Bearing
It is the interface between two major
components of bridge structure i.e.
super structure and sub structure.
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

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


Translation can be permitted by the
following modes of action :
–
–
–
–
By sliding action
By rolling action
By shearing strain
By racker and pinion devices (as in gears)

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).
Classification

Based on Degree of Freedom
1.
3.
Fixed
Sliding
Rocker and roller

Based on material used
1.
Steel
Bronze
Synthetic material
2.
2.
3.
•
•
Elastomeric pads
PTFE-Poly Tetra Fluoro Ethylene
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.
Even when bearing has been
selected
Choose the large components
 Specify the highest grade of material
 Insist upon strictest tolerance possible
i.e

First cost shouldn’t be the criteria
SLIDING BEARINGS
DIFF. SLIDING BEARINGS

a)
b)
Common materials being used and their
co-efficient of friction are
Material

Mild steel over mild steel
0.2 to 0.3
Mild steel over phosphor
bronze
c) PTFE over stainless steel
0.15
Less than 0.08
PTFE
It is a
 Linear chain polymer of high molecular
strength
 Chemically inert
 Low coeff. of friction
 Not oxidized easily
 Remains stable at extreme atmospheric
temperatures
 Resistant to all common solvent.
MAINTENANCE – GREASING
Equipments required for greasing of sliding
bearings :
 Jacks (50 ton capacity) 2 nos
 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.
DESIGN ASPECTS

As per clause 3.16 of IRS : Steel Bridge
Code the allowable bearing pressure for
different materials is as under :
– Stone Masonry = 36 kg/sqcm
– PCC (1:2:4) = 31.6 kg/sqcm
– RCC = 0.2 * fck (average pressure)
= 0.3 * fck (local max. pressure)
ROLLER BEARING
ELASTOMERIC BEARING
ELASTOMERIC BEARINGS

The elastomeric bearings offer a number of
advantages as listed below:
1.
2.
3.
4.
5.
Minimum maintenance- as no moving parts
Installation is easy
Permits movement in all directions
Occupies small space
Serves as a shock absorber due to anti-vibrations
properties of elastomer
6. As an aid to better management of longitudinal forces.
IMPORTANT FINDINGS by ORE
(office for research & experiments) of FRENCH
Elastomer doesn’t follow hook’s law & E
≠constant
 But G = Constant & G is more relevant than
E
 µ between elastomer & base material
unaffected by nature of contact surface
 µ reduces with increased normal loads(N)
µ=0.1+0.6/N

IMPORTANT FINDINGS by ORE
contd.
Performance of elastomer not affected by
temp variation with in range of (-15 to
+50oC)
 Under cyclic load, it becomes flexible
 Repetitive vertical loading more
detrimental than repetitive shear loading
 Tendency to slip, when normal pressure is
less than 2MPa.

PROPERTIES OF ELASTOMER
Property
Unit
Test method I.S.
Specification reference
Value
specified
1. Physical Properties :
1.1 Hardness*
1.2 Mini. tensile strength
1.3 Mini. elongation at break
2.0 Max comp. set
3 Accelerated ageing
IRHD IS:3400 (Part II)
60 + 5
MPa IS:3400(Part I)
17
%
%
%
IS:3400 (Part I)
IS:3400 (PartX)
duration 24hr +0, -2
temp
100 + 1 oC
400
35
IS:3400 (Part IV)
duration 70 h
temp
100 + 1 oC
3.1 Maximum change in hardness
IRHD
+ 15
3.2 Maximum change in tensile
strength
3.3 Maximum change in elongation
%
-15
%
-30
DIMENSIONAL TOLERANCES
SN
1.
2.
3.
4.
ITEMS
Overall plan dimensions
Total bearing thickness
-0, +6 mm
-0, +5%
Parallelism
A. Of top surface of bearing with respect to
the bottom surface as datum
B. Of one side surface with respect to the
other as datum
1 in 200
1 in 100
A.
B.
5.
TOLERANCES
A.
B.
C.
Thickness of individual internal layer of
elastomer
Thickness of individual outer layer
Plan dimensions of laminates
Thickness of laminate
Parallelism of laminate with respect to
+ 20% (max. of 2
mm
-0, + 1 mm
-3 mm, +0
+ 10 %
1 in 100
DESIGN ASPECTS OF
ELASTOMERIC BEARINGS

Dimensional check
–
–
–
–
–
–
L0 ≤ 2b0
b0/10 ≤ H ≤ b0/5
Shape factor = 6 to 12
For no slip condition  min = 2 Mpa
 max = 10 Mpa
 = 0.1 + 0.6/ m
DESIGN ASPECTS
contd.
Distortion limit = 70% of total elastomer ht
 Bed block bearing stress ≤ 0.25 x fck
 For no uplift & condition

 ei
under DL
 tanα dead
b/6
under LL
 ei
b/6
 tanα dead  1.5 tan  live
DESIGN ASPECTS




contd.
Shear stress due to
Comp load + horizontal load + rotation < 5 G
Due to comp load = 1.5/S [(Pc + 1.5 Ps)/ l x b]
Due to Horizontal = H /l x b
Due to rotation = Gb2/2hi x H (tan c + 1.5 tan s)
DESIGN ASPECTS

contd.
Thickness of steel plate
2(hi + hi+1) (Pc + 1.5 Ps)
hs > ------------------------------l x b x s
hi , hi+1 = thickness of elastomer layers on both
sides of steel plate.
INSPECTION AND QC CERTIFICATE

The bearings should be accompanied by an authenticated
coy of the test certificate with folowing information :
– Name of Manufacturer
– Date of manufacture
– Grade of elastomer
– Bearing Dimensions
– Production Batch no.
– Acceptance lot no.
– Date of testing
– Specific bridge location
– Explanation of markings on the bearing.
PERIODICAL INSPECTION
AND MAINTENANCE

The inspecting official should look for the following aspects :
–
–
–
–
–
–
–
–
Correct position
Excessive shear (movement)
Excessive bulging
Separation of rubber from steel lamination plate
Cracking/tearing of Elastomer
Flattening out
Off loading of one edge (Excessive rotation )
Clean surroundings
Anti slip devices
Suitability of Bearing for spans
Type of bearing
Bridge & Spans (mt)
Remarks
Both end steel
plates.
Plate girders 9.15,
12.2, 18.3, 24.4, 30.5
Elastomeric and
PTFE on trial basis
phosphor bronze Composite 9.15, 12.2, For U/S O/W - 30.5
18.3, 24.4, 30.5
(With one end fixed)
Rocker & Roller O/W through girders30.5, 45.7(2 rollers),
61.0, 76.2 (4 rollers)
Rocker & Roller O/W through girders
with oil bath
more than 76.2
One end fixed &
other free
-do-
Load range & movement in
Bearing
Type of bearing
Load (T)
Steel sliding plates.
20-133
Movement
(mm) one way
25
Roller bearing
60-266
100
Elastomeric Bearing
30-220
60
Pot bearing
20-1780
No limit
POT BEARING
PTFE
It is a
 Linear chain polymer of high molecular
strength
 Chemically inert
 Low coeff. of friction
 Not oxidized easily
 Remains stable at extreme atmospheric
temperatures
 Resistant to all common solvent.
WHY POT 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.
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 comp. deflection of elastomer as it is
encased
 Sliding component can be added at top.

THANKS