Inside the staad model is a lattice truss beam. My understanding on the truss system is that the diagonal, vertical & horizontal bracing are subjected to axial force only.
When I assigned all bracings as TRUSS member, it causes instability. Refer to this B2 staad model - B2.STD
When I assigned the diagonal and vertical bracing as TRUSS member only, the lattice truss beam has no issue. (Understand that the horizontal bracing must take moment at the end connections so that the overall structure is stable). Refer to this B1 staad model - B1.STD
So which method of assignation is correct?
Usually, Truss specification is assigned to the web (bracing) members only as top and bottom chords can take some moment and should not have a Truss specification assigned.
I did not assign top & bottom chords as truss member. Apologies that my question is unclear, do I assign the bracing with blue line (diagonal brace) as Truss member only?
Or should I assign both bracing with redline (horizontal and vertical brace) and bracing with blueline (diagonal brace) as Truss member?
Sorry, I missed those cantilever parts on the truss. These cantilevers create torsion on the top chord. If you assign Truss spec to all members joining those nodes with cantilevers, then nothing can resist that moment (as chords also have pinned supports), so these cantilevers become unstable. So some bracings (vertical or horizontal) should have some rotational fixities to resist those moments from the cantilevers.