Normal boundary condition

Dear all,

I have the same problem as @michaellevitin : I have a 3D elasticity problem with symmetry conditions on a equilateral triangle. Displacement vector is [ux,uy,uz]. On the base “1” of the triangle (y=0, x=[-a/2 a/2]) I can simply add …+on(1, uy=0). However on the other two sides of the triangle, being not aligned to x or y axis, I tried …+on(2, uxN.x+uyN.y) but of course is not working. In fact I should put into relations ux with uy and therefore it is not possible to simply putting the diagonal elements to a tgv value.
Is there a way to impose symmetry conditions (e.g., Dirichlet boundary condition for the normal component of a vector field) on boundaries that are NOT aligned to x, y and z axis?
Thanks very much in advance