ICC Prestressed Concrete SI common mistakes
ICC Prestressed Concrete SI · Understand the traps and how to avoid them.
Treating the specified minimum breaking load of a strand as the force used in elongation calculations, so the computed elongation comes out low and the candidate flags an acceptable jacking operation as a failure.
Candidates conflate nominal area times ultimate tensile strength with the actual seating/tare load used in the field. They forget that elongation is computed from the jacking force after allowance for seating, friction, and slip, and that the strand's specified breaking load is a capacity value, not a stressing value.
What to do instead
Always compute elongation from the actual jacking force (target prestress force times the appropriate factors), using the strand's nominal area and modulus from the referenced standard. Keep the breaking load separate as a capacity check. When reviewing a field record, compare measured elongation to the value computed for the same jacking force, and apply the recognized tolerance band before judging acceptance.
Accepting an elongation that falls outside the recognized tolerance band without investigating, or conversely rejecting a tendon whose elongation is within tolerance but whose force gauge reading looks unusual.
Candidates memorize a single tolerance number without understanding that elongation and force must be evaluated together. They do not recognize that a deviation in one parameter while the other is acceptable usually points to friction, seating loss, or a gauge/calibration issue rather than a defective tendon.
What to do instead
Evaluate force and elongation as a pair. If elongation is outside the band, check for excessive friction, tendon misalignment, or a jacking equipment problem before rejecting. If elongation is within band, accept the tendon and document the gauge reading. Know the tolerance values from the governing standard and state them explicitly in the answer.
Confusing the roles of the prestressing steel's specified minimum yield strength and its ultimate tensile strength when judging whether a strand was overstressed during jacking.
Candidates see high jacking stresses and assume any value above yield is unacceptable, forgetting that prestressing steel is intentionally stressed into the range between yield and ultimate, with a code-limited maximum jacking stress expressed as a fraction of the specified tensile strength.
What to do instead
Learn the code maximum jacking stress as a fraction of the specified tensile strength (and the lower limit for the initial stress after short-term losses). Distinguish yield strength (a material property used for design checks) from the allowable temporary jacking stress. State the fraction and the basis (specified tensile strength) in every answer.
Describing anchorage seating loss and elastic shortening as the same phenomenon, leading to double counting or omission of one loss in a loss-calculation question.
Both reduce the effective prestress shortly after transfer, and candidates group them because they occur at similar times. They miss that seating loss is a mechanical take-up at the anchorage proportional to the wedge/seat movement, while elastic shortening is deformation of the concrete under the applied force.
What to do instead
Separate losses by mechanism and timing: instantaneous losses (anchorage seating, elastic shortening, friction) versus time-dependent losses (creep, shrinkage, relaxation). For seating loss, use the seat movement divided by tendon length and relate it to the affected length. For elastic shortening, use the modular ratio and the concrete stress at the tendon level.
Assuming a post-tensioned tendon's force is uniform along its length, ignoring wobble and curvature friction so the calculated force at the far end equals the jacking force.
Candidates apply the simple P = stress times area relationship uniformly and forget the exponential friction relationship that includes both a curvature coefficient and a wobble coefficient per unit length.
What to do instead
Use the friction loss relationship with both the curvature (angle) coefficient and the wobble (length) coefficient, and compute the force at the point of interest from the jacking force. Remember the two coefficients act together and that wobble loss accumulates with length even on straight tendons.
Specifying or accepting a concrete compressive strength at transfer that is below the minimum required for the prestress level, and then not checking the corresponding tension/compression stress limits at transfer.
Candidates focus on the 28-day design strength and overlook that transfer strength governs the allowable prestress at the time of release. They also forget that stress limits at transfer are checked against the strength at that age, not the specified 28-day value.
What to do instead
At transfer, verify the concrete strength by cylinder testing or by the specified maturity method before release, and confirm it meets the minimum required by the design. Then check the extreme fiber stresses at transfer against the transfer-age limits. Report both the strength at transfer and the stress check.
Confusing pretensioned transfer of prestress (release of the strand from the abutments) with post-tensioned stressing (jacking against the anchorage), and applying the wrong inspection steps to each.
Both are called 'stressing' in casual use, but the equipment, the force measurement method, and the inspection records differ. Candidates who memorize one procedure apply it to the other system.
What to do instead
For pretensioned members, inspect strand tensioning before casting, verify the release strength and the release sequence, and record the detensioning method. For post-tensioned members, verify the jacking force and elongation, the anchorage seating, the grouting, and the tendon profile. Keep the two checklists distinct.
Judging a prestressed member's acceptability by surface appearance alone, missing internal defects such as voids in grout, inadequate cover, or strand slippage at the anchorage.
Candidates rely on visual inspection because it is the most familiar method, and they do not know which nondestructive or destructive checks apply to grouted tendons and anchorage zones.
What to do instead
Combine visual inspection with the applicable tests: grout void detection, anchorage zone inspection, cover measurement, and documentation review of the stressing records. State which defect each test is intended to find and what acceptance criterion applies.
Ignoring the effect of storage and handling on camber and cracking, for example stacking members so that the prestress-induced camber is restrained or the lifting points are wrong.
Candidates treat handling and storage as logistics rather than structural events. They forget that a prestressed member is under permanent load from its own prestress and that support and lifting point locations affect the stresses during handling.
What to do instead
Specify support locations and lifting points from the design, avoid stacking that induces reverse moments, and check lifting stresses against the member's capacity at the lifting age. Document the storage arrangement and the lifting equipment used.
Citing a standard by a remembered clause number or an outdated edition, or asserting an inspector's authority to approve a deviation from the specified prestress without engineering review.
Candidates memorize clause numbers without the edition context, and they overstate the inspector's role, treating inspection as approval authority rather than verification and reporting.
What to do instead
Cite the standard by title and edition, and describe the requirement rather than relying on a memorized clause number. Frame the inspector's role as verifying, recording, and reporting nonconformances to the engineer of record, who holds approval authority. When a requirement is uncertain, state it as needing verification rather than asserting it.