Defining a displacement/rotation boundary condition

Display the displacement/rotation boundary condition editor using one of the following methods:

• To create a new displacement/rotation boundary condition, follow the procedure outlined in Creating boundary conditions (Category: Mechanical; Types for Selected Step: Displacement/Rotation).

• To edit an existing displacement/rotation boundary condition using menus or managers, see Editing step-dependent objects. To edit the region to which the boundary condition is applied, see Editing the region to which a prescribed condition is applied.

If you are creating the boundary condition in a buckling step, select the Use BC for option that specifies the calculations for which you want the boundary condition used. For more information, see “Boundary conditions,” in Eigenvalue buckling prediction.

By default, the global coordinate system is used to define the boundary condition. To change the coordinate system in which the boundary condition is applied, click for the CSYS option and do one of the following:

• Select an existing datum coordinate system in the viewport.

• Select an existing datum coordinate system by name.

  1. From the prompt area, click Datum CSYS List to display a list of datum coordinate systems.

  2. Select a name from the list, and click OK.

• Click Use Global CSYS from the prompt area to revert to the global coordinate system.

This coordinate system editing option is available only in the step in which the boundary condition is created.

If a Method field appears toward the top of the editor, click the arrow to the right of the field, and select one of the following:

• Select Specify Constraints if you want to specify values for particular degrees of freedom.

• Select Fixed at Current Position if you want to fix the degrees of freedom at their final values from the previous general step.

The Method option appears in the editor only if both methods are valid for the selected step.

Click the arrow to the right of the Distribution field, if available, and do the following:

• Select Uniform to define a uniform boundary condition.

• Select User-defined to define the boundary condition in user subroutine DISP. See the following sections for more information:

  • Specifying general job settings

  • DISP

• Select an analytical field, labeled with an (A), or a discrete field, labeled with a (D), to define a spatially varying boundary condition. Only analytical fields and discrete fields that are valid for this boundary condition type are displayed in the selection list.

  Alternatively, you can click to create a new analytical field. (See The Analytical Field toolset for more information.)

The Distribution option appears in the editor only if you are creating the boundary condition or editing the boundary condition in the step in which it was created. This option is unavailable if you select the Fixed at Current Position method.

If you selected the Uniform, analytical field, or discrete field distribution option, perform the following steps:

1. Use the appropriate method to define the boundary condition:

   If no text fields appear next to each degree of freedom:

   • Toggle on a degree of freedom to fix the value either at zero (if you are defining a degree of freedom in the initial step) or at its final value at the end of the previous analysis step (if you are defining the degree of freedom in the second analysis step or in a later step.)

   • Toggle off a degree of freedom to leave the degree of freedom unconstrained.

   If text fields appear next to each degree of freedom:

   • Toggle on a degree of freedom to constrain the degree of freedom. The text field becomes available in which you can specify a value for the degree of freedom. If you are creating the boundary condition in this step, a default value of zero appears in the text field. If you are modifying the boundary condition in this step, the value propagated from the previous step appears in the text field.

   • Toggle off a degree of freedom to leave the degree of freedom unconstrained. If you toggle off a degree of freedom after modifying the default or propagated value in the text field, the modified value is lost. If you toggle that degree of freedom back on, the default or propagated value reappears in the text field.

   • If an Amplitude appears at the bottom of the editor, click the arrow to the right of the field, and select the amplitude of your choice from the list that appears. Alternatively, you can click to create a new amplitude. (See The Amplitude toolset for more information.)

2. Click OK to save your data and to exit the editor.

If you selected the User-defined distribution option, perform the following steps:

1. If desired, use the following techniques to define displacement:

   • Toggle on a degree of freedom to constrain the degree of freedom. If you are in the initial step, that degree of freedom is set to zero. If you are in any step other than the initial step, a text field becomes available in which you can specify a value for the degree of freedom.

   • Toggle off a degree of freedom to leave the degree of freedom unconstrained.

   Data that you enter in the editor for particular degrees of freedom are passed into the user subroutine.

2. Click OK to save your data and to exit the editor.
3. Enter the Job module and display the job editor for the analysis job of interest. (For more information, see Creating, editing, and manipulating jobs.)
4. In the job editor, click the General tab, and specify the file containing the user subroutine that defines the boundary condition. For more information, see Specifying general job settings.

   Note:

   You can specify only one user subroutine file in the job editor; if your analysis involves more than one user subroutine, you must combine the user subroutines into one file and then specify that file.