modifying the eff mass fraction in modal analysis

Sayed GHALLEBY_20842

hello everyone I am trying to run a modal analysis using Hyperworks-optistruct, in the output file I am getting the results  below

           MODAL EFFECTIVE MASS FRACTION FOR SUBCASE        1
 RIGID BODY MODES BASED ON REFERENCE POINT AT ORIGIN OF BASIC COORDINATE SYSTEM
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Mode Frequency   X-TRANS    Y-TRANS    Z-TRANS    X-ROTAT    Y-ROTAT    Z-ROTAT 
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   1 4.319E+00  6.628E-04  7.561E-04  5.217E-01  9.286E-01  4.759E-01  1.889E-03
   2 1.971E+01  2.426E-03  6.075E-03  1.279E-01  2.522E-02  1.030E-01  1.334E-05
   3 2.534E+01  3.621E-01  6.083E-04  5.660E-04  6.483E-04  5.799E-02  4.976E-01
   4 2.686E+01  1.524E-03  8.835E-03  6.534E-02  1.383E-02  7.862E-02  5.118E-05
   5 3.316E+01  1.901E-01  5.524E-04  1.743E-04  5.844E-04  6.866E-04  1.882E-01
   6 4.643E+01  5.904E-05  3.293E-03  5.925E-02  4.978E-03  5.161E-02  1.127E-03
   7 5.282E+01  1.722E-04  1.936E-03  1.674E-02  1.379E-03  1.540E-02  4.333E-04
   8 5.716E+01  3.978E-06  1.182E-04  4.273E-03  3.599E-04  4.634E-03  3.127E-05
   9 6.216E+01  1.429E-03  6.587E-05  2.916E-03  1.802E-04  5.716E-03  6.297E-04
  10 6.351E+01  5.081E-04  2.477E-06  1.163E-04  3.698E-06  2.461E-04  3.024E-04
  11 6.627E+01  3.100E-05  1.808E-03  1.406E-02  9.676E-04  6.560E-03  1.699E-03
  12 6.911E+01  1.883E-03  2.867E-06  2.050E-05  1.162E-06  9.190E-06  1.079E-04
  13 7.504E+01  1.320E-03  1.949E-04  9.176E-03  6.293E-04  8.941E-03  7.485E-04
  14 7.734E+01  2.343E-04  1.079E-03  2.303E-02  1.261E-03  2.395E-02  6.670E-05
  15 8.627E+01  4.822E-03  1.125E-04  4.562E-03  2.535E-04  4.264E-03  1.481E-03
  16 8.784E+01  8.605E-04  3.486E-04  1.209E-02  2.930E-04  1.239E-02  1.809E-05
  17 8.994E+01  3.324E-08  3.722E-04  1.314E-03  9.525E-05  1.509E-03  7.126E-05
  18 9.291E+01  9.158E-04  1.087E-03  5.985E-03  2.777E-04  5.982E-03  1.175E-05
  19 9.438E+01  3.264E-03  7.094E-04  4.461E-03  2.063E-04  7.366E-03  8.099E-04
  20 9.787E+01  1.472E-01  2.544E-04  1.561E-04  6.941E-05  4.578E-03  1.468E-02
  21 1.007E+02  2.742E-02  3.381E-03  9.268E-03  2.708E-04  5.672E-03  6.501E-04
  22 1.026E+02  1.623E-03  2.757E-06  7.991E-05  6.566E-06  7.983E-05  2.169E-04
  23 1.039E+02  4.037E-04  1.160E-04  1.123E-03  4.500E-05  2.151E-03  5.372E-05
  24 1.061E+02  1.031E-02  7.686E-04  6.787E-04  1.034E-05  9.338E-05  4.084E-05
  25 1.078E+02  2.614E-03  6.722E-05  5.320E-04  2.585E-05  4.342E-03  1.392E-05
  26 1.122E+02  3.419E-03  1.899E-04  4.923E-03  2.191E-04  7.207E-03  3.563E-04
  27 1.130E+02  7.295E-04  1.057E-04  1.699E-03  9.133E-05  9.677E-04  5.254E-04
  28 1.141E+02  1.248E-06  5.534E-05  1.530E-04  5.604E-06  4.626E-05  5.740E-05
