General Mechanical

General Mechanical

about modal analysis results..

    • 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 


       

    • 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.


       

    • 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 


       

    • 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.


    • byungchun
      Subscriber

      thanks for giving me informative info.


       

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