Bridge Bearings
PREFACE TO THE SECOND EDITION
The book ‘Bearings for Railway Bridges’ was first published in1996. The book had been well taken by the field engineers asthis was perhaps the first book exclusively devoted on bridgebearings.
A need was felt to revise the book thoroughly in view of thefact that numerous developments have taken place in the fieldof bearings. New types of bearings are being conceived andtried on the bridges. These are aimed at mitigating theproblems arising out of increased seismic activities andlongitudinal forces. On existing bridges, these types ofbearings have proved to be a boon because the substructurecan be retained merely by replacing the conventional bearingswith these new bearings in order to cater for the higherlongitudinal forces on account of introduction of higher axleloads.
A revised & enlarged edition of the book has been compiled byShri Ghansham Bansal, Professor/Bridges. A new chapter on‘Emerging trends in bearings’ has been added whichincorporates the new types of bearings which are being used inadvanced countries. Sample design problems have beenincorporated in the chapters of ‘Elastomeric Bearings’ & ‘POT-PTFE Bearings’ for better understanding of the subject.
Although every effort has been made to bring out the latest andpresent the book in error free manner, yet if there is anysuggestion or discrepancy, kindly do write to us.
Shiv Kumar Director
ACKNOWLEDGEMENTS TO THESECOND EDITION
The first edition of the book ‘Bearings for Railway Bridges’published in 1996 was very popular among the field engineersprobably because this was the first book available exclusivelyon bearings for railway bridges. Shri S.M. Vaidya, Ex.Professor/Bridges, IRICEN had made great efforts in publishingthe first edition. Nevertheless, with numerous developmentstaking place in this field, there was a crying need to bring outthe latest on the subject.
In this second enlarged and revised edition, a new chapter on‘Emerging trends in bearings’ has been added to incorporatethe new types of bearings. In addition to this, design of POT-PTFE bearings has been dealt in greater detail with onesample design problem for better understanding. Likewise, onesample design problem has also been included for Elastomericbearings. The critical steps have been explained with the helpof sketches and derivation of formulae.
Many a times, due to increased longitudinal loads on accountof traffic as well as seismic activities, the bridge substructuresconstructed in the past with conventional bearings are found tobe inadequate. New types of bearings like ‘ShockTransmission Units’ and ‘Seismic Isolation Bearings’ havecome to the rescue of bridge engineers and the same oldsubstructures, thus, can be retained by provision of these newbearings.
Efforts have been made to make the book more useful for thefield as well as the design engineers. In this effort, the IRICENfaculty and staff have contributed immensely, notably amongthem are Mrs. Lata Sridhar, Mr. Ganesh and Mr. Sunil Pophale.In fact, Shri Ajit Pandit, Ex. Dean/IRICEN had initiated the ideaof updating this book, which was taken to its logical end. I amalso thankful to Shri A.K. Yadav, Senior Professor Bridges forproof-checking and valuable suggestions.
Above all, the author is grateful to Shri Shiv Kumar, Director/IRICEN for his initiative, encouragement and necessaryguidance for the publication.
Ghansham BansalProfessor Bridges
ACKNOWLEDGEMENTS TO THEFIRST EDITION
While covering the subject of Bridge Bearings during variouscourses at IRICEN the absence of a document covering allaspects of different types of bridge bearings was acutely felt.Information on bridge bearings is available in various technicalliteratures but it is scattered.
This IRICEN publication is a result of the desire to fill the gapand produce a documentation which would be useful for allpracticing civil engineers on Indian Railways. Even though thepublication is primarily aimed at Railway engineers, the basicconcepts are equally applicable to road bridges also.
It would not be out of place to acknowledge the support andassistance rendered by IRICEN faculty and staff in the aboveefforts. Shri N.L. Nadgouda, Associate Professor hascontributed immensely by his experience of handling steelbridges. Word processing of the manuscript and numerouseditings thereof has been done by Mrs. Lata Sridhar. Thedrawing staff of IRICEN have assisted in preparation of thedrawings.
Above all the author is greatful to Shri Vinod Kumar, DirectorIRICEN for his encouragement and guidance for improving thedocument.
