> 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/lines-fea.md). # Lines \[FEA] An FE Line is a one-dimensional element spanning two nodes. It is the workhorse of a bridge model — girders, columns, piles, braces, cross-frame members, stays and hangers are all FE Lines — and it carries the section and material that give it stiffness, together with the end conditions, offsets and property modifiers that tailor its behaviour. An FE Line contributes to the **stiffness matrix**, contributes **mass** (through its self-weight and material density) and can **generate load** (self-weight, and pretension where applicable). ## Geometry **Type:** The element formulation: Beam, Truss, Compression Truss, Tension Truss, Cable, or Inelastic Line. *Engineering influence.* This decides which degrees of freedom the element actually connects. **Beam** carries axial force, shear, bending and torsion — the default and the right choice for girders and columns. **Truss** carries axial force only, so it transmits no moment regardless of the end releases; use it for pin-ended bracing where modelling the release explicitly would be redundant. **Compression Truss** and **Tension Truss** are one-sided: they drop out of the structure when the force reverses, which is how slack bracing or a bearing that cannot pull is modelled. **Cable** is a tension-only member intended for stays and hangers. **Inelastic Line** enables the fibre/hinge formulation for pushover work. The one-sided types (Compression Truss, Tension Truss, Cable) are only honoured in a **nonlinear** analysis. In a linear static or modal run they behave as ordinary truss members and will happily carry the force they are supposed to shed — which is why a cable-stayed model can show compression in stays in a linear case and not in the nonlinear one. Changing Beam to Truss removes the member's bending stiffness entirely and can leave the structure a mechanism if that bending was providing stability. **Node 1 / Node 2:** The joints the element spans. These are **connectivity** parameters; with the nodes' coordinates they set the element length and its local axis direction. The order matters: Node 1 is the "start" or "i" end and Node 2 the "end" or "j" end. End releases, end offsets, and the sign convention of reported end forces are all keyed to this order, so reversing two nodes on an otherwise identical member flips the sign of its reported moments and swaps which end a release applies to. **Section at Start / Section at End:** The cross-section at each end. Setting them differently produces a **tapered** member, with properties interpolated between the two. *Engineering influence.* Section properties are the element's stiffness: axial stiffness scales with area, bending stiffness with moment of inertia, torsional stiffness with the torsion constant. A member with no section assigned has no stiffness and is one of the most common causes of a singular matrix or a wildly flexible model. Leave Section at End empty for a prismatic member. Tapering is linear between the two sections; a member whose real depth varies non-linearly should be subdivided into several elements rather than relying on a single taper. **Beta Angle:** Rotation of the element's cross-section about its own longitudinal axis, in angle units. *Engineering influence.* This orients the section's strong and weak axes relative to the global frame. It does not change the section's properties — it changes which direction each one resists. A girder with its strong axis rotated 90° is enormously more flexible vertically, which typically shows up as deflections an order of magnitude too large, or as a structure whose lowest modes are in the wrong direction. For a vertical member the default orientation is often not what you want, so this is the parameter to check first when a column behaves unexpectedly. Local axes should be displayed and checked visually rather than reasoned about from the number. **Group:** The FE Group used for stage activation and selection. A line whose group is inactive in a stage does not exist in that stage. **Pretension:** The cable jacking force, in force units, applied by a **Cable Stressing Load** as an equivalent initial-tension load. *Engineering influence.* Setting a value here does **not** by itself apply anything: it is the magnitude a Cable Stressing Load will use when it stresses this member. A cable with a pretension and no stressing load carries no prestress; a stressing load pointed at a cable with zero pretension applies exactly nothing. Increasing it increases the cable force and, in a stayed structure, lifts the deck and redistributes moment toward the anchorages. **Length (readonly) / Weight (readonly):** Derived values reporting the element's computed length and self-weight. Useful for verification — a length that is not what you expect indicates a node coordinate error, and a weight of zero indicates a missing section or a material with no density. {% hint style="warning" %} These two columns are derived and cannot be edited, but they are not flagged read-only, so typing into them appears to work and the value is discarded on redraw. {% endhint %} ## Global Offset **Node 1 Offset X / Y / Z** and **Node 2 Offset X / Y / Z:** Rigid offsets, in **global** directions, between the node and the element's actual end. *Engineering influence.