joist model instabilty in Risa 3D

[Johnny pap]

I have a standard OWJ between 2 beams.Risa shows instability in the bottom chord. I have Lyy=1200mm (48in) for the top chord, Lbzz=3000mm (10 ft).
For the bottom chord I've input Lbyy=3000mm and Lbzz=3000mm
Boundary (both ends):
Rx - Reaction
Ry - Reaction
Rz - Fixed
Mx - fixed
My - fixed
Mz - free
What I did wrong?

 

[KootK]

Can you post a sketch or screen capture of your model? I'd start with looking into your torsional boundary conditions.

 

[phamENG]

Is it a roof truss with uplift? And, if so, did you model bridging points? If your bottom chord is in compression and you don't have it braced, it would have a buckling instability.

I am a little confused why you have two of your moments fixed? One for torsion, sure, but not the other.

 

[JoshPlumSE]

It's a torsional instability of the bottom chord, correct? Or, maybe it's and out of plate instability. This is very common. It doesn't mean you did anything wrong. It's just one of the peculiarities of modeling a 2D truss in a 3D program.

Look at it this way. If you applied a lateral point load to the bottom chord, what resists that point load? How does the out of plane loading make its way to the support points?

You're not really going to load your structure that way, but the program doesn't know that. All it knows is that there is a zero in the main diagonal of the stiffness matrix and it has to resolve that somehow.

See the web help topic below. In particular, focus on the part about modeling 2D structures in RISA-3D.

https://risa.com/risahelp/risa3d/Content/3D_2D_Only_Topics/Stability.htm