The lower plastic limit theorem is stated in Wikipedia as follows:
If an equilibrium distribution of stress can be found which balances the applied load and nowhere violates the yield criterion, the body (or bodies) will not fail, or will be just at the point of failure.
I'm particularly interested in this theorem in the context of reinforced concrete.
On this website, design of concrete corbel is described using "Strut and tie" method, which applies the lower plastic limit theorem.
In this method, the concrete joint is assumed to consist of struts in compression, representing parts of concrete in compression, and ties, representing reinforcing bars in tension. The struts and ties are then modeled as a truss, with members in compression and tension.
So, at the heart of my question is this: Do we require previous knowledge about the load paths through the structure before we can apply the lower plastic limit theorem?
The theorem as stated on Wikipedia states that if an equilibrium distribution of stress can be found which balances the external loads which will not fail, the structure will not fail.
So if we first just come up with a placement of reinforcing bars, with concrete assumed to be balancing this "truss", and the rebar does not yield, nor the concrete compressive stress is not exceeded, will our structure hold (not taking into account that it can be unoptimal)?
Considering the picture above: we have placed a bar close to the top and assumed that the locations where we have the compression struts (concrete in compression) are locations where the concrete will and can actually be in compression. Would any such arrangement be correct, assuming the "truss" will not fail? Or do we need some knowledge beforehand that such arrangement is actually close to the real distribution of stresses in the structure?