{"id":310308,"date":"2023-10-06T03:11:02","date_gmt":"2023-10-06T03:11:02","guid":{"rendered":"\/forum\/forums\/topic\/modal-analysis-results-affected-by-local-of-elements\/"},"modified":"2023-10-06T03:11:02","modified_gmt":"2023-10-06T03:11:02","slug":"modal-analysis-results-affected-by-local-of-elements","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/modal-analysis-results-affected-by-local-of-elements\/","title":{"rendered":"Modal analysis results affected by local of elements"},"content":{"rendered":"

I am running a quick test on the modal analysis of a block of concrete supported on four corner nodes on ANSYS apdl. Code is below:<\/p>\n

\/CLEAR               ! Clear all previous data and start fresh
\/PREP7               ! Enter preprocessor phase<\/p>\n

! Element Definitions
ET, 1, SOLID185      ! Define SOLID186 element type for solid modeling<\/p>\n

! Material Properties for the Solid
MP, EX, 1, (32836.56803\/1000)\/0.83   ! Define Young’s Modulus for solid after adjusting value
MP, PRXY, 1, 0.2     ! Define Poisson’s Ratio for the solid
MP, DENS, 1, (23.544\/9810.0)\/10**9   ! Define Density for the solid after adjusting value
MP, ALPX, 1, 10E-006 ! Define Coefficient of Thermal Expansion for the solid<\/p>\n

! Geometry Creation
BLC5, 20000, 20000, 40000, 40000, 50000  ! Create a block starting from origin with the given dimensions<\/strong><\/p>\n

! Meshing Settings for the Solid Block
TYPE, 1               ! Set the element type to SOLID186
MAT, 1                ! Set the material type to the first defined material (for solid)
ESIZE, (5000)         ! Set the mesh size
VMESH, ALL            ! Mesh the created volume<\/p>\n

NSEL,S,NODE,,1
NSEL,A,NODE,,2
NSEL,A,NODE,,10
NSEL,A,NODE,,18<\/p>\n

D,ALL,UX,0
D,ALL,UY,0
D,ALL,UZ,0<\/p>\n

! Re-select everything for subsequent commands
ALLSEL, ALL<\/p>\n

\/SOLU                           ! Enters the solution processor<\/p>\n

DAMPING_RATIO = 0.00            ! Modal Damping Ratio
N_MODES = 25                    ! Number of Modes, When Method = LANPCG, NMODE should be less than 100 to be computationally efficient.<\/p>\n

ANTYPE, MODAL                   ! Perform a modal analysis
MXPAND, N_MODES                 ! Number of modes
DMPRAT, DAMPING_RATIO           ! Modal damping ratio
MODOPT, LANB, N_MODES<\/p>\n

ALLSEL, ALL
\/MKDIR, ‘OUTPUT’
\/OUTPUT, ‘OUTPUT\/Modal_Analysis’, ‘OUT’
SOLVE<\/p>\n

FINISH                          ! Exits solution processor \/SOLU<\/p>\n

We can focus on the rotationalX participation factor printed after the modal analysis. For the above run:<\/p>\n

          ***** PARTICIPATION FACTOR CALCULATION *****ROTX DIRECTION
                                                                                  CUMULATIVE     RATIO EFF.MASS
  MODE   FREQUENCY       PERIOD      PARTIC.FACTOR     RATIO    EFFECTIVE MASS   MASS FRACTION   TO TOTAL MASS
     1     2.32266       0.43054       0.23966E+06    0.673954    0.574382E+11    0.218964        0.218896    
     2     2.32266       0.43054      -0.35561E+06    1.000000    0.126456E+12    0.701038        0.481922    
     3     3.73740       0.26757       0.27703E+06    0.779031    0.767451E+11    0.993603        0.292474    
     4     5.19118       0.19263        0.0000        0.000000     0.00000        0.993603         0.00000    
     5     6.13489       0.16300       -13571.        0.038164    0.184179E+09    0.994305        0.701900E-03
     6     6.13489       0.16300        37987.        0.106824    0.144304E+10    0.999806        0.549939E-02<\/p>\n

If I change the origin of my block to 0,0 by doing this instead: BLC5, 0, 0, 40000, 40000, 50000  ! Create a block starting from origin with the given dimensions<\/strong>, I am noticing my results change. See below results for block created using that command:<\/p>\n

