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Reinforced concrete beams are classified as Over-Reinforced, Under-Reinforced and Balanced based on percentage of reinforcement provided in it. 

Moment of resistance of reinforced concrete beams are calculated based on following aspects 

• Plane sections remain plane in bending up to the point of failure. This means that strains are proportional to distance from the neural axis.

• Ultimate limit state of bending failure is assumed to have been reached when the strain in the concrete at the extreme bending compression fiber reaches 0.0035. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.

Strain diagram of a beam (a) Section (b) Strain diagram

• The stress distribution across compression face will correspond to the stress-strain diagram for concrete in compression.
• The tensile strength of concrete is neglected as the section is assumed to be cracked up to the neutral axis.
• The stress in steel will correspond to the corresponding strain in the steel.

The reinforced concrete section in bending is assumed to fail when the compression strain in concrete reaches the failure strain.

Under-Reinforced Section

Reinforced concrete beam sections in which the steel reaches yield strain at loads lower than the load at which the concrete reaches failure strain are called under-reinforced sections.

Every singly reinforced beam should be designed as under-reinforced sections because this section gives enough warning before failure.

Yielding of steel in under-reinforced beam section does not mean the structure has failed, as when steel yields, excessive deflection and cracking in beam will occur before failure which gives enough time to occupants to escape before the section fails.

The failure in under-reinforced beam section is due to the concrete reaching its ultimate failure strain of 0.0035 before the steel reaches its failure strain which is much higher 0.20 to 0.25.

Over-Reinforced Section

Reinforced concrete beam sections in which the failure strain in concrete is reached earlier than the yield strain of steel is reached, are called over-reinforced beam sections.

If over-reinforced beam is designed and loaded to full capacity then the steel in tension zone will not yield much before the concrete reaches its ultimate strain of 0.0035. This is due to the little yielding of steel, the deflection and cracking of beam does not occur and does not give enough warning prior to failure.

Failures in over-reinforced sections are all of a sudden. This type of design is not recommended in practice of beam design.

Balanced Section

Reinforced concrete beam sections in which the tension steel also reaches yield strain simultaneously as the concrete reaches the failure strain in bending are called balanced sections.