about modal analysis results..

- August 14, 2019 at 12:14 pm
  
  byungchun
  Subscriber
  
  hello
  
  I am doing some modal analysis for simple beam
  
  11 beams in a row and it has same material and stiffness.
  
  total 22 modes for 1st mode(x,z dir)
  
  but only 12 modes for 2nd mode
  
  I do not understand why it has only 12 mode it should be 22 modes for 2nd mode
  
  top and bottom are fixed all
  
  could you explain this?
  
  here is my model and results
  
  *** FREQUENCIES FROM BLOCK LANCZOS ITERATION ***
  
  MODE FREQUENCY (HERTZ)
  
  1 2.202637444537
  
  2 2.202637444537
  
  3 2.202637444537
  
  4 2.202637444537
  
  5 2.202637444537
  
  6 2.202637444537
  
  7 2.202637444537
  
  8 2.202637444537
  
  9 2.202637444538
  
  10 2.202637444538
  
  11 2.202637444538
  
  12 2.202637444538
  
  13 2.202637444538
  
  14 2.202637444538
  
  15 2.202637444538
  
  16 2.202637444538
  
  17 2.202637444538
  
  18 2.202637444538
  
  19 2.202637444538
  
  20 2.202637444538
  
  21 2.202637444538
  
  22 2.202637444538
  
  23 4.962273009864
  
  24 4.962273009864
  
  25 4.962273009864
  
  26 4.962273009864
  
  27 4.962273009864
  
  28 4.962273009864
  
  29 4.962273009864
  
  30 4.962273009864
  
  31 4.962273009864
  
  32 4.962273009864
  
  33 4.962273009864
  
  34 4.962273009865
  
  35 11.90362864830
  
  36 11.90362864830
  
  37 11.90362864830
  
  38 11.90362864830
  
  39 11.90362864830
  
  40 11.90362864830
  
  41 11.90362864830
  
  42 11.90362864830
  
  43 11.90362864830
  
  44 11.90362864830
  
  45 11.90362864830
  
  46 11.90362864830
  
  47 11.90362864830
  
  48 11.90362864830
  
  49 11.90362864830
  
  50 11.90362864830
  
  51 11.90362864830
  
  52 11.90362864830
- August 14, 2019 at 12:27 pm
  
  jj77
  Subscriber
  
  One possibility is that The first two are similar modes, with same freq. but vibration in different planes. (say pipe will have the first 2 bending modes with same freq. in two different planes due to symmetry). In fact though the first 11 are one mode shape, and the others nr. 12-22 are for another mode shape in a different plane with the same freq.
  
  3rd mode (nr. 23 - 34) which is at 4.96 Hz for all beams does not have such a thing hence you get 11 x 4.96 Hz.
- August 20, 2019 at 10:51 am
  
  byungchun
  Subscriber
  
  Thank you for response ^^
  
  I don't understand fully, I may dont have enough knowledge
  
  but they are not connected just single beam
  
  so they have to have their individual mode
- August 20, 2019 at 11:41 am
  
  jj77
  Subscriber
  
  Ok - let's do a simple example.
  
  A beam with a circular cross section (so the cross section has a symmetry), will have the two first bending modes with the same frequency but with a predominant motion of the mode shapes along two different perp. axis (say e.g., z and y). Thus we expect 2 equal frequencies (similar mode shapes also) for 1 beam, but with different main motion (different dir.). If we have two beams we expect 4. Below is an example with two circular beams that are fixed, and as we said they have 4 modes with same frequency (~86 Hz) - this mode shapes are all first bending modes for the two beams - so 2 for 1 beam (but in different planes) and two for the other beam.
  
  The highlighted modes at ~ 780 Hz is a torsional mode and of course one mode for each beam (thus two freq. and two modes total, one for each beam). The second highlighted one is a longitudinal mode and again since they are not bending in different planes we get 2 in total just as for the torsional one.
  
  I can not explain this better - so you need to look at the mode shapes of your model and see what they are.
- August 21, 2019 at 5:44 am
  
  byungchun
  Subscriber
  
  thanks for giving me informative info.