Ineffective Boundary Conditions

If you only have 2 minutes

A boundary condition that has no effect is almost always one that was defined but never actually holds or loads anything: a zero magnitude, a condition that leaves every degree of freedom free, or one placed on degrees of freedom that are already locked. Give it a non-zero value and apply it where it can actually act.

Your boundary condition has no effect

Dr.Q warns when a boundary condition is present but changes nothing:

• Warnings 12–19: a force, moment, pressure, acceleration, or rotational speed of zero, or stabilization springs with zero stiffness. Set a non-zero value, or remove the condition.
• Warnings 10, 11: a displacement condition that leaves every degree of freedom free holds nothing. Lock the DOFs you actually want to constrain.
• Error 274: the condition targets degrees of freedom that another support already locks, so it cannot change the result. Apply it where it can still move the solution, or remove the redundant support.
• Warning 6: the entire deflection field comes out identically zero, so the model is either fully constrained with no load, or not loaded at all. Check that a load is applied and that the part is not locked everywhere.

The solve stops on a boundary condition

These setup errors abort the solve:

• Error 4: the boundary condition derives its direction from the geometry of its own location, but it is pinned directly to nodes with no location. The two contradict each other. Either attach a location, or give the direction as an explicit vector or global axis.
• Error 61: a moment on a line or edge relies on rotational degrees of freedom, which only beam and shell elements have. Solid elements (tets, hexes) have only translational DOFs, so the moment has nothing to act on. Apply the moment to a surface instead, where Dr.Q distributes it as a force couple.
• Error 33: more than one StabilizationSprings boundary condition. Stabilization springs pin down a nearly-free model, but only one is allowed, so use a single one.

The model can move freely

If the model isn't fully constrained, it can move as a rigid body and the solve fails:

• Error 188: the stiffness matrix is singular (a zero pivot), so the solver cannot solve it. Add supports until all six rigid-body degrees of freedom are held, with real supports and/or symmetry planes; see Supports & Constraints.

How degrees of freedom and supports work is explained in Supports & Constraints; how loads (forces, pressures, moments) are applied is covered in Loads & Pressure Faces.