  29 1.158E+02  2.393E-03  1.000E-06  1.515E-03  7.543E-05  2.613E-03  1.416E-04
  30 1.177E+02  1.031E-03  5.651E-05  4.624E-04  2.305E-05  4.914E-04  2.271E-04
  31 1.203E+02  3.595E-04  4.648E-04  2.778E-03  8.792E-05  2.773E-03  3.505E-04
  32 1.221E+02  3.837E-04  2.110E-09  1.753E-03  8.785E-05  1.821E-03  4.739E-05
  33 1.251E+02  1.173E-04  3.204E-04  1.470E-03  7.846E-05  8.162E-04  2.490E-04
  34 1.256E+02  1.102E-05  1.438E-04  2.379E-03  2.786E-05  2.350E-03  1.185E-04
  35 1.269E+02  2.216E-08  3.505E-04  2.872E-06  5.057E-07  7.940E-06  4.411E-05
  36 1.316E+02  8.709E-04  2.034E-03  1.226E-04  5.382E-05  1.430E-06  7.435E-04
  37 1.332E+02  2.390E-03  8.663E-04  5.832E-04  2.810E-06  7.558E-04  9.299E-04
  38 1.343E+02  5.285E-05  3.072E-05  5.778E-04  2.231E-05  4.229E-04  4.283E-05
  39 1.358E+02  4.016E-05  1.938E-04  8.144E-05  1.418E-05  7.039E-05  2.579E-05
  40 1.377E+02  2.183E-04  1.098E-03  2.485E-03  4.345E-05  2.016E-03  5.190E-04
  41 1.380E+02  1.027E-05  7.692E-05  2.717E-04  3.567E-06  1.503E-04  4.228E-05
  42 1.402E+02  3.394E-04  1.623E-03  1.886E-03  3.046E-05  1.519E-03  9.215E-04
  43 1.423E+02  2.619E-04  1.796E-04  9.703E-05  5.060E-07  8.019E-05  1.750E-04
  44 1.437E+02  4.462E-04  7.383E-05  5.058E-05  3.486E-07  2.337E-04  5.247E-05
  45 1.466E+02  1.208E-03  1.416E-04  3.066E-04  5.582E-06  5.746E-04  1.997E-04
  46 1.480E+02  3.469E-04  1.304E-04  5.627E-04  1.710E-05  3.732E-04  2.015E-04
SUBCASE TOTAL   7.766E-01  4.068E-02  9.096E-01  9.811E-01  9.070E-01  7.169E-01
-------------------------------------------------------------------------------- 

,my question is how can we reduce the eff mass fraction for modes 3 and 5

thank you 

BR

Adriano A. Koga

hi @Sayed GHALLEBY 

 

Modal Effective mass will be there, in any mode, and it shows you what is the most participating direction of that particular mode.

This is inherent to the mode shape, and to reduce that you would need to change your design in order to change the mode behavior.

If you look at a fixed mode, you might get some "reduction" if that mode change from 3 to somewhere else, but might not be necessarily an improvement, just a change in the mode number.

Anyway, to change the mass distribution, you might want to play with the way the structure behaves by adding some stiffness in different locations. This would change frequencies and mode shapes, and change the general behavior of the strucutre.

One output that might be useful to you is the EKE.

Take a look at this excelent blog by Mr @Leonardo Rosa :

https://community.altair.com/community?id=community_blog&sys_id=1b5e47361b217050c4dfdbd9dc4bcb67&view_source=searchResult

https://2021.help.altair.com/2021/hwsolvers/os/topics/solvers/os/eke_io_r.htm