Prestressed Concrete Stresses
The top and bottom fibre stresses of a prestressed beam at transfer and at service. Prestress puts the section into compression so the concrete never goes into tension under load — and the two stages have different limits.
| Fibre | Transfer | Service |
|---|---|---|
| Top | 1.2 MPa | -47.6 MPa |
| Bottom | -20.1 MPa | 16.6 MPa |
f = −P/A ± P·e/Z ± M/Z · Compression negative.
Pe = Pi(1 − losses) · After all losses.
A clear path from inputs to results.
The top and bottom fibre stresses of a prestressed beam at transfer and at service. Prestress puts the section into compression so the concrete never goes into tension under load — and the two stages have different limits.
How to use
- Set area a (mm²).
- Set section modulus zb (×10⁶ mm³).
- Set section modulus zt (×10⁶ mm³).
- Set eccentricity e (mm).
- Read the headline metrics, then check the table for the detail.
The formula
Reference: Elastic fibre stresses with effective prestress.
Worked example
At the default inputs
- Area A (mm²): 300000; Section modulus Zb (×10⁶ mm³): 60; Section modulus Zt (×10⁶ mm³): 45; Eccentricity e (mm): 350; Initial prestress Pi (kN): 4000; Losses (%): 18; Self weight moment (kN·m): 600; Service moment (kN·m): 2200
- 1
Bottom at transfer: -20.1 MPa · Bottom at service: 16.6 MPa · Top at service: -47.6 MPa · Effective prestress: 3280 kN
Bottom at transfer: -20.1 MPa · Bottom at service: 16.6 MPa · Top at service: -47.6 MPa
Interpreting the result
At service the bottom fibre is at 16.6 MPa.
Assumptions & limitations
- Elastic uncracked section.
- Losses taken as a single percentage.
Frequently asked questions
Why check at transfer?
The concrete is youngest and the prestress is highest, so the opposite fibre can crack.
What are typical losses?
15–20% for pretensioned members; higher for post-tensioned with long tendons.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.