S.M. VAIDYAPROF. BRIDGES 1
FOREWORD TO THE FIRST EDITION
The subject of bridge bearing is of considerable importance tothe field engineers who are engaged in construction and mainte-nance of railway bridges. Trainee officers have often expressedthe need for a comprehensive documentation on this subject. Itis hoped that this booklet will fulfil this need and disseminatethe knowledge and experience on this subject to the field offi-cials.
This book has been prepared by Professor S.M. Vaidya of thisInstitute. If there are any suggestions for improving the book orif any error/discrepancy is noticed in its contents, kindly writeto the undersigned.
Vinod KumarDirectorIRICEN/Pune
PREFACE TO THE FIRST EDITION
Indian Railways are custodians of one of the largest bridgestock under one management with more than 1,15,000 bridgeson the rail network. It is important to construct new bridges withhighest standard of quality and to maintain them for smoothmovement of railway traffic. Bridge bearings play a veryimportant role in keeping the bridge structure in good health.Tradionally, Indian Railways have been using steel girderbridges and steel bearings. With the advent of new technology,RCC and PSC bridges along with elastomeric and PTFEbearings are gradually replacing steel bridges and conventionalbearings.
The bridge engineers require a comprehensive document deal-ing with all facets of bridge bearings to facilitate procurement,installation and maintenance of the same. I am glad thatIRICEN is bringing out this book exclusively dealing withbridge bearings. I hope that this publication will be very usefulto all civil engineers with the objective of maintaining thebridges in good condition.
V. K. AGNIHOTRIMEMBER ENGINEERING
RAILWAY BOARDNEW DELHI
viii
CONTENTS
CHAPTER - 1 GENERAL1.1 Introduction 1
1.2 Classification of Bearings 4
1.2.1 Degree of Freedom 4
1.2.2 Material Used 5
1.2.3 Types of Bearings 6
1.3 Selection of Bearings 11
1.3.1 Functional Requirement 11
1.3.2 Expected Life 11
1.3.3 Maintenance Efforts 11
1.3.4 Cost 14
1.3.5 Other Factors 14
1.4 Minimizing the Requirement of Bearings 15
CHAPTER - 2 SLIDING BEARINGS
2.1 General 18
2.2 Different Type of Sliding Bearing 18
2.3 Parts 20
CHAPTER - 3 ROCKER & ROLLER BEARINGS3.1 General 23
3.1.1 Parts 23
3.1.2 Connections 25
3.1.3 Segmental Rollers 26
3.1.4 Oil Baths 27
3.2 Design Aspects 29
3.3 Installation 34
ix
CHAPTER - 4 MAINTENANCE OF STEEL BEARINGS4.1 General 38
4.1.1 Lifting of Girders 38
4.1.2 Cleaning and Greasing of Steel Sliding
Bearings 42
4.1.3 Cleaning and Greasing of Rocker & Roller
Bearings of Open Web Through Girders 44
4.1.4 Method of Greasing 45
CHAPTER - 5 ELASTOMERIC BEARINGS5.1 General 47
5.2 Properties of Elastomer 48
5.3 Behaviour of Elastomeric Bearings 51
5.4 Types of Elastomeric Bearings 55
5.5 Design of Elastomeric Bearings 56
5.5.1 Flow Table of Design 56
5.5.2 Input Data Required 59
5.5.3 Output Expected 59
5.6 Manufacture and Quality Control 60
5.6.1 Properties of Elastomer 60
5.6.2 Dimensional Tolerances 60
5.7 Inspection and Testing 62
5.7.1 Lot Size 62
5.7.2 Level 1 Accpetance Testing 63
5.7.3 Level 2 Acceptance Testing 71
5.7.4 Inspection and Quality Control Certificate 71
5.8 Installation 74
5.8.1 General Guidelines 74
5.8.2 Process of Installation 76
5.9 Periodical Inspection and Maintenance 77
5.10 Elastomeric Bearings in Aid of Old Substructures 78
x
5.11 Anti-Slip Devices 81
5.12 Sample Design Problem for Elastomeric Bearings 84
CHAPTER - 6 POT BEARINGS
6.1 General 93
6.2 POT-PTFE Bearing vs Elastomeric Bearing 946.3 Properties of PTFE 95
6.4 Permissible Bearing Pressure on PTFE 100
6.5 Other Recommendations for Design 101
of PTFE Sliding Bearing
6.6 Design Aspects 103
6.7 Installation of POT Bearings 105
6.8 Design Specifications for POT-PTFE Bearings 107
6.9 Design of POT- PTFE Bearings 113
6.10 Design of Guides 119
6.11 Design of Anchoring Arrangement 120
6.12 Sample Design problem for POT-PTFE Bearings 122
CHAPTER - 7 EMERGING TRENDS IN BEARINGS
7.1 General 131
7.2 Shock Transmission Unit 131
7.2.1 Description 133
7.2.2 Advantages 134
7.2.3 Limitations 134
7.2.4 STU on second Bassein Creek Road 135
Bridge, Mumbai.
