If a (fixed) boundary condition is active only during part of an analysis, use *BOUNDARY_PRESCRIBED_MOTION_{OPTION}. This keyword offers more flexibility than *BOUNDARY_SPC_{OPTION}_BIRTH_DEATH which only can be applied once to a given node. You must define a zero-valued curve in order to apply a fixed boundary condition by *BOUNDARY_PRESCRIBED_MOTION_{OPTION}.

For example, the keywords required to keep the NSID 112 constrained between t = 0 and t = 0.5 follow:

*DEFINE_CURVE_TITLE
Zero curve
1001
0.,0.
1.,0.
*BOUNDARY_PRESCRIBED_MOTION_SET
$# nsid dof vad lcid sf vid death birth
112 1 2 1001 1.0 0 0.5
112 2 2 1001 1.0 0 0.5
112 3 2 1001 1.0 0 0.5

In order to constrain rigid parts (a *PART defined as rigid by use of *MAT_RIGID, or a *CONSTRAINED_NODAL_RIGID_BODY), use *BOUNDARY_PRESCRIBED_MOTION_RIGID and a zero-valued curve. This constrains the center of gravity of the rigid part.

If motion is desired around a specific point that does not coincide with the inherent center of gravity of the rigid part, use the _INERTIA option to specify a coordinate or a node ID for the desired center of gravity. For LS-DYNA versions up to and including R11.1, Ansys does not recommended using *BOUNDARY_SPC to constrain nodes of rigid parts, since these constraints then may be ignored by LS-DYNA. In that case, a warning message is printed in the d3hsp file:

*** Warning 60257 (IMP+257) (processor # 0)
skipping spc on rigid body node X
tcode = Y rcode = Z

It can be tempting to constrain rigid parts defined by *MAT_RIGID by using the CMO ≠ 0 and CON1, CON2 options on the *MAT_RIGID card. Normally this enforces the desired constraints for the rigid part. But up to version R11 of LS-DYNA, the reaction forces due to constraints specified on the *MAT_RIGID card cannot be obtained for post-processing. If the corresponding reaction forces are of interest, use *BOUNDARY_PRESCRIBED_MOTION_RIGID and a zero-valued curve.