DEFINITION OF UNDER REINFORCED AND OVER REINFORCED SECTION
Balanced section refers to the section in which the quantity of steel is just adequately arranged so that the concrete in compression zone and steel in tension zone attains its acceptable stresses instantaneously.
In this section, the critical depth is identical to its actual depth. i.e n = Na = Nc
UNDER REINFORCED SECTION:
In this section, the quantity of steel is insufficient to produce the highest concrete fibers in the compression area to be compressed with their highest allowable stress.
In this section, the quantity of steel is insufficient to arrange the concrete for being compressed in compression area to their maximum allowable value. It signifies the steel is arranged below that a balanced section needs. In under reinforced section, the depth of actual Na is under the critical Na.
i.e; Na < Nc
OVER REINFORCED SECTION:In this section, in tension zone, the quantity of steel remains in excess of the quantity of steel necessary to arrange compressive zone concrete to be compressed to their highest permissible value. Alternatively, if the highest compressive stress in concrete attains its permissible limit, the comparing tensile stress in steel will not surpass its allowable value.
So in over reinforced section, the depth of actual Na is over and above the critical Na.
i.e; Na > Nc