressing
mono strands stressing is performed by stressing each strand individually. this method will be suitable for a limited number of strands. usually it is used in stressing building slabs. the strands will be placed side by side inside a flat-oval duct. therefore stressing strands one by one will not cause trapping of subsequent strands.
This method can be used for multi-strand longitudinal strands only if there is no curvature in the tendon profile. the designer should consider the loss of the prestressing force due to elastic shortening. the loss of prestressing force should be considered for all subsequent strands and the stresses loss will increase by the increase of tensioned strands number.
- Single Strand, Single End, and Alternate End Stressing
single end stressing will be sufficient for short tendons. the friction losses for short tendons will be small. therefore no need to go for both end stressing.
alternative end stressing is used to maintain a uniform and even dispersal of post-tensioning force. the alternative end stressing methods performed by stressing all the strands of one tendon from one end then stressing the strands of next tendon from the other end and so on. in special cases, the strands of one tendon can be stressed alternatively from different ends.
- Single Strand, Two-End Stressing
the friction losses depend on the length and angular change of post-tensioning tension profile. for long tendons two-end stressing is used to reduce the friction losses. in this method, the same strand tensioned from both ends. the tensioning can be performed simultaneously using two jacks or the strands will be tensioned at one end then from the second end. Multi-strands jackes are used widely to stress longitudinal tendons. in this method all strands for a single tendon is tensioned at one time used multi-strand jack.
- Multi-Strand, Single End and Alternate End Stressing
single end stressing are referred to tendons that are stressed at one end only. tendons are usually similar and number and symmetrical in the superstructure. therefore alternative end stressing are used similarly as in single strands to improve the stress distribution of the post-tensioning method. The location of applying stress is switched from one end to another in order to apply even post-tensioning force at both ends. in the figure number 2 the tendion T1 is stressed first at east side of superstructure. then T2 and T3 stressed at the west side of the structure. finally T4 is stressed at the east side of the structure. The sequence of the post-tensioning should result in a symmetrical distribution of prestressing force to both ends of the structure.
- Multi-Strand, Two-End Stressing
Two-End stressing used for long tendons. the friction and woble losses is considerably large for long tendons. therefore two end stressing are used to reduce the losses of prestressing force due to friction. Two end stressing can be performed using two methods. first method is by stressing the tendon at one end. Then stressing the tendon from the other end. the total elongation will be the summation of elongation from both ends. The second method two jacks are used. the prestressing force is applied simultaneously at both ends. Each jack will pull approximately half of the elongation. Bar tendons have either fine or coarse threading. bar tendons are anchorded by nut to anchor plate. bar tendons are stressing using special jack as shown in figure 3. Curve bar tendons are rarely used. straight bar tendons are commonly used with a space around the tendon there will be no friction losses. also when the nut are gradually tighten around the bar during the jacking process there will be no or little losses due to anchore seat. The sequence in which PT bars are stressed should be clearly shown on the Contract Plans or approved shop drawings.