$ mmfindfix1  example of massless mechanisms
$ see notes after enddata
nastran system(359)=0 $ block switch from lan to ahou for small problem
DIAG 8
SOL 103 $
Include 'mmfindfixa.v2001' $
CEND
TITLE = massless mechanism example 		mmfindfix1
Subtitle = test all paths through rb discard logic
ECHO = BOTH
METHOD = 1
Disp = all
Subcase 1
Label = no rb modes
Spc = 1
Bc = 1
Subcase 2
Label = one massless mechanism
Spc = 2
Bc = 2
Subcase 3
Label = one MM, one rb mode
Spc = 3
Bc = 3
Subcase 4
Label = two MM, one rb mode
Spc = 4
Bc = 4
Subcase 5
Label = 2 rb modes
Spc = 5
Bc = 5
BEGIN BULK
$Param, usetprt, 0
SPC1,	1,	123456	11	101	$ no rb mode
SPC1,	2,	12356	11	101 $ torsion MM
SPC1,	3,	2356	11
Spc1,	3,	1,	101 $ axial rb mode, torsion MM
SPC1,	4,	2356	11 $ two MM, torsion and disjoint mode,
$ axial rb mode
SPC1,	5,	3456	11	$ axial, lateral rb modes
Spc1,	5,	1,	101 $ axial rb mode, torsion MM
Param, coupmass, 0
Param, mechfix, debug $ force mechfix to run, with more diagnostics
Param, mechprt, yes $ print constraint modes as spcforces
GRID,	11,	,	1.	
GRID,	12,	,	2.
GRID,	13,	,	3.
CBAR,	112,	112,	11	12	0.	0.	1.
CBAR,	123,	112,	12	13	0.	0.	1.
$Conm2,	13,	13,	,	0.0 $ define a torsion rb mode	
$,	1.0 $ I11
PBAR,	112,	11,	1.	2.	3.	4.
MAT1,	11,	1.+10,	,	.3,	1. 
$ use large value of E to separate rb mode freqs from flex mode freqs.
EIGRL,	1,	,	,	6 $  GET 6 MODES
$ disjoint structure, to move size past autoswitch.  Very stiff, light, no 
$ effect on first 4 modes.  Used to avoid autoswitch to AHOU method.
$SPOINT,	1001	THRU	1020
$celas2,	1001,	1.e6	1001
$=,	*1,	=,	*1
$=16
$cmass2,	2001,	1.e-2	1001
$=,	*1,	=,	*1
$=16
$ add a second massless mechanism
grid,	101	
grid,	102
celas2,	10102,	1.,	101,	1,	102,	1
ENDDATA 

Error report

Massless Mechanism Constraint-Constraint of rigid body modes.

The massless mechanism feature (MM) introduced in V2001 first finds DOFs with high maximum ratios in matrix decomposition for [K + s*M].  It then finds shapes associated with these high ratio DOFs.  These shapes may also include rigid body modes.  This is particularly true when the first flexible mode is much greater than 10.0 Hz.  The feature removes the rigid body modes, then constrains only the true massless mechanisms.

There is a logic error in v2001 that does not discard shapes that are rigid body modes, when there are no massless mechanisms.  The alter package mmfindfix.v2001 fixes the error.  It is available on MSC's web site in the SSSALTER directory.