To be equivalent, the transformed cross section must yield the same bending strains as the actual cross section.
Bending strains are defined by:
where y = coordinate measured from the neutral surface and ρ = radius of curvature.
From this relationship, it is evident that the y coordinates of the transformed cross section must be kept identical to those of the actual cross section if the strains are to be the same in both. Therefore, the heights of the bars cannot be modified.