Riks analysis, local buckling?

[vardas]

Hello,

I`m trying to do nonlinear analysis of a column.


I do linear buckling analysis to get the eigenvalues.

I use these material properties(steel):

*Elastic
2.09e+11, 0.3
*Plastic
200.2e+06, 0.0
246.0e+06, 0.0235
294.0e+06, 0.0474
374.0e+06, 0.0935
437.0e+06, 0.1377
480.0e+06, 0.18

Then I introduce imperfections and run static, riks:

*IMPERFECTION, FILE=buckle, STEP=1
1, 5.33

*Step, name=Step-1, nlgeom=YES, inc=40
*Static, riks
1, 1., 1e-05, , ,

But what I get with riks is local buckling(see picture).
It does not matter what BC or column shape i use, I always get local buckling or bending.

What I am doing wrong? Please give me any suggestions.

Thank you very much.

 

[SWComposites]

Maybe nothing is wrong and this is the lowest buckling/collapse mode. Can you post a picture of the eigenvector mode shape?

 

[vardas]

Here is the lowest mode from linear buckling analysis,
I think that the shape should by similar in the riks analysis too.

 

[vardas]

One more thing, the critical load taken from riks analysis is about 4 times lower than bucklings...

 

[bassmanjax]

How did you arrive at the scale factor of 5.33? Could be your scale factor is too large...?

Regards

Martin Stokes CEng MIMechE

 

[vardas]

I used the magnitude of the imperfection L/750.
The length of the column is 4000mm, so I get the imperfection of 5.33 milimeters.