Robot Structural Analysis Forum
Welcome to Autodesk’s Robot Structural Analysis Forums. Share your knowledge, ask questions, and explore popular Robot Structural Analysis topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Matrix is not positive definined

12 REPLIES 12
Reply
Message 1 of 13
AJA14
2637 Views, 12 Replies

Matrix is not positive definined

Hi. I am trying to design a shoring system consisting of diaphragm walls with counterfort walls as stiffeners for a case at hand. I get instability problems and if i don't press escape I receive a dialog saying that the matrix is not positive definitive - critical load may have been exceeded. Could that be due to the location and direction of elastic supports. Or is it because I should be using a different solver. Any ideas or advice. Shoring System.png

Ali Al-Hammoud
Structural Design Engineer
MZ & Partners Engineering Consultancy
12 REPLIES 12
Message 2 of 13
Artur.Kosakowski
in reply to: AJA14

This warning suggests that the load level is excessive (structure becomes unstable under applied load).



Artur Kosakowski
Message 3 of 13
aruser
in reply to: Artur.Kosakowski

Hi,

When getting this message I used to check the full note of Analysis->Calcultion report->full note and make sure that all the load cases are convergent. By doing this, despite the matrix message, could we assume that the calculations, forces, displacements etc are correct? Iguess so. Please confirm.

Message 4 of 13
Artur.Kosakowski
in reply to: aruser

No, as the analysis may converge for the different state of equilibrium of the structure. You need to determine where you are with the current load level first.



Artur Kosakowski
Message 5 of 13
aruser
in reply to: Artur.Kosakowski

Sorry, could you explain yourself a little bit more. How can we do this you are explaining?

Thanks

Message 6 of 13
Artur.Kosakowski
in reply to: aruser

Try to run linear and  non-linear buckling analysis (with no P-Delta) for the case or combination which reports this issue and check the value the critical coefficient. Try to reduce the load and check if the original load converges with no warnings displayed and then investigate the deformation of the model and compare it with the one you have for the original load.



Artur Kosakowski
Message 7 of 13
aruser
in reply to: Artur.Kosakowski

I have an strucrure where I got the "matrix is not positive..." click ye swhen apperars this message to follow with the calculations, when is finished check the full note and verify the load case is convergent and the process parameter reached is 1.
How is that posible?
Message 8 of 13
t.sautierr
in reply to: aruser

Artur is actually explaining this (correct me Artur I'm misunderstanding).

 

you have got a structure that is sensitive to the load case you have are studying in your non-linear buckling analysis. It tends to be unequilibrated easely -> critical factor in the linear buckling analysis is close to 1 or maybe <1.

 

so when performing a non linear analysis on this load case -> cannot converge.

 

1) check the value of critical coeffcient you have with linear analysis

2) If <1, before running a non linear analsys, reduce the value because you are sure that it will not converge.

3) If you tick p-delta analsys maybe even if the coeff is >1 but close to it, it will also crash because the 2nd order effect will have an effect on the critical coeff (reducing it actually). You can easely the this effect when running a buckling analysis on a column with a litlle lateral load : critical coeff will be smaller if you take into account 2nd order effect.

 

4) send your model if lost. 

Message 9 of 13
aruser
in reply to: t.sautierr

The point is that I reach a convergetion in the P-delta analysis with full loas (load factor 1 in the full note). But despite that still appears the matrix not positive message.
The question is, in this scensario, the message of the matrix non positive can be taken as an simple warning and not as an error?
Message 10 of 13
Rafal.Gaweda
in reply to: aruser

On given case set buckling analysis (nonlinear buckling), then after calculations please check non-negative critical coefficients. If there is none of them in the range 0-1 - you can ignore this message, otherwise your structure buckles.

If you will still have doubts please send me model.


Rafal Gaweda
Message 11 of 13
t.sautierr
in reply to: aruser

Check the value in image 2 and not in the calculation note (image1). Check if it is not between 0 and 1 as Rafal hints you.

Message 12 of 13
aruser
in reply to: t.sautierr

I perform a buckling nonlinear PD analysis in that load case. at the end of the anaylisis, i get the message of the matrix not positive, click yes to follow and inmediately apperas a new message saying: execution problema No:3060
Message 13 of 13
Rafal.Gaweda
in reply to: aruser

File please.


Rafal Gaweda

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report