Linear Constraints
|
||||
|
||||
|
||||
Constrain a specific node to subordinate to the movements of certain nodes. Here, the specific subordinate node is referred to as a constraint node, and the nodes to which the specific node is subordinated are referred to as independent nodes. The relationship between the constraint node and the independent nodes is established.
where,
Since the constraint Eq. (1) or (2) allows to constrain any node and any DOF, the range of their applications is quite extensive. The constrain equations are applied to the degrees of freedom defined in GCS.
Fig. 1 shows an application example in which a connection is made between a 3-D structure consisted of solid elements and a thin plate consisted of plate elements. Since solid elements do not have stiffness against rotational DOF, they cannot restrain the rotational behavior of the connected plate. If the rotational DOF of the connected part is restrained using Eq. (3), the plate elements would generally behave perpendicularly to the connection.
Figure 1 Example of constraint equation application
It must be made clear not to confuse the constraint equations with Rigid Link. Rigid Link is one in which a number of nodes are subordinated to the movement of a single node. Whereas a constraint condition by the constraint equations is one in which a single node is subordinated to the movements of a number of independent nodes. |
||||
|
||||
|
||||
|
||||
From the Main Menu select Model > Boundaries > Linear Constraints.
Select Geometry > Boundaries > Linear Constraints in the Menu tab of the Tree Menu. |
||||
|
||||
|
||||
|
|
|
|
|
Example How to constrain translational displacements two nodes which are not aligned along the global axis
Method 1. Using Node Local Axis
If a Node Local Axis is pre-assigned to node with 30˚ angle from the global, the coefficient can become simply 1.
Method 2. Assigning coefficients manually
If node local axis is not assigned, user must compute the coefficients considering position of two nodes. Here, the coefficients equal to 'cos30/cos30' for x2, 'sin30/con30' for y2 and 'sin30/cos30' for y1 respectively according to the following relationship.
Where,
: Translational displacement of constraint node 1 in X-direction
: Translational displacement of independent node 2 in X-direction
: Translational displacement of independent node 1 in Y-direction
: Translational displacement of independent node 2 in Y-direction