parabolic arc diagrams

[edoardo3]

I'm trying to calculate the diagrams of the parabola arc, a simple one, with three pivot. Isostatic with an horizontal distributed load. I've modeled the structure subdividing the arc in 20 frame. rectangular section. What I get is the Shear and Moment diagrams are not 0, moment is 0 in the nodes, and for the single frames M(z) = (q*z/2)-(q*z*z/2) parabolic. deriving you get the corresponding shear, linear.
Is the problem due to the need to refine the frame