Bearing

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Chapter 13

• Bearing

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contents

• END BEARING
• Plate bearings (Sliding hinged bearings).
• Rocker bearings
• Roller bearings
• Bearing adopted by Railway Board.

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END BEARING The bearings are provided at both the
ends of a bridge girder. One end of the bridge
girder is fixed in position while, the other end
is kept free for the horizontal movement. The
bearings are provided for the following
functions 1- The bearings are provided to
transmit the end reaction to the abutments and/
or piers and to distribute it uniformly, so that
the bearing stress does not exceed the allowable
bearing stress of the material. 2- The
bearings are provided to allow the movement in
the longitudinal direction (expansion and
contraction) due to change in temperature and
stresses.
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3-The bearings are provided to allow rotation at
the ends, when the bridge girders are loaded and
deflections take place. For all spans in excess
of 9 m, the provisions are made for change in
length due to temperature and stress variation.
The provisions for expansion and contraction
should be such as to permit movement of the free
bearings to the extent of 10 mm for every 10 m of
length. For spans greater than 15m, on rigid pier
or abutment, the bearings, which permit angular
rotation at the girder ends, are provided, and at
one end, there shall be a roller or other
effective type of expansion bearing.
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In the design of bearings, provision shall be
made for the transmission of longitudinal and
lateral forces to the bearings and the supporting
structures. Provision shall be made against any
uplift to which the bearing may be subjected. All
bearings are designed to permit inspection and
maintenance.
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TYPES END BEARING Depending upon the
magnitude of end reaction, and the span of
bridge, the different types of bearings used for
the bridges are as follows 1-Plate bearings
(Sliding hinged bearings). 2-Rocker
bearings 3-Roller bearings. 4-Bearing
adopted by Railway Board.
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1-Plate bearings (Sliding hinged bearings).

• Plate bearings are simplest type of bearings. The
  plate bearings are used small spans upto 15 m and
  small end reaction of the bridge. Fig. 13-1 shows
  a plate bearing. The plate bearing consists of
  two plates.

