In the design of post-tensioned concrete members, we must consider losses of prestressing forces. The input prestressing force can't be fully applied to the structural member. Losses occur from stressing operations throughout the service life of the structure. They comprise immediate and long-term losses, i.e.
Immediate losses:
Friction
Wedge seating
Elastic shortening
Long-term losses
In our design practice, losses can be computed as a lump sum or a percentage of the initial stressing force. The following value can be used as a guideline for bonded tendon system:
for jacking force = 0.75fpu*Aps
Immediate Losses 7%
Long-term Losses 14%
To reduce friction losses, double-end stressing is recommended for tendon lengths equal to or greater than 30 m. This shall limit up to 60 m, where pour-strips are used to divide the slab or beam for ease of operation.
Another problem that can add up prestressing loss is restraints due to stiff vertical elements, including friction from the ground in slab-on-grade construction. This can be controlled by applying special release joint details and maintaining the minimum precompression stress (F/A).
Detailed Calculation
We have developed Smart Profile Editor for our partner Posteck Prestressing. The program allows the computation of detailed losses for any spans and profile configurations.
