Bearings in bridges

Bearings are important elements
in large infrastructures such as
bridges, heavy buildings and high
rise buildings, ensuring load
transfer, movement, rotation,
vertical or horizontal restraints as
required by the design at critical
locations in a structure

Bridge Bearings are structural devices
positioned between the bridge
superstructure and the substructure.
Their principal functions are as follows:
1. To transmit loads from the
superstructure to the substructure
2. To accommodate relative movements
between the superstructure and the
substructure.

 Self-weight
 Traffic loads
 Wind loads
 Earthquake loads

 Translations ( both transverse and
longitudinal directions ) caused by
Creep, shrinkage and temperature
effects
 Rotations caused by Traffic
loading, Construction tolerances
and uneven settlement of the
foundation

Until the end of the 18th century all
structures of any appreciable size
were built of stone, brick or mixed
masonry. These structures, generally
massive, are little affected by
environmental changes and any slight
movements which may occur are
compensated either by deformation
of the constituent materials or by
small displacement of the supports.

In the 19th century, with introduction of cast iron and
steel larger structural spans accompanied by lightening
of the structure which becomes-more slender and
flexible and loses some of the thermal inertia. They must
then be fitted at their support points with simple
devices, called bearings, which can withstand movement
and, more particularly, the expansion or contraction due
to temperature changes.

Rail Bridge that connects Piermont, NY (Hudson River) to Dunkirk.

Bearings used at that time were metal plates
sliding one on the other or, of rollers or, of a
combination of both. They were gradually
improved by incorporation of swivel
arrangements to provide rotational movements

North Pivot Bearing of Sydney Harbor Bridge

In 20th century, RCC structures developed but, initially they were
as massive as masonry with little significant support
movements.
But at the end of Second World War the necessity of rebuilding
rapidly the structures favored the rapid development of
reinforced concrete and, even more so, of Pre-stressed concrete.
Structures which were more and more slender, needed bearing
devices to allow for movements and rotations

Bearings
Fixed Bearings Expansion Bearings
Allow rotations but
restrict translational
movements
Allow both rotational
and translational
movements

1313
A sliding bearing utilizes one plane metal
plate sliding against another to
accommodate translations.
The sliding bearing surface produces a
frictional force that is applied to the
superstructure, the substructure, and the
bearing itself.
To reduce this friction force, PTFE
(Polytetrafluorethylene) is often used as a
sliding lubricating material.

 Sliding bearings can be used alone or more often used
as a component in other types of bearings.
 Pure sliding bearings can only be used when the
rotations caused by the deflection at the supports are
negligible.
 They are therefore limited to a span length of 15 m or
less by ASHTTO.
 A guiding system may be added to a sliding bearing to
control the direction of the movement.

A rocker bearing is a type of expansion bearing that comes in a
great variety.
It typically consists of a pin at the top that facilitates rotations,
and a curved surface at the bottom that accommodates the
translational movements
Rocker and pin bearings are primarily used in steel bridges.

 The moment and lateral forces induced from the movement of
these bearings are very small and negligible.
 However, metal bearings are susceptible to corrosion and
deterioration. A corroded joint may induce much larger forces.
 Regular inspection and maintenance are, therefore, required.

1818
Roller bearings are composed of
one or more rollers between two
parallel steel plates.
Single roller bearings can
facilitate both rotations and
translations in the longitudinal
direction, while a group of rollers
would only accommodate
longitudinal translations.

 Single roller bearings are relatively cheap to manufacture, but
they only have a very limited vertical load capacity.
 Multiple roller bearings, on the other hand, may be able to
support very large loads, but they are much more expensive.
 Like rocker and pin bearings, roller bearings are also
susceptible to corrosion and deterioration. Regular inspection
and maintenance are essential.

Single Roller Bearing Multiple Roller Bearing

2121
Bearings are formed of horizontal layers of synthetic
or natural rubber in thin layers bound between steel
plates.
These bearings are capable of supporting high
vertical loads with very small deformations.
These bearings are flexible under lateral loads.
 Steel plates prevent the rubber layers from bulging.
Lead cores are provided to increase damping capacity
as plain elastomeric bearings does not provide
significant damping.
They are usually soft in horizontal direction and hard
in vertical direction.

