Last reviewed 30 Sept 2026 · 5 min read
Why prestress a bridge
In a reinforced concrete girder the concrete below the neutral axis cracks under service load and only the steel works. Prestressing puts the concrete in compression before the live load acts, so the whole section stays uncracked and effective. The result is a girder that is lighter, shallower and stiffer, with better durability (no service cracks to let chlorides in). PSC becomes economical from about 25 m and works up to 100 m and beyond for box girders.
Common PSC superstructure forms
| Form | Typical span | Notes |
|---|---|---|
| Precast I-girder + cast-in-situ deck slab | 25–45 m | Pretensioned or post-tensioned; girders cast in a yard, launched and made composite with the slab |
| T-girder / bulb-T | 25–50 m | Wider top flange; used in viaducts |
| Box girder (single cell) | 40–150 m | High torsional stiffness, good for curved alignments; cast in situ or segmental |
| Multi-cell box | Wide decks | Several webs; used for wide roads and metro viaducts |
| U-girder | Metro viaducts | Open trough section carrying the track inside |
| Voided slab | 15–30 m | PSC slab with tubular voids for lightness |
Precast I-girder and slab
Girders are made in a casting yard under controlled conditions, stressed (pre-tensioned on a long bed or post-tensioned with ducts), then lifted, transported and launched by crane or launching truss on the pier caps. A deck slab is cast over the girders on permanent or removable shuttering. Shear connectors (projecting stirrups) make the slab and girder composite.
Advantages: fast construction, quality control, minimal disruption below the bridge, reuse of moulds. Limitations: the girders must be handled and transported; the weight limits the span; the girder and the slab have different ages, so differential shrinkage and creep produce secondary stresses that must be checked.
Diaphragms (cross-girders) at supports and quarter- or mid-span tie the girders and share the load; they should be provided for load distribution and to resist torsion during launching.
Composite action and staged stresses
A composite PSC girder carries load in stages:
- Transfer of prestress — girder alone; stresses check at the ends and at mid-span.
- Self-weight and wet slab — carried by the girder alone (the slab has no strength yet).
- Superimposed dead load (wearing coat, kerbs) — carried by the composite section.
- Live load — composite section.
- Long term — creep and shrinkage redistribute stresses; time-dependent losses reduce the prestress.
Design therefore uses two section properties — girder alone and composite — and checks the limiting stresses at each stage.