Understanding modal frequency

[mcook21VD9DL]

Hi there,

I have some beams idealisations in a structure, I have tried this analysis with gravity on/off, with and without concentrated masses and I get a mode/natrual frequency of nearly every number between my range.

For example, if I have a range of 50 to 100 Hz, and I run 50 modes, I will get a natural frequency at each of the 50 modes. When I see examples, this is not the case. When looking at the frequency vs mode number graph of the structure with no concentrated masses, it is linear. When I look at the graph with offset concentrated masses, I start to see a little movement however it is very close to linear.

I could understand the structure with 0 concentrated masses being linear with frequency, to mean that it is fairly stiff? Each mode shows the structure moving in different areas which is normal, but I would have expected non-linear responses unless our structure is just that stiff (which it actually should be too be honest).

Could someone help me understand this? Cheers

[John_Holtz · Accepted solution]

Hi @mcook21VD9DL 

Can you clarify what the exact analysis type is that you are running? "Modal frequency" could be interpreted as natural frequency analysis (Normal Modes) or Frequency Response.

I suspect you are doing a Modal Frequency Response, and the "50 modes" that you are entering are not calculating 50 natural frequencies, but instead are 50 different frequencies at which you are shaking the model. In other words, it sounds like you have specified a "sine sweep" from 50 to 100 Hz at intervals of 1 Hz. Therefore, you get results at 50 points between 50 and 100 Hz.

[mcook21VD9DL]

Hi John,

All good I figured out the problem.

Cheers