Just a opinion about truss node

[Matrix]

How can I explain the fact that a node in a ideal truss shape have 3 degree of freedom( 2 translations and one rotatios above z axis) but in the considerations for structural analysis we only refers only just two translations degrees of freedom and don´t mention the rotation(I know that the bending moment around the node is zero)?

 

[JAE]

Roughly - you still have three degrees of freedom, its just that the Z rotation is null due to the fact that you have chosen to model your truss with lots of pins (as you said - zero moment). With Hooke's Law - zero force/moment = zero displacement effect and that DOF simply drops out of the equations.