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Fig 13-1
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A sole plate is attached to the bridge. The sole
plate rests on bearing. The bearing plate is
anchored to the concrete. The two anchor bolts
fixed in concrete pass through the bearing plate
and the sole plate. The size of bearing plate is
found by the end reaction and the allowable
bearing pressure on the concrete. The plates are
made rigid to distribute the end reaction as
uniformly as possibly over the required area of
the concrete. When the anchor bolts pass through
the circular holes in the sole plate, then, the
plate bearings act as hinged bearing. One end of
the bridge girder is hinged or anchored to the
concrete through the hinged bearings.
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The hinged bearings are designed for the end
reaction (vertical load) and the lateral forces.
The magnitudes of end reactions used are large.
Therefore, the fixed bearings designed for end
reactions (vertical loads) only strong enough to
take the lateral forces. In order to allow the
longitudinal movement, the slotted holes are
provided in the sole plate. In order to reduce
the friction, the surfaces of sole plate and
bearing plate in contact are well machined and
smoothly finished. The sole plate can slide upon
the bearing plate. The plate bearings act as
expansion bearings of sliding type. In the
expansion bearing, the longitudinal movement
(expansion or contraction) takes place with
change of temperature and loads
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The longitudinal force at any free bearing shall
be limited to the dead load reaction at the
bearings multiplied by the coefficient of
friction. The coefficients of friction for
different surfaces in contact are given in clause
6.10 (Egyptian code for loads). The plate
bearings have bearing two disadvantages. The edge
of plate nearest to the end of span has a
tendency to lift along with the deflection of
bridge girder. Therefore, the end reaction is not
distributed uniformly. Secondly, in order to have
longitudinal movement, the sliding friction is to
be overcome. Therefore, for the large span
bridges, the more efficient devices are
necessary.
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The end reaction is distributed uniformly by
providing a deep cast steel bed block as shown in
Fig. 13-2. Such bed blocks have adequate
rigidity. The sole plate bearings are many times
made curved as shown in Fig. 13-3. The curved
sole plate allows rotation. For large spans, the
plate bearings are not suitable. The hinged
(rocker) bearings and roller bearings are used in
such cases. The sliding bearing is the least
expansive bearing for light and intermediate
reactions.
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Fig 13-2
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Fig 13-3
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Figure 13-4 shows a bearing that makes use of a
rocker between the bearing plate and the beam or
girder.
Fig 13-4
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A similar detail in which the anchor bolts do not
pass through the rocker is shown in Fig. 10.3. In
this case, the beam is held in position by means
of pintles shaped like gear teeth. This type of
support may be used where resistance to uplift
need not be provided. For example, it may be used
for inside beams of the beam bridge, with the
outside beams supported by bearings of the type
as shown in Fig. 13.5.
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Fig 13-5
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Figure 13-6a shows an expansion bearing for
larger bridges. Several variations are shown in
the view at the right. The sole plate may be
bolted to the girder, as at the left of the
centerline, or welded as shown at the right.
Resistance to uplift may be provided by using a
hinge plate, as at the left if such resistance
is needed, lateral movement is prevented by a
plate such as that shown at right. A
corresponding hinged end bearing is shown in Fig.
13-6b.
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Fig 13-6
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Although there is only a line of contact between
an unloaded rocker and its bearing plate,
deformation under load distributes the reaction
over a finite area. Evidently, at a given load
this area increases with increase in radius of
the rocker, since a rocker of infinitely large
radius would have a plane surface to begin with.
The allowable load must be evaluated in terms of
limiting permanent deformation. Thus the yield
point of the material is also a factor. These
bearings consist of An upper sole plate in
rolled steel riveted to the girder. For hinged
bearing the sole plate is provided with two
grooves in which two ribs in the bearing plate in
gage and thus the horizontal movement isnt
available.
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y t1/ 2
t1 ? (3 - 4) cm
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2- Abearing plate of cast steel (or cast iron
for small Roadway Bridges). Fixed to masonry by
ribs. The size of the bearing plate is obtained
from the allowable bearing pressure on masonry
for granite basalt or similar hard stones 40
kg/cm2. For reinforced with circular hoops 70
kg/cm2.
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y t2/ 2
The bearing plates for hinged and movable
bearings are the same size. The bearing plate
shall rest on a 3 mm sheet of lead and shall
provided with masonry ribs to transmit the
horizontal reaction of the bridge.
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2-Hinged (Rocker) bearings
Fig. 13-8 shows a typical rocker bearing.
Fig. 13-8
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The cast steel sole and cast steel bearing block
are used in these types of bearings. A
cylindrical pin is inserted in between the cast
steel sole and the cast steel bearing block. This
pin allows rotations at the ends of bridge
girder. The rocker bearing acts as hinged
bearing. The end reaction of a bridge girder is
transmitted to the pin by direct bearing through
the sole attached with the girder. The vertical
plates are used to transmit the end reaction. The
number of plates (two or three) depends upon the
magnitude of end reaction. The end reaction is
further transmitted to the cast steel bearing
block and then to the supporting structure.
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Two outer vertical plates completely encircle the
pin. In case, the bearing is subjected to an
uplift, then, the uplift is resisted by theses
plates. The middle plates provide only bearing
with the cylindrical surface of the pin. The
required bearing area is provided by the product
of total thickness of plates and the diameter of
pin. The thicknesses of all the plates are kept
equal. Therefore, the end reaction is transmitted
equally by these plates. The value of bending
moment is found by multiplying force transmitted
by outer plate of the sole to the outer plate of
bearing block and center to center distance
between these plates. The size of base plate is
found by the allowable bearing stress in the
concrete and the end reaction.
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The rocker bearing are also bearings are also
subjected to lateral and longitudinal forces in
addition to the end reaction (vertical loads).
The increase of end reaction due to lateral and
longitudinal forces is also taken into
consideration. The lateral forces and the
longitudinal forces are assumed to act at the
level of cylindrical pin of the rocker bearing.
The base plate is subjected to moment along both
the directions. The total bearing stress in the
concrete should not exceed the allowable bearing
stress. The rocker bearings are designed for the
end reaction and then checked for lateral forces
and longitudinal forces. Figure 3.54 shows the
rocker bearing for the hinged end.
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In the rocker bearing for free end of the bridge
girder the underside of sole is curved, which
rotates on the horizontal bearing plates and
allows longitudinal movement. This acts as rocker
type expansion.
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3-Roller bearings.
The roller bearings as shown in Fig. 13-9 are
also used for the long span bridges. Fig. 3.55
(A) shows a single roller used in the bearing.  
The rollers provide the rotation as well as the
longitudinal movement. Fig. 3.55 (B) shows number
of rollers used in the bearing. The bearings act
as roller type expansion bearings. The rollers
are kept in position by means of dowels, lugs or
keys as shown in Fig. 3.55 (A). The roller
bearings for spans above span 35 m should
preferably be protected from dirt by oil or
grease box.
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So long as, the size of rollers is small, the
complete circular rollers are provided. When the
size of rollers become large, then, the sides of
rollers are cut in order to reduce the length of
the sole, and to make the bearings more compact.
These rollers with cut sides are known as
segmental rollers.
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Fig 13-9
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In order to avoid overturning or displacement of
these rollers, these are geared with upper and
lower plates. The spacing between segmental
rollers and the width of rollers may by found as
below It is assumed that the rollers don not
slip but only roll during rolling. When, the
roller rolls to the maximum position, as shown in
Fig. 13-10,
Fig 13-10
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then, the vertical axis of roller turns through
an angle ?, and the center of the roller travels
through a forward motion, B. Then,
(i)
(3.15)
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The distance between adjacent segmental rollers
a, (i.e. the spacing between the segmental
rollers) should be such that the rollers do not
come in contact during the forward motion. Then,
(a d) (db) sec ? (iii)
a b?sec ? d (sec ?- 1) (3.16) Where, b
Least allowable perpendicular distance between
the faces of adjacent, after their revolved
positions. The spacing between adjacent segmental
rollers a, is found, knowing b, d and ?. The
roller bearings are also used to support the cast
steel sole with pin bearings as shown in Fig.
13-11. In such cases the roller also acts as a
hinged bearing.
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Fig 13-11
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The following points are kept in mind while
designing s sole a pedestal for the roller
bearing. 1.      The sole transmits the end
reaction to the pin. The end reaction must be
distributed from the pin to the various rollers
uniformly. 2.      The size and number of
rollers provided should be adequate to have
proper stress and free movement. 3.      The
rollers should be so arranged that these can be
readily cleaned of accumulated dirt and dust. 4-
Segmental rollers (Fig. 13-12) are ordinary
used since they occupy less space than
cylindrical rollers.
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The rollers may be coupled with the sidebars
shown and the entire nest held in position by
tooth guides which engage slots in the shoe and
in the bearing plate. Sidebars may be omitted if
each roller is held by teeth. Lateral movement is
prevented by the tongues shown in the view at the
right. 1.      Resistance to uplift may be
provided by lugs that have projections extending
over the upper surface of the base of the shoe or
by enlarging the base of the shoe and providing
slotted holes for the anchor bolts. The roller
assembly may be enclosed with removable dust
guards they are shown on only two sides in Fig
13-12 to indicate that they are optional.
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The roller bearings consist of the following
parts 1-Upper sole plate in structural steel
or cast steel or cast/ steel riveted to the plate
girder. 2-A lower sole plate (saddle) in cast
steel with a curved upper surface and a plain
lower surface which bears upon the rollers.
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Its dimensions depend upon the number of rollers
their diameter and clearance left between the
rollers. It must project on either side to allow
for longitudinal movement of the bridge. In case
of two rollers the B.M. at center of plate V?S/
4 In case of three rollers or more the saddle
plate acts as a continuous beam of variable
inertia by three rollers the central one will
carry most of the load. For this reason it is
generally preferred to have the number of rollers
either (1 2 4 6 8).
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   The rollers The size of rollers depends upon
the maximum reaction on one roller and the
material of construction. Formula of Hertz for
contact between a plane and cylinder of radius R
and length L is

