How to properly set up symmetry BC for cylindrical shapes

General Questions
1

Hello everyone. I would like to ask for guidance on how to properly set up the boundary conditions for symmetry in a quarter model of a joint of cylindrical pipes. I’ve already set up the no-displacement BC due to symmetry.

I have modeled the full model as well to compare, but the results are very different. I think maybe the BC must be applied in a cylindrical coordinate system, but had no luck guessing the proper way to do this. The circular profile of the application load face is not being maintained, how could I prevent this?

Full model setup:

image
image738×666 90.1 KB

Quarter model setup:

image
image648×534 59.1 KB

Full model results:

image
image1071×683 78.5 KB

Quarter model results:

image
image1076×679 82.5 KB

The files can be found here:
https://drive.google.com/drive/folders/1O7mH7Hy_PVeZUJsuiXLUOS4CDJlEOjw7?usp=sharing

2

It seems to me from the figures the load is not symmetric over the same symmetry planes as the geometry. Is such case the symmetry cannot be applied.

Poslano iz programa Outlook za iOS

2 Likes
3

Indeed, symmetry can be applied only when geometry, material properties, boundary conditions, load and response are all symmetric.

But this joint is somewhat strange. Is the vertical pipe really supposed to go through the horizontal one like this, leaving almost no clearance ?

joint
joint921×660 75.5 KB

1 Like
4

I was too focused on the geometry and BC that forgot about the requirements for load. Thanks for pointing it out!

5

Yes, it is a cross-joint meant only for structural purposes. The horizontal tube is drilled to receive the vertical tube, which will be used to attach to the rest of the structure. a MIG weld of about 4mmx45° is made to joint both.

6

Hello, if i am defining the symmetry BC for this section as shown in your videos it is showing an error “node21 has 2 degree of freedom”

now i have placed BC only in Z rotation is it correct or do i need to modify to get the symmetry effect.
@FEAnalyst

SEMI CUT PIPE.pmx (3.6 MB)

Screenshot 2026-03-12 113654
Screenshot 2026-03-12 1136541608×733 223 KB

7

Symmetry for solids is just fixing the translation in the direction normal to the face (U1 and U2 here). Fixing rotational DOFs has no effect for solids.

But you also have to make sure no nodes have rigid body constraint and boundary condition applied at the same time - hence the error message. You can exclude the edge nodes from one of them via a node set.

SEMI CUT PIPE mod.pmx (3.6 MB)

2 Likes