Concrete · Slabs & post-tensioning
Prestressed beam fibre stresses calculator
Top and bottom fibre stresses at transfer and service for a rectangular section with eccentric prestress.
Standard: AS 3600 Section 8.1.6 (transfer) · elastic stresses
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 |
|---|---|---|
| Width b | 300 | mm |
| Depth D | 600 | mm |
| Initial prestress force Pi | 1000 | kN |
| Eccentricity e (below centroid) | 150 | mm |
| Long-term losses | 20 | % |
| Self-weight / dead moment at transfer | 50 | kNm |
| Additional service moment | 100 | kNm |
| f'cp at transfer | 25 | MPa |
| f'c | 40 | MPa |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| A / Z | 180,000 mm² / 18.0×10⁶ | mm³ |
| Kern distance Z/A | 100 | mm |
| Effective prestress Pe | 800 | kN |
| Transfer top / bottom | 0.00 / -11.11 | MPa (+ tension) |
| Service top / bottom | -6.11 / -2.78 | MPa |
| Limits transfer (C / T) | -12.50 / 1.25 | MPa |
| Limits service (C / T) | -18.00 / 1.58 | MPa |
- ✓ Transfer stresses within limits
- ✓ Service stresses within limits
- ✓ e within section
Method, formulas and assumptions
- σ = −P/A ± P·e/Z ∓ M/Z (compression negative)
- Transfer limits: compression 0.5f'cp (AS 3600 Cl 8.1.6.2); tension 0.25√f'cp (typical design limit)
- Service limits shown (0.45f'c, 0.25√f'c) are common design criteria, not mandatory AS 3600 values
- Ultimate strength must be checked separately
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.