2-D truss in 3-D environment instability

[lawrence_budziszQTYAB]

Hi All, 

Hopefully this is an easy one on this forum. I want to build a 3-D portal frame with some roof trusses. I typically use a 3-D Shell structure type template as it seems to be most versatile. 

As a test I modeled a simple 2-D truss in a 3-D environment. I made supports fixed and changed all members attributes to 'truss member'. I would be grateful if someone could explain to me the reason why I am getting an Instability Type 1 error in all nodes?  Should I approach modelling the truss in a different way? 

Kin Regards, 

[Simau · Accepted solution]

Hi @lawrence_budziszQTYAB 

In 2D there is no UY nor RX RZ, so no instabilities in these directions are possible

Trusses have no flexional rigidity, so in 3D all structure will rotate around X axis.
You may add restaints in Y direction at all nodes, or use lower chord as a beam (not truss) + restaints in upper nodes

[lawrence_budziszQTYAB]

@Simau ,

Thank you for your reply. This is very clear now. 

Thanks,