Precast girder casting and prestressing beds

How bridges get built · Precast concrete decks · Precast girder casting and prestressing beds

Also called: pretensioning bed, precast yard, girder fabrication

How it is done

Prepare the bed: steel abutments anchored for the full strand force, a soffit 100–150 m long so several girders are cast in line, and calibrated stressing equipment.
Stress the strands one by one with elongation checks, correcting for temperature, abutment movement and wedge seating; install hold-downs for draped strands or sleeves for debonded ones.
Place the cages, end-zone steel, lifting devices and inserts, and close the side forms.
Pour high-strength or self-compacting concrete and cure it with steam or heat, keeping the peak temperature within the specified limit.
Release the strands once the release strength is confirmed, in a symmetrical sequence that limits end cracking and sweep.
Inspect end zones, measure camber and sweep, and repair or seal end cracks as specified.
Store the girders on supports at the bearing points, track camber growth until erection, and check lateral stability for handling and transport.

Key numbers

Bed length100–150 m or more
Strand jacking stressup to about 0.75 fpu (about 1,395 MPa for 1,860 MPa strand)
Release strengthtypically 28–45 MPa; design strength 50–70 MPa
Curingpeak concrete temperature commonly limited to about 60–70 °C to avoid delayed ettringite formation
Productionone casting per bed per day
Sweep toleranceabout 3 mm per 3 m of length (PCI, 1/8 in per 10 ft)

Where it fits

All pretensioned girders: viaducts, overpasses, marine trestles, rail bridges.

Where it does not

Storage on the wrong points (creep and sweep), site casting without a proper bed.

Choosing it

Choose pretensioned plant-cast girders for standard spans within transport reach: lowest cost and the most consistent quality.
Prefer post-tensioned or spliced girders for spans beyond single-piece transport limits or when girders must be cast on site.
Choose debonding over draping when hold-down forces or bed safety are a concern; both control top-fibre tension near the girder ends.
Prefer a site casting yard over a plant only for remote sites or very large quantities.

Plant, pace and money

PlantPrestressing bed, steel forms, batching plant, steam curing, gantry cranes.

ProductivityOne girder per form per day.

CostUSD 600-1,000 per m3 of girder.

Risks and controls

What goes wrong

End-zone cracking at release, camber variation between girders, strand slip.

Quality assurance

Strand force records, release strength tests, camber measurement and tracking to erection.

Origins

Long-line pretensioning was developed in Germany by Ewald Hoyer in the late 1930s; after the Walnut Lane Memorial Bridge (1950) introduced prestressed girders to the US, plants casting standard pretensioned girders spread rapidly through the 1950s.

Examples

PCI-certified plants in the US; Korean and Japanese precasters supply most girders by truck.

Case studies

Walnut Lane Memorial Bridge, PhiladelphiaUSA · 1950Gustave Magnel's prestressed I-girder bridge with a main span of about 49 m, the start of prestressed girder construction in the US.

Puyallup River Bridge (southbound I-5), Tacoma, WashingtonUSA · 2021The plant cast 100-inch-deep WF girders 68 m (223 ft) long, beating the 67 m record it had set the year before.