# Dynamic model with structure elements

**URL:** <https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127>\
**Category:** 3DEC\
**Created:** [November 11, 2021, 5:24pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127 "2021-11-11T17:24:42Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![fredrik](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/fredrik/32/46_2.png) [@fredrik](https://forum.itascainternational.com/u/fredrik)\
**Post date:** [November 11, 2021, 5:24pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/1 "2021-11-11T17:24:42Z")

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Hi!  
Running a model with structure shell elements gives a dynamic timestep of 1e-9.  
Running the same model without structures gives a dynamic timestep of 1e-7.

Is there a way to make the model with shell elements using a larger dynamic timestep?

BR  
Fredrik

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**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [November 11, 2021, 7:40pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/2 "2021-11-11T19:40:16Z")

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It looks like the partial mass density scaling does not consider the structural elements. If you have some very small segments, and the shell is likely very stiff, this will definitely reduce the timestep. If you are using STRUCT CREATE FROM-BLOCK-FACE then you will probably by creating tiny segments. You could try instead making the shell from dxf that matches the face before cutting joints - in this way you might avoid some of the small segments. Just make sure that when you make the shell from importing a dxf that the normals are pointing into the tunnel.

Otherwise, maybe @flac3dsupport has some ideas.

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**Author:** ![fredrik](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/fredrik/32/46_2.png) [@fredrik](https://forum.itascainternational.com/u/fredrik)\
**Post date:** [November 12, 2021, 2:39pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/3 "2021-11-12T14:39:36Z")

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Thanks, that helped.

I used STRUCT SHELL IMPORT FROM-FILE ‘ShotcreteLiner.dxf’ and got the dynamic timestep 1e-7

BR  
Fredrik

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**Author:** ![cheng](https://avatars.discourse-cdn.com/v4/letter/c/ecccb3/32.png) [@cheng](https://forum.itascainternational.com/u/cheng)\
**Post date:** [November 15, 2021, 4:44pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/4 "2021-11-15T16:44:41Z")

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In FLAC3D (might be the same in 3DEC), for structures with rigid links, the masses and stiffness of the structure nodes are translated into the target zones and further distributed into the zone gridpoints. The dynamic time step is then calculated from the accumulated masses and stiffness at the gridpoints. To make the time step bigger, the minimum zone/element size can be larger, the stiffness (if possible) can be smaller.  
Also, it depends on damping. Rayleigh damping reduces the time step very much. If possible, use Maxwell damping, or for compromise, using combined (not local) damping can be acceptable to some extent.

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**Author:** ![Damir](https://avatars.discourse-cdn.com/v4/letter/d/7993a0/32.png) [@Damir](https://forum.itascainternational.com/u/Damir)\
**Post date:** [May 2, 2022, 1:59pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/5 "2022-05-02T13:59:28Z")

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Dear,

is it possible to connect **rigid** blocks and **deformable** beams. Deformable vs deformable works fine. See screenshot.

Thanks in advance.

 ![IMG_7246](https://canada1.discourse-cdn.com/flex031/uploads/itasca/original/1X/543c51667719ab19a636ff6ea4eb8ee1c9bf2c0d.jpeg)

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**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\
**Post date:** [May 9, 2022, 10:47pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/6 "2022-05-09T22:47:53Z")

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Not sure what that picture is showing, but you can’t have beams with rigid blocks in 3DEC.

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**Author:** ![Damir](https://avatars.discourse-cdn.com/v4/letter/d/7993a0/32.png) [@Damir](https://forum.itascainternational.com/u/Damir)\
**Post date:** [May 19, 2022, 12:00pm UTC](https://forum.itascainternational.com/t/dynamic-model-with-structure-elements/127/7 "2022-05-19T12:00:47Z")

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Thanks @jhazzard! That is information I needed.