Prestress losses and elongation calculation

Immediate losses

• Friction and wobble: the force drops along the tendon because of curvature (the friction coefficient μ times the angle change) and unintended duct deviation (the wobble coefficient K times length). The coefficients come from the PT system data and depend on whether the tendon is unbonded monostrand or bonded multistrand.
• Anchor set (wedge draw-in): when the jack releases, the wedges seat and the strand slips back a few millimetres. The loss is greatest at the anchorage and fades over a length set by the friction profile.
• Elastic shortening: the concrete shortens as tendons are stressed, which reduces the force in tendons stressed earlier.

Long-term losses

Over time, creep and shrinkage of the concrete and relaxation of the low-relaxation strand reduce the effective prestress further. ACI 318 and PTI guidance allow a detailed time-step calculation or, for typical building slabs, a lump-sum estimate. The effective force after all losses is the value used for load balancing and service stress checks; the design software (we use RAM Concept) reports it along each tendon.

Calculated elongation

Elongation is the strand extension at the jack. It is found from the average force along the stressed length:

Δ = P_avg × L / (A_ps × E_ps)

where P_avg is the average jacking-end force after friction, L the stressed length, A_ps the strand area and E_ps the strand modulus from the mill certificate. Corrections are made for the length inside the jack and for anchor set. Tendons stressed from both ends are split at the point of zero movement.

On site: measured vs calculated

Measured elongations are compared with calculated values within the tolerance in the specification; see PT stressing elongation tolerances and records. Where results fall outside, a lift-off test can confirm the force held in a tendon. Our stressing support covers the calculations, the stressing sequence and the record review.