Integral and semi-integral abutments

How bridges get built · Piers, pylons and abutments · Integral and semi-integral abutments

Also called: jointless bridge, integral bridge, semi-integral abutment

How it is done

Confirm the limits on length, skew, curvature and pile type, and calculate the thermal movement each abutment must accommodate.
Install a single row of flexible piles, typically steel H-piles bending about their weak axis, often in pre-augered holes filled with loose sand to reduce restraint near the head.
Cast the pile cap (lower abutment) with the pile heads embedded, and set temporary or permanent seats for the girders.
Erect the girders and cast the deck slab, leaving the end diaphragm until most of the deck is in place so the dead-load rotations occur before the connection is made.
Cast the end diaphragm (backwall) monolithic with the girder ends, the deck and the pile cap.
Backfill with well-graded granular fill and drainage; some designs add a compressible layer behind the wall to limit pressure build-up.
Cast the approach slab tied to the abutment, ending on a sleeper slab with a joint where the movement is taken up.
For a semi-integral abutment, rest the end diaphragm on bearings on a fixed stem and let it move against the backfill; the joint again goes to the end of the approach slab.

Key numbers

LengthUK guidance (BD 57 and BA 42) made integral design the default up to 60 m; many US states use 90–150 m, and Tennessee's Happy Hollow Creek bridge is 358 m
Skewcommonly limited to 20–30°
Movementeach abutment typically moves 10–40 mm over the annual temperature range
Pilesone row of steel H-piles bending about the weak axis, often in pre-augered holes filled with loose sand
Approach slababout 5–9 m, tied to the abutment, with a sleeper slab and joint at its far end

Where it fits

Bridges up to about 60-120 m long and 30 degrees skew, moderate climates, steel or concrete girders.

Where it does not

Long bridges with large thermal movement, high skews, very stiff foundations that cannot flex.

Choosing it

Choose integral over seat abutments when length, skew and curvature are within the limits and the piles can flex: no joints, no end bearings and better seismic robustness.
Prefer semi-integral when piles cannot flex (rock near the surface, very stiff soil) or bearings must stay replaceable, while still removing the deck joint.
Prefer seat abutments with joints for long bridges with large thermal movements, skews above about 30°, or tall abutments where cyclic earth-pressure build-up would govern.

Plant, pace and money

PlantPiling rig, formwork, standard concrete plant.

ProductivitySimpler and faster than seat abutments.

Cost5-15 % cheaper to build and much cheaper to maintain.

Risks and controls

What goes wrong

Ratcheting of backfill pressures over years, cracking at the deck-abutment junction, approach slab settlement.

Quality assurance

Pile orientation and flexibility checked, junction reinforcement detail inspected.

Origins

Some US states, Tennessee prominent among them, adopted jointless bridges from the 1960s after decades of joint and bearing deterioration; UK guidance (BD 57 and BA 42/96) made integral design the default for decks up to 60 m in the 1990s, and PD 6694-1 now covers the design.

Examples

Standard in the UK, Sweden and much of the US for short bridges; the game bonus for jointless short bridges reflects it.

Case studies

Happy Hollow Creek Bridge (SR 50), Hickman County, TennesseeUSA · late 1990s358 m (1,175 ft) of nine 72-inch bulb-tee spans on a curve, with no expansion joints or expansion bearings, reported as the longest totally jointless integral abutment precast concrete bridge in the US.