7.2.4.1 Type 136
7.2.4.2 Cost 136
7.2.4.3 Basic Requirement of Design 136
7.2.4.4 Critical Factors in Design 137
xi
7.2.5 Load Testing of STU 137
7.2.6 Installation of STU 138
7.3 Seismic Isolation Bearings 139
7.3.1 Types of Seismic Isolation Bearing 140
xii
LIST OF ABBREVIATIONS
AASHTO American Association of State Highway andTransportation Officials
BM Bending Movement
BS British Standard
BSC Bridge Standards Committee
CDA Coefficient of Dynamic Augment
DL Dead Load
EUDL Equivalent Uniformly Distributed Load
HMLS Heavy Mineral Loading Standards
IIBE Indian Institute of Bridge Engineers
IRC Indian Road Congress
IRHD International Rubber Hardness Degrees
IRS Indian Railways Specification
IS Indian Standards
KN Kilo Newton
LF Longitudinal Force
LL Live Load
LUD Lock Up Device
LWR Long Welded Rail
MBG Modified Broad Gauge
MPa Mega Pascal
MS Mild Steel
ORE Office of Research and Experimentation
PSC Pre-stressed Concrete
PTFE Poly Tetra Fluoro Ethylene
RBG Revised Broad Gauge
RCC Reinforced Cement Concrete
RDSO Research Design and Standards Organisation
SEJ Switch Expansion Joint
xiii
SIB Seismic Isolation Bearing
STU Shock Transmission Unit
UIC International Union of Railways (Translated fromFrench)
xiv
LIST OF SYMBOLS
ζα = Shear stress in elastomer due to rotation
ζh
= Shear stress in elastomer due to horizontal force
ζp
= Shear stress in elastomer due to compressive force
σb
= Permissible bearing pressure in Bed block material
σm
= Max. Permissible pressure in elastomer
σs
= Permissible tensile stress in the steel plate
σmin
= Minimum stress in elastomer
σmax
= Maximum stress in elastomer
αc
= Rotation under effect of slow acting loads
αs
= Rotation under effect of quick acting loads
a = length of the bearing along the span
b = width of the bearing across the span
B = width of the girder/beam
Ea = Modulus of elasticity of elastomer
ei = Compression of elastomeric layer
fck = Grade of concrete
G = Shear modulus of elastomer
Hc = Slowly applied horizontal load
Hs = Quickly applied horizontal load
n = number of layers of elastomer
Pc = Slowly applied normal (vertical) load
Ps = Quickly applied normal (vertical) load
S = Shape factor of elastomer
hi = thickness of each layer of elastomer
xv
h = Total thickness of elastomer
ts
= Thickness of steel plate in the elastomeric bearing
Uc
= Horizontal movement due to slow acting load
Us
= Horizontal movement due to quick acting load
D1, D
2= Diameter of contact surfaces in roller bearings
1
CHAPTER 1
GENERAL
1.1 INTRODUCTION
A bridge is assumed to be made up of two majorparts namely, superstructure and substructure.Superstructure consists of track structure, girder/truss and bearing. Substructure consists of bedblock, pier or abutment and foundation as shownin Fig. 1.1.