* An offset inserts a rigid link between the joint and the member end, so the member behaves as though it starts at the offset position while still connecting to the node. This is how a girder is connected to a pier-top node at the correct soffit level, or how the eccentricity between a deck centroid and a girder centroid is represented without adding extra joints. Offsets introduce a **lever arm**, so an axial force in the member produces a moment at the node. That is usually the point — it is the mechanism by which composite eccentricity generates the right behaviour — but it also means an incorrect offset produces spurious moments that are hard to trace, because the member's own forces look perfectly reasonable. Zero (the default) means the member ends at the node. **Related setting:** by default a rigid end offset does **not** expand under uniform temperature, which can lock in force in a restrained member. See **End Offsets Expand Thermally** under Settings. ## Local Offset **Node 1 Offset X / Y / Z** and **Node 2 Offset X / Y / Z:** The same idea expressed in the element's **local** axes rather than global ones. *Engineering influence.* Local offsets follow the member's orientation, so they stay correct when the member is skewed, sloped or rotated — which makes them the better choice for an offset that is defined relative to the section (a soffit offset, an insertion-point offset) rather than relative to the global frame. Global offsets are appropriate when the offset is genuinely a global-direction quantity, such as a fixed vertical drop. Do not enter the same physical offset in both tabs; they add. Use one or the other. ## End Releases **Node 1 Tx / Ty / Tz / Rx / Ry / Rz** and **Node 2 Tx / Ty / Tz / Rx / Ry / Rz:** The connection condition for each degree of freedom at each end. {% hint style="danger" %} **Sign convention — the same as node supports:** Entering `-1` to "release" an end produces a **fixed, moment-carrying** connection. Use `0` to release. Right-click a cell for *Set to Free*, *Set to Fixed* and *Edit Stiffness…* rather than typing the number, which avoids the mistake entirely. {% endhint %} | Value | Meaning | | ------------ | ----------------------------------------- | | **−1** | **Rigid / fully connected** (the default) | | **0** | **Released** — a hinge in that direction | | **positive** | **Semi-rigid** spring of that stiffness | *Engineering influence.* Releasing a rotational degree of freedom converts a moment connection into a pin, which **reduces** the member's end moment to zero and redistributes it into the span and into adjacent members. Releasing makes the structure softer, increasing deflections; over-releasing creates a mechanism and a singular stiffness matrix. Releasing translations is rarer and usually indicates that a spring or a truss element is the better model. A semi-rigid value expresses partial fixity — a bolted connection that is neither pinned nor fixed. What matters is its magnitude relative to the member's own `EI/L`; a value much larger behaves as fixed, much smaller as pinned, and the interesting range is within about an order of magnitude either side. Do not release the same degree of freedom at **both** ends of a member unless something else restrains it: a beam pinned for torsion at both ends is free to spin about its axis. Releases are ignored for Truss-type elements, which already connect only axial force. **Release Coordinate System:** Whether the releases above are interpreted in **Global** (the **default**) or **Local** axes. *Engineering influence.* Local is normally what you want — a hinge is a property of the connection and follows the member — but it is **not** the default, so a release entered on a sloped, skewed or vertical member without changing this is applied about the global axes instead. For a member aligned with the global frame the two agree and the setting is invisible; for anything else it releases a different direction than intended, and the symptom is a member that still carries moment about the axis you thought you had freed. ## Overrides **Section Area / Shear Area Y / Shear Area Z / Torsion Constant / Inertia Y / Inertia Z:** Absolute override values that replace the corresponding property computed from the assigned section. **Material Modulus of Elasticity / Poisson's Ratio:** Absolute overrides for the material properties. **…Factor columns** (Modulus of Elasticity Factor, Poisson's Ratio Factor, Section Area Factor, Shear Area Y/Z Factor, Torsion Constant Factor, Inertia Y/Z Factor): **multipliers** applied to the corresponding property instead of replacing it. *Engineering influence.* These exist for two very different purposes and it is worth being clear which one you are using. **Absolute overrides** substitute a property the section geometry cannot express — a built-up member, a section whose torsion constant has been computed elsewhere, or matching a reference model exactly. **Factors** apply code-prescribed stiffness modifiers: cracked-section properties for concrete in a seismic analysis (commonly 0.35–0.7 on inertia), a reduced torsion constant to reflect concrete cracking in torsion (often 0.1 or lower), or a shear-area factor to tune shear deformation. Reducing an inertia factor softens the member in that direction, lengthens periods, and — because stiffness attracts load in a redundant structure — **sheds** force from that member into stiffer ones. This makes the choice consequential and not conservative in a single direction: softening a pier reduces