Bearings are interfaces between bridge superstructures and substructures that allow for movement caused by thermal expansion, live loads, wind loads, and other factors. The main types of bearings are sliding, roller, elastomeric, and pot bearings. Sliding bearings use steel plates and require regular greasing. Roller bearings use steel rollers and can accommodate more movement than sliding bearings. Elastomeric bearings are made of rubber and allow for movement in all directions with little maintenance. Pot bearings encase an elastomeric pad in a metal pot and can accommodate unlimited movement while preventing compression of the elastomer. Proper selection, inspection, and maintenance of bearings is important for the long
Bridge Bearings has been considered as of huge importance in civil engineering. It plays a significant role in the structure of bridges. This presentation covers the complete study of Bridge Bearings.
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2. 2
Bearing
It is the interface between two major
components of bridge structure i.e.
super structure and sub structure.
3. 3
Functions of bearing:
Load transmission
Permit rotary or rocking movement caused by
deflection of super structure.
Allow horizontal movement of super structure
due to expansion or contraction.
Restrict lateral movement of super structure
4. 4
The factors causing movement in the
bridge superstructure
Thermal expansion and contraction
Deformation under live load
Longitudinal forces- tractive / breaking
Wind loads
Settlement of supports
Seismic forces
Creep and shrinkage of concrete
5. 5
Load range & movement in
Bearings
Type of bearing Load (T) Movement
(mm) one way
Steel sliding plates. 20-133 25
Roller bearing 60-266 100
Elastomeric Bearing 30-220 60
Pot bearing 20-1780 No limit
7. 7
Translation can be permitted by the following
modes of action :
– By sliding action
– By rolling action
– By shearing strain
8. 8
Rotation can be permitted by the following
modes
– By rocking/hinge action
– By differential compression (as in elastomeric pads)
– By bending/flexure (as in tall piers, portals).
9. 9
Classification
Based on Degree of Freedom
1. Fixed
2. Sliding
3. Rocker and roller
Based on material used
1. Steel
2. Bronze
3. Synthetic material
• Elastomeric pads
• PTFE-Poly Tetra Fluoro Ethylene
10. 10
SELECTION OF BEARINGS
The selection depends upon a no. of factors
Functional requirement
Expected life –compatible with life of bridge
Maintenance efforts- should be minimum
Cost
Other factors
– Height of the bearing
– Management of horizontal force transferred to the
substructure
– Performance under seismic loads.
11. 11
Even when bearing has been
selected
Choose the large components
Specify the highest grade of material
Insist upon strictest tolerance possible
– Because the trouble in bearings cause more
problems for the maintenance engineer and the
structure
15. 15
DIFF. SLIDING BEARINGS
Common materials being used and their
co-efficient of friction are
Material
a) Mild steel over mild steel 0.2 to 0.3
b) Mild steel over phosphor
bronze
0.15
c) PTFE over stainless steel Less than 0.08
16. 16
MAINTENANCE – GREASING
Equipments required for greasing of sliding
bearings :
Jacks (appropriate capacity) 2 nos + 1 standby
Hard wooden packing (below and above jack)
Grease graphite grade 3 conforming to IS 508
Kerosene or released black oil for cleaning
6 mm thick steel scrapers
Mortar pan
Cotton waste.
23. 23
Greasing – Care to be taken
To be done after duly informing P-Way, OHE
departments
P Way/ OHE departments shall be available
Proper train protection must be ensured
Girder shall not be lifted excessively
Tested equipment/ standby equipment shall be there
Girder shall be handled from nominated locations only
24. 24
MAINTENANCE ASPECTS
Guide strips and Location strips breakage
Holding down bolts corrosion
Bed block shaking and cracking
– Loose holding down bolts
Sinking of bed plates into bed block
Crack in weld/ flange near the bearing
Corrosion in bearing/ bed plate
36. 38
MAINTENANCE ASPECTS
Excess tilting of rollers
– Rollers not properly inclined at ambient temperature
Flattening of rollers
Cracks in rollers
Corrosion in rollers
– Seizure of rollers
Problems in holding down bolts and bed plates
41. 43
ELASTOMERIC BEARINGS
Elastomer is a form of rubber, a class of polymeric
substance obtained after vulcanization. (making rubber
stronger and more elastic by treating with sulphur at high
temperature)
As a result of vulcanization, rubber molecules are cross
linked with sulphur, making the rubber stronger
It possesses rubber like properties i.e. ability to regain
shape almost completely even after large deformations
These are very flexible in shear but very stiff in bulk
compression
Design as per UIC772-2R 1989 and as per IRC 83 Part II
46. 48
Shape Factor
Compressive strength of the bearing depends
upon the ratio of loaded area to the area of
bearing free to bulge. This is known as Shape
Factor “S” which is a dimensionless parameter
Greater compressive stiffness is ,therefore,
obtained by dividing elastomer in to many layers
by introducing very thin, usually 1 to 3mm steel
reinforcement plates.
