Bearing installation and setting

How bridges get built · Finishing, articulation and maintenance · Bearing installation and setting

Also called: bridge bearings, elastomeric bearings, pot bearings, spherical bearings, bearing plinths

Bearing installation and setting — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Check the bearing schedule against the drawings before ordering: type, design loads, movement directions and ranges, rotations, guide orientation, pre-set table and the jacking points for future replacement. Receive each bearing with its certificate and store it covered with the transport clamps on.
Cast the plinth or pier top low by the bedding thickness (typically 25–50 mm), with pockets for the anchor dowels; scabble and clean the surface and survey its level.
Set the lower plate on levelling screws, wedges or packs, aligned to the bridge axis and to the guide direction, levelled to the specified slope, and with the sliding element offset by the pre-set for the expected installation temperature.
Grout the bedding and anchor pockets in one operation with a flowable non-shrink cementitious grout (epoxy mortar for thin beds), pouring from one side so air escapes; remove packs that would form hard spots and let the grout reach strength before load.
Connect the upper plate: cast it into the soffit of an in-situ deck, sealed so slurry cannot reach the sliding surfaces, or seat precast and steel girders on it with tapered plates for grade and camber.
Release the transport clamps once the deck bears on the bearing and before prestress, shrinkage or temperature can move it. Record the time, deck temperature and measured offset, and correct the pre-set if the temperature has changed.
Fit movement indicators and identification plates, measure offsets and rotations as built, confirm that inspection access and jacking positions are usable, and put the records in the maintenance file.

Key numbers

Laminated elastomeric bearings (AASHTO)average service compressive stress about 7–11 MPa; shear deformation limited to about 50 % of total rubber thickness, giving movements up to roughly ±50 mm
Pot bearingsvertical capacity from about 1 MN to over 50 MN; rotation capacity typically 0.01–0.03 rad
Spherical bearingsrotations of 0.02–0.05 rad and more, at similar loads to pot bearings
PTFE on stainless steelfriction about 1–4 % lubricated at service pressure, higher at low temperature and low pressure
Bedding25–50 mm cementitious grout, or 10–20 mm epoxy mortar; typical setting tolerance ±2–3 mm in level and about 3 mrad in slope (check the specification)
Seismic isolators (lead-rubber, friction pendulum): isolated period about 2–3 s; design displacements commonly ±150–600 mm, up to about ±1 m on large bridges

Where it fits

Every non-integral bridge; the game's bearing choice feeds the upkeep score.

Where it does not

Leaving transport clamps on (locked bearings crack piers), bearings inaccessible for replacement, rocker bearings on new bridges.

Choosing it

Choose laminated elastomeric bearings over pot or spherical bearings when reactions are a few MN or less and movements and rotations are moderate: they are cheap, need no lubrication and tolerate small setting errors.
Choose spherical over pot bearings when rotations exceed about 0.01–0.02 rad or are cyclic, for example at the ends of long steel girders or during launching; pot seals wear under repeated rotation.
Prefer integral or semi-integral abutments to bearings at the ends of bridges up to about 60–120 m long; there is then nothing to replace.
Choose isolation bearings over fixed and guided bearings in high-seismic zones when the longer period cuts pier and foundation forces enough to pay for isolators and larger joints; isolation helps little on soft soil sites, where long-period motion is amplified.

Plant, pace and money

PlantSurvey, grouting kit, small crane.

ProductivityA pier's bearings in a day.

CostElastomeric USD 500-3,000 each; pot and spherical USD 5,000-60,000 each; seismic isolators USD 20,000-150,000 each.

Risks and controls

What goes wrong

Misalignment, grout voids under the plate, wrong pre-set, corrosion of steel bearings under leaking joints.

Quality assurance

Bearing certificates and tests, plinth level within 1-2 mm, grout inspection, clamp removal recorded.

Origins

Steel rockers, rollers and sliding plates served until the 1950s; laminated elastomeric bearings and PTFE sliding pot bearings replaced them during the 1960s. Seismic isolation followed: the lead-rubber bearing was developed in New Zealand in the 1970s and the friction pendulum bearing in the United States in the 1980s.

Examples

Pot and spherical bearings on all the game's long spans; elastomeric on short ones.

Case studies

Bolu Viaduct No. 1Türkiye · 1999 (damage); retrofit early 2000sTwin viaducts of about 38 m simply supported spans on sliding bearings with steel yielding dampers were nearly complete when the Düzce earthquake, whose fault rupture crossed the alignment, exceeded the displacement capacity of the isolation system. They were rebuilt as continuous units on friction pendulum bearings of about ±0.9 m capacity.

Sierra Point OverheadUSA · 1985First seismic isolation of a bridge in the United States: an existing highway overcrossing retrofitted with lead-rubber bearings.