TOPIC RELATED TO BRIDGE BEARINGS

1. 1
Bearings

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

6. 6
EFFECT OF TEMPERATURE ON
GIRDER IF BEARING IS FROZEN
UP

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

12. 12
SLIDING BEARINGS

13. 13

14. 14

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.

17. 17

18. 18

19. 19

20. 20

21. 21

22. 22

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

25. 26
BENDING

26. 27

27. 28
Bearing
Sinking into
Bed Block

28. 30

29. 31
Steel Stool

30. 32
Corrosion near Bearing

31. 33
Corroded Bearing Stiffener

32. 34
ROLLER BEARING

33. 35

34. 36

35. 37

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

37. 39
Rollers Broken and Pitted

38. 40
OIL BATH BEARING

39. 41
ELASTOMERIC BEARING

40. 42

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

42. l
c
c
c
b
laminate

43. 45
SHEAR/ TRANSLATION ACTION

44. 46
ROTATION/ BENDING ACTION

45. 47
ACTION UNDER VERTICAL
LOAD
 Vertical load gets converted to shear in the
elastomer/ laminate interface

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.

50. 52
Anti slip devices

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

54. 56

55. 57

56. 58

57. 59

58. 60
POT BEARING

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.

62. 64
POT
ELASTOMER
BRASS
SEAL
PISTON
PTFE
STAINLESS
STEEL
PLATE
WEARING
SURFACES
DUST
SEAL

63. 65

64. 66
TRANSPORTATION
CLAMP

65. 67

66. 69

67. 70

68. 71

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

74. THANKS