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Dear Stko team,
I am developing a 3D model consisting mainly of forceBeamColumn elements.

After running the analysis, I use the post-processor of STKO to visualize the output obtained using the MPCO recorder. In the specific case of the section.fiber.stress, I obtain a graphic plot with the fiber secions rotated not according to the defined local axis system, although the stress state represented is the expected one (but rotated!).

The problem is graphical and not technical since I have an idea of the expected stress state on the sections, but anyway "annoying".

I hope to have been clear, but I remain available for further clarifications. Thanks a lot.

Best regards.
Nicola

Dear user,
this happens because you are not using STKO as a pre processor. And it does not affect your results, it's only a post-processing issue.

STKO gets all the information that it needs from OpenSees storing them in the MPCO file.
However, the local axes cannot be obtained from OpenSees (though in the latest OpenSees version it seems they added such functionality so we will implement as soon as we verify that they actually work).

In fact, when you use STKO as a pre-processor, it writes a support txt file (with the file extension: MPCO.CDATA) that stores the local axes as defined in STKO.

If you don't want to use STKO as a pre processor, you have 2 options:

1. study the format of the MPCO.CDATA, so that you can write it programmatically according to your model
2. in your TCL file use the Z vector of your geomTransf according to the default ones that STKO would give to your model

Option 1 is more difficult, because you need to know how to write the MPCO.CDATA file, and you need to know how Quaternions work, since we use quaternions to store the element orientation.

Option 2 is quite easy instead. You just have to follow 2 simple rules.
For beams (element in the X-Y plane) use vecZ = [0, 0, 1].
For columns (vertical elements) use the vecZ = [1, 0, 0].

Let us know if it solves your problem.

Thank you for your quick and detailed reply.

I will try to apply option n.2 by modifying the fiber section generator, applying a 90° rotation. This will require applying the geometric transformation with vecz vectors as the default ones in STKO. This should be enough to resolve the problem and plot the sections as I expect.

Thank you again.