If the user enters the steel reinf layers from top to bottom there are instances where the plastic neutral axis calculation fails. Appears to be due to the top most layer of steel falling directly on the PNA location at certain intervals in the calculation.
find_pna class function was using the first bar elevation in lieu of the highest d, therefore causing the bisection method to only search for a solution between the top of the beam and first steel layer.
If the user enters the steel reinf layers from top to bottom there are instances where the plastic neutral axis calculation fails. Appears to be due to the top most layer of steel falling directly on the PNA location at certain intervals in the calculation.