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I need a help in the following topic.

Submitted by DHANYA T M on

I am doing a modal analysis of a 3 D frame.The expected fundamental natural frequency was above 1000Hz.I selected Block Lanczos method as analysis option.The start frequency and end frequency in the appeared window was not given.The fundamental frequency I obtained was very less(about 40Hz) than the expected value.Later,I changed the start frequency and end frequency to 1000Hz and 25000Hz respectively.Then,I got the result as 1018Hz.I want to know whether we should give the start frequency and end frequency?From my experience,I got the idea that if we are not giving those two values,the result obtained will be wrong.Please give me a solution

                                                                                                                                    Thank you                            

                                                                                                                                                                              DHANYA

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Jayadeep U. B.

Hi,

If you specify start and end frequencies in this manner, you might be missing many vibration modes of significance.  I would suggest you to extract more natural frequencies in your first analysis, look at the mode shapes, and make the decisions on correctness etc.  As far as I understand, the start and end frequencies are given for checking the possibility of resonance etc., if you have a critical range of frequencies (may be due to the presence of time varying forces).

Regards,

Jayadeep

Fri, 07/03/2009 - 07:25 Permalink