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BEARINGS AND RAILINGS
BEARINGS
Bearings are structural devices
positioned between the bridgesuperstructure and the
substructure.
Their principal functions are as
follows:1. To transmit loads from thesuperstructure to the
substructure, and
2. To accommodate relative
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Movements should be accommodatedproperly and these result
from
q longitudinal movement due to temperaturechanges-reversible
q rotation due to deflection of girders- live loaddeflection is
reversible
q vertical movement due to settlement of
support-irreversible
q movement due to shrinkage, creep
orprestressing-irreversible
Bearings are categorized in to two as
q Fixed bearings are fixed against translation, butfree to
rotate
q Expansion bearings-are free to translate and
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Selection of the type depends on thetype of superstructure, type
of
substructure supports and the length ofthe span.
qA simply supported span requires
fixed bearings at one andexpansion joints at the other.
qA continuous girder require fixed
bearing at one and expansionbearing on all other supports
qA two span girder will have fixed
bearing at the centre support and
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Bearings are important components ofbridges and call for care in
design,skill in execution, regular attention inmaintenance.
Faulty design or malfunctioningbearings due to clogging by
debrishave often resulted in collapse ofbridges due to the high
stresses
induced due to restrained motion. On certain major bridges
bearing cost
is as high as 10-15% of the total cost.
It is worth designing carefully and
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Types of Bearings
For slab bridges no special bearingsare provided because
thetranslational movement is very small.
Edges of supports are beveled toallow for rotation.
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The following are the different types of bearings for girder
bridgesSliding plate bearing- is the simplest type of bearing. This
utilizes one platesliding on another. This type is used for bridges
up to 30m span. Flat plateswere common but due to freezing and high
frictional forces the surface ofthe upper plate is slightly curved
as shown in Figure below.
Figure: Sliding plate bearing
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Roller bearing- in this type rotation is accommodated by the
pinand translation by the use of a roller or a group of rollers.
SeeFigure below. This is used for spans of moderate length.
Figure: Roller bearing
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Rocker bearing- this is used for longer spans and isused to
accommodate large live load deflections.
See Figure below.
Figure: Rocker bearing
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Rocker bearing- this is used forlonger spans and is used to
accommodate large live loaddeflections. See Figure below.
Figure: Rocker bearing
In seismic areas suitable guides
should be provided to prevent theroller and rocker components
frombeing displaced off abutments and
piers during earthquakes.
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RAILINGS
Railings are provided along edges ofbridges for protection of
traffic andpedestrian. There are three types ofrailings.
qTraffic railings
q Pedestrian railings
q Combination railings The following guideline indicate the
application of various types of rails
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