# Rigid constraints

Rigid constraints define sets of nodes constrained to move as a rigid body. These are commonly use to model rigid diaphragms, where typically the nodes are rigid in the x-y plane but not in the z direction.

Rigid constraints are similar to link elements except that the number of nodes is not limited. The differences between link elements and rigid constraints is discussed elsewhere.

Unlike joints rigid constraints maintain equilibrium in the system.

## Definition​

Name

The name is only used as a convenient way of identifying a rigid constraint.

Stage List

This specifies a stage list using one any of the forms detailed in Lists. If this is set to all the constraint applies to the whole model, irrespective of stage. Otherwise it applies only to the specified stage(s).

Node list

This specifies a node list using any one of the forms detailed in Lists. These include a list definition to select nodes on a specified plane.

Primary

The reference node to which the constrained nodes are attached. The primary node may be included in the node list.

This should be set to 'all' to make the constraint completely rigid. There are however many cases in which it is useful to make a constraint rigid in a plane (for example in modelling floor slabs) or a plate. The following planes can be specified:

allx, y, z, xx, yy, zz
all (pin)x, y, z
xy planex, y, zz
yz planey, z, xx
zx planex, z, yy
xy plane (pin)x, y
xy plane (pin)x, y
yz plane (pin)y, z
zx plane (pin)x, z
x platex, yy, zz
y platey, xx, zz
z platez, xx, yy
x plate (pin)x
y plate (pin)y
z plate (pin)z
customany

The pin linkage types do not include the rotational degrees of freedom at the constrained node in the rigid body displacements. Note that the pin condition applies only to the constrained node - the rotational degrees of freedom at the primary are always retained. The degrees of freedom linked are always in the constraint axis of the nodes and all nodes must be assigned the same constraint axis.

Coupled directions

For most constraint types the couple directions are fixed, but for custom links they must be specified explicitly. This can lead to behaviour that is not consistent with a rigid condition.