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Fixed Constraint

In one sentence: Prevent all translational and rotational motion on a selected face, edge, or vertex — the most common structural boundary condition.

Overview

Workbench: FEM
Menu: FEM → Model → Mechanical Boundary Conditions → Fixed Constraint

Prevents all six degrees of freedom (3 translations + 3 rotations) from moving on the selected geometry. This is the simplest constraint and the most commonly used.

When to use Fixed vs Displacement

Situation Use
The surface truly cannot move at all (bolted base, welded joint to a rigid wall) Fixed
Some motion is allowed (e.g. can slide in one direction) Displacement Constraint
A known non-zero deformation must be applied Displacement Constraint

Step-by-step

  1. Select the face, edge, or vertex to constrain.
  2. Choose FEM → Model → Mechanical BCs → Fixed Constraint.
  3. Click OK. A visual indicator appears on the geometry.

Common mistakes and pitfalls

No constraint → singular stiffness matrix

Every structural analysis must have at least one Fixed or Displacement constraint to prevent rigid body motion. Without it the solver fails with a singular matrix error.

Python API

import ObjectsFem
doc = App.ActiveDocument
analysis = doc.getObject("Analysis")

fix = ObjectsFem.makeConstraintFixed(doc, "ConstraintFixed")
fix.References = [(doc.getObject("Body"), "Face1")]
analysis.addObject(fix)
doc.recompute()

See also