Calculation using branch load factor

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1

Hello,
I am currently a student and conducting an analysis of a column base.

For the steel column base, a so-called load-bearing capacity diagram is to be created.
For this, I would perform a nonlinear stability analysis using the Structural Stability add-on (bifurcation load factor).

I have made the following settings:

  • Incremental method without eigenvalue analysis
  • Load factor increment: 0.5
  • Maximum number of load steps: 9999
  • Refinement of the last load step: 25
  • Maximum number of iterations: 9999
  • Material: Steel, isotropic plastic (surfaces)

During the calculation, it runs up to a load factor of 3.0, but no refinement of the last load step is performed (e.g., a value of 3.18); only the load increments of 0.5 are calculated.

Can I set a plastic strain of, for example, 5%? Since this should be my termination criterion for the calculation, or the system should be calculated up to this strain.
Currently, the calculation already stops at a significantly smaller strain.

I would appreciate your help.

3

Hi muhlbauermarina,

Thank you for your message!

To analyze the issue with the refinement of the load step more precisely, the model file would be very helpful:

:right_arrow: Click on File → Save As and select the following settings to reduce the file size:
image

:right_arrow: Then upload the file here (e.g., *.rf6, *.rs9) – this way the community can also contribute to the solution.

:owl: Don’t want to share the file publicly? No problem – send it to me via direct message: click on my profile picture or my username → Message.

Regarding the calculation up to reaching the plastic strain:
Currently, I suspect that with the material used (bilinear with no or very small second stiffness branch) and the size of the load step from the force at which the first plasticity occurs, no convergence occurs anymore.
For investigating the automatic refinement, the model as described above would be helpful.
You can also investigate two alternatives:

  1. Manually set the initial load factor higher and manually reduce the step size
  2. Instead of calculating with an increasing force, you can also use a prescribed nodal displacement. For this, the displacement direction of the node to be incrementally displaced must be constrained.

Best regards
Thomas Eichner