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# Boundary condition

## Boundary condition

(OP)
Hi everybody! I'm analyzing with Patran and Nastran a multi-arch structure where the arches are made of a sandwich material (so I used 3D elements for the core e 2D elements for the upper and lower faces). The problem that I have is related to the boundary condition because as you can see in the picture at the end of each arch there will be a hinge condition, in order to have a rotation around one axis, and that's why I used also RBE2. The problem is when I've to set the boundary condition for the arches rotated by 45° with respect to the axis Y-axis. I thought to use a different Coord system but it seems useless because when I run the analysis there is a difference between the arches aligned with the principal Coord system of Patran and the other rotated by 45°. I hope that is clear my message.

### RE: Boundary condition

Post a plot of the deformed geometry. Have no idea what your issue is.

### RE: Boundary condition

(OP)
Hi SWComposites, here are two pictures that can help. The structure is symmetric and the load is also symmetric and as you can see from the picture the load is higher in the center and decrease along the arches. In the center, there is a pressure. So if the load is symmetric and the structure is symmetric I suppose that the displacements are symmetric.

### RE: Boundary condition

Do the RBE2s at the base connect to all dofs?

Are the reaction loads at all 8 base nodes the same?

Are your element material coordinate systems all aligned in the same way along the legs?

The base coordinate systems look ok, as do the base constraints. There is something subtly different between the legs. Try changing all of the material properties to purely isotropic.

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