Matrix is not positive definite and missing Eigenvalues

[Eirik.Olastuen]

Hi!

I'm trying to design the cross bracing in a building, but keep geting some warnings I can't figure out. When I use simple bars as brasing and linear analysis it going ok. But if I change to tension only members in brasing and nolinear analysis I get a Warning: "matrix is not positive definite". (see picture) I have searched the forum for simular problems and have read these treads:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Matrix-is-not-positive-definite/m-p/4602589#...

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Matrix-is-not-positive-definined/td-p/356775...

As I understand the model becomes unstable under applied load? 

 - I have tried to model with stiffer memers in bracing (HEB650) didnt solve it. Could it be that some other members should be stiffer? 

 - Then I tried to run buckling analysis (no p-delta) - brings me to the next problem: Missing eigenvalues ( see picture).

I have searched the forum for problems regarding missing eigenvalues and foudn this tread:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Eigenvalues/m-p/3201388

But I can't figure out the problems?

Regards

Eirik

[Artur.Kosakowski]

Try to increase sizes of elements that transfer vertical loads.

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.

[Eirik.Olastuen]

I have now increased the the size of the elements that transfer vertical load. The colums that transfer vertical loads in the brasing is steel with big cross sectrion and the columns in the rest of the buiulding is Concrete wirh cross sectrion variates from 750x650mm to 350x350mm. And I still got the message of Matrix not positive definite.

[Artur.Kosakowski]

Attach the file.

[Eirik.Olastuen]

Link to download from wetransfer: http://we.tl/tMPS8EMbsx

Thanks!

[Artur.Kosakowski · Accepted solution]

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.

[Eirik.Olastuen]

It's working now, thanks!