# The use of U1 and B31 elements in one simulation causes an error

**URL:** <https://calculix.discourse.group/t/the-use-of-u1-and-b31-elements-in-one-simulation-causes-an-error/2911>\
**Category:** Analysis issues\
**Created:** [November 27, 2025, 11:30am UTC](https://calculix.discourse.group/t/the-use-of-u1-and-b31-elements-in-one-simulation-causes-an-error/2911 "2025-11-27T11:30:53Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![acet](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@acet](https://calculix.discourse.group/u/acet)\
**Post date:** [November 27, 2025, 11:30am UTC](https://calculix.discourse.group/t/the-use-of-u1-and-b31-elements-in-one-simulation-causes-an-error/2911/1 "2025-11-27T11:30:53Z")

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Hello,  
I am trying to use U1 and B31 elements in simulation because I found U1 is useful when I connect beam to solid.

However if I add B31 element, even when when it is not connected to U1, the simulation makes error.  
\<\*ERROR in gen3delem: first thickness  
in node 1 of element 1  
is zero\>

Input file is  
\*NODE, NSET=Nall  
1,0.,0.,0.  
2,50.,0.,0.  
3,0.,20.,0.  
4,50.,20.,0.  
\*ELEMENT, TYPE=U1, ELSET=Eu1  
1,1,2  
\*USER ELEMENT,TYPE=U1,NODES=2,INTEGRATION POINTS=2,MAXDOF=6  
\*ELEMENT, TYPE=B31, ELSET=Eb31  
2,3,4  
\*MATERIAL,NAME=steel  
\*ELASTIC  
200.e3,0.3  
\*BEAM SECTION,ELSET=Eu1,MATERIAL=steel,SECTION=GENERAL  
314.000, 7854.000, 0, 7854.000, 0.866  
0.,1.,0.  
\*BEAM SECTION,ELSET=Eb31,MATERIAL=steel,SECTION=Rect  
20,20  
0.,1.,0.  
\*STEP  
\*STATIC  
\*CLOAD  
2,1,1.  
4,1,1.  
\*END STEP

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<div class="post-metadata">

**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:** [November 27, 2025, 4:15pm UTC](https://calculix.discourse.group/t/the-use-of-u1-and-b31-elements-in-one-simulation-causes-an-error/2911/2 "2025-11-27T16:15:46Z")

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It seems that you can’t use U1 and regular beam elements in one analysis. I’m not really surprised since U1 are quite new and specific. CalculiX often doesn’t print precise error messages in such cases.