
Re: Fixedpinned column buckling analysis help
Anthony Botting Feb 2, 2011 7:05 PM (in response to Omid Khandan)Hi Omid:
I got a difference from your formula of less than 3%. I used the BEAM functionality because it's fairly easy to apply the boundary conditions. You may have already tried this but, right click on the solid body in the simulation tree and choose "treat as beam". Next I assigned a pinned end to the wedge tip. I attached a screen shot of the joint settings in the fixture property manager. Essentially I just restricted translations inplane (relative to the selected plane), but allowed a vertical translation of the joint (there is a purplecolored, downward unit load arrow there, too). I also allowed rotations about the joint with the exception of axial rotation. This solves in just a few seconds (presumably because it's using BEAM elements which solve superfast). I hope this helps a little.  Tony
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Re: Fixedpinned column buckling analysis help
Omid Khandan Feb 2, 2011 7:32 PM (in response to Anthony Botting)Thanks a lot Tony. I actually didn't know about the BEAM feature in SW. This helps a lot. Although I am having trouble applying the load on the edge of the wedge tip. Can you please show a print screen of the applied restrictions to the force?
Thanks,
Omid

Re: Fixedpinned column buckling analysis help
Anthony Botting Feb 2, 2011 9:02 PM (in response to Omid Khandan)Yes here is one. I just put a unit load on it, normal to the front reference plane. The "Joint" as they call it, is the olivecolored sphere (it is where the fixtures and loads terminate). The color olive is used to show that the joint is not attached to any other joint (SW uses a fuscia color to show when a joint is attached to another joint). These "Joints" actually have six degrees of freedom: three rotations and three translations. It turnsout, the joint is actually an end node of the beam element. The beam element for this case is an idealization, because it extracts the crosssection of the column (as a constant cross section) and does not take into account the wedgeshaped end that is on your model (hence the joint terminates a little early inside the end of the column where the constant cross section ceases to exist). So, perhaps another try using solid elements would represent the geometry better. With solid (tetrahedron) elements, we could get the actual cross section near the wedgeend modeled more accurately, but I don't believe the results will be closer to the analytic calculation I'm assuming the analytic calcuation assumes constant cross section, too. It's worth a try, though.

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Re: Fixedpinned column buckling analysis help
Anthony Botting Feb 2, 2011 9:43 PM (in response to Omid Khandan)Hi Omid:
One thing I left out  you have to set the eigenvalue extractor to go to six modes for the BEAM model (which presumably correctly approximates the closed form solution). The result varied by only 1.7%. I tried this on a solid model, too, and it found the correct bending (buckling) direction by solving only to the second eigenvalue. However, this default solid mesh produced a solution that was about 10% difference from the closed form solution (assumed constant cross section). I conclude the solid mesh is likely to be more accurate (subject to a convergence check) because it models the geometry more accurately than the BEAM model. I sure hope this helps. Tony
Re: Fixedpinned column buckling analysis help
Omid Khandan Apr 4, 2011 4:45 PM (in response to Anthony Botting)Hi Anthony,
Thanks for all your help on this. I actually figured it out later to solve without using the 'treat as a beam' mode. I applied constraints to the top plane and was able to run my fixedpinned analysis correctly. The modeling corresponded very well with the analytical theory.
Thanks!
Omid




Re: Fixedpinned column buckling analysis help
Omid Khandan Apr 4, 2011 5:11 PM (in response to Omid Khandan)thanks