For the design of the longitudinal tension reinforcement, the effective depth of a section, d is defined as the distance from the extreme concrete fibre in compression to the centre of gravity of the longitudinal tension reinforcement.

For the design of the longitudinal compression reinforcement, the effective depth in compression, d₂ is defined as the distance from the extreme fibre in compression to the centre of gravity of the longitudinal compression reinforcement.

The designs for the longitudinal and shear reinforcement follow an iterative process.

For the initial design iterations the values of d and d₂ are given by;

d = (h – c_nom,i – φ_sw,u – 0.5*φ_i,u)

d₂ = c_nom,i + φ_sw,u + 0.5*φ_i,u

where

h = the overall depth of the concrete section including any slab

c_nom,i = the nominal cover to the reinforcement in outermost layer in the top or bottom of the beam as appropriate

φ_sw,u = the minimum diameter of the shear reinforcement specified by the user

φ_i,u = the minimum diameter of the reinforcing bars in the outermost layer in the top or bottom of the beam as appropriate as specified by the user

As the iterations proceed, the values of c_nom, φ_sw and φ_i vary accordingly and as 2nd and subsequent layers of longitudinal reinforcement are added, the value of d and d₂ become the distance to the centre of gravity of the reinforcement.