Symmetry contraint with dymanic/modal analysis 3

[jean15paul]

Hi all, this is my first thread here.

In the past I've used symmetry constraints to only model a portion of a structure when running a static analysis. I just want to make sure that there are no other considerations or "gotchas" when using that approach with dynamics or modal analysis. (Specifically I need to run a normal modes analysis right now.)

FYI ... I have ~5 years of experience with FEA of structures, but so far all of my experience has been with linear static analysis. I recently started a new job where my primary responsibility is dynamic and modal analysis. Also at this new job there aren't really any more experienced people that I can go to with FEA questions, so I'll probably be asking a number of basic questions here. Thanks in advance for your help.

 

[ESPcomposites]

Good question.

I would not recommend that you use symmetric boundary conditions as you did for static analysis. You can easily miss anti-symmetric modes this way. Once you become familiar with structural dynamics and your specific model, you may consider symmetric B.C. (but still be careful).

Example: Consider a beam with 3 pinned supports (two on ends, one in the middle). For a static problem, you could use symmetry and would end up with a single span with 1 simple and 1 clamped end. This would clearly have a different natural frequency and mode shape than the original problem had.

Brian

http://www.espcomposites.com

 

[henki]

An extract from Cook's book:

"In static analysis, symmetry of geometry, material properties, boundary conditions, and loads makes it possible to analyze part of a structure as representative of the whole. Symmetry can also be exploited in vibration analysis, but it is easy to overlook modes. For example, let the left half of a simply supported beam be modeled. If the right end of this model is simply supported, only antisymmetric modes of the original beam are represented. To capture symmetric modes, the left-half model must be analyzed again, this time with its right end allowed to displace laterally but not allowed to rotate. A shell of revolution might be modeled as axisymmetric in static analysis, but most of its vibration modes will display waves around the circumference. It is recommended that symmetry conditions not be used in vibration problems or other dynamic analysis."

 

[jean15paul]

Thanks. I appreciate the feedback.

@henki, Question: What is "Cook's book"? Sounds like it's potentially a good reference.

 

[henki]

"Concepts and Applications of Finite Element Analysis" by Robert D. Cook et al.

It's a good book, one of the most recommended ones for FEM.