Updated README.md with my understanding of how to compute the PM inte…

…raction diagram.

README.md

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+I have been fumbling around this for a while now, say a couple of years off and
+on. I have played with different ideas and approaches. Read some stuff, read
+some other stuff, re-read the original stuff. Talked with people about how it
+works. Doodled endlessly and wrote pages of notes. All to discover that it was
+right in front of me the whole time and it really is fairly straight forward. 
+
+P-M interaction diagrams determine the capacity envelope of a reinforced concrete
+member with a combination of axial force and moment applied at a section of the
+member. 
+
+The maximum usable concrete strain is given from experimentation as 0.003. Then
+it is a matter of iterating over a range of curvature values or neutral
+axis locations, either one works because they are related by a single equation.
+The strain in the steel is determined based on distance from the neutral axis.
+The stress in the steel is then calculated based on the strain in the steel and
+modulus of elasticity. Alternatively, a value directly from the stress-strain
+diagram could be use. Also depending on the material model it could be
+elastic-perfectly plastic or a more exact model. Finally, the forces and moments
+on the cross-section are summed. For each iteration a corresponding P-M pair are
+added to the array. 
+
+The one thing I am still unsure about is how a cracked section analysis plays
+into the development of P-M interaction diagrams. The answer to this is that the
+cracked section is taken care of by the stress-strain diagram of the concrete.
+Instead of using a Whitney stress block one can find the actual strain at
+discrete points, calculate the stress based on the strain and the stress-strain
+diagram, multiply the stress by the assumed area the discrete point represents
+to get a force for use in the force and moment equilibrium.