Two-Way Slab Design Procedure

Step 1: Aspect Ratio

Aspect Ratio = ly / lx < 2

Step 2: Depth of Slab

Assume overall depth of slab = D
Assume clear cover for both side spans
Assume diameter of main bars φ = 8, 10, 12 mm
Effective cover = clear cover + φ / 2

Effective depth d = D - C.C - φ / 2

Alternate Method:

d = Span / (Base Value × Modification Factor)

Base Value: Refer Clause 23.2.1, Page 37
Modification Factor: Typically between 1–2 (Generally 1.5)

Step 3: Effective Span of Slab

Leff = clear span + effective depth = l + d
Leff = clear span + (support width + support width) / 2

Step 4: Design Load Calculation

Dead Load (DL)

Self-weight = 25 × thickness of slab (kN/m²)
+ Superimposed dead load
Total DL = sum of above

Live Load (LL)

Imposed live load
+ Other live loads
Total LL = sum of above

Factored load, Wu= 1.5(DL + LL)

Step 5: Design Moment Calculation

Mx = αx × w × lx²

My = αy × w × ly²

Refer Table 26, Page 91 for αx and αy (bending moment coefficients for rectangular panels supported on four sides with torsion at corners).

Note:

Check panel type based on slab location
Consider negative moment at continuous edges for top reinforcement
Consider positive moment at mid-span for main steel

Step 6: Main Reinforcement Calculation

Along x-direction

Astx = (0.5 fck / fy) × [1 - √(1 - (4.6 Mx / fck bd²))] × bd

Along y-direction

Asty = (0.5 fck / fy) × [1 - √(1 - (4.6 My / fck bd²))] × bd

Assume bar diameter φ = 8, 10, 12, 16 mm
Area of 1 φ bar = (π / 4) × φ² (mm²)
Spacing = (Area of 1 φ bar × 1000) / Ast (required)

Note: Max spacing between main bars in two-way slabs should not exceed 3 × effective depth or 300 mm (whichever is smaller) as per Clause 26.3.3(b)(1). Refer Amendment 3.

Step 7: Torsion Reinforcement

Refer Clause D-1.8 to D-1.11, Page 90

Ast, torsion = (3 / 4) × Ast (maximum)

Assume torsion bar diameter = 8–10 mm
Area of 1 φ bar = (π × φ²) / 4
Spacing = (Area of 1 φ bar × 1000) / Ast, torsion (required)

Note: Torsion reinforcement is provided at extreme corners (not at continuous edges), within 1 m from the corner and extending up to 1 m in both directions.