Defining a base motion boundary condition

You can create a base motion boundary condition to specify the motion of nodes in a modal dynamic step, steady-state dynamic (mode-based or subspace-based) step, or random response step. The base motion can be acceleration, velocity, or displacement.

See Also
Creating and modifying prescribed conditions
Understanding symbols that represent prescribed conditions

Context:

For more information on base motion, see the boundary conditions discussions in the following sections:

  1. Display the boundary condition editor using one of the following methods:

    • To create a new displacement base motion boundary condition, follow the procedure outlined in Creating boundary conditions (Category: Mechanical; Types for Selected Step: Acceleration base motion or Velocity base motion or Displacement base motion).

    • To edit an existing base motion boundary condition using menus or managers, see Editing step-dependent objects.

  2. In the Basic tab, select the Degree-of-freedom to specify the direction for which the base motion is being defined. This direction is always a global direction: U1, U2, U3, UR1, UR2, or UR3.
  3. For a modal dynamic step or steady-state dynamic (mode-based or subspace-based) step, specify the following options:
    1. If the base motion is to be applied to a secondary base, toggle on Secondary base and choose the name of the secondary base boundary condition defined previously in the eigenfrequency extraction step.
    2. Choose an Amplitude curve that defines the time history (modal dynamic step) or frequency spectrum (steady-state dynamic step) of the base motion.
    3. Enter a new value for the Amplitude scale factor, if desired. The default value is 1.0.
    4. Base motions are always specified in global directions, regardless of the use of nodal transformations. You specify the global direction (Degree-of-freedom) for which the base motion is being defined. If a rotation is specified about an origin that is not the origin of the coordinates, you must specify the Center of rotation by clicking and picking a point in the viewport or entering the coordinates.
    5. For a steady-state dynamic step, you can toggle on Define imaginary (out-of-phase) portion given by amplitude if the specified amplitude curve defines the imaginary part of the base motion (instead of the real, in-phase portion).
  4. For a random response step, click the Correlation tab and toggle on Specify correlation to define the cross-correlation as part of the definition of random loading. See Defining the Correlation, for complete information about correlation options. To define a correlation, fill out the cells in a row of the table as follows:
    1. Click in the Approach column, and select one of the following approaches:

      • Select Correlated if all terms in the correlation matrix should be included.

      • Select Uncorrelated if only diagonal terms should be used.

      • Select User to indicate that user subroutine UCORR will be called to obtain the scaling factors for the correlation matrix.

    2. Click in the PSD column cell, and select the PSD definition amplitude that you defined previously. This amplitude represents the power spectral density function for the random noise source.
    3. In the Real and Imaginary columns, enter the real and imaginary parts of the scaling factor. These scaling factors specify whether the real and imaginary terms will be included in the spatial correlation matrix.

    You can add rows in the table to create additional correlations to define the random loading.

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