Balanced, Under reinforced, Over reinforced Beam Sections | Classification of RCC Beam Sections

“If you don’t get out of the box you’ve been raised in, you won’t understand how much bigger the world is.”

A beam is a horizontal or inclined structural member that is subjected to transverse loads which cause bending, torsion, and shear.

RCC beams are briefly classified as Balanced Section, Under-reinforced Section, Over-Reinforced Section.

Table of Contents

Balanced, Under reinforced, Over reinforced Beam Sections

Assumptions in Analysis of a beam section

1.Tensile stresses are taken up by reinforcement alone and none by concrete. That is why in tensile zone concrete only helps in maintaining the shape and providing cover to steel.

2.Stresses from concrete are transferred to steel so that strains in the steel and concrete remain the same.

3.Any section plane before bending remains plane after bending.

4.The modulus of elasticity E remains constant for all stresses.

5.Steel reinforcement is free from initial stresses when it is embedded in concrete.

6.Modular ratio m = (280/3σcbc)

Direct Tension

σt = { Ft \over { Ac + mAst } }

Direct Compression

σc = { P \over { Ac + mAsc } }

Balanced, Under reinforced, Over reinforced Beam Sections – Classification

Balanced Section

The section in which stresses in steel and concrete reach their maximum permissible limits simultaneously is known as Balanced Section.

Balanced section also known as Economic or Critical Section.

• As stresses in steel and concrete reach their permissible limits simultaneously therefore concrete and steel will fail at the same time.
• One can easily design a balanced section theoretically.
• But practically it is impossible to have a balanced section.
• The balanced section undergoes failure by primary compression.
• The depth of neutral axis in a Balanced reinforced section is: Actual depth of neutral axis is equal to the critical depth of the neutral axis. i.e. xa=xc

Under-reinforced Section

Section in which stresses in steel reaches first to their maximum permissible limit even before concrete is known as under-reinforced section.

• Steel in RCC steel section is less than amount of steel which is required to have a balanced section then the section is called as under-reinforced section.
• Most of the sections practically are under-reinforced sections.
• As stresses in steel reaches first to their maximum permissible limit therefore failure occurs in steel first.
• It is ductile failure which gives a clear warning before failure.
• That warning gives enough time to residents to escape from the room.
• Under-reinforced section undergoes failure by primary tension.
• Depth of neutral axis in an under reinforced section is: Actual depth of neutral axis is less than critical depth of neutral axis. i.e. xa<xc
• Moment of resistance of under reinforced section will be less than that of balanced section.

Over-reinforced Section

Section in which stresses in concrete reaches to its maximum permissible limit first even before steel is known as over-reinforced section.

• Steel in the RCC section is more than the amount of steel that is required to have a balanced section then the section is called an over-reinforced section.
• As stresses in concrete reaches first to its maximum permissible limit therefore failure occurs in concrete.
• Which is brittle failure and which does not give any clue or a warning sign.
• That is sudden failure does not give any warning to residents and there will not be any chance to escape.
• The over-reinforced section undergoes failure by primary compression.
• Depth of neutral axis in an over reinforced section is: Actual depth of neutral axis is greater than critical depth of neutral axis. i.e. xa>xc
• Moment of resistance of over reinforced section will be greater than that of balanced section.

FAQs on Balanced, Under reinforced, Over reinforced Beam Sections

What is Xumax?

Xumax is maximum depth of neutral axis at failure condition.

Why Over-reinforced section not preferred?

In under-reinforced RCC section as stresses in steel reaches first to their maximum permissible limit therefore failure occurs in steel first. It is ductile failure which gives a clear warning before failure.
That warning gives enough time to residents to escape from the room.
In over-reinforced as stresses in concrete reaches first to its maximum permissible limit therefore failure occurs in concrete. Which is brittle failure and which does not give any clue or a warning sign.
That is sudden failure does not give any warning to residents and there will not be any chance to escape.

That is why Over-reinforced sections are not preferred.

In an under reinforced section the depth of actual neutral axis is?

Depth of neutral axis in an under reinforced section is: Actual depth of neutral axis is less than critical depth of neutral axis. i.e. xa<xc

In an over reinforced section the depth of actual neutral axis is?

Depth of neutral axis in an over reinforced section is: Actual depth of neutral axis is greater than critical depth of neutral axis. i.e. xa>xc

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