For actual depth of neutral axis,
C= T
(Area of stress block diagram)xb = T
0.36 fck Xu b = 0.87 fy Ast
Note : Position of Neutral Axis is directly proportional to amount of steel in tension.
Note : Based on amount of tension steel in section following type of section is defined.
Balanced Section
It is type of section in which the amount steel is such that strain in concrete is 0.0035 and strain in steel (tension)is 0.002+0.87fy/Es at the time of failure. It means both steel and concrete fails at the same time. Neutral axis at this section is equal to limiting neutral axis (Xu,Lim = Xu).
Under-reinforced Section
It is type of section in which amount of steel is such that strain in concrete at the failure (i.e, 0.0035) and in tension steel is more than 0.002+0.87fy/Es at the time of failure. In this section, Steel fails first then concrete hence the failure is ductile. It is the known as best section for design. Neutral axis in this section is less than maximum or limiting neutral axis (Xu<Xu,lim).
Over-reinforced Section
It is type of section in which amount of steel is such that the strain in concrete is 0.0035 & strain in tension steel is less than 0.002+0.87fy/Es at the time of failure. That means, concrete fails first which is brittle failure. since, concrete fails the used of steel is wasteless. This type of design is not recommended.
