Last reviewed 30 Sept 2026 · 7 min read
Two design ideas: working and ultimate
Prestressed members are designed twice:
- Service (working) stage — elastic stress checks at transfer and at service load; controls cracking, deflection and durability.
- Ultimate stage — strength against collapse in flexure and shear with load factors and a check of ductile behaviour.
The section is usually proportioned by the service stage and then checked for ultimate strength, and the steel supplemented if it falls short.
Classes of prestressing
| Class | Tension in concrete at service | Cracking |
|---|---|---|
| Type 1 (full prestress) | No tension | None — for aggressive or water-retaining conditions |
| Type 2 (limited prestress) | Tension allowed but below the modulus of rupture | No visible cracks |
| Type 3 (partial prestress) | Tension allowed with cracks of limited width | Small controlled cracks (crack width of the order 0.1–0.2 mm) |
Class is chosen from the exposure and the type of structure: bridges in severe environments and tanks use Type 1 or 2; buildings often use Type 2 or 3.
Permissible stresses (working stage)
The code fixes:
- At transfer: compression in concrete not more than about 0.5 ; tension limited (small in the top near the supports if no auxiliary steel, larger with mild steel to carry the tension).
- At service: compression not more than about 0.33–0.4 (for combined dead and live load); tension according to the class.
- Tendon stress: limited to about 0.8 of characteristic strength at jacking and less after transfer.
(Exact values depend on the code edition — IS 1343, IRC:112 — and must be taken from the code in use.)
Minimum section modulus and prestress
For a simple beam the design starts with the section modulus. Let be the ratio of effective to initial prestress, the self-weight moment and the moment from the other loads, so the service moment is . Let
- = permissible tension at transfer (top fibre) and = permissible compression at transfer (bottom fibre);
- = permissible compression at service (top fibre) and = permissible tension at service (bottom fibre).
Writing the four stress conditions and eliminating and gives the minimum section moduli:
The numerator is the moment range between the two stages, ; the denominator is the stress range the fibre is allowed to travel. A section with less modulus than this cannot satisfy both stages, however the tendon is placed. Once the section is chosen, the prestress and eccentricity follow from the stress conditions at the critical section; the check is repeated at other sections along the span.
A section has = 0.45 m², = 0.09 m³. The maximum service moment is 2200 kN·m; = 0.8; = 0.55 m at mid-span. For no tension at the bottom under service load:
m⁻².
kN, so .
The top fibre at transfer and the sections near the supports are then checked; the tendon is raised towards the ends if tension appears.