47. 49
PROPERTIES OF ELASTOMER
(915.2)
Property Test method
I.S.
Specification
reference
Value specified
1. Physical Properties :
1.1 Hardness* IS:3400(Part II) 60 + 5 IRHD
1.2 Mini. tensile strength IS:3400
(Part I)
17 MPa
1.3 Mini. elongation at break 400%
2.0 Max comp. set
(comp strain of 25 % given for
duration 24 (+0, -2) hr & temp
100o C (+ 1o C)
IS:3400
(Part X)
35 % of initial
48. 50
PROPERTIES OF ELASTOMER
(915.2)
Property Test method
I.S.
Specification
reference
Value
specifie
d
3.0 Accelerated ageing (When it is heated for a
duration of 70 h & at temp 100 ± 1o C)
IS:3400 (Part
IV)
3.1 Maximum change in hardness +15
IRHD
3.2 Maximum change in tensile strength -15 %
3.3 Maximum change in elongation -30 %
4.0 Ozone Test
20% strain, 40o C, 50pphm ozone conc., for 96 hours
There should be no
cracking
49. 51
ELASTOMERIC BEARINGS
The elastomeric bearings offer a number of
advantages as listed below:
1. Minimum maintenance- as no moving parts
2. Installation is easy
3. Permits movement in all directions
4. Occupies small space
5. Serves as a shock absorber due to anti-vibrations
properties of elastomer
6. As an aid to better management of longitudinal forces.
51. 53
Limitations of Elastomeric
Bearings
Ordinary elastomeric bearing can’t be used as
a fixed bearing
Translation allowed by elastomeric bearing is
restricted by its thickness – 0.5 to 0.6 of
thickness
Thick elastomeric pads are rather unstable
Limit of vertical load which can be placed
safely on elastomeric pads
– It causes excessive compression & bulging.
52. 54
Maintenance Aspects
Bulging of neoprene
Tearing of neoprene
Tilting of bearing
Disintegration of bearing
Soft bearings (excess vibrations)
53. 55
General Guidelines for
Inspection
Shear deformation more than 50% of height
of Elastomeric Pads
Rotation leading to off loading of an edge
Compression more than 5% of height of the
pads
59. 61
POT BEARINGS
Pot bearing – It take beneficial properties of
elastomer in fixed bearings & design of large
expansion bearings
Rotational movement permitted by shear
deformation of an elastomeric pad
Translational movement restraint by completely
encasing the elastomeric pad in a POT
No compressive deflection of elastomer as it is
encased
Sliding component can be added at top.
60. 62
POT BEARINGS
Friction between PTFE and stainless steel is
highly susceptible to intrusion of dust
Hence Silicon grease is generally used as
lubricant
Dust seals are also provided around PTFE
bearings
61. 63
PTFE
It is a
Linear chain polymer of high molecular
strength
Chemically inert
Low coefficient of friction
Not oxidized easily
Remains stable at extreme atmospheric
temperatures
Resistant to all common solvent.
69. 72
Pot Bearings used for the first time for Steel open web Girders of 120m
span of Indian Railways in Mandovi and Zuari Bridges under Konkan
Railway.
70. 73
Advantages of POT/PTFE
When limitation of overall height of the bridge
Large vertical force involved
Large movements- in rotational as well as in
translation anticipated
More life
Less maintenance efforts
71. 74
Load range & movement in
Bearings
Type of bearing Load (T) Movement
(mm) one way
Steel sliding plates. 20-133 25
Roller bearing 60-266 100
Elastomeric Bearing 30-220 60
Pot bearing 20-1780 No limit
72. 75
Suitability of Bearing for Steel spans
Type of bearing Bridge & Spans (m) Remarks
Both end steel
plates.
Plate girders 9.15,
12.2, 18.3, 24.4, 30.5
Elastomeric on
local conditions
phosphor bronze Composite 9.15, 12.2,
18.3, 24.4, 30.5
For U/S O/W - 30.5
(With one end fixed)
Rocker & Roller O/W through girders-
30.5, 45.7(2 rollers),
61.0, 76.2 (4 rollers)
One end fixed &
other free
Rocker & Roller
with oil bath
O/W through girders
more than 76.2
-do-
POT PTFE bearings are being used for longer spans
73. 76
Suitability of Bearing for PSC spans
Type of bearing Bridge & Spans
(m)
Remarks
Elastomeric
Bearing
I section/ BOX
section girders
As per Cl. 16.9.13 of Concrete
Bridge Code, Elastomeric
bearings shall be restricted for
clear spans more than 30.5 m
POT / PTFE
bearing
All PSC spans ≥
30.5 M
Earthquake restraint shall be provided for longer
spans