FIG. 1.1 PARTS OF BRIDGE
Thus, a bridge bearing is an element ofsuperstructure which provides an interfacebetween the superstructure and substructure.This interface is vital because superstructureundergoes dimensional changes anddeformations due to various factors which arelisted as follows:
TRACK STRUCTURE
GIRDER
BEARING
BED BLOCK
PIER / ABUTMENT
FOUNDATION
}}
SUPERSTRUCTURE
SUBSTRUCTURE
↑
↓↑
↓
2
a) Thermal expansion/contractionb) Elastic deformation under live loadc) Seismic forcesd) Creep and shrinkage of concretee) Settlement of supportsf) Longitudinal forces - tractive/ breakingg) Wind loads.
Most of these movements are bi-directional butsome, like creep of concrete may result inirreversible unidirectional movement. Themagnitude of these movements depends upon anumber of factors like span of the bridge,magnitude of loads, extent of temperaturevariation etc.
If the movement between the superstructure andsubstructure are not allowed to take place freely,large amount of forces may develop in the girderor the substructure. If the ability to move is notbuilt into the bridge (span), it will push thesupports until it achieves the freedom requiredand in the process causing damage to thesupports. It is, therefore, necessary to permitrelative movement between the girders and thesubstructure.
Since the bearing is introduced betweensuperstructure and substructure for acco-mmodating the various permitted movements, ithas to transfer the entire load from superstructureto the substructure of bridge. We can say that‘Bearings’ assume the functionality of a bridge byallowing translation and rotation to occur whilesupporting the vertical loads. In nutshell, the
3
various functions of bearings can be summarisedas given below:
(1) To allow the permitted’ movements.(2) To prevent the ‘not permitted’ undesirable
movements.(3) To transfer the load from superstructure to
substructure.
The ‘permitted’ and ‘not permitted’ movements inthe bridge in relation to bearing can be betterappreciated if we analyse the degree of freedomin 3-D as shown in Fig. 1.2.
FIG. 1.2 DEGREE OF FREEDOM IN 3-D
In the era of stone and brick masonry bridges, thespans were limited and the superstructures usedto be massive, primarily developing onlycompressive stresses under the loadingconditions. Such bridges did not need specialbearings since the movements were very small.
ACROSS TRACK
ALONG TRACK
VERTICAL
PERMITTED MOVEMENT
NON-PERMITTED MOVEMENT
ACROSS TRACK
VERTICAL
ALONG TRACK
4
With the advent of steel, RCC and PSC forconstruction of bridges, the spans became largeand the girders longer. The longer spans coupledwith higher elastic deformations led to the needfor and development of various forms of bridgebearings.
1.2 CLASSIFICATION OF BEARINGS
Bearings can be classified depending upon
a) Degree of freedom
b) Material used
These are discussed below.
1.2.1 Degree of freedom : There are possible 6degrees of freedom at any support as describedearlier. These are translation in three directionsand rotation about these three axes. A bearingmay permit movement in any of these 6 degreesof freedom or in none. During the structuraldesign of the bridge girders, each support point isidealised in a specific manner by the designengineer. The bearing has to fulfill thisassumption.
Translation can be permitted by the followingmodes of action :i) by sliding actionii) by rolling actioniii) by shearing strainiv) by racker and pinion devices (gears)
5
Rotation can be permitted by the followingmodes:
i) by rocking/hinge actionii) by differential compression (as in
elastomeric pads)iii) by bending/ flexure (as in tall
piers, portals)
Therefore based upon degree of freedomrequirements, different degree of freedom can begiven at the support point and bearings may beclassified as:
(1) Fixed - Translation not permitted,Rotation permitted
(2) Free - Translation permitted,Rotation permitted
(3) Rocker & Roller - Roller end free, Rocker end fixed
1.2.2 Material used : A number of different materialshave been used for making bearings such assteel of various types, phosphor bronze, syntheticmaterial like rubber (elastomer) and PTFE etc.Out of these materials steel, rubber and PTFEare the most commonly used materials, today, forbearings. In certain forms of bearings, acombination of two materials is also used.Table 1.1 lists various materials used infabrication and installation of bridge bearings.
6
TABLE 1.1 MATERIALS USED IN BRIDGEBEARINGS
Material Components of bearing wherematerial used
1) Steel a) Plates-MS, HTS, Stainless steelb) Cast and forged productsc) Gearsd) Anchor bolts, rivets, pins etc.
