Prestressing [FEA]

A Prestressing entry schedules a tendon stressing operation at a specific construction stage, so the analysis reproduces the real jacking sequence and the losses that depend on it.

It uses the same underlying object as Prestressing Loads — the difference is context. In staged construction the stage decides what structure the prestress acts on and when in the timeline it is applied, both of which change the result substantially.

Prestressing entries generate load. The magnitude comes from the Tendon; this object decides when it is applied, from which ends, and whether elastic-shortening loss is included.

Staging is what makes prestress analysis realistic. A tendon stressed against a bare precast girder produces very different stresses from the same tendon stressed after the deck is composite, because the section resisting it differs. Only the stage assignment expresses that.

Stressing

Load Case: The stage's case the prestress is applied in.

Engineering influence. This places the stressing operation in the sequence, and it is the parameter that matters most on this object. Stressing a tendon in a stage before the concrete it acts on has been cast, or before it has gained the strength the design assumes, is not physically meaningful — and the analysis will not object. Stressing later than intended leaves the member unprestressed for loads applied in between, which is often the most critical condition for a precast girder.

The stage also fixes the concrete age, which determines the modulus and therefore the elastic shortening, and the starting point from which creep and relaxation accumulate.

Tendon: The tendon being stressed. With none assigned, nothing is applied.

Stressing at Start / Stressing at End: The proportion of the tendon's jacking force applied from each end in this operation, entered in this sheet as a percentage.

These are proportions, not yes/no switches. Enter 100 (or 100%) to stress fully from an end and 0 not to stress from it. The sheet divides what you type by 100, so entering 1 applies 1% of the jacking force from that end — a 100× under-application that produces no error and quietly leaves the member almost unprestressed.

Engineering influence. Friction loss accumulates away from a stressing end, so one-end stressing gives an asymmetric force profile with its minimum at the far end, and both-end stressing gives a symmetric profile with substantially more force delivered to the far end. On a long tendon this materially changes the prestress at midlength. Anchor-set loss is applied at each end actually stressed.

Intermediate values are how a staged jacking sequence is expressed — taking a tendon to part of its force in one stage and the remainder in a later one. Be clear whether your values are cumulative totals or increments.

Setting both to 0 leaves the tendon unstressed in this stage.

These must be consistent with the Tendon's own Jacking Method and jacking forces; nominating an end that carries no jacking force delivers nothing from it.

Instantaneous Elastic Shortening Loss: Whether the immediate loss from the member shortening under compression is included.

Engineering influence. In a staged, multi-tendon member this is a sequence effect: each stressing operation shortens the member and relieves strain in the tendons already stressed, so tendons stressed early lose the most and the last one stressed loses nothing from itself. Ignoring it overstates the prestress remaining, which is unconservative for serviceability tension checks and for camber. The effect grows with the number of stressing operations, so it matters most in heavily post-tensioned members.

Verification

• Step through the stages and confirm each tendon is stressed in the intended one, on a structure that exists and has gained strength.
• Open Prestressing Details… on the tendon and confirm the force profile, including the anchor-set dips at the ends actually stressed.
• Check the member's camber after each stressing stage; it should increase as tendons are stressed.
• Confirm the concrete stresses at each stage stay within the allowable limits for the age at that stage — the critical check is often at transfer, not at service.
• Compare the effective prestress at the final stage against a hand loss calculation.
• Toggle elastic shortening loss and confirm the effect is a plausible percentage.
• Confirm prestress carries forward into subsequent stages rather than being re-applied.

Common mistakes

• Stressing in a stage before the concrete exists or has gained strength, which the analysis accepts without complaint.
• Entering 1 in a stressing cell to mean "stress this end", applying 1% of the jacking force. The cells are percentages — enter 100.
• Both stressing ends set to 0, so the tendon is never stressed in that stage.
• Scheduling the same tendon in more than one stage unintentionally, applying its prestress twice.
• Ignoring elastic shortening loss in a member with several stressing operations.
• Checking stresses only at the final stage, missing the transfer condition where a precast girder is most highly stressed and least mature.
• Stressing ends inconsistent with the Tendon's jacking definition, delivering less than intended.
• Assuming one-end stressing is adequate on a long tendon, leaving the far end under-stressed.