# Expansion of B31 elements with pretensions

**URL:** <https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477>\
**Category:** Uncategorized\
**Created:** [February 4, 2023, 7:45pm UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477 "2023-02-04T19:45:43Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![jbr](https://avatars.discourse-cdn.com/v4/letter/j/ecccb3/32.png) [@jbr](https://calculix.discourse.group/u/jbr)\
**Post date:** [February 4, 2023, 7:45pm UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477/1 "2023-02-04T19:45:43Z")

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Hello dear CalculiX community,  
I’be been trying to set this model up with \*PRE-TENSION on a 1D beam element (B31). One of the error messages is:  
`*ERROR in gen3dfrom1d: element X is a linear beam element with circular cross section Please use quadratic elements for beams with circular cross section. `  
But when I change to the B32 elements, I get the following:  
`*ERROR reading PRE-TENSION SECTION: element X is not a linear beam element`  
Has anyone experienced this before?

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**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\
**Post date:** [February 4, 2023, 8:05pm UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477/2 "2023-02-04T20:05:04Z")

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Try with a rectangular cross-section. The beam element used for pre-tension is deleted and replaced with MPCs anyway.

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**Author:** ![xyont](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/xyont/32/3553_2.png) [@xyont](https://calculix.discourse.group/u/xyont)\
**Post date:** [February 5, 2023, 4:22am UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477/3 "2023-02-05T04:22:30Z")

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> [@Calc\_em](#):
>
> Try with a rectangular cross-section

right, it can be replaced by rectangle sections as provide in official [example](https://feacluster.com/CalculiX/ccx_2.18/doc/ccx/input_deck_viewer.php?input_deck=pret3.inp). but required pre-calculation of beam width/height dimension based on equivalency of areas. direct way defining by it’s diameter could be better.

> [@jbr](#):
>
> One of the error messages is:

can it be more specific, solver versions and operating system?

i’m using latest versions on windows OS, simple changed section type from `rect` to `circ` above input examples is not a problem here.

also someone out there [reported](https://juejin.cn/post/6864175805161799688) similar succeed to solve.

\*edited  
i checked again by rerun varying models, linear beam element only for easiness defining pretension.

it’s a dummy, the section and dimension are for completeness purpose. changing the type and values will not lead any differences in result.

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**Author:** ![jbr](https://avatars.discourse-cdn.com/v4/letter/j/ecccb3/32.png) [@jbr](https://calculix.discourse.group/u/jbr)\
**Post date:** [February 5, 2023, 7:24am UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477/4 "2023-02-05T07:24:23Z")

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> [@Calc\_em](#):
>
> Try with a rectangular cross-section.

You read my mind, that’s what I did, and it worked. Just catching up now. Thank you!

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<div class="post-metadata">

**Author:** ![jbr](https://avatars.discourse-cdn.com/v4/letter/j/ecccb3/32.png) [@jbr](https://calculix.discourse.group/u/jbr)\
**Post date:** [February 5, 2023, 7:27am UTC](https://calculix.discourse.group/t/expansion-of-b31-elements-with-pretensions/1477/5 "2023-02-05T07:27:08Z")

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> [@xyont](#):
>
> . direct way defining by it’s diameter could be better.

Yes, much easier.

> [@xyont](#):
>
> can it be more specific, solver versions and operating system?

I ran Pastix and Pardiso on 2.18 in Ubuntu and 2.20 Pastix on Windows… They all fail in the analysis, so I need to debug this model more, but I think the pretension part is now working with the RECT workaround.