Boundary condition to apply plane of symmetry?
Boundary condition to apply plane of symmetry?
(OP)
Hi,
I have a simple linear-elastic model I'm trying to run. It has a plane of symmetry, so I decided to put that plane on rollers. I first did this by constraining all nodes on that plane to have zero displacement in the z-direction, and let it free in the x- and y- directions. This failed.
I realized that I didn't constrain it enough. It's been a while since I learned the fundamentals, but I do remember having to constrain one node on that plane in all directions, and another node in two directions. I just don't remember where the second node needs to be in respect to the first node (fixed in x- y- and z-), and in which directions I need to constrain the second node to achieve a plane of symmetry BC.
Can someone explain/refresh me on this? Thanks
I have a simple linear-elastic model I'm trying to run. It has a plane of symmetry, so I decided to put that plane on rollers. I first did this by constraining all nodes on that plane to have zero displacement in the z-direction, and let it free in the x- and y- directions. This failed.
I realized that I didn't constrain it enough. It's been a while since I learned the fundamentals, but I do remember having to constrain one node on that plane in all directions, and another node in two directions. I just don't remember where the second node needs to be in respect to the first node (fixed in x- y- and z-), and in which directions I need to constrain the second node to achieve a plane of symmetry BC.
Can someone explain/refresh me on this? Thanks





RE: Boundary condition to apply plane of symmetry?
For instance take a simply supported beam of length L subjected to a UDL along its length. The beam can be modelled as a an equivalent beam of Length L with a rotational restraint at midspan to mimic this effect. This because there is maximum moment and no rotation.
As another example take a portal frame fixed at its base subjected to a horizontal point load at the top corner. At the midspan of the beam there is no overall net vertical deflection, therefore apply a roller support.
RE: Boundary condition to apply plane of symmetry?
Initially, I take a 10 x 10 x 30 beam and fix it at both z=0 and z=30. I apply a 10N force at z=15. The reaction forces at z=0 and z=30 are 5N each. This is my baseline reference to check my symmetry BC in the next model.
Then, I take a 10 x 10 x 15 beam and fix it at z=0. At z=15, I constrain all nodes such that they cannot displace in the z-direction, but otherwise leave them free to move around in the x-y plane. I then apply a 5N force at z=15. The result I get from my code (reaction forces at z=0) is all 0N, when I'm expecting 5N. This makes me think I'm not constraining it enough.
RE: Boundary condition to apply plane of symmetry?
Post a sketch of the model you've got it you would so I can have a look.
I did a quick LUSAS run to make sure I didn't give you erroneous advice for your 3-point bending problem, for both the full model and symmetrical half model. I think you might be going wrong with working with a 3D rather than a 2D problem and have a mechanism of some means. Note I overlooked applying only half the load only!
For the first model, why did you put a roller on it? What was it thata you're trying to model, a portal frame subjected to a lateral load?
RE: Boundary condition to apply plane of symmetry?
RE: Boundary condition to apply plane of symmetry?
translations: -,-,0
rotations: 0,0,-
where
'-' = free
'0' = fixed