The document discusses the design of reinforced concrete (RCC) beams, categorizing them into three types: balanced, under-reinforced, and over-reinforced sections, each with distinct failure characteristics and behaviors under load. It highlights the differences in the placement of neutral axes, moment resistance, and failure modes, emphasizing the economic and safety implications of different reinforcement strategies. Additionally, it includes references for further reading on the topic.

1. Balanced, Under Reinforced
and Over Reinforced Section
Name - Navdeep
L-2020-AE-051-BIV

2. Content
• Introduction
• Balanced section
• Under Reinforced section
• Over Reinforced Section
• Failure in Balanced, Under and Over Reinforced section
• Comparison
• Moment of resistance
• References

3. Introduction
• When it comes to designing RCC beams, there are several options to
consider when it comes to the amount of reinforcement used
• we have three types of sections:
1) Balanced Section
2) Under Reinforced Section
3) Over Reinforced Section

4. Balanced Section
• Stress in concrete and steel
reach their permissible value at
the same time
• Percentage of steel
corresponding to this section is
called as balanced steel
• Neutral axis is called as critical
neutral axis 𝑛𝑐

5. Under Reinforced Section
• Percentage of steel provided is
less than that provided in
balanced section
• Actual neutral axis will shift
upwards i.e., nc > n
• Stress in steel first reaches it
permissible value, while the
concrete is under stressed

6. Over Reinforced Section
• Percentage of steel provided is greater
than the balanced section
• Actual neutral axis shift downward i.e.,
n>nc
• Stress in concrete reaches its permissible
value while steel is not fully stressed
• Concrete is brittle and it fails by crushing
suddenly
• Steel is not fully utilised, the over
reinforced section is uneconomical (steel is
much costlier than concrete)

7. Failure in Balanced Sections
• Balanced failure
• Both the materials will fail at
the same time due to same
permissible value

8. Failure in Under Reinforced Section
• Ductile or Under Reinforced
Failure
• The permissible strength of steel
is less as compared to concrete
• The application of load on this
particular member, the failure of
steel takes place prior to the
failure of concrete

9. Failure in Over Reinforced Section
• Brittle or Over Reinforced
Failure
• Permissible strength of steel is
greater as compared to concrete
• Application of load on such
member, the failure of concrete
will occur prior to the failure of
steel

10. Comparison
sr
no.
Balanced section Under Reinforced
section
Over Reinforced
Section
1)
Steel and Concrete both are fully
stressed
Steel is fully stressed Concrete is fully stressed
2)
n = nc Neutral axis lies above the
critical neutral axis
n < nc
Neutral axis lies below the
critical neutral axis
n > nc
3)
Percentage of steel axis lies on the
natural axis
Percentage of steel is less than
balanced section
Percentage of steel is more than
balanced section

11. Sr no. Balanced section Under Reinforced section Over Reinforced section
4) The moment is balanced at the
natural axis
The moment of resistance is less
than balanced section
The moment of resistance is
more than a balanced section
5) Uneconomcial Economical Uneconomical
6) Sudden failure
Safe failure
Ductile failure
Sudden failure
Brittle failure – crushing failure
7) Not Preferred by designers Preferred by designers Not Preferred by designers
continued......

12. Parameters
• Ast= Area of Steel in Tension Zone
• fst or σst= Stress in Steel
• fst.u or σst.u= Ultimate Stress in Steel
• fc or σc = Stress in Concrete
• fy = Yield strength of steel
• fck = Characteristics strength of concrete
• ϵc= Strain in concrete
• ϵst = Strain in Steel
• Nc or nc or Xu = Critical Neutral Axis
• N or n= Actual Neutral Axis
• D= Total Depth of Beam
• d= Effective depth of beam (from centroid of
steel in tension zone to topmost fiber of
concrete in compression zone).

13. Moment of resistance
Balanced section Under Reinforced section Over Reinforced section
(Moment of Resistance) Mr =
Compressive Force*(d-0.42d)
Mr= Tensile Force*(d-0.42d)
(Moment of Resistance)
Mr=fst.Ast(d-n/3)
(Moment of resistance)
Mr=b.n.(fst/2) *(d-n/3)

14. References
Sr no. Slide number Reference
1) 3 ,7-9 https://civiltutorofficial.com/why-do-we-prefer-under-reinforced-
sections-over-balanced-or-over-reinforced-sections/
2) 4-6 https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=web&c
d=&ved=0CAMQw7AJahcKEwjQhveztdH-
AhUAAAAAHQAAAAAQAg&url=https%3A%2F%2Fcivilengineering.blo
g%2F2017%2F11%2F14%2Fbalanced-sections-under-reinforced-
section-over-reinforced-section%2F&psig=AOvVaw2Oh-H4W6z-
5abbhLF16LmN&ust=1682937637971857
3) 10-11 https://civilexperiences.com/balanced-sections-under-reinforced-
section-over-reinforced-section/
4) 12-13 https://civilideas374921463.wordpress.com/2021/09/11/balanced-
section-under-reinforced-section-over-reinforced-section/

15. Thank You