Instability type 3 and missing eigenvalue

[Anonymous]

Hi,

I am a relatively new user of Robot and I experience the following issues for which I would much appreciate any help I can get.

I have a rather big model in Robot and I get instability type 3 for a number of nodes. I have checked the values of the stiffness matrix and I believe it is caused by the difference in stiffness. Does this mean that I can ignore these instabilities? is there any way to fix them??

I also get a message for a missing eigenvalue while performing a modal analysis to identify the fundamental period of the structure. Does this affect the analysis results?

Please see the model attached.

Thank you in advance for your help.

[Artur.Kosakowski]

Hi @Anonymous

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@Anonymous wrote:

I am a relatively new user of Robot and I experience the following issues for which I would much appreciate any help I can get.

I have a rather big model in Robot and I get instability type 3 for a number of nodes. I have checked the values of the stiffness matrix and I believe it is caused by the difference in stiffness. Does this mean that I can ignore these instabilities? is there any way to fix them??

Looking at the shape of deformation I'd consider having some bracing first. 

I also get a message for a missing eigenvalue while performing a modal analysis to identify the fundamental period of the structure. Does this affect the analysis results?

It is unlikely that it is the 1st one (or global one) but the program asks you if you want to search for it while running the modal analysis case. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski

Thank you for your reply.

Unfortunately I cannot add bracings or amend the structure as it is existing.

I managed to find the "missing eigenvalue" but I still get warnings about instabilities type 3.

The only way I managed to run the analysis without any warnings was when I erased the rigid diaphragm.

Could you please explain how is this affecting the results and if I can keep the rigid diaphragm and ignore the isntabilities??

Thank you in advance.

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

Unfortunately I cannot add bracings or amend the structure as it is existing.

I managed to find the "missing eigenvalue" but I still get warnings about instabilities type 3.

The only way I managed to run the analysis without any warnings was when I erased the rigid diaphragm.

Could you please explain how is this affecting the results and if I can keep the rigid diaphragm and ignore the isntabilities??

The instability of type 3 can be due to the large difference of the elements of the stiffness matrix and when you define a diaphragm you actually introduce elements which are very rigid comparing to the elements of the model. It can be ignored provided you have made thorough check of the correctness of the obtained results (see the recording of the 16th RSA webinar available from here.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski

Thank you for your reply.

I have decided to model the floor with a concrete panel instead of a rigid diaphragm and it seems to work.

However, for some reason I do not understand the model does not lock after the analysis is complete.

Moreover, when I check the results of the modal analysis, it looks like all modes are activating mass predominately on y direction. I would expect to see first mode on y, second on x and third to be rotational (Speaking of which, I don't seem to be able to locate results for rotational modes...)

Could you please advise?

Please see attached the revised model

[Artur.Kosakowski]

HI @Anonymous

Before I start to investigate this deeper could you tell me what you mean by:

However, for some reason I do not understand the model does not lock after the analysis is complete.

[Anonymous]

Hi @Artur.Kosakowski

Thank you for your reply. It must have been  a glitch in the program causing a graphic issue which made me wonder whether the analysis was actually performed or not. When I ran the analysis again the next day this issue was resolved so please ignore this comment.

Regarding the modal analysis and sorting out the mode order, I managed to bring the mode activating the mass in x direction higher in the list by omitting a beam which does not contribute to the lateral resistance mechanism of the structure so it can be considered as a "secondary element" according to ASCE 41. However, the modal analysis results still are not as I would expect them to be and they don't match the results from Etabs which show the first mode in X direction, the second one in Y and the third one around Z.

Please see attached the updated model.

[Artur.Kosakowski]

Hi @Anonymous

I'm not able to compare results with ETABS. Perhaps the difference is to the way the mass distribution is assumed (e.g. concentrated in the center of the gravity in Etabs). 

[Anonymous]

I think that there is something not enough stiff in your structure. Why dont you simplify the model using symetry? the calcultation are too long...You can do first a simple model to fix at list some dimensions of your structure...