PSC Girders & Slabs Notes

Last reviewed 30 Sept 2026 · 6 min read

PSC girders

Choosing the section

Choice	Guidance
Shape	I-section for economy in simple spans; T or bulb-T for wider flange; box for torsion and long spans; rectangular only for small members
Depth	About span/15 to span/20 for simply supported girders; span/20 to span/25 for continuous spans
Top flange	Wide enough to carry the deck slab and the compression at service; thick enough for temporary stresses and to take shear connectors
Bottom flange	Wide enough to house the tendons in ducts and to carry the compression at transfer
Web	As thin as the shear, the duct diameter and the placing of concrete permit (about 150–250 mm for I-girders); thickened at the ends

Efficiency of the section $ρ = \frac{r ^{2}}{y _{t} y _{b}}$ (with $r$ the radius of gyration): a higher value means a bigger kern and a smaller prestress for the same load. I-sections have $ρ \approx 0.5$ –0.6, rectangles 0.33.

Design procedure

FormulaSteps for a simply supported PSC girder

Loads: self-weight, superimposed dead load (deck slab, wearing coat), live load with impact; moments at mid-span and critical sections.
Trial section from the depth ratio and the minimum section moduli $Z_{t}$ and $Z_{b}$ (previous notes).
Prestress and eccentricity: from the stress requirement at mid-span; maximum $e$ within the cover and the duct arrangement.
Tendon profile: draped or parabolic profile with the eccentricity reduced towards the supports, keeping the cable within the limiting zone; number of cables and strands.
Losses: friction, slip, elastic shortening (sequence), shrinkage, creep, relaxation → effective prestress.
Stress checks: transfer and service; top and bottom fibres; several sections along the span.
Ultimate flexure: $M_{u} \geq$ factored moment and $\geq 1.2 M_{cr}$ .
Shear and torsion: stirrups; inclined-tendon component; web crushing.
End block and anchorage zone: bursting and bearing.
Deflection and camber: at transfer and long term.
Detailing: ducts, vents, grout, bearings, diaphragms, lifting points.

Composite girders

The girder and the deck slab (cast later) act as a composite section. The girder alone carries its self-weight and the wet slab; the composite section carries the superimposed load and the live load. Two sets of section properties and four stress checks (girder top/bottom, composite top/bottom) are needed. The horizontal shear at the interface is resisted by roughened surface and stirrups extending into the slab. The differential shrinkage between slab and girder produces additional stresses that are added at the final stage.

Continuity

Simply supported spans are converted into a continuous girder by casting a connection at the pier. Advantages: fewer joints, lower moments, smaller deflections, improved riding quality and durability. Secondary moments arise from the prestress in continuous girders (the prestress deflects the girder and the supports restrain it); they must be included in the ultimate check and they change the stresses at service.

Camber and lifting

A prestressed girder cambers upward when the prestress is transferred, before any superimposed load: the camber is $δ = 5 P e L^{2} / (48 E_{c} I)$ (parabolic tendon) minus the self-weight deflection. The camber is time-dependent (creep increases it) and must be limited so that the deck level is correct; the slab thickness is adjusted to compensate. Lifting points are placed at $L /5$ (or a suitable position) from the ends and the girder is checked for the reversed moment; the girder must also be stable laterally on the transport and on the launcher.

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