Last reviewed 30 Sept 2026 · 6 min read
Why bridges need bearings and joints
A bridge deck is not fixed rigidly to its piers. Temperature, creep, shrinkage, prestress shortening and live load make the deck change length and rotate at its ends. If these movements are prevented, huge forces build up. Bearings are placed between the superstructure and the substructure to:
- transmit the vertical load and the horizontal forces (braking, wind, seismic);
- permit rotation of the girder end;
- permit translation (movement) where required, and restrain it where not.
Expansion joints are gaps in the deck surface, bridged in a way that lets traffic pass smoothly while the deck moves.
Movement and rotation
- Temperature: , with = 1.2×10⁻⁵ per °C for concrete and steel.
- Shrinkage and creep (concrete) and elastic shortening from prestress — cause the deck to shorten.
- Live-load rotation at girder ends and, in longitudinal direction, braking and tractive forces.
- Construction tolerances and settlement — add a margin.
For a simply supported girder under a uniformly distributed load, the end rotation is ; bearing design uses the code values or the computed rotation with an allowance for tilt during erection.
A 30 m long PSC girder with the fixed bearing at one end experiences a temperature range of ±25 °C (total 50 °C).
at the free end. A design margin of about 20–25 % is added for shrinkage, creep and construction.
Types of bearings
| Bearing | How it works | Use |
|---|---|---|
| Sliding (plane) | Two plates slide; lubricated or PTFE | Short spans (up to about 15 m) |
| Roller | Steel cylinder(s) between plates; allows rotation and translation | Older steel and long-span bridges |
| Rocker | Curved top allowing rotation; sometimes combined with rollers | Steel girders and older railway bridges |
| Elastomeric (laminated rubber) | Rubber layers bonded to steel plates; deforms in shear and compression | Most highway girders (span up to about 30–35 m) |
| Pot bearing | Elastomeric disc confined in a steel pot with piston; allows rotation, with PTFE slider for translation | Heavy loads, spans up to about 60 m |
| Spherical bearing | Sliding surface curved (spherical) with PTFE; permits large rotations | Curved decks, long spans, high loads |
| Disc bearing | Polyether-urethane disc | High vertical loads, modest rotation |
| Seismic isolation bearings | Lead-rubber or friction-pendulum type; reduce seismic force | Earthquake-prone regions |
Fixed, free and guided bearings
On a simply supported span one bearing is fixed (no translation) and the other free (longitudinal movement permitted). In continuous bridges the fixed bearing is placed at the pier with the most suitable stiffness, and the free bearings elsewhere. Guided bearings allow movement in one direction only. Horizontal forces (braking, wind, seismic) are shared by fixed and guided bearings.
Design of an elastomeric bearing
An elastomeric (laminated rubber) bearing consists of layers of natural or chloroprene rubber separated by steel shims. It carries vertical load through compression, accommodates rotation by differential compression, and accommodates horizontal movement by shear deformation.
- Shape factor for a rectangle and internal layer thickness . A high shape factor makes the bearing stiff in compression.
- Compressive stress limited by the code (about 10–15 N/mm² depending on shape factor).
- Shear deformation with = total rubber thickness and = shear modulus (about 1.0 N/mm²). The rubber thickness is chosen so that the movement does not exceed about 50–70 % of .
- Stability: limit on total thickness relative to plan dimension to avoid buckling.
- Sliding check: the horizontal force must not exceed the friction resisting force .
Elastomeric bearings are maintenance-free, cheap and forgiving, but they age and creep; their rubber must be replaced when it hardens or splits.