Continuity over piers for precast girders: stitch pours, diaphragms and link slabs

How bridges get built · Precast concrete decks · Continuity over piers for precast girders: stitch pours, diaphragms and link slabs

Also called: continuity diaphragm, made-continuous precast girders, link slab, jointless deck over piers, stitch pour

How it is done

Choose the system: full continuity for superimposed dead and live load (continuity diaphragm plus deck reinforcement over the pier), an integral diaphragm cast with the pier cap, or a link slab that joins only the deck.
Analyse the creep and shrinkage restraint moments for the expected girder age at continuity, and detail the positive moment connection at the girder ends.
Erect the girders as simple spans on their bearings, with extended strands or bent bars projecting from the girder ends into the diaphragm zone.
Fix the diaphragm reinforcement and the negative moment bars in the deck over the pier, plus any dowels into an integral pier cap.
Pour the span regions of the deck first and the diaphragm and deck over the pier last (or follow the sequence the design assumes), so the slab weight is carried simply supported before continuity is made.
For a link slab, debond the slab from the girder ends over the designed length on each side of the pier, reinforce it for crack control (or use ECC or UHPC), and keep the girders on their simple-span bearings.
Cure, map cracks at the diaphragm and over the pier at 28 days and after the first winter, and record girder ages and cambers for the as-built file.

Key numbers

Girder age at continuityAASHTO LRFD allows creep and shrinkage restraint moments to be neglected, under stated conditions, if girders are at least 90 days old when made continuous
Positive moment connection (extended strands or bent bars into the diaphragm): commonly designed for at least 1.2 Mcr (NCHRP 519)
Link slab debonded zoneabout 5 % of each adjacent span on each side of the pier (Caner and Zia)
Link slab thicknessas the deck, about 200–250 mm (Grove Street ECC link slab 225 mm)

Where it fits

Multi-span precast I, T and U girder viaducts and overpasses, where deck joints over piers would leak onto bearings and caps
De-icing salt climates and marine sites
Retrofit of existing simply supported spans, by replacing pier joints with link slabs
Seismic regions where integral diaphragms with the pier caps improve continuity and redundancy, with suitable detailing

Where it does not

Very long continuous units without checking thermal movement at the end joints and bearings
Making continuity with very young girders without designing for the positive restraint moment
Link slabs over piers with large differential settlement or large girder end rotations

Choosing it

Choose full continuity (diaphragm plus deck reinforcement) over simple spans with joints for new multi-span girder bridges: it removes the pier joints and reduces live-load moments, at the cost of restraint-moment design and detailing.
Choose link slabs when the girders should stay simply supported (existing bridges, uncertain creep behaviour, settlement-prone foundations) and only the joint must go.
Choose integral diaphragms cast with the pier cap where frame action or the removal of bearings is wanted; keep bearings under a continuity diaphragm where piers are stiff and thermal movement must be released.

Plant, pace and money

PlantOrdinary deck plant: diaphragm formwork, reinforcement, concrete pump; ECC or UHPC batching where used for link slabs.

ProductivityAdds one to two weeks to the deck sequence of each continuous unit; a link slab retrofit of one joint takes a few days per stage under traffic.

CostIndicative: about USD 1,500–4,000 per metre of deck width at each pier line for a continuity diaphragm or link slab; the saving comes from joints and bearings that no longer need replacing.

Risks and controls

What goes wrong

Cracking at the diaphragm-girder interface from positive restraint moments
Negative moment cracking in the deck over the piers
Bearings and diaphragms not designed for the rotations and longitudinal forces of the continuous unit
Link slab cracks wider than designed, letting water through

Quality assurance

Girder age and camber at continuity recorded, extended strand and bar placement checked, pour sequence enforced, crack mapping at 28 days and after the first winter, and debond length and material tests on link slabs.

Origins

The Portland Cement Association tested precast girders made continuous with cast-in-place diaphragms and deck reinforcement around 1960, and the detail became common US practice; NCHRP Reports 322 (1989) and 519 (2004) set out the current design approach, link slabs were proposed by Caner and Zia (1998), and a ductile ECC link slab was first used in the field in Michigan in 2005.

Examples

Standard US practice for multi-span precast girder bridges since the 1960s; jointless examples include the Happy Hollow Creek Bridge in Tennessee, and the ECC link slab on the Grove Street Bridge in Michigan shows the retrofit route.

Case studies

Grove Street Bridge over I-94, Ypsilanti, MichiganUSA · 2005First field application of a ductile ECC link slab (225 mm thick, about 5.5 m by 20 m) to replace a deck joint, built in two phases with the bridge open to traffic.

Happy Hollow Creek Bridge (SR 50), Hickman County, TennesseeUSA · late 1990s358 m of nine precast bulb-tee spans connected to the bent caps and integral abutments with no expansion joints or expansion bearings anywhere.