🔗 Constraints

A constraint makes the motion of one node depend on the motion of another. Where a Support holds a node still, a constraint ties nodes to each other.

Constraints remove independent degrees of freedom from the model, which is what makes them cheap — and what makes them easy to over-use. Two constraints that close a loop, or that both claim the same DOF, will make the system singular.

Components in this tab

Component	Nickname	Ties
Rigid Diaphragm	Rigid Diaphragm	A group of nodes into a plate that is rigid in its own plane.
Rigid Link	Rigid Link	One node to another, as a rigid body.
Equal DOF	Equal DOF	Chosen degrees of freedom of one node to another, ignoring the offset between them.

Choosing between them

You want	Use
A floor slab that is stiff in-plane	Rigid Diaphragm, one per storey
Two offset nodes to move together as a rigid body	Rigid Link, beam
Some degrees of freedom to read the same at two nodes — a shear key, a slider, a symmetry tie	Equal DOF
A join with real flexibility	a Spring Link instead
A node held still	a Support instead

Note

Constraints are solved by the Constraint handler set on Analysis Settings. The default, Transformation, handles rigid diaphragms and rigid links correctly; Plain does not and will quietly give the wrong answer. Only change it if you know why.