> 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/fe-surface-loads-fea.md). # FE Surface Loads \[FEA] An FE Surface Load applies load **to a shell element** — either a uniform pressure over the whole element face, or a concentrated force or moment at a position within it. The program converts it into equivalent nodal forces during assembly. Use it for deck surfacing, water or earth pressure, snow, and any distributed action over an area, as well as for a wheel or bearing load that lands inside an element rather than at a node. FE Surface Loads **generate load**. They add nothing to stiffness or mass. **Load Case:** The analysis case the load belongs to. With no case assigned it applies nowhere. **Element:** The FE Surface, or group of surfaces, the load acts on. This is a **connectivity** parameter, and for a distributed pressure it is normally a group covering the whole loaded area. *Engineering influence.* The total load applied is the pressure times the **summed area of the elements in the group** — so the extent of the group is exactly as important as the pressure value. A group that misses a strip of deck under-applies the load; one that extends past the loaded region over-applies it. Neither is flagged. **System:** Whether Direction is interpreted in the element's **local** axes or in **global** axes. *Engineering influence.* A **local** direction follows the shell, so a pressure stays normal to the surface as it slopes or curves — correct for water pressure, earth pressure, and wind on a face. A **global** direction is fixed in world axes — correct for gravity-type loads such as surfacing weight, whose intensity is measured per unit of plan area. On a sloped or curved deck the difference is significant: a surfacing load entered as a local (normal) pressure is reduced by the cosine of the slope and gains an in-plane component. **Direction:** Which axis of the chosen system the load acts along. Combined with the sign of the magnitude, this sets where the load goes; a downward load in a Z-up model with a global system is a **negative** value. ## Point Load **Uniform Force Magnitude:** A uniform pressure over the element face, in force per unit area. *Engineering influence.* This is the main way area loads enter the model. The total force delivered is pressure × element area, so the value scales the response linearly. Note it is applied per unit of the **element's own area**, not per unit plan area, so on a sloped deck a pressure derived from a plan quantity must be converted first. The uniform-load lumping is translation-only, which is worth knowing when triangles are involved: the ANDeS triangle formulation is inconsistent under distributed load and its error grows under mesh refinement, so distributed loads on triangular elements should use the default DKGT formulation. Increasing pressure increases deflection and moment proportionally in a linear analysis. A value of zero applies nothing. **Point Force Magnitude:** A concentrated force applied at a position inside the element. **Point Moment Magnitude:** A concentrated moment applied at a position inside the element. **X Coord. (in elem. local) / Y Coord. (in elem. local):** The position of the concentrated load, in the element's **local** coordinates. *Engineering influence.* These let a wheel load or a bearing reaction be placed accurately without putting a node there. The coordinates are local to the element, so they depend on the element's node ordering and orientation — a position that is correct on one element is not the same physical point on its neighbour if the local axes differ. Check local axes before relying on a precise position. A concentrated load on a shell produces a local **stress singularity**: the stress directly under it grows without bound as the mesh is refined, so any stress read at that point reflects mesh size rather than the structure. Read stresses a short distance away, or spread the load, whenever local stress matters. Global response — deflection, reactions, overall moment — is unaffected by this and remains reliable. ## Verification * Sum the case's reactions and compare against pressure × total loaded area computed by hand. This is the definitive check and catches group-extent errors, which are the most common problem here. * Display the applied loads and confirm the loaded region matches the intended area, with no gaps or overlaps at its boundary. * Confirm the direction: a pressure entered as local should be visibly normal to a sloped surface, a global one visibly vertical. * Compare a uniformly loaded simply supported plate against the closed-form deflection. * For a concentrated load, confirm its position by inspection rather than trusting the local coordinates, and check that reported stresses stabilize away from the application point. * Refine the mesh and confirm the global response is unchanged; if it moves, the mesh rather than the load is governing. ## Common mistakes * **A group that does not cover the intended area**, silently under- or over-applying the total load. * **Sign error**, applying pressure upward. * **Confusing local (normal) with global (vertical) pressure** on a sloped or curved deck, changing both magnitude and direction. * **Using a plan-area pressure on a sloped deck** without converting to the element's own area. * **Reading peak stress directly under a concentrated load** and treating a mesh-dependent singularity as a real result. * **Placing a concentrated load by local coordinates** without checking the element's local axes. * **Applying distributed load to ANDeS triangles**, where the lumping is inconsistent and refinement makes it worse. * **No Load Case assigned**, so the load does nothing. --- # Agent Instructions This documentation is published with GitBook. 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