The depth to the neutral axis is solved for by an iterative strain-compatibility procedure that takes into account the force in each individual strand, bar or wire. The concrete stress profile is modeled through the Whitney stress block. This assures that multiple reinforcement layers and types are properly accounted for in the final flexural resistance moment.
The depth of the Whitney stress block is related to the depth of the neutral axis by the equation:
Where β1 is calculated as per ACI Table 22.2.2.4.3.
Phi is computed using the net tensile strain in the reinforcement, and is based on whether the section is compression controlled, tension controlled, or in the transition zone between the compression and tension controlled zones. Phi is calculated as per ACI Table 21.2.2.
Deflections
The maximum horizontal deflection is checked against the allowable deflection criteria input by the user.
Minimum Steel
ACI 318 requires that the total prestressed and non-prestressed reinforcement be adequate to develop a design strength of at least 1.2 times the total moment required to crack the concrete element. This provision is assumed to apply only at critical sections, and is waived for components where the flexural resistance is at least twice that required by analysis. This value is displayed on the flexural results graphs.