Why does modal analysis repeat each natural frequency?

- October 15, 2021 at 7:12 pm
  
  Amirmkr
  Subscriber
  
  Hi,
  The following macro is for a cantilever beam and I was wondering why the frequencies are repeated in the results?
  
  /PREP7
  
  ET,1,BEAM188
  
  MPTEMP,,,,,,,,
  MPTEMP,1,0
  MPDATA,EX,1,,2.068e11
  MPDATA,PRXY,1,,0.3
  MPTEMP,,,,,,,,
  MPTEMP,1,0
  MPDATA,DENS,1,,7830
  SECTYPE,  1, BEAM, RECT, Beam01, 0
  SECOFFSET, CENT
  SECDATA,0.01,0.01,2,2,0,0,0,0,0,0,0,0
  K,1,0,0,0,
  K,2,1,0,0,
  LSTR,    1,    2
  
  LESIZE,ALL, , ,8, ,1, , ,1,
  LMESH,    1
  FINISH
  /SOL
  
  ANTYPE,2
  
  MODOPT,LANB,8
  EQSLV,SPAR
  MXPAND,8, , ,0
  LUMPM,0
  PSTRES,0
  
  MODOPT,LANB,8,0,0, ,OFF
  FLST,2,1,1,ORDE,1
  FITEM,2,1
  
  /GO
  D,P51X, , , , , ,ALL, , , , ,
  /STATUS,SOLU
  SOLVE
  
  The results are as follows:
  
  ***** INDEX OF DATA SETS ON RESULTS FILE *****
  
    SET  TIME/FREQ  LOAD STEP  SUBSTEP CUMULATIVE
     1 8.3119       1     1     1
     2 8.3119       1     2     2
     3 53.804       1     3     3
     4 53.804       1     4     4
     5 160.84       1     5     5
     6 160.84       1     6     6
     7 349.86       1     7     7
     8 349.86       1     8     8
- October 15, 2021 at 11:29 pm
  
  peteroznewman
  Subscriber
  
  When a structure has symmetry, the the vibration in the X direction will have the same frequency as that in the Y direction (assuming symmetry in the XY plane). This is true for all modes that exhibit symmetry.
- October 30, 2021 at 4:42 pm
  
  hollow4444
  Subscriber
  
  Hey was hunting for a solution for this. I have a fixed hanging bar 1m long and 500sqmm cross sec area. When I do a modal analysis IÔÇÖm getting 1st and 2nd frequency as 18 and 3rd and 4th Frequency as 116.
  
  any idea how I can remove the repeats?
  
  thanks
- October 30, 2021 at 8:41 pm
  
  peteroznewman
  Subscriber
  
  Remove them from what? Why do you need to remove them? Why is it not sufficient to simply ignore the repeats? What is wrong with leaving them in the solution results?
- November 1, 2021 at 11:55 am
  
  hollow4444
  Subscriber
  
  Hey peteroznewman.
  
  I have been asked to provide the first 4 natural frequencies and compare it with analytical finite element method. so in the analytical Method i get the Following:
  
  F1:18.611, f2:116.638, f3: 326.59123, f4: 639.988
  so im worried My Modal analysis is missing something. Excuse the ignorance as i have just started this course.
  thanks
- November 1, 2021 at 9:35 pm
  
  peteroznewman
  Subscriber
  
  In ANSYS, request 12 frequencies. Delete the duplicates and keep the lowest four.