Method of joints & forces between joints

[Shakarri]

Part of my assignment is to find the stresses in each member of a truss bridge, but unlike the problems I have been taught before, this involves a load between the joints of the bridge. Me and my group are using the method of joints to find the forces acting through each member, the problem is that there are forces that do not act through the joints so we cannot sum them so easily. I would like to know how to set up an equation describing the vertical force on the joints beside the force.
I attached an image which should help make sense of my query. We are meant to assume that each member is in pure tension or compression i.e. there is no deflection or bending moments.

 

[SteamKing]

If the members can sustain no deflections and the joints cannot experience bending moments, then all that can be done is to write an equation for vertical force equilibrium involving the joints on either side of the applied load. The reactions due to the loads must then be transmitted into whatever members meet at a particular joint.