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Valued Mentor
AJA14
Posts: 491
Registered: ‎11-01-2011

Matrix is not positive definined

655 Views, 12 Replies
08-06-2012 11:03 PM

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
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Artur.Kosakowski
Posts: 4,965
Registered: ‎12-17-2010

Re: Matrix is not positive definitive

08-06-2012 11:56 PM in reply to: AJA14

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



Artur Kosakowski
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Matrix is not positive definitive

01-22-2013 01:20 AM 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.

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Artur.Kosakowski
Posts: 4,965
Registered: ‎12-17-2010

Re: Matrix is not positive definitive

01-22-2013 01:23 AM 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
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Matrix is not positive definitive

01-22-2013 01:34 AM in reply to: Artur.Kosakowski

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

Thanks

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Artur.Kosakowski
Posts: 4,965
Registered: ‎12-17-2010

Re: Matrix is not positive definitive

01-22-2013 02:03 AM 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
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Matrix is not positive definitive

01-16-2014 09:13 AM 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?
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Valued Mentor
t.sautier
Posts: 442
Registered: ‎12-14-2012

Re: Matrix is not positive definitive

01-16-2014 04:11 PM 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. 

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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Matrix is not positive definitive

01-16-2014 11:47 PM in reply to: t.sautier
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?
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Rafal.Gaweda
Posts: 5,315
Registered: ‎04-26-2010

Re: Matrix is not positive definitive

01-16-2014 11:52 PM 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
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