2) Bronze a) Sliding platesb) Bushings
3) Synthetic a) Elastomermaterials b) PTFE (Poly Tetra Fluoro Ethylene)
4) Other a) Concretematerials b) Wood and timber
5) Lubricants a) Graphiteb) Grease, oils and silicones
6) Packing a) Lead sheetsand levelling b) Bitumen impregnated felt padsmaterials c) Cement / Epoxy grouts
1.2.3 Types of bearings : Based upon degree offreedom and types of materials used, the varioustypes of bearings used on bridges are shown inFig. 1.3 to 1.13.
7
OUTERBEARINGPLATES
SLIDING SURFACE
FIG. 1.3 PLAIN SLIDING BEARING
OUTERBEARINGPLATES
ROLLERS
OUTERBEARINGPLATES
CIRCULAR ROLLER
FIG. 1.4 SINGLE ROLLER BEARING
FIG. 1.5 MULTIPLE ROLLER BEARING
OUTERBEARINGPLATES
CIRCULAR ROLLER
ROLLERSOUTERBEARINGPLATES
OUTERBEARINGPLATES
SLIDING SURFACE
8
OUTER BEARINGPLATE
PIN
OUTER BEARINGPLATES
PIN
LEAVES
O R
OUTERBEARINGPLATES
FIG. 1.6 KNUCKLE PIN BEARING
FIG. 1.7 LINEAR ROCKER BEARING
FIG. 1.8 KNUCKLE LEAF BEARING
SOUTERBEARINGPLATES
OUTERBEARINGPLATES
OUTERBEARINGPLATES
PIN
CYLINDRICALROCKERCYLINDRICAL
ROCKER
PIN
LEAVES
9
PISTON
POT
ELASTOMERSEAL
OUTER BEARINGPLATE
OUTER BEARINGPLATES
SPHERICAL ROCKER
FIG. 1.9 POT BEARING
FIG. 1.10 SPHERICAL KNUCKLE BEARING
FIG. 1.11 POINT ROCKER BEARING
S
ELASTOMER
PISTON
POTSEAL
OUTERBEARINGPLATES
OUTERBEARINGPLATES
SPHERICAL ROCKER
10
OUTER BEARINGPLATES
FIG. 1.13 CYLINDRICAL KNUCKLE BEARING
OUTERBEARINGPLATES
ELASTOMER STEEL REINFORCING PLATES
FIG. 1.12 REINFORCED ELASTOMER BEARING
ELASTOMER STEEL REINFORCING PLATES
11
1.3 SELECTION OF BEARINGS
For a given bridge structure there could be anumber of different solutions for providingbearings. However, in each case there will be onemost appropriate choice of the bearing. Theselection will depend on a number of factors.These are listed and discussed below:
1.3.1 Functional Requirement : The bearing must fulfillthe functional requirement in terms of permittedmovements, load bearing and load transmission.The various functions performed by differenttypes of bearings are reproduced in Table 1.2from BS: 5400 part-IX. Table 1.3 may also bereferred for selection of bearings as this tablegives load ranges and movement capacities ofvarious types of bearings.
1.3.2 Expected life : An attempt should be made toselect a bearing whose expected life iscompatible with that of the bridge itself. Failingthis, replacement of the bearing will have to beplanned for during the life of the bridge. It shouldhowever be acknowledged that any scheme forreplacement of bearings will invariably requiresuspension of traffic, which is very costly andtroublesome.
1.3.3 Maintenance efforts : The importance of properfunctioning of the bearing for the health of bridgecan not be overemphasized. In many cases, thebearing is not in a easily accessible position. It is,therefore, preferable to opt for a bearing whichrequires minimum maintenance effort. Bearings
12
Ty
pe o
f bea
ring
Tra
nsla
tion
perm
itted
Rot
atio
n pe
rmitt
ed
Lo
adin
g re
sist
ed
Long
itudi
nal
Tra
nsve
rse
Long
itudi
nal1
Tra
nsve
rse2
Pla
nV
ertic
alLo
ngitu
dina
lT
rans
vers
eR
olle
rS
ingl
e cy
lindr
ical
!X
!X
X!
XS
Mul
tiple
cyl
indr
ical
!X
XX
X!
XS
Non
-cyl
indr
ical
!X
!X
X!
