# GROUNDCHECK

Subcase Information Entry The GROUNDCHECK command can be used to perform a Grounding Check Analysis on the stiffness matrix to expose unintentional constraints by moving the model rigidly.

## Format

GROUNDCHECK(PRINT,GID=GRID,THRESH=THRESH, SET=G, DATAREC=NO) = OPTION

## Definitions

Argument Options Description
PRINT <PRINT, NOPRINT>
PRINT (Default)
Write output to the .out file.
NOPRINT
Do not write output to the .out file.
GRID <GID>

Default = geometric center of the structure.

Grid Point ID: Reference grid point for the calculation of the rigid body motion.
THRESH <e >

Default = largest term in the stiffness matrix, divided by 1.0E10.

Value of the maximum strain energy which passes the check. 5
OPTION <YES, NO>
YES
Grounding check is performed.
NO (Default)
Grounding check is not performed.
SET <G, N, F, A, All> Selects the Degree Of Freedom (DOF) set(s):
G (Default)
All DOFs.
N
All DOFs that are not constrained by MPCs.
F, A
Unconstrained structural DOFs.
All
Includes G, N, F, A.
DATAREC <YES, NO> Requests the recovery of data for the grounding forces.
YES
NO (Default)

1. GROUNDCHECK must be specified before the first SUBCASE.
2. Grounding check is performed on all degrees-of-freedom of the model and all degrees-of-freedom that are not constrained by SPC’s.
3. Any MPC that will be violated due to rigid body modes is reported. An equivalent energy magnitude is also calculated between MPC violation and the strain energy. The equivalent energy from MPC violation is added to the strain energy when performing the grounding check.
4. GROUNDCHECK is output for G-Set, N-Set, F-Set and A-Set. The following descriptions give possible problems from a matrix. Currently F-Set and A-Set are the same.
G-Set
CELAS elements with one end grounded or misaligned CELAS elements. Convert misaligned CELAS elements to CBUSH elements.
N-Set
MPC is violated with rigid body motion.
A-Set and F-Set
Single point constraint which prevent rigid body motion.
5. The THRESH option can be used to set a user-defined strain energy limit along the translational DOFs only.
The strain energy limit along the rotational DOFs is internally calculated and is given by:(1)
$\left(Factor\right)*THRESH$
The Factor is calculated by the ratio of absolute energy summation for each element along the rotational DOFs to the translational DOFs.(2)
$\frac{\sum \left(‖{E}_{RX}‖,‖{E}_{RY}‖,‖{E}_{RZ}‖\right)}{\sum \left(‖{E}_{TX}‖,‖{E}_{TY}‖,‖{E}_{TZ}‖\right)}$
Where,
$‖‖$
Denotes the absolute value of energy.
Subscripts $RX$ , $RY$ and $RZ$
Denote the X, Y and Z Rotational DOFs, respectively.
Subscripts $TX$ , $TY$ and $TZ$
Denote the X, Y and Z Translational DOFs, respectively.