Diaphragm Design Calculation Sheet – Collector and Shear Checks

1. Collector (Drag Strut) Design

Given:

- Axial force from ETABS (Fx) = 220 kN (tension)

- Overstrength factor (Ω₀) = 2.5 (special shear wall system)

- φ = 0.9 (ACI tension strength reduction)

- Steel yield strength (fy) = 500 MPa

Step 1: Amplify collector force

Pu = Fx × Ω₀ = 220 × 2.5 = 550 kN

Step 2: Required steel area

As = Pu / (φ × fy)

= 550 × 10^3 / (0.9 × 500 × 10^6) = 1.22 × 10⁻³ m² = 1222 mm²

Step 3: Rebar selection

Use 4-#20 bars (4 × 314 mm² = 1256 mm²) – OK

Given:

- Total shear at wall = 320 kN

- Effective contact length = 1.2 m

- Slab thickness = 0.2 m

Step 1: Shear stress

v = V / (b × t) = 320 / (1.2 × 0.2) = 1333.3 kN/m²

Step 2: Allowable shear capacity

Depends on interface shear provision (roughened, dowels, or friction).

Check against μ × φ × Vn or dowel capacity per ACI 318 §22.9.4

Given:

- Diaphragm shear = 250 kN at 3 m from wall

- Effective width = 10 m

- Slab thickness = 0.2 m

- f'c = 30 MPa

Step 1: Shear stress

v = V / (b × t) = 250 / (10 × 0.2) = 125 kN/m²

Step 2: Concrete shear capacity

Vc = 0.17 × √f'c × b × d

= 0.17 × √30 × 1000 × 160 = 149 kN

φVc = 0.75 × 149 = 112 kN/m (use b = 1 m)

→ 125 > 112 → Provide reinforcement to increase shear strength.

Given:

- Diaphragm span = 12 m - Diaphragm shear = 300 kN - Diaphragm depth = 10 m

Step 1: Compute moment in diaphragm (treated like a beam)

M = V × L / 2 = 300 × 12 / 2 = 1800 kNm (diaphragm force is treated as concentrate load at CM of diaphragm, normally at mid span)

Step 2: Compute chord tension/compression

T = M / c = 1800 / 10 = 180 kN

Step 3: Required steel area

As = T / (φ × fy) = 180 × 10³ / (0.9 × 500 × 10⁶) = 0.0004 m² = 400 mm²

Step 4: Select rebar

Use 2-#20 bars (2 × 314 mm² = 628 mm²) – OK