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Concrete Slab Thickness Design Calculator

Design concrete slab thickness per IS 456, ACI 318, or BS 8110. Computes minimum thickness, reinforcement, deflection, and fire resistance checks.

Reinforced Concrete Design Structural engineers, civil engineering students Commercial Intent: HIGH

Engineering Formulas

Span/Depth Ratio Method

d = L / (basicratio × modificationfactor)
IS 456 Table 14: simply supported 20, continuous 26, cantilever 7
ACI 318 Table 7.3.1.1: simply supported 20, continuous 28, cantilever 10

d: Effective depth (mm)

L: Span (mm)

basic_ratio: Basic span/depth ratio from code

Design Moment (One-Way)

Mu = wu × L² / 8
wu = 1.5 × (DL + LL + SW)
SW = D/1000 × 25 kN/m³

Mu: Design moment (kNm/m)

wu: Factored load (kN/m²)

SW: Self-weight (kN/m²)

Design Moment (Two-Way)

Mux = αx × wu × Lx²
αx from IS 456 Table 26 (0.062–0.113 depending on ly/lx)
Continuous slabs: ~80% of simply supported values

Mux: Mid-span moment in short direction

αx: Moment coefficient (0.062–0.113)

Lx: Short span (m)

Area of Steel Reinforcement

Ast = (Mu × 10⁶) / (0.87 × fy × d × (1 — 0.42 × xu/d))
xu/d = 1 — √(1 — 4.6 × Mu / (fck × b × d²))
Ast,min = 0.12% of b×d (Fe415), 0.15% (Fe250)

Ast: Area of steel (mm²/m)

xu/d: Neutral axis depth ratio

b: Unit width (1000 mm)

Deflection Check

Check: L/dactual ≤ L/dbasic × 1.3
For cantilevers: L/dactual ≤ L/dbasic × 0.8

L/d_actual: Actual span/effective depth ratio

Worked Example

One-Way Simply Supported Slab (IS 456)

slabStandard: ISslabType: one-wayspanShort: 4spanLong: 5liveLoad: 3deadLoad: 1.5concreteGrade: 25steelGrade: 415exposureCondition: moderatefireRating: 2supportCondition: simply-supported

Basic L/d ratio

20 (simply supported one-way per IS 456 Table 14)

Effective depth

d = 4000 / (20 × 0.95) ≈ 211 mm

Total depth

D = d + 30 + 5 = 246 mm → round to 250 mm

Self-weight

0.250 × 25 = 6.25 kN/m²

Factored load

wu = 1.5 × (1.5 + 3 + 6.25) = 16.125 kN/m²

Design moment

Mu = 16.125 × 4² / 8 = 32.25 kNm/m

Result: D = 250 mm, d = 215 mm, Ast = ~470 mm²/m, Deflection: PASS, Fire: PASS

Engineering Notes

For two-way slabs, check moments in both directions — the short-span moment typically governs.

Flat plates have lower span/depth ratios than beam-supported slabs due to punching shear concerns.

For cantilever slabs, deflection is more critical — consider doubling the basic ratio check.

Fire resistance: 2-hour rating typically requires 100mm minimum thickness and adequate cover.

Long-term deflection due to creep and shrinkage should be checked for spans > 6m.

Assumptions

• Slab is prismatic with uniform thickness

• Linear elastic behavior for deflection check

• Two-way moment coefficients per IS 456 Table 26/27 interpolated by ly/lx ratio

• No redistribution of moments considered

• Cracked section properties for deflection

Common Mistakes

✕ Forgetting to include self-weight in dead load

✕ Using overall depth instead of effective depth for reinforcement calculation

✕ Applying one-way moment formula for two-way slabs

✕ Not checking minimum reinforcement requirements

✕ Ignoring exposure condition for cover requirements

Frequently Asked Questions

What is the minimum slab thickness?

Per IS 456, minimum slab thickness is 100mm (general) and 75mm for sunshades. Per ACI 318, minimum is 90mm (3.5 in) for one-way slabs.

What is the difference between one-way and two-way slabs?

One-way slabs span in one direction (Ly/Lx > 2). Two-way slabs span in both directions (Ly/Lx ≤ 2) and have reinforcement in both directions.

What is the typical span/depth ratio for slabs?

One-way simply supported: 20. One-way continuous: 26. Two-way simply supported: 25 (Fe415). Two-way continuous: 31. Cantilever: 7.

How much cover is needed for concrete slabs?

Mild exposure: 20mm. Moderate: 30mm. Severe: 45mm. For fire resistance, cover may need to be increased per code requirements.

What is the minimum reinforcement in slabs?

0.12% of gross cross-sectional area for Fe415 steel, 0.15% for Fe250 steel. Maximum reinforcement is 4% of gross area.

How do I check deflection in slabs?

Deflection is checked by ensuring the actual span/effective depth ratio does not exceed the basic ratio multiplied by a modification factor based on steel percentage and stress.