> 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/geometry-fea/tendons-fea.md). # Tendons \[FEA] A Tendon defines a prestressing strand group or bar — its path through the concrete, its material and friction properties, and the force it is jacked to. From these the program computes the force profile along the tendon after friction, wobble and anchor-set losses, and applies the resulting equivalent loads to the structure. A Tendon is **load-generating** and **geometry-defining**. It does not add its own stiffness element to the model; the prestress reaches the structure as an equivalent load system distributed onto the nodes or lines you nominate. {% hint style="info" %} Right-click any cell for **Prestressing Details…**, which displays the full computed force profile along the tendon including every friction and anchor-set loss. That dialog is the primary verification tool for this object — it turns the parameters below into the number the structure actually receives. {% endhint %} ## Geometry **Path Variation:** The tendon's profile through the member — the shape the strand follows between its anchorages. Right-click for **Path Points…** to edit control points along the profile, or **Path Equation…** to define it with an expression. *Engineering influence.* The profile is what makes prestressing work: the **curvature** of the tendon, combined with its force, produces the upward equivalent load that balances applied load, and the **eccentricity** relative to the section centroid produces the moment. A tendon draped low at midspan and high over supports generates hogging where it is needed; a straight tendon at the centroid generates axial compression and no balancing load at all. Changing the profile changes both the equivalent load pattern and the friction losses, because friction accumulates with the total angle turned. Define together with the start/end stations and offsets, which anchor the profile in space. Verify by displaying the tendon in 3D and confirming the drape looks like the design drawing. **Start Station / End Station:** The stations along the alignment where the tendon begins and ends. These place the tendon longitudinally, define its length, and therefore determine which members it prestresses. *Engineering influence.* A tendon that stops short of its intended anchorage leaves the end region unprestressed — usually visible as tension at a location the design assumed was in compression. Stations also determine which construction stage the tendon can be stressed in, since the members it crosses must exist by then. **Start Offset / End Offset:** Transverse offsets from the alignment at each end. Used to place a tendon in the correct web or at the correct transverse position in a wide deck. *Engineering influence.* Transverse position controls the **transverse** distribution of prestress. A tendon offset into one web prestresses that web and produces transverse moment across the deck; the effect is missed entirely if all tendons are modelled on the centreline. **Start Elevation / End Elevation:** Vertical offsets at each end, which together with the path variation set the tendon's eccentricity relative to the section centroid. *Engineering influence.* This is the single most influential geometric quantity for a prestressed member. Eccentricity multiplies the tendon force into a moment, so a small error in elevation produces a proportional error in the balancing moment. The sign matters: a tendon below the centroid produces hogging (upward) camber and relieves sagging moment; above it, the reverse. An elevation error that flips the eccentricity reverses the prestress moment entirely while still reporting a plausible tendon force. **Rotation:** Rotation angle of the tendon about its own axis, for profiles whose orientation matters. **FEGroup (Load Dist. Elems):** The group of elements the equivalent prestress load is distributed onto. This is a **connectivity** parameter and it decides *what receives the prestress*. *Engineering influence.* If the group omits members the tendon physically passes through, those members receive no prestress and will appear under-compressed. If it includes members the tendon does not cross, prestress is applied where it does not exist. This is the most common reason a correctly-defined tendon produces the wrong force distribution. **Load Dist. Elem. Type:** Whether the load is distributed onto **FENodes** (0) or **FELines** (1). *Engineering influence.* Node distribution applies discrete equivalent forces at joints; line distribution applies the equivalent load along members. Line distribution generally gives a smoother and more realistic force profile in a beam model, while node distribution suits a model whose prestressed region is represented by joints. Choosing the type that does not match how the region is meshed produces local disturbances at the application points rather than a smooth prestress field. ## Properties **Material:** The prestressing steel. Supplies the modulus used to convert strain to force and the relaxation properties used in time-dependent analysis. **Prestressing Type:** Pre-tension (0) or Post-tension (1). *Engineering influence.* This is a fundamental behavioural switch, not a label. **Pre-tensioned** strands are stressed against an external bed before the concrete is cast, so there is no duct friction and the force transfers by bond over a transfer length. **Post-tensioned** tendons are stressed after the concrete has gained strength, against the member itself, and therefore lose force to duct friction and anchor set. Selecting pre-tension makes the friction and wobble coefficients below irrelevant; selecting post-tension makes them decisive. **Post-tension Type:** Bonded or Unbonded. Only meaningful for post-tensioned tendons. *Engineering influence.* A **bonded** tendon is grouted, so it strains compatibly with the adjacent concrete and its stress increases locally as the member is loaded; a section analysis can treat it as part of the section. An **unbonded** tendon strains over its whole free length, so its stress rises much less under load, and its ultimate-strength contribution is correspondingly lower. Choosing bonded for an unbonded tendon overstates flexural capacity. **Exposure:** Internal (0) or External (1) to the concrete section. *Engineering influence.