# \*BOUNDARY on DOF 8 in a structural model silently fixes the rotation about x (DOF 4) instead of giving an error **URL:** \ **Category:** Analysis issues\ **Created:** [October 19, 2026, 7:32am UTC](https://calculix.discourse.group/t/boundary-on-dof-8-in-a-structural-model-silently-fixes-the-rotation-about-x-dof-4-instead-of-giving-an-error/4191 "2026-10-19T07:32:14Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Sandervdb](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Sandervdb](https://calculix.discourse.group/u/Sandervdb)\ **Post date:** [October 19, 2026, 7:32am UTC](https://calculix.discourse.group/t/boundary-on-dof-8-in-a-structural-model-silently-fixes-the-rotation-about-x-dof-4-instead-of-giving-an-error/4191/1 "2026-10-19T07:32:14Z")
**Summary** The manual documents DOF 8 in only two places: - as the static pressure in 3-D CFD (§6.9.19); - as the electric potential in electromagnetic calculations (§6.9.22). For structures, \*BOUNDARY (§7.4) lists DOFs 1–6 and 11. Internally ccx maps 8 onto its DOF 4 (§10.2.1), the slot that a structural node uses for the rotation about x. In a structural model ccx accepts a \*BOUNDARY on DOF 8 without any message, and the result depends on the node: - **Shell or beam node:** the rotation about x is held, or prescribed if a value is given. - **Solid node:** the line is ignored. - **Plane-stress, plane-strain or axisymmetric node:** ccx usually crashes. The outcome varies from run to run. ### Version ccx 2.23 (conda-forge build pl5321h484372f\_2, macOS arm64). The DOF 8 mapping in bounadd.f is unchanged on master (9ba5d76c). ### Already reported or fixed? Not reported: there is no matching GitHub issue (open or closed) or open pull request in Dhondtguido/CalculiX, and no calculix.discourse.group topic (searched 17 Oct 2026). Not fixed: unchanged on master. conda-forge’s latest 2.23 build (pl5321hd17ca8b\_8) gives the same results on shell, beam and solid nodes. On plane and axisymmetric nodes it no longer crashes (its fix-2d-rotational-dofs patch; open PR #200 covers that crash upstream). ### Minimal input `job.inp`: ```auto ** One S4 shell, clamped along the edge x=0 (nodes 1 and 4), corner load at node 3. ** The rotation about x at nodes 1 and 4 is given as DOF 8 instead of DOF 4. *NODE 1,0.,0.,0. 2,1.,0.,0. 3,1.,1.,0. 4,0.,1.,0. *NSET,NSET=NALL 1,2,3,4 *ELEMENT,TYPE=S4,ELSET=EALL 1,1,2,3,4 *MATERIAL,NAME=STEEL *ELASTIC 210000.,0.3 *SHELL SECTION,ELSET=EALL,MATERIAL=STEEL 0.1 *BOUNDARY 1,1,3 1,5,6 1,8 4,1,3 4,5,6 4,8 *STEP *STATIC *CLOAD 3,3,1. *NODE PRINT,NSET=NALL U *END STEP ``` ### How to run ```auto OMP_NUM_THREADS=1 ccx job ``` ### Expected An \*ERROR, since DOF 8 has no meaning in a structural analysis. For example: ```auto *ERROR reading *BOUNDARY: degree of freedom 8 can only be used in CFD and electromagnetic calculations ``` At the least there should be a \*WARNING that the line is ignored. The solution would then be that of the deck without the two DOF 8 lines: U3 = 1.569850E-02 at node 3. ### Actual The job finishes with exit code 0 and no warning: ```auto STEP 1 Static analysis was selected Job finished ``` The .dat file is byte-identical to that of the same deck with DOF 4 in place of DOF 8: | \*BOUNDARY lines at nodes 1 and 4 | U3 at node 3 | | --- | --- | | `1,8` / `4,8` (this deck) | 1.450110E-02 | | `1,4` / `4,4` | 1.450110E-02 | | no DOF 4 or DOF 8 line | 1.569850E-02 | | `1,8,8,0.01` / `4,8,8,0.01` | 1.494961E-02, the same as `1,4,4,0.01` / `4,4,4,0.01` | Other element types: - **B31:** DOF 8 acts like DOF 4. An L-frame of two B31 elements is a torsional mechanism without a DOF 4 constraint at its support, yet it solves when DOF 8 is given, with the same .dat as with DOF 4. - **C3D8:** `7,8,8,0.5` on a node changes nothing and gives no message. - **CPS4, CPE4, CAX4:** `4,8` on a node ends differently from run to run. Over 6 runs of the CPS4 deck there were 5 bus errors (exit code 138) and 1 normal finish. Other runs gave a segmentation fault (139) or `*ERROR in usermpc: mpc of type ... is unknown` (201). This is the uninitialised mean-rotation path that DOFs 4–6 take on plane nodes, which open PR #200 addresses. ### Where it goes wrong bounadd.f maps DOF 8 onto DOF 4 for every node (master lines 109–110). It does not check the type of analysis or the elements at the node. ### Workaround Do not use DOF 8 in structural \*BOUNDARY cards. If the rotation about x is meant, use DOF 4. ---
**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:** [October 19, 2026, 8:42am UTC](https://calculix.discourse.group/t/boundary-on-dof-8-in-a-structural-model-silently-fixes-the-rotation-about-x-dof-4-instead-of-giving-an-error/4191/2 "2026-10-19T08:42:34Z")
Confirmed (using official build of ccx 2.23 on Windows), needs a GitHub ticket. ```auto CSC Conversion Time: 0.005352 Init Time: 0.064804 Factorize Time: 0.006282 Solve Time: 0.086747 Clean up Time: 0.000000 --------------------------------- Sum: 0.163185 Total PaStiX Time: 0.163185 CCX without PaStiX Time: 0.039652 Share of PaStiX Time: 0.804514 Total Time: 0.202836 Reusability: 0 : 1 ________________________________________ Using up to 1 cpu(s) for the stress calculation. Job finished ________________________________________ Total CalculiX Time: 0.218138 ________________________________________ ``` ---
**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:** [October 19, 2026, 9:25am UTC](https://calculix.discourse.group/t/boundary-on-dof-8-in-a-structural-model-silently-fixes-the-rotation-about-x-dof-4-instead-of-giving-an-error/4191/3 "2026-10-19T09:25:46Z")
@Sandervdb This one is left to report on GitHub. ---
**Author:** ![Sandervdb](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@Sandervdb](https://calculix.discourse.group/u/Sandervdb)\ **Post date:** [October 19, 2026, 9:27am UTC](https://calculix.discourse.group/t/boundary-on-dof-8-in-a-structural-model-silently-fixes-the-rotation-about-x-dof-4-instead-of-giving-an-error/4191/4 "2026-10-19T09:27:54Z")
Sorry, I posted it on github but forgot to report it back here 😃 [\*BOUNDARY on DOF 8 in a structural model silently fixes the rotation about x (DOF 4) instead of giving an error · Issue #219 · Dhondtguido/CalculiX · GitHub](https://github.com/Dhondtguido/CalculiX/issues/219)