XS
Ro
cker
Lin
ear
XX
!X
X!
!S
Poi
ntX
X!
!!
!!
!
Kn
uck
lePi
nX
X!
XX
!!
SLe
afX
X!
XX
!!
!C
ylin
dric
alX
S!
XX
!!
SS
pher
ical
XX
!!
!!
!!
Pla
ne
Slid
ing
!!
XX
X!
!S
Ela
sto
mer
icU
nrei
nfor
ced
!!
!!
!!
!!
lam
inat
ed!
!!
!!
!!
!
Po
tX
X!
!S
!!
!
Gu
ide
Long
itudi
nal
!X
!S
XX
X!
Tra
nsve
rse
X!
S!
XX
!X
Key
:!!!! !
suita
ble
Xno
t sui
tabl
eS
Spe
cial
con
side
ratio
n re
quire
d
Not
e :
1. R
otat
ion
abou
t tra
nsve
rse
axis
.2.
Rot
atio
n ab
out l
ongi
tudi
nal a
xis.
TAB
LE
1.2
F
UN
CT
ION
S P
ER
FO
RM
ED
BY
DIF
FE
RE
NT
TY
PE
S O
F B
RID
GE
BE
RA
ING
S
13
SN
TAB
LE
1.3
G
UID
EL
INE
S F
OR
SE
LE
CT
ION
OF
BR
IDG
E B
EA
RIN
GS
Typ
e of
bea
ring
Ver
tical
load
reco
men
ded
rang
e(K
N)
Mov
emen
tca
pa
city
‘‘One
Way
’’(m
m)
Rot
atio
nin
radi
ans
Sei
smic
perf
or-
man
ce
Mai
nten
ance
requ
irem
ent
Hor
izon
tal*
forc
e on
supp
orts
(% o
fsu
pers
truct
ure
dea
dlo
ad)
Bea
ring
heig
htap
prox
.ra
nge
(mm
)
Typ
ical
app
licat
ion
Str
aigh
tC
urve
dS
teel
Con
c.
1 2 3 4 5 6 7
Ste
el ro
ller
Ste
el S
lidin
g
Pot
Dis
c
Sph
eric
al
Pla
inE
last
omer
ic
Lam
inat
edel
asto
mer
ic
600
to 2
660
200
to 1
330
200
to 1
7800
400
to 1
7800
800
to 2
6700
100
to 4
50
300
to 2
200
100
25
No
limit
No
limit
No
limit
10 60
0.19
0.08
0.04
0.04
Neg
ligib
le
0.02
5
Poo
r
Poo
r
Goo
d
Goo
d
Goo
d
Goo
d
Goo
d
3 15
3 to
5
3 to
5
5 to
10
3 to
12
3 to
12
Max
imum
Max
imum
Min
imum
Min
imum
Min
imum
Non
e
Non
e
360
to 6
60
50 to
100
60 to
180
60 to
180
125
to 2
50
10 to
209
10 to
20
! ! ! ! ! ! !
- - ! ! ! ! !
! ! ! ! ! - -
- - ! ! ! ! !
14
with moveable parts require greater maintenanceeffort as well as those made of steel due to thepossibility of corrosion, and consequent freezingof the bearing.
1.3.4. Cost : The capital cost includes cost of design,fabrication and installation of bearing. Generally,this will be a fraction of the cost of the bridge. Assuch, the initial cost alone should not be aconsideration in choice of the bearing. Manybearings which had attractive initial cost proved tobe a liability later on during maintainance.Therefore, life cycle cost should be the criteria forselection of bearing.
1.3.5 Other factors : Factors which may be relevant inour quest for the most suitable bearing are:
a) Height of the bearing : This may be critical incase of regirdering works where maintainingexisting rail / road level is the main constraint .
b) Management of horizontal force transferred tothe substructure : This is an importantconsideration while upgrading the load carryingcapacity/gauge conversion works. The bridgerule stipulates that with properly designedelastomeric bearings, the dispersion of thelongitudinal forces to the approaches can beincreased from 25% to 35%.
c) Performance under seismic loads : Some-times seismic consideration may alter thechoice of bearing particularly in zone IV & V.
