ETABS JOINT TYPE 3

[Nadifa]

Hello everyone! anyone know why is it happen like this? the only difference between two models is only the joint type. First model, I assign joint restraint with no movement allowed at all (as you can see below) and the second model I assign column (the real condition at the site). Which one the result of design is true?

for more information and result, the file is attached.

I really hope I could find an answer through this forum. Thank you!

 

[XR250]

Probably neither. Try a rough hand calc to get an estimate.

 

[JoshPlumSE]

If you want to model a truss by itself, the normal boundary condition assumption is Pinned on one end and roller on the other. When you model in the column, you get closer to this assumption than what your other model (with overly restrained boundary conditions) shows.

Why doesn't a fully restrained (against translation) boundary condition work for trusses? Well, our assumption is that when the truss is subject to vertical load, the primary load resistance will be through axial tension in the bottom chord and axial compression in the top chord. Right? For this to happen, the chords must be allowed to elongate (when in tension) or shorten (when in compression) and your extra restraint prevents that.