The strain distribution used in section 18.7 calculations is used (see "Section 18.7 Design Flexural Resistance (Prestressed)" for details).
The neutral axis depth is limited to:
(0.36/0.85) × d
where
d
=
depth of tensile reinforcement centroid
To be rational (instead of literally following the code), "d" is taken as the depth of the total tension reinforcement, not just the depth of the PT
For details on how the neutral axis depth limit is related to the code criterion, see "Unified Design Recommendations for Reinforced...." by Antoine Naaman in ACI Structural Journal, pp 200-210, Vol 89, no.2, April-March 1992
