Expansion joints are provided if the length of concrete structures exceeds:
Correct Answer: D. 45 m
📚 Detailed Explanation: Expansion Joints Required When Structure Exceeds 45 m
Why D (45 m) is correct: IS 456:2000 and general structural engineering practice specify that expansion joints should be provided in buildings where the length exceeds approximately 45 metres. Ordinary buildings below this length can accommodate thermal and shrinkage movements through the inherent flexibility of their structural elements. Beyond 45 m, the accumulated thermal movement becomes too large for the structure to absorb without cracking or damage.
Thermal Expansion Calculation
Thermal expansion of a 45 m concrete structure:
ΔL = α × L × ΔT
α (concrete) = 10 × 10&sup-;&sup6; per °C (IS 456)
L = 45 m = 45,000 mm
ΔT = 30°C (typical seasonal range in India)
ΔL = α × L × ΔT
α (concrete) = 10 × 10&sup-;&sup6; per °C (IS 456)
L = 45 m = 45,000 mm
ΔT = 30°C (typical seasonal range in India)
ΔL = 10 × 10&sup-;&sup6; × 45,000 × 30 = 13.5 mm
A 30 m structure with ΔT = 10°C:
ΔL = 10 × 10&sup-;&sup6; × 30,000 × 10 = 3 mm (as mentioned in IS code commentary)
→ Beyond 45 m, cumulative movement cannot be safely absorbed without expansion joints.
Expansion Joint Provision in Buildings (IS 456:2000)
| Criterion | Guideline |
|---|---|
| When to provide | When plan dimension exceeds 45 m; or at changes of plan (L, T, H, E, U shapes) |
| Spacing | Every 30–45 m in long buildings; 30 m is common practice in India |
| Width of expansion joint | Typically 20–25 mm; filled with compressible filler (cork, rubber, foam, bitumen) |
| Plan configurations requiring joints | At re-entrant corners of L, T, H, E, U-shaped buildings; at abrupt changes in plan or height |
| Structural arrangement at joint | Twin columns / separate columns; continuous roof/floor slab interrupted at joint |
Why Other Options Are Wrong
| Option | Why Incorrect |
|---|---|
| A. 10 m | Far too short; all but the smallest concrete structures would need joints; normal structural flexibility handles 10 m easily |
| B. 15 m | Still too short; IS 456 does not require expansion joints at 15 m |
| C. 25 m | Closer but still below the IS 456 threshold; movements at 25 m are typically manageable |
| D. 45 m | Correct IS 456 threshold; beyond 45 m expansion joints are required |
- IS 456:2000: expansion joints required when building length exceeds 45 m.
- Provided every 30–45 m in large buildings; gap filled with compressible material.
- When temperature rises 10°C, a 30 m structure expands ≈3 mm (IS 456 commentary example).