Out of plane constrains of a 2D beam model

Grasshopper Karamba3D
1

Hi there,

I am working with a 2D beam model and I am not sure why certain degrees of freedoms need to be constrained in order to avoid rigid body modes. I would value if someone can provide me some background.

To figure out what I need to constrain I made a simplified model, see attached. The triangular shaped frame consisting of two beams is simply supported and joined with a hinge. To avoid rigid body modes I must constrain the rotation around the x-axis (direction of span) for both supports and also for the joint.

I understand that constraining the rotation around x is required for the supports, else the frame would just tilt over, why is a constraint in the joint required?

I tried to replace the rotation constraint around the x axis in the support with a support in the out of plane direction at the node where the two beams meet. I would expect that this leads to the same result, consequently making the constraint of the rotation around the x axis superflous at the supports. However, when removing the named constraints at the supports rigid body modes occur, so my expectation is wrong. Where is my fault?

I appreciate your perspective!

image
image478×618 10.5 KB

triangle_2Dcheck.gh (44.4 KB)

2

Hi, can you show the error message that you see? When I open your script I do not see any errors.

In any case, Karamba gives a warning if the structure is potentially kinematic. In this case, if you have supports defined at 0 and 1, only in Tx and Tz, it will give this warning, but calculate accordingly. If you were to then apply loads out of the xz plane, then the structure will fail.

3

Thank you for your response, Matthew. Please apologize that I only respond now, I am in the in final stage of my master thesis.
I always assumed that the warning means something is wrong with my model, but after checking what you said I agree. I am glad that this is also true for my bigger model and not only the triangle. I enjoy working with Karamba3D.

Have a good day!