1.3.8. Constraints

The parts of the structure that has any type of external constraint in their movements, are called constraints. These constraints can be applied for every degree of freedom (X, Y, Z and rotations for beams and shells), and for every node in the model In this condition, the local axes have no relationship with the beam local axes defined in the properties section. The GLOBAL option means to prescribe related to the global axes of the problem. Local axes are used to prescribe the displacement or rotation in a direction not coincident with any of the global axes. The values part of the condition is used to prescribe a fixed amount of displacement or rotation. Default units are meters for the X, Y and Z displacements and radians for the prescribed rotations. X Constraint, Y Constraint and Z Constraint mean the displacements along the axes. Theta x Constraint, theta y constraints and theta z constraints mean the rotations around the axes. Signs are as follows (right hand rule):

This condition can be applied to either points, lines or in the solid analysis, to surfaces. Note: 3D solids have only three degrees of freedom: displacements in X, Y and Z.

1.3.8.1. Fixed constraints

Constraints > Fixed constraints

1.3.8.2. Elastic constraints

Constraints > Elastic constraints

The elastic constraints are similar to the constraints but instead of prescribing the displacement or rotation of a point, an elastic spring is attached to that node for each prescribed degree of freedom. The first three constraints: X-constraint, Y-constraint and Z-constraint are the prescriptions for the three displacements. If any is set, a value must be given that represents the stiffness of that spring. The last three constraints: theta-X-constraint, theta-Y-constraint, theta-Z-constraint, are prescriptions for the three rotations This condition can be applied to either:

Points: Units for the stiffness in IS are: \(\left[N/m\right]\) and \(\left[N·m/rad\right]\)
Beam lines: Units for the stiffness in IS are: \(\left[N/m^2\right]\) and \(\left[N·m/(rad·m)\right]\)
Surface shells: Units for the stiffness in IS are: \(\left[N/m^3\right]\) and \(\left[N /(rad·m)\right]\)
Surfaces that are contour of volumes: Units for the stiffness in IS are: \(\left[N/m^3\right]\) and \(\left[N·m/(rad·m^2)\right]\)

It is possible to use a combination of normal and elastic constraints for the same point. The only condition is that every degree of freedom must have prescribed only a displacement or an elastic movement. This constraint can be used in the analysis of foundations and interactions with the ground and terrain.

1.3.8.3. Rigid constraints

1.3.8.4. Connections

Constraints > Connections

As defined in last section, constraints are the restrictions applied externally to the model in order to avoid some movements or prescribe some of these movements. We define connections as a way to change the relative movements between different parts of the model. By default, all the elements and parts of the model are attached together as completely rigid. This condition is used to disconnect some degrees in one or several nodes between different parts of the structure. In this way, it is easy to define Rotules, that permit free rotation between several parts of the structure. The disconnection of several degrees of freedom can be made in several ways. The most simple ones are those known as rotules.

1.3.8.4.1. Disconnect all

Constraints > Connections > Disconnect all

This condition defines a group, identified by a name, that work together as a part. This group will have the marked degrees disconnected of the rest of beams or shell elements that share the same node. The elements that belong to the group must be marked with condition Disconnect Group. If local axes are defined, the degrees disconnected are related to that local axes. If not, they are related to the global axes.

1.3.8.4.2. Disconnect group

Constraints > Connections > Disconnect group

This condition is used in collaboration with the condition Disconnect All and is used to mark the elements that belong to a common part with a common Group.

EXAMPLES:

Beams:

In this example, one part, called Id1, is defined with the two bars that have a rigid connection between them. Degree y rotation is disconnected from the rest of the bars. So, the third bar has free rotation related to these two.

Typical results in momentum for that node.

Shells:

In this example, one part, called Id1, is defined with all the elements that belong to one of the two surfaces. Degree z rotation and x displacement are disconnected for all the elements that belong to one of the surfaces and contain one node or 2 on the connection line. So, the elements of the other surface have free rotation related to the elements of this surface

1.3.8.4.3. Virtual Connections

Constraints > Connections > Virtual connections

It creates a rigid connection among a point and other geometric entities as points, lines or surfaces.

This option is available only if beams or shells are selected in General data > Analysis > Element types. To define the connection, it is necessary to enter a point in Connect point and a group of points, lines or surfaces depending on the pressed button.