15
Having chosen the type of bearing for a givenstructure, the following guidelines may befollowed in order to minimize the life cyclecost.
i) Choose larger size of rollers in rocker & rollerbearing, since smaller components are moreprone to accumulating dust and moisture. Alarger roller will overcome debris more easilythan smaller roller. Larger components alsofacilitate inspection and maintenance.
ii) For the material selected, specify the highestgrade of mechanical properties and thestrictest tolerance that can be practicallyattained. Maintenance efforts, thus, can begreatly reduced.
These recommendations only underscore the factthat the initial cost is not a consideration for goodbearing design and specification.
1.4 MINIMIZING THE REQUIREMENT OFBEARINGS
Bearings are unavoidable evils. In bridges of verysmall spans, however, the bearings are notrequired e.g. in slab bridges. Here, the interfacebetween the slab and the abutment-top or bedblock functions as a ‘bearing’. The coefficient offriction between concrete and concrete can betaken as 0.50 to 0.60 depending upon the surfacecondition. Generally speaking, spans shorter than9 m do not need bearings.
16
The various ways, which can be used to minimizethe number of bearings are given below:
1. Adopt continuous construction through anumber of spans. Superstructure is supportedon the intermediate piers with one bearing oneach pier. Thus the number of bearings on eachpier is reduced by one half.
2. On long and tall piers, the bridge movement canbe accommodated by flexible piers and therebyusing fixed bearings only. The fixed bearingsare relatively less problematic as compared tofree bearings.
3. The superstructure and substructure can bemade monolithic, thus totally eliminating theneed for any bearings. In such type of multispanstructures, the entire movement isaccommodated at the abutments, wherebearings capable of providing large movementsare required.As per AASHTO specifications, insliding bearings up to span 50 feet, no provisionfor deflection of the spans need be made.
Excluding these special cases, all other forms ofbridges require bearings. Though bearing is a tinypart of the bridge, both physically as well ascostwise, the entire load is transmitted through it.Therefore, great attention must be paid onselection, design, fabrication, installation andmaintenance of the bridge bearings.
Theoretically, the bearings can be avoided for anytype of bridge, but the design of substructure will
17
have to be modified to bear the entire loads. Thismodification will result into high cost ofsubstructure. Therefore, provision of bearings isthe economical solution.
A bearing is a negligibly small part of a bridgeand unfortunately the attention it receives fromthe engineers is also negligibly small. In fact, theimportance of this small part should have beeninversely proportional to its size, as the entireload is transmitted through this tiny componentand any mis-behaviour of bearing may lead tocatastrofic results both for substructure as wellas superstructure. Therefore, selection, fabrica-tion, installation and maintenance of bearingsshould be on the top of list as far as the bridgesare concerned.
18
CHAPTER 2
SLIDING BEARINGS
2.1 GENERAL
A system of two plates, one sliding over the othermakes one of the simplest type of bearings.These bearings permit translation in longitudinaland transverse directions, unless specificallyrestrained in any of these directions. No rotationis permitted unless specially provided in the formof articulation and only vertical loads are resisted /transmitted by these bearings.
Common materials that have been used assliding surfaces and their coefficients of frictionare:
a) Mild steel over mild steel - 0.2 to 0.3b) Mild steel over phosphor bronze - 0.15c) PTFE over stainless steel - less than
0.08
Generally, plain sliding bearings are providedwhere span is less than 30 m, because themovement capacity of these bearings is generallysmall.
2.2 DIFFERENT TYPES OF SLIDING BEARINGS
There has always been an endeavor to reducethe coefficient of friction. The longitudinal forcetransmitted to substructure depends uponcoefficient of friction. In an effort to reduce the
19
coefficient of friction, different materials havebeen tried and different types of sliding bearingshave been created. These are as given below:
(a) Steel over steel : Steel over steel slidingbearings transmit considerable horizontal force tothe substructure because coefficient of friction isvery large. In addition to the type of material thecoefficient of friction also depends upon thecondition of the contact surface. Bridge Rulesstipulate that the coefficient of friction should betaken as 0.25 for the lubricated steel surface.Entrapment of dirt, debris and corrosion of steelplates can increase the coefficient of frictionconsiderably, and in the limiting case it maycause the bearings to freeze. These bearings,therefore, require periodic cleaning and greasingso that the superstructure is allowed to expand/contract freely without transmitting excessivelongitudinal force to the substructure.
