Hi, I'm using weldments to create a rectangular pyramid and wish to conduct a stress/strain analysis. (see figures)

I have a couple of questions and problems..

- Wish to have all joints as "ball hinges" with dof's in all directions. What is my best go? Trusses or Beams? If I treat all profiles as a Truss, are the joints acting like 3d hinges?

- I also wish to have a rigid corner between beam AB and AD. For this one, I thought I have to set the profiles as Beams. Just in order to let me choose what kind of connection there is.

- Also, running a Beam-analysis  with a force of 500N perpendicular to the top-plane gives an "Model is unstable"-error.

How can I achieve this in the best way? I tried to search the internet and forums, but most of the analyses use rigid connections. And that is really not what I need. Thanks in advance!