> 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.md).

# Geometry \[FEA]

The Geometry branch holds the objects that make up the analytical model: the joints, the elements that connect them, and the constraints and groups that organise them.

**Nodes** carry the degrees of freedom and the support conditions. **Lines** are the one-dimensional elements - girders, columns, braces, cables. **Surfaces** are shell elements for decks, slabs and webs. **Volumes** are solid elements for regions where a full three-dimensional stress state matters. **Springs** connect two joints with a defined stiffness, which is how bearings and isolators are modelled.

**Equal Constraints** and **Rigid Diaphragms** tie degrees of freedom together without adding stiffness - the first as a strict same-DOF tie, the second as an in-plane rigid body with lever arms. **Tendons** define prestressing geometry, and **FE Groups** collect objects into named sets used for staging, constraints, loads and result extraction.

One convention runs through this branch and is worth learning once: for supports and end releases, **0 is free/released, a negative value is fixed/rigid, and a positive value is a spring stiffness.**


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