Generalised restraints

Generalised restraints allow a set of restraint conditions to be applied to a list of nodes, saving the effort of specifying restraints for individual nodes in the Nodes > Restraints table. The generalised restraints and nodal restraints work together to apply restrain to the model. Car must be taken to avoid conflicting constraints.

Definition

Name

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

Stage list

This specifies a stage list using one any of the forms detailed in Lists. If this is set to all the restraint 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 of the forms detailed in Lists. These include a list definition to select nodes on a specified plane.

Restraints

Specifies the directions that are to be restrained. Restraints are applied in the constraint axes of each individual node, as specified in the Nodes module.

Generalised restraints act in addition to Nodal restraints. The user should remain aware of this to avoid duplication and general confusion.

Generalised restraints are not affected by the Sculpt Modify nodes operation – it adjusts the nodal restraints. A warning is given when the user is about to modify nodal restraints in Sculpt when generalised restraints are specified.

In Output views, the Nodes table reports the nodal restraints and the generalised restraints separately. The Supports table reports the total restraints (nodal + generalised).

Restraint hierarchy

Restraints are a constraint on the model where the displacement at a node in a particular direction is set to zero. Restraints can be applied to a model in several ways and it is important to understand the different restraint methods.

Implied restraint due to structure type

For a Plane structure type the active degrees of freedom are x, z and yy. So the remaining three degrees of freedom are ignored in the analysis. This has the same results are restraining all the out-of-plane degrees of freedom. This feature is set by the program when the user selects a structure type from the General specification.

Global restraints

In some it is useful to restrain all the degrees of freedom in model in a particular direction. For example modelling a slab with plate elements, where the plate elements have no stiffness in the x and y directions, it may be useful to apply global restraints in the x and y directions. These are also set on the Analysis specification.

Nodal restraints

Nodal restraints are the most commonly used restraint type. Here particular degrees of freedom at a node are restrained. So, for example, to create a pinned support, the user can apply restraints to a node in the x, y and z directions. These are set in the Restraints page of the Nodes table view or in the Node wizard.

Generalised restraints

Generalised restraints are similar to nodal restraints except that they are applied to a list of nodes. It is therefore possible to use a list specifying all the nodes on a plane so as to apply restraint across all of the base of a structure.

The total restraint on a node is the sum of all of the restraints acting on it.