
Re: Why is there no symmetry in this linear buckling simulation?
J. Mather Mar 19, 2014 6:42 AM (in response to Philip Eddy Eriksen)Can you attach the file here?
Also, to get a good screen capture
go to the Windows globe icon in lower left corner of screen
start typing Snipping Tool
and start the Windows Snipping Tool
you can use this to capture the screen
or
hit Print Screen on the keyboard and then start MS Paint and Ctrl V to paste into Paint.
Save the file.
My first thought (without seeing the file) is the difference in symmetry is not significant.

Re: Why is there no symmetry in this linear buckling simulation?
Philip Eddy Eriksen Mar 19, 2014 7:35 AM (in response to J. Mather)Here is my file:)

x1.SLDASM.zip 89.9 KB

Re: Why is there no symmetry in this linear buckling simulation?
Eric Irwin Mar 19, 2014 1:02 PM (in response to Philip Eddy Eriksen)You attached the assembly file, but not the part files.

Re: Why is there no symmetry in this linear buckling simulation?
J. Mather Mar 19, 2014 1:04 PM (in response to Philip Eddy Eriksen)Shouldn't an assembly include parts (*.sldprt) files?

Re: Why is there no symmetry in this linear buckling simulation?
Shaun Densberger Mar 19, 2014 1:06 PM (in response to Philip Eddy Eriksen)Philip, use the "Pack and Go" function under "File" to save your assembly (and all corresponding part files) as a zip.



Re: Why is there no symmetry in this linear buckling simulation?
Jared Conway Mar 19, 2014 10:17 AM (in response to Philip Eddy Eriksen)If the load was purely symmetric would it buckle?

Re: Why is there no symmetry in this linear buckling simulation?
Philip Eddy Eriksen Mar 19, 2014 2:39 PM (in response to Jared Conway)As far as I know, the load is purely symmetric. I have tried a non linear study to find the buckling load. (see my latest post). I`m not sure how to find the load at initial buckling of the web. Is that possible? And is it possible to get a visual result of the buckling from NLstudy?


Re: Why is there no symmetry in this linear buckling simulation?
Shaun Densberger Mar 19, 2014 12:43 PM (in response to Philip Eddy Eriksen)I'm a little confused on what your question is, so please bear with me. Are you asking why the solution varies when you use symmetry compared to when you don't use symmetry? If so, then remember that a linear buckling analysis is an eigenvalue problem (like a modal analysis). If you use symmetry you must keep in mind that the symmetric constraint will remove all nonsymmetric modes of buckling (or vibration in the modal analysis) from the solution.

Re: Why is there no symmetry in this linear buckling simulation?
Philip Eddy Eriksen Mar 19, 2014 2:28 PM (in response to Shaun Densberger)The symmetryproblem concerns the visual bucklingresult (see picture above). Why is the bucklingdeformation different on each side of the transversal webstiffener?
Another thing:
I have tried an nonlinear static study using arclenght control to find the buckling load. Is it possible to find the critical load exactly when the initial buckling of the web occurs?
I`m sorry for the uploading of the file. I tried the "pack and go", so I hope this new file will work.

x1.zip 534.2 KB

Re: Why is there no symmetry in this linear buckling simulation?
Jared Conway Mar 19, 2014 4:27 PM (in response to Philip Eddy Eriksen)have you gone through the examples in the training manual on arc length? it should do exactly what you're describing.
as for the assymmetry of your results, i'm asking a more physical question. if everything was perfectly symmetric, would the system buckle? it would be very hard right? in the simualtion, the prediction for buckling somewaht comes from the fact that numerically it isn't perfectly symmetric and to me it is expected that what you see won't be perfectly symmetric. If your real world is symmetric perfectly, the only way to guarantee that is to add symmetry. but heed the warning from shaun.

Re: Why is there no symmetry in this linear buckling simulation?
Shaun Densberger Mar 19, 2014 5:46 PM (in response to Philip Eddy Eriksen)The symmetryproblem concerns the visual bucklingresult (see picture above). Why is the bucklingdeformation different on each side of the transversal webstiffener?
If you look at Eularian buckling, you'll see that the solutions to the classic problems are sinusoidal in nature. That being said, you might not be seeing the buckling mode shape that you would expect because:
 The geometry, loads, and constraints are such that a nonintuitive mode is the weakest. This mode seems to be characterized by a twisting of the column.
 There are multiply buckling modes that are very close in value to one another, and due to numerical variations, the one you're seeing happens to be the "first". The "second" mode of buckling could be 0.001% greater than the "first" (as calculated by the solver) and have the buckling mode shape that you'd expect. Have you looked at the other modes of buckling? Remember, this is a numerical solution (as opposed to an analytical or exact solution) to an approximated system.
To illustrate item (2) better, imagine you're doing a modal analysis on a cube that doesn't have any constraints applied, and you've asked the solver for the first six modes of vibration. The solver would return six nonzero values; something like:
 2.49E03 Hz
 2.56E03 Hz
 2.60E03 Hz
 3.46E03 Hz
 3.83E03 Hz
 4.24E03 Hz
Now, intuitively, we know all the modes should be zero and equal in value. However, we get nonzero and unequal values because we're using numerical methods to solve our system. The fact that first mode (in this case) is characterized by rigid body rotation about the zaxis is completely arbitrary; it could have easily be translation in the ydirection (or any of the other four remaining degrees of freedom). The same is true for a buckling analysis; if there are several buckling modes that are almost identical in terms of the buckling load factor, then numerical variability will determine the order of the buckling modes. This becomes an issue if you only look at the first mode of buckling.
Another thing:
I have tried an nonlinear static study using arclenght control to find the buckling load. Is it possible to find the critical load exactly when the initial buckling of the web occurs?
Yes, you should be able to do this. As Jared stated, you should go through some of the arclength (and more over, general nonlinear) examples before you tackle your model. I also recommend that you use an enforced displacement (instead of an applied load) for numerical stability.

