I am running a normal modes analysis of an unconstrained model. The modal has 800K DOF. The f06 file lists 361 roots found with 6 rigid body modes (frequencies << 1) but the op2 and pch files contain only 359 modes with 4 RBMs. Here is an excerpt from the f06 file:
E I G E N V A L U E A N A L Y S I S S U M M A R Y (READ MODULE)
BLOCK SIZE USED ...................... 7
NUMBER OF DECOMPOSITIONS ............. 5
NUMBER OF ROOTS FOUND ................ 361
NUMBER OF SOLVES REQUIRED ............ 108
1 NORMAL MODES JANUARY 2, 2017 NX NASTRAN 5/ 5/15 PAGE 26
0 SUBCASE 1
R E A L E I G E N V A L U E S
MODE EXTRACTION EIGENVALUE RADIANS CYCLES GENERALIZED GENERALIZED
NO. ORDER MASS STIFFNESS
1 1 -9.803721E-08 3.131089E-04 4.983284E-05 1.000000E+00 -9.803721E-08
2 2 -3.928240E-08 1.981979E-04 3.154417E-05 1.000000E+00 -3.928240E-08
3 3 9.201642E-09 9.592519E-05 1.526697E-05 1.000000E+00 9.201642E-09
4 4 1.158430E-07 3.403572E-04 5.416953E-05 1.000000E+00 1.158430E-07
5 5 1.661520E-07 4.076175E-04 6.487434E-05 1.000000E+00 1.661520E-07
6 6 2.147217E-07 4.633807E-04 7.374933E-05 1.000000E+00 2.147217E-07
7 7 4.318702E+03 6.571684E+01 1.045916E+01 1.000000E+00 4.318702E+03
8 8 2.199661E+04 1.483125E+02 2.360467E+01 1.000000E+00 2.199661E+04
9 9 3.198790E+04 1.788516E+02 2.846512E+01 1.000000E+00 3.198790E+04
10 10 3.519191E+04 1.875951E+02 2.985668E+01 1.000000E+00 3.519191E+04
11 11 5.078170E+04 2.253480E+02 3.586524E+01 1.000000E+00 5.078170E+04
What could cause this anomoly?
Solved! Go to Solution.
Yes, the f06 file shows 6 rigid body modes. But the op2 and pch files contain only 4 RBMs. When I read the op2 file into FEMAP, the first mode listed in the results has a mode number of 3 and has the same frequency as the third mode listed in the f06 file. So the first 2 modes are omitted for some reason. The same thing occurs when I read the pch file into Matlab. Only 4 RBMs are read and the first 2 listed in the f06 are not there.
I figured out the problem. The model file I was running was created from a SOL 111 modal frequency response file which I manually editted to run a SOL 103 to compute normal modes. The original file had multiple subcases for applied loads which I neglected to delete. When I removed all the SUBCASE case control commands the modal solution yielded 6 RBMs in the op2 and pch files as it should. Interestingly, when I left a single subcase in the model it also correctly computed 6 RBMs. But when I included 2 subcases, only 5 RBMs were computed. And including 3 subcases resulted in 4 RBMs. So the moral of this story is be careful when you edit a model file to perform a different analysis.