Modal Analysis

[Maxhbk]

Hi all,

I have an out file with me for the Modal analysis of a frame and i can see the frequencies are continuously increasing with the number of modes requested. It would be great help if some one can explain the exact reason behind this.


Thanking you,
Harsha

 

[Rhei]

What do you mean when you say " the frequencies are continuously increasing with the number of modes requested" ? Can you post an example?

 

[Maxhbk]

Hi Olgola,

This is the out file, for a mode there is a frequency and it goes on increasing with the modes. If i request further modes the frequency will be ever higher..... Why is it so?

And what is "Generalized Stiffness" and "Generalized Mass"


Thanks,
Harsha

 

[Rhei]

Well, the modal analysis compute the natural frequencies and the mode shapes of the structure.

The natural frequencies are the frequencies at which the structure naturally tends to vibrate if subjected to a disturbance with that frequency content.

The mode shape is the deformed shape at a specific natural frequency (and it is also called normal mode).
Each mode is associated with a specific natural frequency, therefore if you ask for more modes you will also have more natural frequencies and the Nastran output list them from the smallest to the biggest

 

[Tuw]

The reason you are getting 25 modes, is because you have specified to get 25modes in your program before solving. In FEA, the modes are limited by the simulation model's total DOF if I am not mistaken. For example, if your simulation model have 25DOF (degree of freedom), or 25 equations, then the maximum number of mode you can get is 25. But in actual environment, everything have infinite modes.

So how many modes is sufficient you may ask? It depends on your analysis objective. For example, If you are doing Sol105 (buckling) analysis, you only interested on the first mode. If you are doing Modal analysis, most probably the same, the first mode. However, if you are doing modal transient or modal frequency analysis, you will need more, something like 25modes, in order to capture the mass participation of the vibration to be about 80% in every DOF, in order to fully capture the vibration effect.

Tuw

 

[Maxhbk]

Thanks Olgola, Tuw for your replies.

It would be even more help full if any one can explain what is "Generalized mass and stiffness" in the out file i have posted above????


Thanks,
Harsha