| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Slab thickness (simply supported) | Span / 20 to Span / 25 | 5m span → 200–250mm | Conservative: 1cm per foot of span |
| Slab thickness (continuous) | Span / 25 to Span / 30 | 5m span → 170–200mm | Continuous slabs are thinner |
| Slab thickness (cantilever) | Span / 7 to Span / 10 | 1.5m cantilever → 150–215mm | |
| Minimum slab thickness | 100mm (one-way), 100mm (two-way) | Practical minimum: 125mm | |
| Two-way slab: check Ly/Lx | If Ly/Lx ≤ 2 → two-way; else one-way | 4m × 6m → 6/4 = 1.5 → two-way | |
| Steel qty (slab) | 0.7–1.0% of concrete volume | 55–78.5 kg/m³ | Residential typical |
| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Beam depth (simply supported) | Span / 10 to Span / 15 | 6m span → 400–600mm | 7–8cm per metre of span |
| Beam depth (continuous) | Span / 12 to Span / 18 | 6m span → 333–500mm | |
| Beam depth (cantilever) | Span / 5 to Span / 7 | 2m cantilever → 285–400mm | |
| Beam width | Depth / 1.5 to Depth / 2 | 450mm deep → 230–300mm wide | Min 230mm for residential |
| T-beam flange width | Lo/6 + bw + 6df (or b/b spacing) | Lo = distance between zero moment pts | |
| Steel qty (beam) | 1.0–2.0% of concrete volume | 78.5–157 kg/m³ |
| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Column size (rough) | Load / (0.4×fck + 0.67×fy×p/100) | 800kN M20 1.5% → 230×450mm | From IS 456 Cl 39.3 |
| Tributary area method | Interior: 12 kN/m²/floor, Edge: 17, Corner: 22 | Interior col 22m² × 3 floors × 12 = 792 kN | From TN PWD Handbook |
| Minimum column size | 230×230mm (residential) | 9"×9" minimum | |
| Maximum column spacing | 4–5m (residential), 6–8m (commercial) | Governs beam depth | |
| Slenderness check | Leff / least dimension ≤ 12 (short) | 3m height / 230mm = 13 → slender! | > 12 → slender column |
| Steel qty (column) | 1.0–5.0% of gross area (avg 2.5%) | 300×400: 1.5% = 1800mm² | 1–2% is economical |
| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Footing area | Total Load / SBC | 600kN / 200kN/m² = 3.0m² | Add 10% for self-weight |
| Square footing side | √(Area) | √3.0 = 1.73m → use 1.8m | Round to next 150mm |
| Footing depth (one-way shear) | ≈ (L − column width) / 4 | 1.8m footing, 300mm col → ~375mm | Minimum 300mm |
| Footing depth (practical min) | 300mm (isolated), 150mm at edge | Per IS 456 Cl 34 | |
| PCC below footing | 75–100mm M10 PCC | Leveling + cover protection |
| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Steel per sqft (residential G+2) | 3.5–4.5 kg/sqft | 1000 sqft → 3.5–4.5 tonnes | |
| Steel per sqft (commercial) | 5.0–7.5 kg/sqft | Heavier loads & spans | |
| Concrete per sqft | 0.038–0.04 m³/sqft | 1000 sqft → 38–40 m³ | Including footings |
| Cement per sqft (total construction) | 0.4–0.5 bags/sqft | 1000 sqft → 400–500 bags | Including plaster, flooring |
| Shuttering area | 6 × concrete volume (m²/m³) | 40m³ → 240 m² formwork | OR 2.4 × plinth area |
| Shuttering cost | 15–18% of total construction cost | ||
| Binding wire | 8 kg per tonne of steel | 4 tonnes → 32 kg wire | |
| Chair / spacer bars | 1% of total steel weight |
| Rule | Formula / Value | Example | Note |
|---|---|---|---|
| Total floor load (residential) | DL + LL + finish ≈ 8–10 kN/m² | 150mm slab: 3.75+1.0+0.4+2.0 = 7.15 kN/m² | Conservative: use 10 kN/m² |
| Wall load on beam (230mm brick, 3m) | ≈ 13–15 kN/m | 4.4 kN/m² × 3.3m + plaster = 15.6 kN/m | Including plaster both sides |
| Parapet load | ≈ 4–5 kN/m | 230mm × 1.0m = 4.4 kN/m | |
| UDL on beam from slab | Slab load × tributary width / 2 | 10 kN/m² × 4m / 2 = 20 kN/m | For interior beam |
| Total load for column estimation | 10–12 kN/m² × tributary area × floors | 10 × 20m² × 4 = 800 kN | Quick check |