Ground check comparison 2D Shell and 3D solid

[FrSpace]

Hi all,

I don't understand my ground check result in NX Nastran..

Here my simple study case :

I try to have the same ground check result with 2D shell mesh and 3D tetra mesh.

1) With tetra elements :

- Fem

and results :

*** USER INFORMATION MESSAGE 7570 (GPWG1S)
RESULTS OF RIGID BODY CHECKS OF MATRIX KNN+AUTO (N+AUTOSPC-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 1.000000E-03
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 6.435819E-07 PASS
2 1.039633E-05 PASS
3 1.325356E-06 PASS
4 1.350626E-08 PASS
5 5.839262E-09 PASS
6 4.894699E-10 PASS
SOME POSSIBLE REASONS MAY LEAD TO THE FAILURE:
1. MPC EQUATIONS WHICH DO NOT SATISFY RIGID-BODY MOTION;
2. RBE3 ELEMENTS FOR WHICH THE INDEPENDENT DOF CANNOT DESCRIBE ALL POSSIBLE
RIGID-BODY MOTIONS;
3. PARAM,AUTOSPC HAS OVER-CONSTRAINED THE MODEL.

all check pass.

2) With shell:

- Fem

and results :

*** USER INFORMATION MESSAGE 7570 (GPWG1S)
RESULTS OF RIGID BODY CHECKS OF MATRIX KNN+AUTO (N+AUTOSPC-SET) FOLLOW:
PRINT RESULTS IN ALL SIX DIRECTIONS AGAINST THE LIMIT OF 1.000000E-03
DIRECTION STRAIN ENERGY PASS/FAIL
--------- ------------- ---------
1 1.406297E-07 PASS
2 2.579764E-07 PASS
3 2.093613E-06 PASS
4 5.228651E+03 FAIL
5 1.123782E-08 PASS
6 6.363437E-09 PASS
SOME POSSIBLE REASONS MAY LEAD TO THE FAILURE:
1. MPC EQUATIONS WHICH DO NOT SATISFY RIGID-BODY MOTION;
2. RBE3 ELEMENTS FOR WHICH THE INDEPENDENT DOF CANNOT DESCRIBE ALL POSSIBLE
RIGID-BODY MOTIONS;
3. PARAM,AUTOSPC HAS OVER-CONSTRAINED THE MODEL.

I don't understand why, and I really need to modeling my structure in shell...(restriction on the number of nodes..)

Any ideas ?

Thanks !