Define boundary conditions - fixed supports, displacements, symmetry. Use when user mentions fixed, pinned, clamped, supported, or constrained. Does NOT handle loads or forces.
This skill defines boundary conditions (BCs) in Abaqus models. BCs constrain motion and prevent rigid body movement.
Route here when user mentions:
Route elsewhere:
/abaqus-load/abaqus-interaction/abaqus-field| User Describes | BC Type | DOFs Constrained | Physical Meaning |
|---|---|---|---|
| "Fixed", "clamped", "welded" | Encastre | All 6 | Fully rigid connection |
| "Pinned", "hinged" | DisplacementBC | U1, U2, U3 only | Rotation allowed |
| "Roller", "sliding" | DisplacementBC | 1 translation | Free in-plane motion |
| "Half model", "symmetric" | XsymmBC/YsymmBC/ZsymmBC | Normal + 2 rotations | Symmetry plane |
| "Move it 5mm" | DisplacementBC | Specified value | Prescribed motion |
Default choice: Encastre for fixed supports (most common).
| BC Purpose | Apply In | Reason |
|---|---|---|
| Fixed support | Initial | Active before loads |
| Prescribed displacement | Load step | Applied with loading |
| Released BC | Later step | Use FREED to release |
Default: Apply supports in 'Initial' step.
For 3D static analysis, constrain at least 6 DOFs total:
| Configuration | Stability |
|---|---|
| One face Encastre | Fully constrained |
| Three pinned points (non-collinear) | Fully constrained |
| One vertex + symmetry planes | May be sufficient |
"Zero pivot" error = insufficient constraints.
| Symmetry BC | Apply When | Constrains |
|---|---|---|
| XsymmBC | Symmetric about YZ plane (X=const) | U1, UR2, UR3 |
| YsymmBC | Symmetric about XZ plane (Y=const) | U2, UR1, UR3 |
| ZsymmBC | Symmetric about XY plane (Z=const) | U3, UR1, UR2 |
Apply symmetry BC to the face AT the symmetry plane.
If unclear, ask:
Where is it supported?
What type of support?
Is the model symmetric?
Any prescribed motion?
Before running analysis:
After analysis:
| Mistake | Symptom | Fix |
|---|---|---|
| Missing BC | Zero pivot error | Add Encastre to a face |
| Over-constraint | Warning in .dat file | Remove redundant BC |
| BC on wrong region | Model flies away | Verify findAt coordinates |
| Symmetry without symmetric load | Wrong results | Ensure loads are also symmetric |
| Pinned beam (no rotation) | Unrealistic stress | Use Encastre or add rotational stiffness |
| Error | Cause | Solution |
|---|---|---|
| "Zero pivot" | Insufficient constraints | Add more BCs |
| "Negative eigenvalue" | Unstable / buckling | Check supports, may need stabilization |
| "Face not found" | Wrong findAt coordinates | Use bounding box method |
| "Over-constraint" | Conflicting BCs | Remove duplicate BC on same DOF |
For API syntax and code examples, see: