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Abaqus Boundary Conditions Skill

abaqus-bc

Define boundary conditions - fixed supports, displacements, symmetry. Use when user mentions fixed, pinned, clamped, supported, or constrained. Does NOT handle loads or forces.

SKILL.md

Full skill instructions

Abaqus Boundary Conditions Skill

This skill defines boundary conditions (BCs) in Abaqus models. BCs constrain motion and prevent rigid body movement.

When to Use This Skill

Route here when user mentions:

  • "fixed", "encastre", "clamped", "welded"
  • "pinned", "hinged", "simply supported"
  • "roller", "sliding support"
  • "symmetry", "half model", "quarter model"
  • "constrain", "prevent movement"
  • "prescribed displacement", "move by X mm"
  • "rigid body motion error"

Route elsewhere:

  • Forces, pressures, gravity → /​abaqus-load
  • Contact between parts → /​abaqus-interaction
  • Initial temperature/​stress → /​abaqus-field

Key Decisions

Step 1: What Type of Support?

User DescribesBC TypeDOFs ConstrainedPhysical Meaning
"Fixed", "clamped", "welded"EncastreAll 6Fully rigid connection
"Pinned", "hinged"DisplacementBCU1, U2, U3 onlyRotation allowed
"Roller", "sliding"DisplacementBC1 translationFree in-plane motion
"Half model", "symmetric"XsymmBC/​YsymmBC/​ZsymmBCNormal + 2 rotationsSymmetry plane
"Move it 5mm"DisplacementBCSpecified valuePrescribed motion

Default choice: Encastre for fixed supports (most common).

Step 2: Which Step to Apply?

BC PurposeApply InReason
Fixed supportInitialActive before loads
Prescribed displacementLoad stepApplied with loading
Released BCLater stepUse FREED to release

Default: Apply supports in 'Initial' step.

Step 3: Rigid Body Motion Check

For 3D static analysis, constrain at least 6 DOFs total:

  • 3 translations (X, Y, Z)
  • 3 rotations (about X, Y, Z axes)
ConfigurationStability
One face EncastreFully constrained
Three pinned points (non-collinear)Fully constrained
One vertex + symmetry planesMay be sufficient

"Zero pivot" error = insufficient constraints.

Step 4: Symmetry Plane Selection

Symmetry BCApply WhenConstrains
XsymmBCSymmetric about YZ plane (X=const)U1, UR2, UR3
YsymmBCSymmetric about XZ plane (Y=const)U2, UR1, UR3
ZsymmBCSymmetric about XY plane (Z=const)U3, UR1, UR2

Apply symmetry BC to the face AT the symmetry plane.

What to Ask User

If unclear, ask:

  1. Where is it supported?

    • "Which face/​edge is fixed?"
    • "Where does it mount to the frame?"
  2. What type of support?

    • "Fully fixed (welded) or can it rotate (pinned)?"
    • "Free to slide in any direction?"
  3. Is the model symmetric?

    • "Can we use half symmetry to reduce model size?"
    • "Is the loading also symmetric?"
  4. Any prescribed motion?

    • "Does anything move by a known amount?"
    • "Is this a displacement-controlled test?"

Validation Checklist

Before running analysis:

  • At least one region has fixed support
  • All 6 rigid body modes constrained
  • BCs applied in correct step
  • Symmetry planes match actual symmetry (geometry AND loads)
  • No conflicting BCs on same DOF

After analysis:

  • Reaction forces at supports balance applied loads
  • No "zero pivot" or "rigid body motion" warnings
  • Displacements at fixed regions are zero

Common Mistakes

MistakeSymptomFix
Missing BCZero pivot errorAdd Encastre to a face
Over-constraintWarning in .dat fileRemove redundant BC
BC on wrong regionModel flies awayVerify findAt coordinates
Symmetry without symmetric loadWrong resultsEnsure loads are also symmetric
Pinned beam (no rotation)Unrealistic stressUse Encastre or add rotational stiffness

Troubleshooting

ErrorCauseSolution
"Zero pivot"Insufficient constraintsAdd more BCs
"Negative eigenvalue"Unstable / bucklingCheck supports, may need stabilization
"Face not found"Wrong findAt coordinatesUse bounding box method
"Over-constraint"Conflicting BCsRemove duplicate BC on same DOF

Code Patterns

For API syntax and code examples, see:

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