Assuming equal distribution of the reaction V on
all rollers
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For Cast Iron E 1000 t/ cm2, ?max 5.0 t/cm2
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For Rolled Steel E 2100 t/ cm2, ?max 6.50
t/cm2 For Cast Steel E 2200 t/
cm2, ?max 8.50 t/cm2 For Forged Steel E
2200 t/ cm2, ?max 9.50 t/cm2



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The rollers are provided with wider discs to take
up the lateral reaction. The rollers are coupled
together by strong side bars, serving as spacers
allowing (2 - 4) cm between every two rollers.
The diameter of the rollers shall be not less
than 12 cm and not more than 35 cm. 4-The
lower bearing plate It distributes the
concentrated
reaction of the rollers upon a wider bearing area
of the abutment. We generally assume uniform
upward pressure and the plate acts as a beam with
over hanging ends.
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4-Hinged bearings with a bearing block

• It used for longer spans and consist of an upper
  sole plate riveted to the girder and a bearing
  block.
• The bearing block is made of cast steel (or cast
  iron for small Roadway bridges) with longitudinal
  and transverse ribs.
• For vertical reaction only the pressure on the
  abutment

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? ?perm 40 kg/ cm2 for Basalt and Granite 70
kg/cm2 for Reinforce Concrete
Including the effect of horizontal reactions in
the longitudinal and transverse directions then
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? 1.15 ??perm
The height ht of the hinged bearing is
practically taken equal to that of the opposite
movable bearing The maximum stressed section is S
S, it is equivalent to a T section with a web 4
t1.
and
For cast iron Ft all 400 kg/cm2 Fc all
1000 kg/cm2
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For cast steel F all 1800 kg/cm2 Thickness
of lower flange (1/3 1/5) ht Total n t1
(1/4 1/5) of the total width b Thickness of
central web ? 1/6 ht The upper surface of the
bearing block must be curved to a count for end
slop of the girder. The lower surface of the sole
plate may be either straight or curved. The face
of the contact is a line in the unloaded
condition. Under the load it becomes a rectangle.
The width which (b) increased with increase at
loads. Hertz formula for contact between two
curved surfaces
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b width of area of contact
r1 r2 radii of upper and lower surfaces
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where E ? 1/m are the modulus of elasticity
and Poisson ratio of the two materials.
m 3 for steel m (2 4) for all the
materials Assuming the elliptical pressure
distribution over the narrow strip b
For the case E1 E2 E, and ?1 ?2 ? 1/3
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For a flat lower surface of sole plate and 1/r1
0
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The allowable pressure ?max can be taken much
higher than the working stress In compression
?max 5.0 t/cm2 Cast Iron ?max 6.5 t/cm2
Rolled Steel ?max 8.5 t/cm2 Cast
Steel ?max 8.5 t/cm2 Forged Steel Forged
Steel Rolled Steel but subjected to
temperatures up to 800 900 1000? C
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