          ***** PARTICIPATION FACTOR CALCULATION *****ROTX DIRECTION
                                                                                  CUMULATIVE     RATIO EFF.MASS
  MODE   FREQUENCY       PERIOD      PARTIC.FACTOR     RATIO    EFFECTIVE MASS   MASS FRACTION   TO TOTAL MASS
     1     2.32266       0.43054       0.25505E+06    0.739835    0.650502E+11    0.350567        0.350486    
     2     2.32266       0.43054      -0.34474E+06    1.000000    0.118844E+12    0.991041        0.640326    
     3     3.73740       0.26757        0.0000        0.000000     0.00000        0.991041         0.00000    
     4     5.19118       0.19263        0.0000        0.000000     0.00000        0.991041         0.00000    
     5     6.13489       0.16300        17294.        0.050166    0.299092E+09    0.992653        0.161149E-02
     6     6.13489       0.16300        36443.        0.105713    0.132813E+10    0.999810        0.715586E-02<\/p>\n

Thats one issue, another is why do my rotational mass participation not match with a software like SAP2000. Is there somethjing special about how ansys deals with rotational mass?<\/p>\n

For reference, same model on SAP2000 produces this:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n<\/colgroup>\n\n\n\n\n\n\n\n\n\n\n
TABLE:  <\/span>Modal Participating Mass Ratios<\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n <\/td>\n<\/tr>\n
OutputCase<\/td>\nStepType<\/td>\nStepNum<\/td>\nPeriod<\/td>\nFrequency<\/td>\nUX<\/td>\nUY<\/td>\nUZ<\/td>\nSumUX<\/td>\nSumUY<\/td>\nSumUZ<\/td>\nRX<\/td>\nRY<\/td>\nRZ<\/td>\nSumRX<\/td>\nSumRY<\/td>\nSumRZ<\/td>\n<\/tr>\n
Text<\/td>\nText<\/td>\nUnitless<\/td>\nSec<\/td>\nHz<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\nUnitless<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n1<\/td>\n0.380409<\/td>\n2.62875<\/td>\n0.74<\/td>\n0<\/td>\n0<\/td>\n0.74<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0.26<\/td>\n0<\/td>\n0<\/td>\n0.26<\/td>\n0<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n2<\/td>\n0.369457<\/td>\n2.706675<\/td>\n0<\/td>\n0.68<\/td>\n0<\/td>\n0.74<\/td>\n0.68<\/td>\n0<\/td>\n0.32<\/td>\n0<\/td>\n0<\/td>\n0.32<\/td>\n0.26<\/td>\n0<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n3<\/td>\n0.232478<\/td>\n4.301482<\/td>\n0<\/td>\n0<\/td>\n1<\/td>\n0.74<\/td>\n0.68<\/td>\n1<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0.32<\/td>\n0.26<\/td>\n0<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n4<\/td>\n0.176128<\/td>\n5.677689<\/td>\n0<\/td>\n7.47E-20<\/td>\n5.216E-20<\/td>\n0.74<\/td>\n0.68<\/td>\n1<\/td>\n2.06E-20<\/td>\n4.894E-20<\/td>\n0.99<\/td>\n0.32<\/td>\n0.26<\/td>\n0.99<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n5<\/td>\n0.149814<\/td>\n6.674944<\/td>\n0.26<\/td>\n0<\/td>\n0<\/td>\n1<\/td>\n0.68<\/td>\n1<\/td>\n0<\/td>\n0.73<\/td>\n0<\/td>\n0.32<\/td>\n1<\/td>\n0.99<\/td>\n<\/tr>\n
MODAL<\/td>\nMode<\/td>\n6<\/td>\n0.141023<\/td>\n7.091042<\/td>\n0<\/td>\n0.32<\/td>\n0<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n0.68<\/td>\n0<\/td>\n0<\/td>\n1<\/td>\n1<\/td>\n0.99<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

You will notice for the SAP2000 output, it shows most of the rotationalX mass participating in the 6th mode with a frequency of 7.1 Hz while ANSYS is indicating the first 2 modes are the ones with the high rotational X mass participation.<\/p>\n","protected":false},"template":"","class_list":["post-310308","topic","type-topic","status-closed","hentry","topic-tag-ansys-apdl","topic-tag-apdl","topic-tag-mechanical"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t