The force the structure actually gets is the jacking force minus losses. IS 1343 quantifies short-term losses (elastic shortening, anchorage slip, friction & wobble in post-tensioning) and long-term losses (concrete creep & shrinkage, steel relaxation). The relaxation component depends on the strand class — low-relaxation strand (IS 14268) loses far less — so the assumed strand class must match the supplied tendon.
Effective prestress = jacking force minus the sum of all losses
P_eff = effective prestress forceP_jack = jacking forceΔP_* = individual short- and long-term loss components
Practical Notes
✓Relaxation is the strand-property loss — the IS 1343 budget assumes a class (usually low-relaxation, IS 14268); supplying normal-relaxation strand silently under-prestresses the member.
✓Friction/wobble loss is verified on site by the elongation reconciliation (IS 8543) — measured elongation vs computed, within ≈ ±7%.
Common Mistakes
⚠Substituting normal-relaxation strand where the loss calc assumed low-relaxation (IS 14268).
⚠Ignoring friction/wobble on long post-tensioned tendons.
⚠Using the jacking (not effective) force in deflection/crack checks.