Flexural capacity of bending members is calculated by the OpenBrIM library object designated for Building Design Suite.
The computation of flexural strength is based on “Whitney Rectangular Stress Block Distribution” replacing nonlinear stress distribution in concrete by a rectangular compressive stress distribution (Whitney, 1937).
Main assumptions of Whitney’s approach:
An average stress of 0.85f'c is used with a rectangle of depth a =β1*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)