I was working earlier on an example problem of a trussed beam. Above you can see a picture illustrating the situation. On top there is a simply supported beam, with a stiffening truss below. The truss consists of 3 members, all connected through pins to each other, the supports, and the middle member pin connected to the beam. The beam has a uniform load on top.
I'm having a hard time calculating the degree of indeterminacy of this system. I'm told that this system is first degree indeterminate.
If we take the left support as point $A$ and right support as point $B$, we should have supporting reactions $A_x$, $A_y$ and $B_y$, as well as 3 normal loads for the truss members (since they only carry normal loads, no moments or shear). Using statics, we can form 3 equations: $\Sigma F_x$, $\Sigma F_y$ and $\Sigma M_z$. Because we have 6 unknowns, I would say this is 3rd degree indeterminate.
My reasoning is quite likely wrong, I'm having hard time for some reason understanding static determinacy beyond simple examples. Could somebody walk through the process of calculating the degree of indeterminacy for this structure in detail? Thank you very much!