* External tendons sit outside the concrete and are connected only at deviators and anchorages, so their eccentricity does not follow the section as it deflects and they are treated as unbonded. Internal tendons follow the section. **Anchor Set Length:** The slip that occurs at the anchorage when the jack is released and the wedges seat, in length units. *Engineering influence.* Anchor set removes force from the tendon **near the stressing end**, over a length that depends on the friction present — high friction confines the loss to a short zone, low friction spreads it further. Increasing it reduces the effective prestress near the anchorage, and on a short tendon the loss can extend over its whole length and reduce force everywhere. A value of zero means no seating loss, which is unrealistic for a wedge anchorage; typical values are a few millimetres and are supplied by the anchorage manufacturer. This parameter is ignored for pre-tensioned strands. **Wobble Friction Coefficient:** Friction arising from unintentional deviation of the duct from its theoretical profile, per unit length. *Engineering influence.* Wobble loss accumulates with **length**, so it dominates on long, nominally straight tendons. Increasing it reduces the force reaching the far end. Setting it to zero models a perfectly-placed duct and overstates the force delivered at the dead end — a non-conservative assumption for the region furthest from the jack. It is ignored for pre-tensioned strands. **Curvature Friction Coefficient (1/rad):** Friction between the strand and the duct as the tendon turns, per radian of angle change. *Engineering influence.* Curvature loss accumulates with **total angle turned**, so it dominates on sharply draped or reverse-curved profiles. Increasing it reduces the force reaching the far end, and the effect compounds with the profile: making a tendon more sharply draped increases the balancing load per unit force but also increases the friction loss, so the two partly cancel. Zero models a frictionless duct. Ignored for pre-tensioned strands. Results sensitive to both friction coefficients: the force profile along the tendon, and therefore the distribution of prestress moment along the member. They do not change the jacking force itself, only what survives at each point. ## Jacking **# of Strands:** The number of strands in the tendon. **Strand Area:** The cross-sectional area of one strand. *Engineering influence, both together.* Their product is the tendon's steel area, which converts the jacking force into a stress and governs how much force the tendon can carry. Increasing either increases the available force and the axial stiffness the tendon contributes in a bonded section analysis. An inconsistent pair — total area entered as the strand area, with the count also set — overstates the steel by the count factor, which shows up as an implausibly low stress at jacking. **Jacking Method:** Where the tendon is stressed from — Start, End, Start then End, or End then Start. *Engineering influence.* Friction loss accumulates **away from the jacking end**, so the force profile is highest at the jack and falls along the tendon. Stressing from one end produces an asymmetric profile; stressing from both ends produces a symmetric profile with the minimum near midlength and substantially higher force at the far end than one-end stressing achieves. For long tendons this choice materially changes the prestress delivered to the middle of the member. The order in "Start then End" matters only in combination with anchor set, which is applied as each end is released. **Start Jacking Force / End Jacking Force:** The force applied at each end. *Engineering influence.* These set the magnitude of everything the tendon does — the balancing load, the axial compression, and the prestress moment all scale with them. Only the force at an end actually jacked is used, so entering a force at the End while jacking only from the Start has no effect. Force is usually limited by code to a fraction of the strand's ultimate strength; entering an unrealistically high value produces a model that satisfies equilibrium but a tendon that could not be stressed in practice. A value of zero produces no prestress at all — the most common reason a tendon appears to do nothing. ## Verification * Open **Prestressing Details…** and read the computed force profile. Confirm the force at the jacking end matches what you entered, that it decays in the right direction, and that the anchor-set dip is where you expect. * Check the member's camber under prestress alone. An upward deflection confirms the eccentricity sign is right; downward usually means the tendon is on the wrong side of the centroid. * Confirm the concrete is in compression where the design intends, particularly at the extreme fibre at midspan and over supports. * Display the tendon in 3D and compare its drape against the drawing. * Check that every member the tendon crosses is in the load-distribution group. * Compare the equivalent balancing load against a hand calculation: for a parabolic profile, `w = 8·P·e/L²`. * Run with friction coefficients set to zero and compare; the difference is the total friction loss, which should be a plausible percentage of the jacking force. * In a staged model, confirm the tendon is stressed in the intended stage and that its force carries forward correctly. ## Common mistakes * **Zero jacking force**, so the tendon exists geometrically and prestresses nothing. * **Eccentricity sign reversed**, producing prestress moment in the wrong direction while the tendon force itself looks correct. * **Omitting members from the load-distribution group**, leaving parts of the member unprestressed. * **Leaving friction and wobble at zero for a post-tensioned tendon**, overstating force at the dead end. * **Selecting Bonded for an unbonded tendon**, overstating ultimate flexural capacity. * **Entering the total strand area as the Strand Area** while also setting the strand count, multiplying the steel area by the count. * **Stressing from one end on a long tendon** and not checking the force that survives at the far end. * **Stressing the tendon in a stage before the concrete it acts on exists**, or before it has gained the strength the design assumes. --- # Agent Instructions This documentation is published with GitBook. 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