Frequency Response results have one more load case than modal analysis

[Anonymous]

I'm doing some Frequency Response analysis of a welded steel structure (a shaker table).

The modal analysis gave me 40 modes at frequencies ranging from 21Hz to 275Hz (see image).

I setup the frequency response to excite at 69Hz since that is the frequency of the applied force from a vibrator. I end up with 41 Load Cases. Is this expected? (i.e. 40 modes + 1 excitation?)

Does that mean I might mainly be interested in the single load case that is linked to this applied 69Hz? (this is where my next question comes in....)

As each result says "Frequency: Not Available" upon it, how do I know which load case relates to which frequency/mode?

Thanks

[Stefan2653]

Hi,

If you only want to apply one excitation frequency (e.g. 69 Hz), you need to make sure that you uncheck the "include natural frequencies" button in the "exciting frequencies" tab when entering analysis parameters. If that box is checked, it will apply your load at 69 Hz plus all the modes you solved for. That is why you have 41 load cases. Also make sure that you are only entering the one frequency (e.g. 69) in the frequency table under that same tab. Hope that helps.

Stefan

[Anonymous]

Thanks for that. Unckecing the "Include Natural Frequencies" box does mean we only end up with the 69Hz result.

But if I leave it checked, does the result for the 69Hz load case insert itself between the frequencies in the modal analysis? In this case slotting in between modes 5 (at 62Hz) and 6 (at 70Hz) in the modal analysis. 

[Stefan2653 · Accepted solution]

Yes it will. If you click on the "report" tab, open the summary file near the bottom of the tree. Once that is displayed, you can scroll down until you see "START OF LOAD 1", "START OF LOAD 2", etc. Each load has the corresponding applied frequency listed. You can then see which loads correspond to which frequencies. I'm not sure why the results window doesn't display the frequencies.

Just note that if you do include natural frequencies in your solution, but you actually only have a 69 Hz single frequeny input load, all the other load cases (at the various natural frequencies) will be fictitious. They will only be relevant if your input load is actually sweeping over the full range of solved modes.

Stefan