## *BASE MOTION

Keyword type: step

This option is used to prescribe nonzero displacements and/or accelerations in *MODAL DYNAMIC and *STEADY STATE DYNAMICS calculations. The parameters DOF and AMPLITUDE are required, the parameter TYPE is optional.

The prescribed boundary condition applies to the degree of freedom DOF in all nodes in which a homogeneous *BOUNDARY condition has been defined for this degree of freedom. If using *BASE MOTION it is good practice to define these *BOUNDARY conditions BEFORE the step in which *BASE MOTION is used, else the effect may be unpredictable. The parameter DOF can only take the values in the range from 1 to 3. With the parameter AMPLITUDE the user specifies the amplitude defining the value of the boundary condition. This amplitude must have been defined using the *AMPLITUDE card.

The TYPE parameter can take the string DISPLACEMENT or ACCELERATION. Default is ACCELERATION. Acceleration boundary conditions can only be used for harmonic steady state dynamics calculations, displacements can be used for any modal dynamic or steady state dynamic calculation. Since only three degrees of freedom are at the user's disposal, defining more than three *base motion cards does not make sense.

The *BASE MOTION card is more restrictive than the *BOUNDARY card in the sense that the same amplitude applies to a specific degree of freedom in all nodes in which a homogeneous boundary condition for exactly this degree of freedom has been defined. It is, however, less restrictive in the sense that for steady state dynamics calculations also accelerations can be applied.

First line and only line:

• *BASE MOTION
• Enter any needed parameters and their value.

Example:

*BASE MOTION,DOF=2,AMPLITUDE=A1


specifies a base motion with amplitude A1 for the second degree of freedom for all nodes in which a homogeneous boundary condition was defined for precisely this degree of freedom.

Example files: beamdy10bm.