2222
It consist of a laminated elastomeric bearing equipped
with a lead cylinder at the center of the bearing.
The function of the rubber-steel laminated portion of
the bearing is to carry the weight of the structure and
provide post-yield elasticity.
The lead core is designed to deform plastically, thereby
providing damping energy dissipation.
Lead rubber bearings are used in seismically active areas
because of their performance under earthquake loads.

2323
A curved bearing consists of two matching
curved plates with one sliding against the
other to accommodate rotations.
The curved surface can be either cylindrical
which allows the rotation about only one axis
or spherical which allows the bearing to
rotate about any axis.
Lateral movements are restrained in a pure
curved bearing and a limited lateral resistance
may be developed through a combination of
the curved geometry and the gravity loads.

2424
To accommodate lateral movements, a PTFE slider must be attached to the
bearings.
The lower convex steel plate that has a stainless steel mating surface is recessed in
the masonry plate.
The upper concave plate with a matching PTFE sliding surface sits on top of the
lower convex plate for rotations.
Between the sole plate and the upper concave plate there is a flat PTFE sliding
surface that will accommodate lateral movements.

2525
A pot bearing comprises a plain elastomeric
disk that is confined in a shallow steel ring, or
pot. Vertical loads are transmitted through a
steel piston that fits closely to the steel ring
(pot wall).
Flat sealing rings are used to contain the
elastomer inside the pot.
The elastomer behaves like a viscous fluid
within the pot as the bearing rotates.

2626
Because the elastomeric pad is confined, much
larger load can be carried this way than through
conventional elastomeric pads.
Translational movements are restrained in a pure
pot bearing, and the lateral loads are transmitted
through the steel piston moving against the pot wall.
To accommodate translational movement, a PTFE
sliding surface must be used.
Keeper plates are often used to keep the
superstructure moving in one direction.

2727
A disk bearing utilizes a hard elastomeric
(polyether urethane) disk to support the vertical
loads and a metal key in the center of the bearing
to resist horizontal loads.
The rotational movements are accommodated
through the deformation of the elastomer. To
accommodate translational movements, however,
a PTFE slider is required.
In this kind of bearings, the polyether urethane
disk must be hard enough to resist large vertical
load without excessive deformation and yet
flexible enough to accommodate rotations easily.

2828
Loads acting on bearings are calculated and suitable types are decided
For the various qualified bearing alternatives, preliminary designs are performed to
determine the approximate geometry and material properties in accordance with
design specifications.
It is likely that one or more alternatives will be eliminated now because of an
undesirable attribute such as excessive height, oversize footprint, resistance at low
temperature, sensitivity to installation tolerances, etc.
At the end of this step, one or more bearing types may still be feasible and they will
be included in the bid package as the final choices of the bearing types.

2929
Where a simple span deck is supported over rigid supports and the span is less than
7.5m, no special bearing devices are necessary. Only tar paper or a felt layer is
adequate.
For spans betweens 7.5m to about 15m, mild steel plate bearings, sliding type over
free supports and rocking type over the fixed support, may be used.
For spans in excess of 15m, metallic rocker type bearing is provided over the fixed
support and a roller cum rocker type bearing is provided over the free supports.
Alternatively suitably designed laminated neoprene bearings may be provided.

3030
Power-wash steel sliding plates and steel rocker bearings (especially at
abutments) annually when washing decks. Make sure to concentrate efforts
between rocker surfaces and base plates to remove dirt that could accumulate
and impede movement.
Touch up steel with high-quality epoxy paint as needed.
Neoprene bearings require little maintenance except occasional power washing
to keep dirt and debris from building up around bearings

3131
1) Raina VK (1994) , ‘ Concrete Bridge Practice – Analysis, Design and Economics’,
Second edition, Tata McGraw-Hill Publishing Company Limited, New Delhi
2) Standard Specifications and Code of Practice for Road Bridges, Section-1 General
Features of Design (Eighth Revision), IRC-5:2015, Indian Road Congress, New Delhi
3) Bridge Engineering Handbook, CRC Press, New York Washington DC

Bearings in bridges

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Bearings in bridges