Bridges · Girders & decks
Box girder section calculator
Single-cell box: area, centroid, I, section moduli and fibre stresses, with optional prestress.
Standard: Section properties · AS 5100 concepts (simplified)
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Top flange width | 6000 | mm |
| Top flange thickness | 220 | mm |
| Bottom flange width | 3500 | mm |
| Bottom flange thickness | 200 | mm |
| Overall depth | 2000 | mm |
| Web thickness (each) | 350 | mm |
| Simple span | 40 | m |
| Superimposed dead load | 30 | kN/m |
| Live load (equivalent UDL) | 40 | kN/m |
| Live point load at midspan | 300 | kN |
| Prestress | Include | |
| Effective prestress Pe | 25000 | kN |
| Tendon height above soffit | 250 | mm |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Area A | 3.126 | m² |
| Self-weight | 78.1 | kN/m |
| Centroid above soffit | 1,171 | mm |
| I | 1.7593 | m⁴ |
| Zt / Zb | 2.122 / 1.503 | m³ |
| Service M | 32,630 | kNm |
| Ultimate M* (1.2G + 1.5Q) | 42,456 | kNm |
| Ultimate V* | 4,021 | kN |
| Top / bottom service stress | -12.53 / -1.60 | MPa |
- ✓ Bottom fibre tension ≤ 0.25√f'c (≈ 1.6 MPa for 40 MPa)
- ✓ Top compression ≤ 0.5 f'c (20 MPa for 40 MPa)
Method, formulas and assumptions
- Section = top flange + bottom flange + two webs (fillets ignored)
- yb = ΣAy/ΣA; I = Σ[bh³/12 + A(y − yb)²]
- Service M = (G + sw + Q)L²/8 + PL/4; M* = 1.2(G + sw) + 1.5Q (AS 5100 uses SM1600 traffic and its own factors; enter equivalent loads)
- σ = −Pe/A ± Pe·e/Z ∓ M/Z (compression negative), e = yb − tendon height
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.