# Joints

Joints allow nodes to be tied in the specified translational or rotational directions. Unlike rigid constraints, joints do not impose equilibrium on the model.

As with all constraints, care must be taken to avoid conflicting constraints.

## Definition​

### Name​

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

### Stage list​

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

### Primary​

The reference node to which the constrained node is attached.

### Constrained​

The node coupled to a primary node so that it shares the coupled degrees of freedom.

### Directions​

Specifies the translational and rotational directions that are linked by this joint.

If the constrained and primary nodes are coincident there are no problems with joints, however if the constrained and primary nodes are not coincident there can be moment imbalance since forces are transferred directly to the primary node and no account is taken of the moment implied by the force times offset. If this is to be modelled, rigid constraints should be used instead.

## Joint limitations​

Joints are a facility to allow two nodes to be coupled in particular directions. Thus to create a pin joint, two nodes can be defined at the two parts of the structure to be joined. These are included in a joint, which is linked in the required directions (e.g. x, y and z). In a joint as in a link element there is a constrained and primary node and the linked degrees of freedom are simply shared between the constrained and the primary node. The axis directions of a joint are the constraint axis directions of the constituent nodes. This means that if a direction is linked this may be different for the two nodes in the joint. In this case it is the directions associated with the constrained node that are linked, through an internal constraint equation, to the primary node. In the case of GsRelax analysis this is treated as an error. When running an explicit time history or explicit nonlinear analyses, joints only work with the global axis: the analyses will stop with a data error if a constrained node's constraint axis does not use the global axis.

While a link element or rigid constraint uses the geometrical relationship between constrained and primary to ensure equilibrium, this is not true for joints. Thus joints in which the nodes are not coincident will not in general be in equilibrium. To guard against this problem the user is warned if the nodes are not coincident.