Boundary Conditions: Restraint Without Over-Constraint
Best-practice guidance on applying boundary conditions that follow the physical load path — avoiding over-constraint, under-constraint, rigid-body modes and artificial stiffness.
Principle
Boundary conditions are not simply numerical necessities — they are part of the structural model. A support that is too rigid alters the load path, stiffness, stress distribution, natural frequencies, joint forces and buckling behaviour. The boundary conditions must represent the physical restraints of the real structure.
Why it matters
Over-constraint is one of the most common and consequential errors in FEA. Fixing a surface that is bolted with a finite number of bolts makes the model stiffer than reality, underestimates stress at the bolts and overestimates stress at the fixed edge. Under-constraint produces rigid-body modes, meaningless displacements and convergence failure. Both are avoidable with careful thought about the physical support.
Good practice
- Constrain only the DOF that are physically restrained — a pinned support fixes translations, not rotations
- Use the minimum constraints to prevent rigid-body motion: 6 DOF in 3D (3 translations + 3 rotations)
- Model bolted joints with bolt elements or connector stiffness, not as fully fixed surfaces
- Use distributed coupling or RBE3 (reference-point-to-surface) to distribute constraint without local over-stiffening
- Consider the flexibility of the support structure — a bracket bolted to a thin panel is not connected to a rigid wall
- Apply symmetry constraints only when both geometry and loading are symmetric
- Use soft springs (stiffness < 1% of structural stiffness) to stabilise mechanisms without affecting results
Symmetry boundary conditions
Symmetry can reduce model size by up to 87.5% (1/8 model). However, symmetry constraints must be applied correctly for both geometry AND loading.
Never apply symmetry constraints "by default" without verifying that both the geometry and every applied load are symmetric about the chosen plane. Mixed symmetric/anti-symmetric loading requires superposition of two half-model analyses.
| Constraint Type | DOF Fixed | Use When |
|---|---|---|
| Symmetry (XZ plane) | UY, RX, RZ | Geometry and load symmetric about XZ plane |
| Anti-symmetry (XZ plane) | UX, UZ, RY | Geometry symmetric, load anti-symmetric |
| Cyclic symmetry | Match circumferential DOF | Repeating geometry and loading (e.g. bladed disc) |
Warning signs
- A bolted surface is modelled as fully fixed (encastre) — this over-constrains the model
- The reaction force at a support is much larger than expected — the support may be carrying load that should go elsewhere
- Stress is concentrated at the fixed boundary edge — likely a singularity from over-constraint
- The model has rigid-body modes (zero-frequency modes in modal analysis) — under-constrained
- A thin panel is connected to a rigid support via a rigid coupling (RBE2) — the coupling over-stiffens the panel locally
Verification checks
- Check reaction forces: sum in each direction must balance applied loads to within 1%
- Check that the deformed shape is physically sensible — no spurious rotation or rigid-body motion
- Run a modal analysis and verify no zero-frequency (rigid-body) modes (unless intended)
- Compare reaction force distribution with the expected load path
- Vary the support stiffness and check sensitivity — if the result changes dramatically, the BC is controlling the answer