Re: Why is there no symmetry in this linear buckling simulation?
Philip Eddy Eriksen Mar 21, 2014 4:56 AM (in response to Shaun Densberger)Thanks for the nice feedback!
I have checked the training manual, but i dont find anything on the arc lenght control (ALC) for non linear (NL) simulation. Can it have something to do that I use a educational version of the program?
For point nr. 2; I have looked at other modes of buckling in the linear buckling study. The bucklingform is a litle different, but the buckling load factor (BLF) just increases in value.
For the linear buckling(B) the BLF seems way to high. How can I get a more valid result using linear B. ? Is a load eccentricity the way to go here? If so  how do i set this, and how big should the eccentricity of the force be?
I have tried the NL and ALC, but i think it is a litle bit to compicated for my experience. I`m very close on the NL study though. The bucklingdeformation seem more correct in comparison to the LABresult. But I get so many load factors (LF), and I don`t know which LF that is the most correct. If I go with the load factor that matches my LABresult, the bucklingdeformation is almost equal to nothing. If I choose a plot step wich the bucklingdeformation is more close to the LABresult, then the LF is again way to high
My purpose with this is to find the bucklingform and the design buckling load for the web, localy. The slenderness of the web is very high, so I can`t use the yieldstrenght as a reference to when the web will buckle. I assume that the buckling will occure before the steel reaches it`s yieldingpoint, which is most likely because of the slendernessratio .
If I must go with the Euler buckling load in the linear B.study, does it excist some kind of a safety factor that i can use that gives me the approx. design buckling load?
Thanks!

Re: Why is there no symmetry in this linear buckling simulation?
Shaun Densberger Mar 24, 2014 2:09 PM (in response to Philip Eddy Eriksen)I have checked the training manual, but i dont find anything on the arc lenght control (ALC) for non linear (NL) simulation. Can it have something to do that I use a educational version of the program?
Maybe, but I'm not sure; Jared might know. What exactly do you want to learn about it?
For the linear buckling(B) the BLF seems way to high. How can I get a more valid result using linear B. ? Is a load eccentricity the way to go here? If so  how do i set this, and how big should the eccentricity of the force be?
It very well could be. If you look at my response to your other post, you'll see that LBA give higher BLF values. The eccentricity of your load does have an effect on the BLF from a LBA, so if you know this then go ahead and add it. Please keep in mind the two points I made in the other post (especially the second one) when you decide how to use these results.
I have tried the NL and ALC, but i think it is a litle bit to compicated for my experience. I`m very close on the NL study though. The bucklingdeformation seem more correct in comparison to the LABresult. But I get so many load factors (LF), and I don`t know which LF that is the most correct. If I go with the load factor that matches my LABresult, the bucklingdeformation is almost equal to nothing. If I choose a plot step wich the bucklingdeformation is more close to the LABresult, then the LF is again way to high
Are you using displacement control for your NL analysis? If so,then you shouldn't need to use ALC (it's typically needed when you're using force control). What do you mean by, "I get so many load factors"? Are you talking about the LBA?
My purpose with this is to find the bucklingform and the design buckling load for the web, localy. The slenderness of the web is very high, so I can`t use the yieldstrenght as a reference to when the web will buckle. I assume that the buckling will occure before the steel reaches it`s yieldingpoint, which is most likely because of the slendernessratio .
This is going to be difficult to do (and risky) if you base everything off of FEA. I'd get your model working such that it matches what yuo've tested in the lab, and then use FEA to test different designs and look at the trends. When you have a design that you like, go and do the physical test and see how well it compares.
If I must go with the Euler buckling load in the linear B.study, does it excist some kind of a safety factor that i can use that gives me the approx. design buckling load?
Not that I'm aware of, but maybe there is a standard out there that have more information.

Re: Why is there no symmetry in this linear buckling simulation?
Jared Conway Mar 25, 2014 1:23 AM (in response to Philip Eddy Eriksen)what training manual did you check? arc length is in the nonlinear training available from your reseller. i don't think there is an example in the tutorials though if you're looking in the software.




Re: Why is there no symmetry in this linear buckling simulation?
Roland Schwarz Mar 19, 2014 5:52 PM (in response to Philip Eddy Eriksen)Is your mesh perfectly symmetric? Doubtful.