See "General Design Approach" for general information on cross section calculations.
The neutral axis depth (kou) is limited to 0.36 per section 8.1.5. Reinforcement is added to minimise the neutral axis depth.
If the reinforcing bar covers are such that the compression bar is outside of the 0.36 d zone, then a solution may not be possible.
In certain circumstances it may not be possible to simultaneously provide positive moment ductility and negative moment ductility. This most commonly happens if there is a large post-tensioning tendon near the center of the cross section.
