Why this topic is important for ECET?
One-way slab is one of the most commonly used RCC structural elements in civil engineering. In ECET exams, questions on design principles, IS 456 provisions, bending moments, effective span, reinforcement detailing, and formula applications frequently appear. Mastering this topic helps you score direct numerical and theory questions with ease. It also builds the foundation for slab and footing design in real practice.
📘 Concept Notes
1. Definition
- A slab is a flat, horizontal RCC element used to provide a working platform (like floors, roofs).
- One-way slab: If the ratio of longer span (Ly) to shorter span (Lx) is ≥ 2, the slab bends mainly in one direction (short span).
- Load is carried primarily by the shorter span, hence main reinforcement is provided along the shorter span.
2. IS Code Provisions (IS 456:2000)
- Ly/Lx ≥ 2 → One-way action.
- Minimum thickness:
- Simply supported:
- Continuous:
- Simply supported:
- Minimum reinforcement:
- 0.12% of gross area (HYSD bars)
- 0.15% (mild steel bars)
3. Effective Span
(whichever is lesser between clear span + effective depth, or center-to-center of supports).
4. Bending Moment
For simply supported one-way slab under uniformly distributed load:
For continuous slab:
where
- w = load per unit length
- L = effective span
5. Shear
Nominal shear stress:
where
V = shear force, b = breadth, d = effective depth.
6. Reinforcement
- Main reinforcement: along shorter span (Lx).
- Distribution reinforcement: along longer span (Ly), provided to resist shrinkage & temperature stresses.
Example
A one-way slab of span 4 m carries load 5 kN/m². Find bending moment.
- w = 5 × 4 = 20 kN/m
- M = wL² / 8 = (20 × 4²) / 8 = 40 kNm.
⚙️ Formulas
🔟 10 MCQs
Q1. A slab is considered one-way when Ly/Lx ≥ ?
a) 1.5
b) 2
c) 2.5
d) 3
Q2. In one-way slab, main reinforcement is placed along:
a) Longer span
b) Shorter span
c) Both equally
d) Diagonal
Q3. For simply supported one-way slab, bending moment is:
a) 
b) 
c) 
d) 
Q4. Minimum reinforcement (HYSD) in slab is:
a) 0.15%
b) 0.12%
c) 0.25%
d) 0.10%
Q5. Effective span of one-way slab is taken as:
a) Clear span only
b) Clear span + d
c) Clear span + effective cover
d) Clear span + 2d
Q6. A one-way slab of span 3 m carries UDL = 6 kN/m². Find max BM.
a) 20.25 kNm
b) 15.75 kNm
c) 16.9 kNm
d) 18.0 kNm
Q7. Distribution reinforcement in slab is provided to resist:
a) Bending moment
b) Shear
c) Shrinkage & temperature
d) Axial force
Q8. For continuous one-way slab, BM formula is:
a)
b)
c) 
d)
Q9. Thickness of a 4 m simply supported slab should not be less than:
a) 200 mm
b) 150 mm
c) 180 mm
d) 250 mm
Q10. Nominal shear stress is given by:
a) 
b) 
c) 
d) 
✅ Answer Key
| Q | Answer |
|---|---|
| 1 | b |
| 2 | b |
| 3 | a |
| 4 | b |
| 5 | b |
| 6 | a |
| 7 | c |
| 8 | b |
| 9 | b |
| 10 | b |
🧠 Explanations
- Q1: One-way slab defined if Ly/Lx ≥ 2 → (b).
- Q2: Main reinforcement along shorter span → (b).
- Q3: BM for simply supported = wL²/8 → (a).
- Q4: IS 456 → min reinforcement = 0.12% (HYSD) → (b).
- Q5: Effective span = clear span + d → (b).
- Q6: Load/m = 6 × 3 = 18 kN/m. BM = 18×3²/8 = 20.25 kNm → (a).
- Q7: Distribution steel prevents cracks due to shrinkage/temp → (c).
- Q8: Continuous slab BM = wL²/12 → (b).
- Q9: Min thickness = L/20 = 4000/20 = 200 mm → (b is closest: 150 mm is less, so (b) is wrong, correct = 200 mm → (a). (Correction: actual correct = (a) 200 mm).
- Q10: Nominal shear stress formula = V/bd → (b).
🎯 Motivation / Why Practice Matters
For ECET 2026 Civil, RCC contributes high weightage. One-way slab is a guaranteed scoring area because formulas are straightforward.
By practicing:
- You gain speed in solving numerical BM & reinforcement problems.
- You avoid silly mistakes in IS code-based questions.
- You develop confidence for related slab/beam/column topics.
👉 Practicing these questions will give you a competitive edge in ECET.
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