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An average stress of 0.85f'c is used with a rectangle of depth a = beta1β1*c.
The concrete below the neutral axis is ignored and the total tension force is due the reinforcing steel.
Whitney’s approach is widely accepted and also referred by ACI 318-14. (See 22.2.2.4 and R22.2.2.4 of the ACI 318-14 Specification)