> 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/staged-construction-fea/tendon-path-points-fea.md). # Tendon Path Points \[FEA] A Tendon Path Point is one control point on a tendon's profile. A sequence of them describes the drape of the tendon through the structure, each located **relative to a reference element and node** rather than in absolute space, so the profile follows the structure as the geometry changes. Path points are **geometry-defining**. They add nothing to stiffness or mass, but through the tendon's curvature and eccentricity they determine both the equivalent load the prestress produces and the friction losses along it. {% hint style="info" %} Curvature and eccentricity are what make prestressing work: the **curvature** between points produces the upward balancing load, and the **eccentricity** relative to the section centroid produces the prestress moment. Path points are therefore not a drawing convenience — they are the load definition. {% endhint %} **Reference Element:** The FE Line the point is positioned relative to. *Engineering influence.* This anchors the point to the structure. Because positions are relative, the profile follows the member if the model geometry changes — the reason to use path points rather than absolute coordinates. A point referencing an element the tendon does not actually pass through places the profile somewhere unintended. **Alignment:** An optional alignment the point is related to, for profiles defined by station and offset along the roadway. ## Geometry **RefNode:** Which end of the reference element the position is measured from — by default `@I`, the start node. *Engineering influence.* Sets the origin for the longitudinal coordinate. Measuring from the wrong end mirrors the point along the member, which on an asymmetric drape reverses the profile — the tendon ends up high where it should be low. On a symmetric profile the error is invisible, which is why it survives into models undetected. **RefLocY / RefLocZ:** The reference datum for the transverse and vertical offsets — by default `@COG`, the section centre of gravity. *Engineering influence.* This is what the offsets below are measured **from**, and it determines the sign and magnitude of eccentricity. Measuring from the centroid means a Z offset *is* the eccentricity; measuring from a soffit or top-fibre datum means the same typed number represents a completely different eccentricity. Because prestress moment is force times eccentricity, a datum mistake scales or reverses the prestress moment while the tendon force itself stays correct — which makes it hard to spot. **X:** The longitudinal position along the reference element, measured from RefNode. *Engineering influence.* Places the point along the member. The spacing of points controls how finely the drape is described: too few points and a parabolic profile is approximated by long chords, which misstates curvature and therefore the balancing load. Points are normally placed at the anchorages, at the low point of the drape, at the high points over supports, and at any deviator. **Y:** The transverse offset from the RefLocY datum. *Engineering influence.* Positions the tendon across the section — which web it sits in, or its position in a wide deck. Transverse position produces transverse prestress moment and governs how prestress distributes across the section; modelling all tendons on the centreline loses that entirely. **Z:** The vertical offset from the RefLocZ datum — normally the **eccentricity**. *Engineering influence.* The most influential single value on this object. Prestress moment is `P × e`, so this scales the balancing effect directly, and its **sign** decides whether the tendon hogs or sags the member: below the centroid it produces upward camber and relieves sagging moment; above it, the reverse. A sign error reverses the entire prestress effect while every force value in the model still looks correct. **R:** The radius associated with the point, for curved segments. *Engineering influence.* Controls how tight the curve through the point is. A tighter radius produces more curvature and therefore more balancing load per unit of tendon force — but also more **curvature friction loss**, since friction accumulates with total angle turned. The two effects partly offset, which is why sharpening a drape delivers less additional prestress than the geometry alone suggests. Radii must also respect the duct's minimum bend radius; a tighter one is not buildable. **Segment Type (from prev. point):** Whether the tendon runs straight or on a curve from the previous point. *Engineering influence.* A **straight** segment has no curvature and therefore produces **no balancing load** along its length — it contributes axial force and end effects only. A **curved** segment produces distributed load proportional to its curvature. Describing a genuinely draped tendon as a series of straight segments removes the distributed balancing load entirely and leaves only the end forces, substantially misrepresenting the prestress effect. **Tangent Angle:** The tangent direction of the profile at this point. *Engineering influence.* Controls the slope the tendon passes through the point with, keeping the profile smooth across segments. Discontinuous tangents produce a kink, which is both unbuildable and a source of concentrated friction loss and local force in the analysis. ## Verification * Display the tendon in 3D and compare the drape against the design drawing — low at midspan, high over supports, passing through the intended web. * Open **Prestressing Details…** on the tendon and check the force profile; sharp local drops indicate unintended kinks or excessive curvature. * Check the member's camber under prestress alone. Upward deflection confirms the eccentricity sign is right; downward usually means Z or the datum is wrong. * Confirm the eccentricity at midspan by measuring against the section centroid, not the drawing datum. * Compare the equivalent balancing load against `w = 8·P·e/L²` for a parabolic profile. * Confirm segment types are curved where the tendon is draped. * Check that radii are within the duct system's minimum bend radius. ## Common mistakes * **Eccentricity sign reversed**, producing prestress moment in the wrong direction while the tendon force reads correctly. * **A RefLocY / RefLocZ datum other than the centroid** while entering values as if they were eccentricities. * **Measuring from the wrong RefNode**, mirroring an asymmetric profile. * **Straight segments used for a draped tendon**, removing the distributed balancing load. * **Too few path points**, approximating a parabola with long chords and misstating curvature. * **Radii tighter than the duct system allows**, giving an unbuildable profile and overstated friction. * **Discontinuous tangent angles**, creating kinks with concentrated losses. * **All tendons on the centreline**, losing transverse prestress distribution. --- # Agent Instructions This documentation is published with GitBook. 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