instability type 3 rx

[aruser]

Hi,

I've been reading some posts about instability type 3 as I have a model in which I have that kind of instability.

Curious things:

- In a reduced version (is a very repetitive structure with a small substructures repeated several times) doesn't show any instability issue.

- When I delete the node of the instability, in the following calculation the instability appears in other node...and so on.

- I select all the bars and apply the same section and the instability issue disappear (in the big model I mean).

I guess that all about this instability has to do with the difference between the min/max matrix diagonal elements...and obviously every change in the structure change the matrix and thus, these values. But some questions:

- When can be ignored that message of instability type 3 rx?

- What means the "precision" indicated below the min/max matrix diagonal elements?

- Any special care for modal analysis, second order...when appears this instability?

- Is it logical, that a simple structure doesn't show any instability issue and when you repeat it several times appears the problems?

Thanks

[teixeiranh]

"- When can be ignored that message of instability type 3 rx?"

Well that's not easy to answer this...instabilities are always instabilities and I guess we should avoid them. I tend to ignore this one when the deformation of the structural modal for the simple load cases are just fine.

"- What means the "precision" indicated below the min/max matrix diagonal elements?"

Nice question.

"- Any special care for modal analysis, second order...when appears this instability?"

Guess not. The same care that you take in a simple linear elastic analysis.

"- Is it logical, that a simple structure doesn't show any instability issue and when you repeat it several times appears the problems?"

If the repeat procedure was done correctly, I guess not.