abaqus-bc

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 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).

Step 2: Which Step to Apply?

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.

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)

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.

Step 4: Symmetry Plane Selection

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.

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

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

Troubleshooting

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

Code Patterns

For API syntax and code examples, see:

• API Quick Reference
• Common Patterns
• Troubleshooting Guide