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Critical Load - Buckling analysis

17 REPLIES 17
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Message 1 of 18
skilak
1542 Views, 17 Replies

Critical Load - Buckling analysis

Hi,

 

I am carrying out a buckling analysis (see attached model) and I have a few concerns, namely:

- For two the same load cases I have different Crit Coef,

- I have modeled the structure in a different soft and for the same geometry, restraints, members etc it gives me a positive results.

 

Please could you see the attached file and investigate?

 

Thanks in advance,

 

Tom

17 REPLIES 17
Message 2 of 18
tony.ridley
in reply to: skilak

case 1 has self weight and case 2 does not.
Message 3 of 18
tony.ridley
in reply to: tony.ridley

positive results? your results look OK?
Message 4 of 18
skilak
in reply to: tony.ridley

Hi,

 

I have turned off the self weight but it seems returned to value 1.

 

than is ok.

Message 5 of 18
skilak
in reply to: tony.ridley

when I changed all the members including bracings into HEB400 it still fails, is it correct then?

Message 6 of 18
tony.ridley
in reply to: skilak

still fails? the matrix is still exceed critical value. If you remove releases on the horizontal beams the analysis runs ok
Message 7 of 18
skilak
in reply to: tony.ridley

Hi,

 

I have checked the options with releases and you are right but the simple beams shall be modeled as pinned. The other soft I have used confirmed it as well that fixing beams solvs the problem. I am concerned that making it so rigid e.g. using all elements as HEB400  makes it still unstable.

Message 8 of 18
Rafal.Gaweda
in reply to: skilak

check results:

- case1 - original loads and sections -> cc=0,0216

- case1 - original loads and sections = HEB400 -> cc=0,21

so it is 10 times better but still low

 

Other approach example

 

cc14.jpg



Rafal Gaweda
Message 9 of 18
skilak
in reply to: Rafal.Gaweda

Dzieki Rafal,

 

Do you think the results for HEB400 are correct, I mean the soft gives what it gives, but my feeling is that in reality it should be rigid enough to pass? Is there any chance to figure out which element is the weekest link or the result is only for the whole system?

 

Best regards,

 

tom

Message 10 of 18
tony.ridley
in reply to: skilak

check eigenvectors
Message 11 of 18
skilak
in reply to: Rafal.Gaweda

Hi,

 

I have attached a screenshot from MasterFrame shwoing that it passes. I am concern as we use both softs;)

 

Cheers

Message 12 of 18
Rafal.Gaweda
in reply to: skilak

Maybe, releases defined  in MasterFrame are not fully released - elastic ones?

The interesting thing is that cc in this soft is eq 15 sharp.

 

cc15.jpg



Rafal Gaweda
Message 13 of 18
skilak
in reply to: Rafal.Gaweda

In MasterFrame the releses are normal pinned as I modeled in Robot. The value 15 means that it is either 15 or more. On the main model I have checked the results for buckling and it looks that some of the beams have eff length in km.

I will check the model in S-Frame during the week,

 

Cheers,

 

Tom

Message 14 of 18
Pawel.Pulak
in reply to: skilak

The reason of anomalies is defining X bracings as tension-only truss members and running buckling analysis for load cases which do not result in any forces in these bracings. So in practice buckling analysis is run for the model without any X bracings.

The possible ways of proceeding:

1/ or  switching off "tension only" attribute for X bracings (leaving only "truss" attribute) - but in this case buckling will be run considering stiffness of bracings in both directions so the stiffness will be overestimated

2/ or defining in load case 2 some very small horizontal load to activate bracings in one direction (for instance FX=0.01 kN in node 7)

 

The effect is resulting from the linearisation of model during buckling eigenvalue analysis (model linearised to structure without bracings).

It is the effect analogous to effects observed in dynamic modal analysis for non-linear models - see this forum post: http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Barre-de-treillis-traction-compression-vs-s%...

 

---------------------------------------------
If this post answers your question please click the "Accept as Solution" button. It will help everyone to find answer more quickly!

 

Regards,

 


Pawel Pulak
Technical Account Specialist
Message 15 of 18
skilak
in reply to: Pawel.Pulak

Thanks,

 

But isn't it that the P-Delta analysis applies a horizontal load to unbalance the model? Of course, the soft cannot decide which way to go, left or right;) I have managed to to get the "positive" results adding a little bit of rotational stiffness 1% to the top beam left node. The critic coef is 6.02 Inc. P-Delta but I do not know if this is correct that the buckling length of member 3 (lower beam) is 140.69m. I have to make sure I know what is going on as I want to use the buckled model as an initial deformation according to EC3.

 

Thanks in advance for your help.

 

Best regards,

 

Tom

 

Message 16 of 18
Pawel.Pulak
in reply to: skilak


@skilak wrote:

But isn't it that the P-Delta analysis applies a horizontal load to unbalance the model? 


No, P-Delta analysis is not adding any horizontal loads. It can be seen in the table of reactions checking FX for "Sum of forc."

 

Returning to my previous answer - generally when running buckling analysis for model with tension-only elements it is important to check which of these tension-only elements are active in this specific load case. Knowing it you know for what structure the buckling is actually calculated. See some examples without and with some small lateral load to activate bracings:

buckling1.png

buckling2.png

 


@skilak wrote:

 The critic coef is 6.02 Inc. P-Delta but I do not know if this is correct that the buckling length of member 3 (lower beam) is 140.69m.


It was already explained in the message 2 of this post that using buckling length calculated by buckling analysis is in the most of cases useless.

 

Regards,

 


Pawel Pulak
Technical Account Specialist
Message 17 of 18
skilak
in reply to: Pawel.Pulak

Dzieki Pawel,

 

I think I am getting now more knowledge about Robot. A final question, is it posible to use buckling mode of a tension only bracings frame e.g. the one we used in the post, as the initial deformation (the last tab in Robot analysis window) to account for global analysis imperfection as required by EC3 5.2. I can do it using Notional Loads but using buckling mode as initial deformation would be much easier fot the type of the structures we design. A helpsheet would be a fantastic mean.

 

Thank you very much in advance for your kind help.

 

best regards,

 

Tom

Message 18 of 18
gutitocal
in reply to: Pawel.Pulak

Hi Pawel,

 

Just coming back to this old post I was just checking what would happen if I introduce a horizontal load in the node 7 to make the tension-bracings work. So that is what I did, I put a horizontal load of 0.1 kN in node 7 for the buckling case 1 and 2.

I've kept the horizontal beams with the releases pinned-pinned and after running the calcs I got a critical coefficient of 218.5 and it has nothing to do with what you got in previous messages (around 15).

 

Don't know if I am doing it properly or there is something I am doing wrong.

 

Find attach the model with the horizontal loads included.

Thanks in advance.1.jpg

 

2.jpg

 

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