Expansion joints

How bridges get built · Finishing, articulation and maintenance · Expansion joints

Also called: strip seal, modular joint, finger joint, asphaltic plug joint, jointless design

How it is done

Calculate the movement range from temperature, shrinkage, creep, prestress shortening, braking and seismic displacement, add the rotation and vertical movement at the joint, and select the joint type and size for that range.
Form the blockout in the deck end and backwall with the anchorage reinforcement, and where practicable install the joint late, after most shrinkage and creep shortening has occurred.
Lift the joint assembly into the blockout on its setting beams, set the gap for the deck temperature at installation from the pre-set table, and fix it to the pavement profile and crossfall.
Fix the anchorages to both sides during a period of stable temperature, then release the setting brackets so the deck movement is not restrained; cast the nosings in high-strength or elastomeric concrete, fully compacted under the edge beams.
Fit the seal or drainage trough, lap the deck waterproofing into the joint, and lead the drainage away from bearings and pier tops.
Modular joints: install the factory-assembled unit with its support bars, sliding springs and bearings, check the equal-spacing (control) mechanism, and set the specified gap in every cell.
Water test the seal, check level and ride across the joint, record the gap and temperature, and set the first inspection date.

Key numbers

Thermal movementabout 0.4–0.6 mm per metre of expansion length for a 40–50 °C effective temperature range
Buried and asphaltic plug jointsup to about 40–50 mm total movement
Strip sealsone gap up to about 80–100 mm
Modular jointsabout 80 mm per cell, from 160 mm to about 2,000 mm total
Finger jointscantilever fingers commonly up to about 400–600 mm; supported sliding fingers more
Indicative coststrip seal USD 1,500–3,000 per m; modular USD 8,000–20,000 per m

Where it fits

Bridges over 60-100 m, or shorter ones with high skew or curvature; joint type by movement range.

Where it does not

Joints over piers and bearings without drainage (every leak corrodes the bearing below); modular joints where a series of strip seals would do.

Choosing it

Choose a buried or asphaltic plug joint over a mechanical joint when total movement is below about 40 mm and traffic is not heavy and slow: cheap and quiet, but it ruts and cracks in heat and cold.
Choose a strip seal over a modular joint whenever one gap can take the movement (up to about 100 mm): fewer parts, less fatigue and less noise.
Choose finger joints for large longitudinal movements where modular joint noise and fatigue are concerns and the drainage trough below can be maintained; choose modular joints where large rotations, skew or multi-directional seismic movement must be accommodated.
Prefer no joint at all (integral abutments, link slabs, continuous decks) whenever length and skew allow.

Plant, pace and money

PlantSaws and breakers for the blockout, small crane for modular units.

ProductivityA joint in two to five days.

CostStrip seal USD 1,500-3,000 per metre; modular USD 8,000-20,000 per metre; the game's upkeep score penalises modular and finger joints most.

Risks and controls

What goes wrong

Snowplough and truck impact damage, seal failure, noise complaints, fatigue of modular joint bars.

Quality assurance

Gap setting recorded with temperature, water test of the seal, drainage inspection.

Origins

Early bridges used steel sliding plates and open comb joints; elastomeric compression seals, strip seals and modular joints came into use from the 1960s. Fatigue cracking of modular joint beams and welds in the 1980s and 1990s led to dedicated fatigue design rules (NCHRP Report 402, 1997) and performance tests (NCHRP Report 467, 2002).

Examples

Every seat abutment in the game; integral bridges avoid them entirely.

Case studies

Millau ViaductFrance · 2004A single continuous 2,460 m steel box deck with movement joints only at the two abutments.

Sunniberg BridgeSwitzerland · 1998526 m curved cable-stayed bridge built without expansion joints or bearings at the abutments; the plan curvature lets thermal length changes deflect the deck sideways.