> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/openbrimfea/loads-fea/static-loads-fea/prestressing-loads-fea.md). # Prestressing Loads \[FEA] A Prestressing Load applies the force of a **Tendon** to the structure. The Tendon object defines the geometry, material, friction and jacking force; this object decides **which analysis case** receives the resulting prestress, **which ends** are stressed, and whether elastic-shortening loss is included. Prestressing Loads **generate load**. They add nothing to stiffness or mass; the prestress reaches the structure as an equivalent load system derived from the tendon force profile and its curvature. {% hint style="info" %} The magnitude comes from the **Tendon**, not from this object. A tendon with zero jacking force produces nothing here, no matter how this load is configured. Use **Prestressing Details…** on the Tendon to see the force profile that this load will apply. {% endhint %} **Load Case:** The analysis case the prestress belongs to — normally a case classified as prestress (PS) so combinations and code checks treat it correctly. With no case assigned, the prestress is applied nowhere. *Engineering influence.* In a **staged** model, the case determines when the tendon is stressed, and therefore what structure the prestress acts on: a tendon stressed before the deck is composite acts on the bare girder and produces a very different stress distribution than the same tendon stressed afterwards. In a non-staged model this simply separates prestress from other actions so it can be factored independently. **Tendon:** The tendon whose force is applied. This is a **connectivity** parameter — it selects the geometry, profile and force that will be converted into equivalent loads. A Prestressing Load with no tendon assigned does nothing. ## Stressing **Stressing at Start:** The proportion of the tendon's jacking force applied from its start end, **entered in this sheet as a percentage**. **Stressing at End:** The same, for the end. {% hint style="danger" %} **These are proportions, not yes/no switches, and the cells are percentages.** Enter `100` (or `100%`) to stress fully from that end and `0` not to stress from it. The sheet divides what you type by 100 and displays it back with a `%` suffix, so entering `1` applies **1%** of the jacking force from that end — a 100× under-application with no error. The value multiplies the tendon's jacking force, so intermediate values express partial stressing from an end. {% endhint %} *Engineering influence, both together.* Friction loss accumulates **away from a stressing end**, so the force profile peaks at each end that is jacked and dips in between. Stressing from **one** end gives an asymmetric profile with the lowest force at the far end; stressing from **both** gives a symmetric profile whose minimum is near midlength and whose far-end force is substantially higher than one-end stressing achieves. For a long tendon this choice materially changes the prestress delivered to the middle of the member, and therefore the balancing load and the concrete stresses there. For a short tendon the difference is small. Anchor-set loss is applied at each end actually stressed, so stressing both ends introduces the seating dip at both. Setting **both to 0** leaves the tendon unstressed, which produces no prestress at all. {% hint style="warning" %} The **View Tendon Short Term Losses** preview applies `Stressing at End` to *both* ends when it builds its preview, so for a tendon with different values at each end that dialog shows a force profile that does not match the analysis. The analysis itself uses both values correctly; trust the results over the preview in that case. {% endhint %} These settings must be consistent with the Tendon's own **Jacking Method** and jacking forces — nominating an end here that carries no jacking force on the tendon delivers nothing from that end. ### Instantaneous Elastic Shortening Loss **Instantaneous Elastic Shortening Loss:** Whether the immediate loss caused by the member shortening as it is compressed is included. *Engineering influence.* When a tendon is stressed, the member shortens elastically, and that shortening relieves some of the strain in tendons already stressed. Including it **reduces** the effective prestress, most for tendons stressed early in a sequence and least for the last one stressed — the final tendon experiences no elastic-shortening loss from itself in a post-tensioned member, while a pre-tensioned strand loses immediately at transfer. Ignoring it overstates the prestress force, which is unconservative for serviceability tension checks and for camber prediction. The magnitude grows with the number of stressing operations and with how heavily the member is compressed, so it matters most in heavily prestressed members with many tendons. ## Verification * Open **Prestressing Details…** on the referenced tendon and confirm the force profile is what this load will apply, including the anchor-set dips at the ends actually stressed. * Check the member's camber under prestress alone — upward deflection confirms the eccentricity and the force are both being applied. * Confirm the concrete is in compression where the design expects it, particularly at the extreme fibre at midspan and over supports. * Compare the force at midlength between one-end and both-end stressing to confirm the setting took effect. * Toggle elastic shortening loss and confirm the effective prestress changes by a plausible percentage. * In a staged model, confirm the prestress appears in the intended stage and carries forward. * Compare the equivalent balancing load against `w = 8·P·e/L²` for a parabolic profile. ## Common mistakes * **Entering `1` in a stressing cell to mean "yes, stress this end"**, which applies 1% of the jacking force. The cells are percentages — enter `100`. * **Both stressing ends set to 0**, so the tendon is never stressed. * **No tendon assigned**, or a tendon with zero jacking force, producing no prestress while the load object exists. * **Stressing ends inconsistent with the Tendon's jacking method or forces**, delivering less than intended. * **Ignoring elastic shortening loss** in a heavily prestressed member, overstating the effective prestress and the resulting compression. * **Applying prestress in the wrong stage**, acting on a structure that is not yet composite or not yet erected. * **Assuming one-end stressing is adequate on a long tendon**, leaving the far end substantially under-stressed. * **No Load Case assigned**, so nothing is applied. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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