> 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/nodal-disp-loads-fea.md). # Nodal Disp. Loads \[FEA] A Nodal Displacement Load imposes a **prescribed movement** at a node rather than a force. Use it for support settlement, jacking a bearing to a known lift, an imposed deformation at a boundary, or any condition where the displacement is known and the force required to cause it is the unknown. This is the inverse of a Nodal Load: a force load asks "what movement results from this force?", a displacement load asks "what force is needed to produce this movement?". Nodal Displacement Loads **generate load**. They add nothing to stiffness or mass. {% hint style="warning" %} **The degree of freedom must be restrained for the prescribed displacement to mean anything.** A prescribed movement is enforced through the support at that node; on a **free** degree of freedom there is nothing to push against, and the imposed value is either ignored or produces a rigid-body movement rather than the intended settlement. Check the node's DOF settings before entering values here. {% endhint %} **Load Case:** The analysis case the imposed displacement belongs to. With no case assigned it applies nowhere. *Engineering influence.* Settlement is normally kept in its own case so it can be combined at the factor the code requires, and so its effects can be examined separately — settlement effects are often a significant fraction of design moments in continuous structures and need to be visible. **Node:** The joint the displacement is imposed at, normally a support node. ## Displacements **Tx / Ty / Tz:** Prescribed translations along each global axis. **Rx / Ry / Rz:** Prescribed rotations about each global axis. *Engineering influence, all six.* The response is driven entirely by the **restraint the movement is imposed against**. In a **statically determinate** structure a support settlement produces movement but **no internal force at all** — a simply supported span whose abutment settles simply tilts. In a **redundant** structure the same settlement forces the structure to deform out of its natural shape, generating large internal moments and shears and redistributing reactions between supports. This is why differential settlement matters enormously in continuous bridges and not at all in simple spans, and it is the most important thing to understand about this object. The forces generated scale with the structure's stiffness as well as the settlement, so a stiffer structure attracts **more** force from the same imposed movement. Anything that softens the structure — cracked-section factors, creep — reduces settlement effects, which is why codes often permit long-term settlement to be assessed with reduced stiffness. Sign follows the global axis directions: a downward settlement in a Z-up model is a **negative** Tz. A value of zero means no imposed movement in that component — not "free". **Differential** settlement is what generates force; imposing the same settlement at every support translates the whole structure rigidly and generates nothing. Cases are therefore normally set up to settle one support, or a subset, at a time, and the envelope of those cases is what governs design. Results sensitive to these: support reactions (which redistribute markedly), moments over interior supports, and bearing and joint movements. ## Verification * Check that the node actually moves by the amount you prescribed. If it does not, the degree of freedom is unrestrained and the load is not being enforced. * Check the reactions: settling one support of a continuous beam should **reduce** its reaction and increase the adjacent ones, summing to the same total. * Confirm that in a determinate structure the same settlement produces essentially zero internal force — a useful sanity check on both the model and your expectations. * Compare the moment induced over an interior support against a hand calculation for a two-span beam with a known central settlement. * Confirm settlement is applied differentially rather than uniformly, unless a rigid movement is genuinely intended. * Check the sign by looking at the deflected shape. ## Common mistakes * **Imposing a displacement on an unrestrained degree of freedom**, where it cannot be enforced. * **Sign error**, settling a support upward. * **Applying the same settlement at every support**, which produces a rigid-body movement and no internal force — and then concluding settlement does not affect the structure. * **Combining settlement with full uncracked stiffness** where the code expects a reduced long-term stiffness, overstating the induced forces. * **Putting settlement in the dead-load case** rather than its own, so it cannot be factored separately or examined on its own. * **Expecting a determinate structure to develop force** from settlement. * **Forgetting to envelope** the individual support-settlement cases, and checking only the all-supports-settle condition, which is the least onerous. --- # Agent Instructions This documentation is published with GitBook. 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