Supports & Constraints
How supports work in FEM: fixed supports, symmetry, and degrees of freedom.
Supports and constraints are one of the two kinds of boundary condition; the other is loads (forces, pressures, moments — see Loads & Pressure Faces). This article covers supports: where and how the structure is held. Without them, the structure is free to move as a rigid body, and the solver cannot find a unique solution and will fail.
Degrees of freedom
Every node in the FEM mesh has degrees of freedom (DOF): three translations (X, Y, Z) and three rotations. A support removes one or more of these DOFs at the selected nodes.
| DOF | Description |
|---|---|
| UX, UY, UZ | Displacement in X, Y, Z |
| RX, RY, RZ | Rotation about X, Y, Z axes |
Solid elements (Tet, Hex) only have translational DOFs. Shell and beam elements also have rotational DOFs.
This matters for moments. A moment applied to an edge or point acts on rotational DOFs, so it only works on shell or beam elements. On a solid, apply the moment to a surface instead: Dr.Q then distributes it as a force couple on the translational DOFs, which every element has.
Common support types
Fixed support
Locks all translational DOFs at a surface. No displacement in any direction. In Dr.Q this is the standard "fixed" constraint.
Use it to model: bolted flanges, welded connections, rigid clamps.
A fully fixed support is stiffer than reality. If the support deflects even slightly in practice, your model will overestimate stiffness and underestimate stress near the constraint.
Roller / pin support
Locks displacement in one direction only, leaving others free. Useful for modelling simple supports (beams resting on bearings) or guided supports (a rail slot).
Symmetry
A special case: when geometry, material, and loads are all symmetric about a plane, you can model only half (or a quarter) and apply a symmetry constraint to the cut plane instead of real supports. Fewer elements, same accuracy. It has its own rules and pitfalls, explained in Symmetry.
What happens without supports
The stiffness matrix is singular: the solver cannot invert it. You will get an error about "rigid body motion" or "singular matrix." The structure has no way to resist the applied load.
Practical guidelines
- Place supports as far from the region of interest as possible. Stresses concentrate near constraints and are less accurate close to them (Saint-Venant's principle).
- When in doubt, use more supports and check that reaction forces match the applied load.
- Avoid over-constraining: locking DOFs that should be free introduces artificial stiffness and distorts results.