How to Select Steel Beam Size for Your Span — IS 800 Guide
Selecting the right steel beam is one of the most common tasks in structural engineering. Too small and it fails; too large and you waste money. This guide walks you through the preliminary beam selection process as per IS 800:2007, from load estimation to final section choice.
Skip the Math: Use the Beam Selection Helper to instantly find the lightest adequate section for your span and load. Input span, load type, and support conditions — get a ranked list of suitable sections with deflection checks.
Step 1: Determine the Load
Loads on a beam come from IS 875 (Parts 1–5):
| Load Type | IS Code | Typical Values |
|---|---|---|
| Dead Load (DL) | IS 875 Part 1 | Slab self-weight + finishes: 3–5 kN/m² |
| Live Load (LL) | IS 875 Part 2 | Residential: 2 kN/m², Office: 3 kN/m², Storage: 5+ kN/m² |
| Factored Load | IS 800 Table 4 | w = 1.5 × (DL + LL) for limit state design |
Example: For a floor beam in a residential building with 150mm RCC slab:
- Slab DL = 0.15 × 25 = 3.75 kN/m², finishes = 1.0 kN/m² → Total DL = 4.75 kN/m²
- LL = 2.0 kN/m²
- Total unfactored = 6.75 kN/m²
- Factored = 1.5 × 6.75 = 10.125 kN/m²
- If beam spacing = 3m: w = 10.125 × 3 = 30.4 kN/m (UDL on beam)
Step 2: Calculate Bending Moment
For a beam with span L (metres) and UDL w (kN/m):
| Support Condition | Maximum Moment | Formula |
|---|---|---|
| Simply Supported | At midspan | M = wL²/8 |
| Fixed-Fixed | At supports | M = wL²/12 |
| Cantilever | At fixed end | M = wL²/2 |
| Point Load (SS) | At midspan | M = PL/4 |
Example (continued): Simply supported beam, span 5m:
M = 30.4 × 5² / 8 = 95 kN·m
Step 3: Calculate Required Section Modulus (Zxx)
From IS 800:2007, Clause 8.2.1.2:
Zxx required = M × γm0 / fy
Where: M = bending moment (N·mm), fy = yield stress (250 MPa for Fe 250, 410 MPa for Fe 410W), γm0 = 1.10 (partial safety factor)
Example: For Fe 250 steel:
Zxx = 95 × 10⁶ × 1.10 / 250 = 418 cm³
Step 4: Select a Section
Choose the lightest section with Zxx ≥ required Zxx:
| Section | Weight (kg/m) | Zxx (cm³) | Status |
|---|---|---|---|
| ISMB 250 | 37.3 | 410.5 | 418 needed — just short |
| ISMB 300 | 44.2 | 573.6 | OK (1.37× margin) |
| ISLB 350 | 44.2 | 862.8 | OK but heavier depth |
| ISWB 250 | 37.3 | 517.6 | OK — wider flanges |
Result: Use ISMB 300 (44.2 kg/m, Zxx = 573.6 cm³) — lightest adequate section.
Step 5: Check Deflection
IS 800 Table 6 limits deflection for beams supporting floors:
Maximum deflection = Span / 300 (for beams with brittle finishes)
δ = 5wL⁴ / (384 × E × I) for simply supported UDL
Where E = 200,000 MPa, I = Ixx in mm⁴
Example: ISMB 300, unfactored load = 6.75 × 3 = 20.25 kN/m:
δ = 5 × 20.25 × 5000⁴ / (384 × 200000 × 8603.6 × 10⁴) = 9.6 mm
Limit = 5000/300 = 16.7 mm → 9.6 < 16.7 ✓ OK
Quick Selection Chart
For simply supported beams with typical residential loading (10 kN/m factored UDL), Fe 250 steel:
| Span (m) | Suggested Section | Weight (kg/m) | Deflection Check |
|---|---|---|---|
| 3 | ISMB 150 | 15.6 | OK |
| 4 | ISMB 200 | 22.9 | OK |
| 5 | ISMB 250 | 37.3 | OK |
| 6 | ISMB 300 | 44.2 | OK |
| 7 | ISMB 350 | 52.4 | Check |
| 8 | ISMB 400 | 61.6 | Check |
| 10 | ISMB 500 | 86.9 | Check |
Note: This chart is for preliminary selection only. Actual loads, support conditions, and lateral bracing affect the final choice. Always verify with detailed design per IS 800:2007.
Related Standards
- IS 800:2007 — General construction in steel — Code of practice
- IS 808:2021 — Dimensions of hot rolled steel sections
- IS 875 Parts 1–5 — Code of practice for design loads
- IS 2062:2011 — Hot rolled steel for general structural purposes
For the full interactive tool, use the Beam Selection Helper — input your exact span, load, and support conditions to get a ranked list of suitable sections with deflection checks.