,I would be glad if you could also answer my query in this thread.
You mentioned, "The solution results in all internal forces being in equilibrium with the external applied forces and reaction forces from the constraints.", so I have read that the reaction forces at the boundary conditions are the unknowns, we know this. So basically, the solution to stiffness matrix equation gives us the unknown displacements as well as the unknown forces (at the boundaries), right? Well, to be more exact, the reaction forces from the constraints are going to be in equilibrium with the external applied forces. I just want to understand that the internal forces that you mentioned, are these the forces at the nodes, or within an element? Secondly, if these internal forces are at the nodes, then what are they compared against to check equilibrium? Because the externals are balanced by the forces at constraints. These nodes (which are neither the constraints nor any external forces are applied there), we know that we find the displacements of them by solving the stiffness matrix equation, but how do we find the internal force at these?