Why this topic is important for ECET
In ECET 2026 Civil, questions from Reinforced Cement Concrete (RCC) are frequently asked, and beams are a high-weightage area. Singly reinforced beams are strong in tension but fail when compression zone is insufficient. That’s where Doubly Reinforced Beams (DRB) come in.
They are essential when:
- The section is restricted in size (architectural limits).
- Moments to resist are very high.
- Both sides (tension & compression) require reinforcement.
👉 Understanding DRB helps you score direct formula-based as well as tricky numerical questions in ECET.
📘 Concept Notes
1. What is a Doubly Reinforced Beam?
- A beam reinforced with steel in both tension and compression zones.
- Provides additional moment resistance beyond what a singly reinforced section can carry.
2. Why Doubly Reinforced?
- Section size (depth) restricted.
- High bending moments.
- Control of deflection and cracking.
- Economical design in some cases.
3. Components of DRB
- Tensile Steel (Ast): In the tension zone.
- Compression Steel (Asc): In the compression zone.
- Concrete: Resists compression.
4. Assumptions (as per IS 456:2000 Limit State Method)
- Plane sections remain plane after bending.
- Max compressive strain in concrete = 0.0035.
- Stress-strain curve for steel is bilinear (idealized).
- Bond between steel and concrete is perfect.
5. Moment of Resistance in Doubly Reinforced Section
Total ultimate moment of resistance:
→ Moment of resistance of a singly reinforced section.
→ Additional moment resisted by compression steel + balancing tension steel.
6. Important Relations
- Limiting depth of neutral axis:
- Additional moment:
where:
= yield strength of steel
= additional tensile steel
= effective cover to compression steel- Stress in compression steel (if yielded):
7. Example
A beam section 300 mm × 500 mm, effective cover to compression steel = 50 mm, 
. Additional tensile steel provided = 800 mm².
So, total moment of resistance = 
.
⚙️ Formulas
🔟 10 MCQs
Q1. A doubly reinforced beam is used when:
a) Only tensile reinforcement is required
b) Section depth is restricted
c) Only compressive reinforcement is required
d) None
Q2. The total moment of resistance of DRB is:
a) 
b) 
c)
d) None
Q3. Compression steel in DRB helps in:
a) Reducing tension steel
b) Increasing moment capacity
c) Reducing shear stress
d) None
Q4. A DRB has d = 500 mm, d’ = 50 mm, 
, 
. Find .
a) 87.5 kNm
b) 100 kNm
c) 109 kNm
d) 120 kNm
Q5. Which code gives DRB provisions?
a) IS 800
b) IS 456
c) IS 1343
d) IS 3370
Q6. If depth of neutral axis < limiting depth, section is:
a) Balanced
b) Over-reinforced
c) Under-reinforced
d) None
Q7. Poisson’s ratio of concrete ≈ ?
a) 0.1
b) 0.2
c) 0.3
d) 0.4
Q8. For Fe415 steel, maximum depth of neutral axis / d = ?
a) 0.48
b) 0.53
c) 0.46
d) 0.5
Q9. In DRB, additional tensile steel balances:
a) Stress in compression steel
b) Stress in concrete
c) Shear stress
d) Bond stress
Q10. Which strain is maximum in concrete as per IS 456?
a) 0.002
b) 0.003
c) 0.0035
d) 0.004
✅ Answer Key
| Q | Answer |
|---|---|
| 1 | b |
| 2 | c |
| 3 | b |
| 4 | c |
| 5 | b |
| 6 | c |
| 7 | b |
| 8 | a |
| 9 | a |
| 10 | c |
🧠 Explanations
- Q1: DRB is used when section depth restricted → (b).
- Q2: Total resistance = limiting + additional → (c).
- Q3: Compression steel increases moment capacity → (b).
- Q4:
→ (c). - Q5: IS 456 deals with RCC beams → (b).
- Q6: Neutral axis < limiting → under-reinforced section → (c).
- Q7: Poisson’s ratio of concrete = 0.2 → (b).
- Q8: For Fe415, xu,max/d = 0.48 → (a).
- Q9: Additional tensile steel balances compression steel force → (a).
- Q10: Max compressive strain = 0.0035 → (c).
🎯 Motivation / Why Practice Matters
In ECET 2026, RCC beams questions are sure-shot scoring.
- DRB problems test whether you remember formulas and can apply them fast.
- Speed matters – because MCQs are often direct substitution type.
👉 Practicing these daily will help you avoid confusion between singly vs doubly reinforced concepts and give you an edge over competitors.
📲 CTA
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