A post-tensioned tendon longer than the stroke of the available hydraulic jack must be stressed in two or more sequential pulls, with each pull's measured elongation reconciled against the theoretical value before the next pull begins [S1][S2].
For very long tendons, the elongation at the jacking end and at each intermediate pull stage is tracked against the well-known stress-strain relationship, with friction losses and anchor set accounted for at every step [S1][S2].
Why a Single Jack Stroke Is Not Enough
A standard monostrand stressing jack has a ram stroke of roughly 8 inches, and a 100-foot unbonded tendon elongates approximately 8 inches at full design load, so any tendon longer than that cannot be stressed in one pull [S5]. A documented field example shows a 130-foot unbonded tendon being stressed with a hydraulic jack of 8-inch stroke, requiring two complete strokes to reach the target force and elongation [S5]. Multi-strand jacks used on bridge and slab tendons have larger piston travels, but on long external or continuity tendons the total required elongation still routinely exceeds the available stroke, forcing the crew to stroke, set the chair, re-grip, and stroke again [S1][S8]. FHWA's Post-Tensioning Tendon Installation and Grouting Manual flags multi-pull tendons as a category where elongation records are essential because small per-stroke errors compound across the full tendon length [S1].
Elongation Math and Per-Stroke Targets
Theoretical elongation for a pull is computed as ΔL = P·L / (A·E), where P is the jacking force, L the free length, A the steel area, and E the elastic modulus of the strand or bar, and the same formula is reapplied to each partial stroke until the cumulative ΔL matches the design value [S2]. PTI Technical Note 10 frames this as the basis for verifying that the in-place tendon is behaving as designed, since measured-versus-theoretical elongation is the primary acceptance check on a working site [S2]. The Post-Tensioning Institute's "Field Procedures Manual for Unbonded Single-Strand Tendons" (Chapter 6) is the cited field reference for recording the elongation and gauge pressure after each stroke on a monostrand [S5]. For a 0.5-inch or 0.6-inch diameter 7-wire low-relaxation strand at 270 ksi ultimate, the typical jacking force is on the order of 33 kip per strand, and the per-foot elongation at full design stress works out to roughly 0.08 in/ft before losses, which is why the 8-inch ram is the limiting component for any tendon past about 100 ft [S3][S5].
How Multi-Pull Stressing Is Executed in the Field
The crew stresses to a calculated intermediate force, marks the tendon, measures the partial elongation, then de-pressurizes, sets the anchor wedges or chair against the bearing plate, repositions the jack, and re-tensions for the next stroke until the full design force and total elongation are achieved [S1][S8]. FHWA guidance treats this as a documented sequence: each pull is logged with its starting and ending gauge pressure, the corresponding elongation, and the time, so that the field engineer can reconcile the sum of partials against the single theoretical value [S1]. DYWIDAG-style multi-strand jacks allow the operator to overstress and release to compensate for friction losses along the tendon, which is a common adjustment on long draped profiles where wedge slip and seating loss are non-trivial [S8]. On long tendons, anchor set of roughly 6 mm at the active end and friction loss along the duct are deducted from the theoretical elongation before the partial-stroke targets are computed, otherwise the cumulative reading will overshoot the design by the set amount [S2][S8].
Stressing Sequence and Tendon Geometry
The general stressing sequence for one-way post-tensioned slab systems runs uniformly distributed tendons first, then beam tendons, then girder tendons, because stressing in the wrong order redistributes stress into concrete that has not yet been pre-compressed and can cause unwanted cracking at the anchorage zone [S7]. On long tendons, sequencing also governs which end is stressed first; for symmetric ducts, the crew typically stresses from one end only, but for tendons exceeding roughly 130 ft of duct length the contractor often splits the pull between both ends to keep individual jack forces and per-stroke elongations within the equipment envelope [S1][S5]. For unbonded systems, each monostrand is stressed independently because there is no grout bond to redistribute force after the fact, so a missed or under-recorded stroke on one strand is a permanent force defect, not a redistribution event [S3][S5]. Bonded multi-strand tendons tolerate small per-strand variation because the grout fixes the final force profile once set, but the jacking-end record still must reconcile to within the AASHTO / PTI tolerance band, commonly 7 percent of theoretical elongation, before the tendon is approved [S1][S2].
Where Multiple Strokes Cause Real Problems
Failure modes concentrate at the chair / wedge interface and at the elongation record itself: if a stroke is performed without re-measuring from a fixed reference, the partial elongations stack on top of an unknown starting point and the final reading cannot be reconciled against theory [S1]. Wedge seating losses of 6 to 10 mm per anchor are re-introduced on every re-grip, so the per-stroke elongation target must be increased by that set amount, otherwise the operator will keep chasing a cumulative elongation that is physically impossible to reach [S2][S8]. On long draped tendons, friction loss along the duct can consume 15 to 25 percent of the jacking force by the time the force reaches the dead end, which is why overstressing to 80 percent of ultimate followed by controlled release is a standard compensation method for the multi-pull case [S2][S8]. In unbonded monostrand slabs, an under-stroked tendon shows up later as a slab that deflects more than the design predicted, because the active compressive force is below the locked-in target; in bonded bridge tendons, the same defect is masked by grout but shows up as lower effective prestress in a lift-off test [S3][S5].
Acceptance Criteria and Documentation
Acceptance on a multi-pull tendon requires the sum of measured partial elongations to fall within 7 percent of the theoretical total, with gauge pressure reconciled at each step and a calibrated jack with a current pressure gauge chart [S1][S2]. PTI's field manual is the working reference cited by monostrand crews for the per-stroke log, while FHWA's installation manual governs the multi-strand bonded case on U.S. bridge work [S1][S5]. A complete multi-pull record includes the jack serial, gauge calibration date, strand area and modulus, duct layout, friction coefficients used, per-stroke force, per-stroke elongation, anchor set observed, and the signed-off comparison against theory, and this packet is what the engineer of record accepts before grouting or cap-casting [S1][S2]. For more on the math that drives these per-stroke targets, the pressure transmitter instrumentation that reads the jack gauge is itself a 0.1 to 0.25 percent-of-span device on a calibrated hydraulic line, and on long tendons the flow meter on the grout line later confirms that the duct is fully filled after stressing. Crews working on hydraulic power for the stressing pump routinely size against the PLC that sequences the stroke-and-set cycle, since a missed interlock is what produces an undocumented partial pull.
Trackable signals for the next reporting window: any update to PTI's "Field Procedures Manual for Unbonded Single-Strand Tendons" Chapter 6 figures, and any revision to FHWA HIF-13-026's elongation reconciliation table for multi-pull tendons.
Background reading: CSI 07 13 00 vs 07 14 00: Sheet vs Fluid-Applied Waterproofing.