What is the difference between over reinforced beam and under reinforced beams ?

To start with you should first know what is a beam — Beam is basically a structural member, constructed horizontally which resists the load applied on the it. The primary mode of failure is bending. We mostly go for a reinforced beam construction. A combination of steel and concrete is used for construction.

To know more about reinforcement and the use of it, check this post.

On the basis of the tensile strength of steel considered for the analysis and designs of beams we classify three types of sections, they are :

  1. Under – reinforced section
  2. Balanced section
  3. Over – reinforced section

Under Reinforced beam section

An under reinforced section is a type of section in which we use steel for its ultimate tensile strength. Under reinforced beam section undergoes a tensile failure. The percentage of tensile reinforcement is less than the amount of reinforcement provided for a balanced section. As in this type of section reinforcement fails first i.e it undergoes a ductile failure. The main standout feature of a ductile failure is that it gives sufficient amount of safe time before failure.

Balanced beam section

In this type of section the ultimate strength of concrete and steel are reached simultaneously. At the same point of time concrete and steel fails and ultimately structure fails by crushing of concrete.

Over Reinforced beam section

An over reinforced section is a type of section in which we use the concrete for its ultimate compressive strength strength. The yield strain of concrete is reached before the ultimate strength of steel. Over reinforced beam section undergoes a compressive failure. The percentage of tensile reinforcement is more than the amount of reinforcement provided for a balanced section. As in this type of section concrete fails first i.e it undergoes a brittle failure. The main disadvantage of this section is it undergoes a sudden failure without warning. We will not advice to select this type of section for the structure as it adds an extra cost, by increasing the percentage of reinforcement.

It is always advisable to go for a under reinforce beam section as it is safer when we compare all the three beam section. Because of its ductile failure it provides sufficient time before collapse and reduces the intensity of hazard.

Beam design

  • RCC Beam Design is a free app for designing reinforced concrete beams as per Indian Standards.
  • RCC Design and detailing could be performed by Limit State Method specified in IS456:2000
  • Option to save the design projects in local storage.
  • Detailed calculation steps presented for verification and validation.
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