August 14, 2019 at 12:14 pmbyungchunSubscriber
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)
August 14, 2019 at 12:27 pmjj77Subscriber
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 ambyungchunSubscriber
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 amjj77Subscriber
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 ambyungchunSubscriber
thanks for giving me informative info.
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