In the equivalent rectangular concrete stress distribution, the distance from the fiber of maximum strain to the neutral axis shall be measured in a direction parallel to the neutral axis
Here are some conditions for equivalent rectangular concrete stress distribution;
Concrete stress shall be assumed uniformly distributed over an equivalent compression zone bounded by edges of the cross section and a straight line located parallel to the neutral axis.
Distance from the fiber of maximum strain to the neutral axis, shall be measured in a direction perpendicular to the neutral axis.
