What is the difference between under reinforced section and over reinforced section?

What is the difference between under reinforcement section and over reinforcement section?

difference between under and over reinforce section 

Under Reinforce Section

If less amount of steel used in the section than that of a balance section is called under reinforced section. In such condition the stress the stress in steel will reaches its concrete remain below the permissible stress. Under such condition the beam will fails due to over stress in the steel. The steel provided in the beam is such that steel reach the limiting values of strain prior to concrete. This results in yielding of the steel and the steel could yield till it attains the ultimate strain at which point it breaks. It is important to notice that it is the yielding stain that is considered as failure strain in steel and not the ultimate fracture strain. The steel still has to undergo strain hardening, softening and then fracture. In under reinforced section, the depth of actual Na is less than the critical Na.

i.e.; Na<Nc.

Over Reinforce Section

If more steel used in the section than that of a balance section is called overreinforce section. The steel is not fully stressed to its maximum permissible tensile stress, which the comprehensive stress in concrete at extreme fiber reaches its maximum permissible value in this case the beam will fails initially due to over stress in concrete.So, in over reinforced section, the depth of actual Na is greater than the critical Na.

i.e.; Na > Nc

Xumax Equation: Xumax / 0.0035 = d-xumax / (0.87fy/Es + 0.002),,,

value of Xumax and Mlimit