How to use Z88 in FEM?

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bernd
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Re: How to use Z88 in FEM?

Post by bernd »

May be both are correct in sense what the solver analyses and may be even none of them is correct in sense of the original example taken from Z88. This because the rotations might be wrong ... I will have a look if I can find the original example ...
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Re: How to use Z88 in FEM?

Post by bernd »

lt is example 2 of Z88. In German:

BSP 2: Kranträger: Räumliches Fachwerk mit Element 4

But I did not found not yet the examples for download.
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NewJoker
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Re: How to use Z88 in FEM?

Post by NewJoker »

From here you can download all the files related to example problems (models and projects): https://en.z88.de/examples/

I ran this simulation in Abaqus and the result is 136.634 mm so apparently Z88 Aurora gives correct values here.

Update: previously I recreated this analysis in Abaqus and obtained aforementioned result. Now I imported the ccx input file directly to Abaqus and the value is 133.921 mm.

Update 2: finally, I submitted the previously exported inp file in ccx outside of FreeCAD (just to be sure) and the result is 64.6 mm:

ccx.JPG
ccx.JPG (193.71 KiB) Viewed 1841 times

Maybe it's some bug in CalculiX. I've heard about such issues regarding beam elements in this software (although it was about user beam element U1).
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Re: How to use Z88 in FEM?

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Re: How to use Z88 in FEM?

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In the original example, a different cross-section and load magnitude is used. Other settings should be the same. Anyway, it seems that there's a bug in CalculiX that causes the results to be around 50% smaller in this particular case.
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Re: How to use Z88 in FEM?

Post by bernd »

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Re: How to use Z88 in FEM?

Post by NewJoker »

I've finally found a reason why CalculiX results were incorrect in this case. It was all about mesh (as it usually is in FEA). After changing the element order to 2nd and increasing the number of elements, the results are good:

ccx_new.PNG
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ccx_new2.PNG
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Re: How to use Z88 in FEM?

Post by bernd »

ha same problem here, first I used Z88 got correct results with just one truss element foreach bar. Than used CalxuliX and wondered why it was to stiff. Than I got remembered on this post from you. Calculix needs 4 second order elements foreach bar.
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Re: How to use Z88 in FEM?

Post by Kunda1 »

If anyone is inspired, there is still FEM_SolverZ88 wiki page to complete ;)
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Laurie Hartley
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Re: How to use Z88 in FEM?

Post by Laurie Hartley »

NewJoker wrote: Mon Aug 02, 2021 1:57 pm I've finally found a reason why CalculiX results were incorrect in this case. It was all about mesh (as it usually is in FEA). After changing the element order to 2nd and increasing the number of elements, the results are good:
I just noticed there are only 5 bottom chords on one side of the first truss image. ???
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