*BASE MOTION

Define the base motion for linear, eigenmode-based, dynamic procedures.

This option is relevant only during linear dynamics procedures that use the natural modes of the system (STEADY STATE DYNAMICS without the DIRECT parameter, MODAL DYNAMIC, and RANDOM RESPONSE).

This page discusses:

See Also
In Other Guides
Natural Frequency Extraction
Transient Modal Dynamic Analysis
Mode-Based Steady-State Dynamic Analysis
Random Response Analysis
Transformed Coordinate Systems

ProductsAbaqus/Standard

TypeHistory data

LevelStep

Required parameters

DOF

Set this parameter equal to the direction (1–6, including rotations) for which the base motion is being defined. For primary base motion, this direction is always specified in the global coordinate system. For secondary base motion, you can specify the degree of freedom in either the global (default) or local direction at a node with applied coordinate transformations, depending on the value of the DIRECTION parameter.

Set this parameter equal to 8 for the acoustic degree of freedom, which is for use for secondary base motions only in the high-performance implementation of the modal analysis procedures.

Required parameter for MODAL DYNAMIC and STEADY STATE DYNAMICS analyses

AMPLITUDE

Set this parameter equal to the name of the AMPLITUDE option that defines the time history (MODAL DYNAMIC) or frequency spectrum (STEADY STATE DYNAMICS) of the motion. This parameter is irrelevant for the RANDOM RESPONSE procedure. The parameter DEFINITION=SOLUTION DEPENDENT cannot be used in an AMPLITUDE referenced by this option.

Optional parameters

BASE NAME

Set this parameter equal to the name of the base if this base motion is to be applied to a secondary base. The base name is defined with the BASE NAME parameter on the BOUNDARY option in the FREQUENCY step.

DIRECTION

For primary base motion, the base motion is always applied in the direction of the global coordinate axes. For secondary base motion, use this parameter to define the coordinate system in which mechanical degrees of freedom (1–6) are applied.

Set DIRECTION=GLOBAL (default) to apply the base motion in the direction of the global coordinate axes. To specify secondary base motion in a global direction at a node at which a coordinate transformation is applied, you must specify all three translational degrees of freedom (1–3) or all three rotational degrees of freedom (4–6) in the named boundary condition in the frequency extraction procedure if the base motion is translational or rotational, respectively.

Set DIRECTION=LOCAL to apply the secondary base motion in the local direction along one of the coordinate axes of the transformed coordinate systems at the base nodes as defined by the TRANSFORM option.

LOAD CASE

Set this parameter equal to the load case number. This parameter is used in RANDOM RESPONSE analysis, where it is the cross-reference for the load case on the CORRELATION option.

SCALE

Set this parameter equal to the scale factor for the amplitude curve. The default is SCALE=1.0. This parameter applies during MODAL DYNAMIC and STEADY STATE DYNAMICS procedures.

TYPE

Set TYPE=ACCELERATION (default), VELOCITY, or DISPLACEMENT.

Optional, mutually exclusive parameters for steady-state dynamics analysis

IMAGINARY

Include this parameter to define the imaginary (out-of-phase) part of the base motion record given by the amplitude definition.

REAL

Include this parameter (default) to define the real (in-phase) part of the base motion record given by the amplitude definition.

There are no data lines associated with this option unless a primary base motion defines rotation about a point that is not the origin of the coordinate system

 

Data line to define the center of rotation for a prescribed rotation

First (and only) line

  1. X-coordinate of the point about which the rotation is applied.

  2. Y-coordinate of the point about which the rotation is applied.

  3. Z-coordinate of the point about which the rotation is applied.

This data line is relevant only for a primary base motion defined in the MODAL DYNAMIC and STEADY STATE DYNAMICS procedures.