FEA Software to check a structures frequency

[eit09]

I currently use RISA3D to check the frequency of model being designed to avoid resonance. I usually break down the RISA model of the whole structure and isolate the elevation supporting the equipment to find the frequency of that area, compare the frequency of the isolated model and design for +/- 20% away from the exciting frequency. Is there any other software out there that is geared more toward checking the frequency for these types of applications? RISA3D is great but I believe the frequency feature is geared more toward earthquake design.

 

[Twoballcane]

There are many, but what are you finding modes for?

Tobalcane
"If you avoid failure, you also avoid success."

 

[eit09]

I am finding the mode that has the most participation and comparing that frequency to avoid excesive excitation between a structure and a machine.

 

[Twoballcane]

ANSYS is the do all for dynamic studies. However, I have only seen it used on things like cars, airplanes, and electronic equipment, but never on a building, but I am sure they have. But in any case, you should be separating your frequency by at least an octave. So if you have a forced frequency of say 10 Hz, you should design your add on with a Fn of 20 Hz.

Tobalcane
"If you avoid failure, you also avoid success."

 

[Drej]

@ Twoballcane

ANSYS - along with several other COTS codes such as ABAQUS, NASTRAN... - is indeed a tremendous code for dynamics. Just to clarify, ANSYS is used in numerous applications in order to assess a structure's dynamic response, buildings included. The analyses can be frequency-domain based (modal-based transient, harmonic, response spectrum, etc.) or fully integrated time-domain analyses (transient). A particular example of modal-based transient is response spectrum analyses of buildings under seismic loads (for example), which uses the eigenvalues/vectors of the structure to determine the dynamic response (with a corresponding spectra).

@ eit09

If the code you refer to (RISA3D) is capable of undertaking eigenvalue analyses and extracting modes and participating masses, then it is probably appropriate to use (given you've verified the software against benchmarks - normal best practice). Of course, you need to make sure you're not "isolating out" parts of the structure that may be playing an important role in the dynamic response.


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[ThomasH]

Hi

If you are looking for the eigenfrequency and nothing else I would say that most of the FEM softwares can solve that problem. If you want to take the analysis further then the selection might decrease depending on the task.

I would be surpriced if RISA 3D couldn't solve that problem. As far as I know STAAD, SAP, Robot etc can do it.

If you look at general FEM softwares like ANSYS, ABAQUS, Nastran etc they can do that, and a lot more.

Regards

Thomas

 

[johnhors]

Calculix is also capable of solving the following:-

frequency (linear)

dynamic - linear

transient modal dynamics
steady state dynamics
harmonic periodic loading
non-harmonic periodic loading

dynamic - nonlinear implicit and explicit


see

http://www.dhondt.de/ov_calcu.htm

and of course it costs nothing !

http://www.Roshaz.com

 

[ESPcomposites]

I recently discovered CalculiX and am very impressed with it. Since it uses an ABAQUS input deck (or similar), your learning curve may be very small. The developers are helpful and I have had a great experience with them.



Brian

http://www.espcomposites.com

 

[JoshPlumSE]

I've seen these types of questions before. To me, the initial response is a bit too simplistic:

Compare the frequency of your equipment to the frequency of your structure and make sure that you stay out of a certain range.

The question becomes more difficult when you follow it up with the following:

1) What happens when I have frequency (with extremely low mass participation) occuring within the range of frequencies I want to avoid. How should EIT09 evaluate the acceptability of the structure in that case?

2) What about cases where your structures frequency is exactly 50% or 200% of your equipment's frequency? Do you risk resonance in those cases?

There are hand calc solutions to these questions and FEM solutions. The hand calc methods come right from textbooks on mechanical vibrations. There are dynamic amplification factors that you can use to judge the increase in amplitudes due to near resonance.

The FEM solutions (in my opinion) are usually the same. Response history analysis of the structure given an applied forcing function. Though, you can often use the hand calculation techiques to justify ignoring this (much more complex) method of analysis.