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Negative critical coefficients in buckling analysis

13 REPLIES 13
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Message 1 of 14
Pawel.Pulak
8653 Views, 13 Replies

Negative critical coefficients in buckling analysis

Question from the user:
What does it mean with a negative critical coefficient?


Pawel Pulak
Technical Account Specialist
13 REPLIES 13
Message 2 of 14
Pawel.Pulak
in reply to: Pawel.Pulak

The basic results of buckling analysis are critical coefficients (eigenvalues) and buckling shapes (eigenvectors) corresponding to these critical coefficients.
Each critical coefficient is the factor by which the loads of appropriate load case should be multiplied to obtain appropriate loss of stability (buckling shape or mode).

Negative critical coefficient for some buckling mode means that the loads of appropriate load case shoud have opposite direction to result in such buckling shape. Of course in practice buckling modes with negative critical coefficients should be neglected.

 

In some situations (for instance optimized truss girders with slender tendons working in tension for standard load conditions) positive critical coefficient can be preceded by a lot of negative ones. It is so because by default analysis is searching for critical buckling coefficients starting from zero and considering absolute values of them (so both positive and negative ones). It may result in necessity to calculate a lot of critical coefficients before finding the first positive one. It can be avoided setting non-zero shift to start searching from some positive value - see the attached screen capture.


Pawel Pulak
Technical Account Specialist
Message 3 of 14

An interesting thing: after selecting ‘Block subspace iteration’ and defining the positive value of the shift and then returning to the selection of ‘Subspace iteration’, the program still runs calculations as with the ‘Block subspace’ method, or possibly applies the initial shift to the ‘Subspace’ method?

 

Message 4 of 14

Probably you have not noticed that "Block subspace iteration" method was also used on the very beginning - prior to switching from "Subspace iteration" to  "Block subspace iteration".

By default automatic choice of solver is active (see the attached screen capture) and in case of bigger models it results in using Sparse solver for static analysis. When Sparse solver is used then buckling analysis is run using "Block subspace iteration" method ignoring settings from parameters of buckling analysis.


Pawel Pulak
Technical Account Specialist
Message 5 of 14
maris.bauers
in reply to: Pawel.Pulak

In Robot  it is written that shifting clould cause problems. How to know what value for shift should I choose? 0.1? 1.0? 5.0?

Shift value for Buckling analysis.PNG

Message 6 of 14
Pawel.Pulak
in reply to: maris.bauers

This information in Help is related to the possible risk of loosing some small positive critical coefficients when setting too big shift.

 

For instance if your model contains a lot of slender tendons in tension and because of it reports a huge number of negative critical coefficients only in the range of for instance -0.1 to - 0.6 setting the shift equal to +2 is correct only if for the specified number of buckling modes Robot finds the possible buckling modes with the critical coefficients from +0.3 to +2 range.

That is why in such case it is safer to set shift to +0.3 and to specify a big number of searched modes.

This topic was also discussed in this forum post.

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 7 of 14
maris.bauers
in reply to: Pawel.Pulak

Thanks, that helped. Especially for filtering out large critical coeficcients larger than 10
Message 8 of 14
eear_gabriel
in reply to: Pawel.Pulak

How can I set the critical coefficients to only positive values?

Negative eigenvalues are usually problematic. They will revert loads in the model, which usually is not possible in real problems.

Analyzing any shell structure will be a very frustrating task if values cannot be switched to positive only.

 

Message 9 of 14
Rafal.Gaweda
in reply to: eear_gabriel

Hi @eear_gabriel 

 

determine1.jpg

 

If You do not see that option it means You robot is too old.

 



Rafal Gaweda
Message 10 of 14
eear_gabriel
in reply to: Rafal.Gaweda

Thanks Rafal,

 

Just had a look at 2021 version and it doesn't have this option. What version are you using?

Message 11 of 14
Rafal.Gaweda
in reply to: eear_gabriel

Hi @eear_gabriel 

 

determine2.jpg



Rafal Gaweda
Message 12 of 14
eear_gabriel
in reply to: Rafal.Gaweda

Found it here, sorry.

Thanks

Message 13 of 14

@Rafal.Gaweda 

 

Hi,

 

is there a way to make multiple combination selection and set "determine positive critical ........" all at once, or the only way is selection case by case ?

 

Kind regards!

 

 

 

 

 

Message 14 of 14

Hi @mark.martin.se 

 

Unfortunately one-by-one.

 



Rafal Gaweda

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