its own moment but increases the demand on its neighbours. A factor of 1 leaves the property unchanged; a factor of 0 removes that stiffness entirely and will usually make the model unstable. The torsion constant is the property most often modified in concrete bridge work, because an uncracked `J` grossly overstates the torsional stiffness a cracked concrete member actually delivers, and an overstiff torsional path attracts load away from the bending path where the design expects it. {% hint style="warning" %} None of the parameters on this tab are declared in the object type. They are still writable from the agent API and `set_parameter` — those resolve any field the object exposes in its sheet — but they carry no unit category and no authorability from a library template, and scripted writes bypass the sheet's own conversion. {% endhint %} ## Settings **Is Rigid:** Marks the member as rigid, so it transfers force without deforming. *Engineering influence.* A rigid member connects its two ends without adding flexibility, which is useful for representing a stiff connection block, a rigid arm, or a pier cap idealized as non-deforming. It is an idealization: real members are not rigid, and making a member rigid attracts force to it and to whatever it connects, which can materially change the distribution in a redundant structure. Prefer real stiffness where you have it. **Geometric Nonlinear:** Whether the element's geometric stiffness is included, so that axial force affects its bending stiffness. Defaults to YES. *Engineering influence.* This is the P-delta effect at member level: compression reduces the effective bending stiffness and amplifies deflection and moment, while tension stiffens the member. It is only active in a **nonlinear** analysis; a linear static case ignores it regardless of the setting. It matters most for slender compression members and for cables, where tension stiffening is the dominant source of stiffness — a stay analysed without it is far too flexible. **End Offsets Expand Thermally:** Whether a rigid end offset is treated as expanding under uniform temperature. Defaults to NO. *Engineering influence.* With the default, the rigid offset does **not** change length under a temperature change, so a member with offsets in a restrained structure locks in force under uniform temperature that the real structure would not develop. Setting it to YES makes the thermal-free length the node-to-node span, releasing that spurious force. Turn it on where offsets are long enough for the difference to matter and where uniform temperature governs; leave it off to preserve legacy behaviour. ## Time Dependent **Casting Day:** The day the member is cast, used by time-dependent analysis to compute concrete creep, shrinkage and modulus development. *Engineering influence.* Age at loading drives creep: concrete loaded young creeps substantially more than mature concrete, so a casting day that is wrong by a significant margin misstates long-term deflections and the redistribution of force between stages. It only has effect when the analysis includes time-dependent effects and the material is configured for them; in an ordinary static run it is inert. ## Verification * Display local axes and confirm the strong axis of each member points the way you intend. This is the fastest check on Beta Angle. * Check the **Length** and **Weight** columns against expectation; a wrong length means a node coordinate error, and a zero weight means a missing section or density. * Read end moments at any end you released — a released rotation must report **zero moment**. This is the definitive check on the release sign convention. * Confirm a Truss-type member reports no moment. * Check deflections against a hand calculation for a simple span (`5wL⁴/384EI`) to confirm section and material are being picked up. * Where property factors are used, compare the member's stiffness contribution against the unfactored model to confirm the factor took effect. * For a one-sided type (Cable, Tension/Compression Truss), run the nonlinear case and confirm the member drops out when its force reverses. * Where offsets are used, check for unexpected moments at the connected node. ## Common mistakes * **Entering −1 to release an end**, which fixes it instead. Verify by reading the end moment. * **Wrong Beta Angle**, orienting a girder's weak axis vertically. Deflections come out an order of magnitude too large. * **No section assigned**, giving a member with no stiffness. * **Reversing Node 1 and Node 2** on some members but not others, so reported end-force signs are inconsistent across a run of otherwise identical members. * **Releasing the same rotation at both ends** and creating a mechanism. * **Entering the same offset in both the Global and Local tabs**, doubling it. * **Using Truss where Beam was needed**, silently removing bending stiffness that was holding the structure up. * **Applying cracked-section factors to steel members**, or leaving concrete torsion at its uncracked value and attracting load into a torsional path the real structure cannot sustain. * **Setting Pretension but never creating a Cable Stressing Load**, so no prestress is applied. * **Expecting one-sided types or geometric nonlinearity to act in a linear analysis**, where both are ignored. --- # Agent Instructions This documentation is published with GitBook. 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