(b) Steel and phosphor bronze : Since thecoefficient of friction between steel and phosphorbronze is considerably low, it is advantageous toprovide these in lieu of steel sliding bearings.Phosphor bronze bearings also require lessermaintenance than steel bearings as no greasingis required. This eliminates the need to jack upthe girders for greasing operation. Moreover, useof the grease which attracts dust and sandparticles is avoided. Only outside area (other thanthe contact area) needs to be cleaned.
(c) Steel and PTFE : Use of PTFE (Poly TetraFluoro Ethylene) more widely known as Teflon
20
also offers many advantages. The coefficient offriction between PTFE and stainless steel is thelowest between any two materials within thenormal temperature range. A peculiar feature ofPTFE is that the coefficient of friction reduces asthe applied load increases. The value ofcoefficient of friction at 5 MPa is 0.08 where as at30 MPa the value reduces to 0.03, which is veryclose to rolling friction. Thus we are able toachieve near-rolling friction without having tomaintain the rolling arrangements. PTFE is alsohard, durable and possesses high chemicalresistance. It is routinely used in POT bearingswhere very large translational movements,required for large span bridges can be achieved.
2.3 PARTS
Stopper plates : When both ends of a span aresupported on such sliding bearings, the girdermay have a tendency to creep under theinfluence of predominantly unidirectionalmovement. To prevent the girders from falling offthe bearings, stopper plates are provided.
Guide strip : To regulate the movement of thegirder in the correct alignment, guide strips areprovided parallel to the span.
Size of bearing plate : The size of the bearingplate is governed by the total vertical load (DL +LL + CDA) on the bearing and the allowablebearing stress in the bed block material. The soleplate of the bearing is directly connected to thegirder by welding or countersunk rivets / bolts.The base or bed plate is held in position on the
21
bed block by anchor bolts. Since the slidingbearings do not allow free rotation, unequaldistribution of load takes place due to the rotation/deflection of the girder. This leads to stressconcentration under the bearing towards inside ofthe span. There have been instances of failure ofthe bed block material due to this deflection. Inorder to overcome this deficiency, the inside edgeof the top plate of the sliding bearing ischamfered. This is commonly known as thecentralised articulated bearing. Fig. 2.1 shows atypical sketch of the standard centralisedarticulated bearing adopted on Indian Railways,These bearings are used in steel plate girders,composite girders and underslung girders.
On a reference made by RDSO to BritishRailways it was learnt that the PTFE bearings arein use on British Railways since middle ofseventies without any maintenance problems.The British Railways have provided thesebearings in through type girder bridges of spansupto 40 m and concrete bridges of various typesupto 90 m spans.
Sliding bearings are the simplest type ofbearings, used up to 30.5 m span girders. Theirregular maintenance is very important, to keep atab on friction otherwise the value of horizontalforce transmitted to sub-structure will increasetremendously. It may so happen that the value ofhorizontal force becomes so large that cracksmay develop at the bottom of bed block.Therefore, the frequency of lubrication has beenprescribed as once in three years. It may beincreased to once in two years in case trackconsisting of long welded rails is provided overthe bridge.
22
CLEAR SPAN
THEORETICAL SPAN
OVERALL LENGTH
PLATE GIRDER
GENERAL ARRANGEMENT
A - EXPANSION GAP 12 TO 20 mm B - INSTALLATION GAP 1.5 TO 2 mm
NOTE: FOR ENSURING THESE GAPS, THE ANCHOR BOLTS SHOULDBE INSTALLED ACCURATELY IN PROPER POSITION.
FIG. 2.1 CENTRALISED ARTICULATED BEARING
LOCKINGSTRIP
C.L. OF GIRDER
BEARINGPLATE
ANCHORE BOLT
GUIDE STRIP(ON OPPOSITE SIDE ON OTHER END OF GIRDER)
PLAN
A
B
SE
CTI
ON
AT
'X X
'
X
X
BEARINGPLATES
CSK RIVETS
BEARING FLAT
BEARING PLATE
ELEVATION
CSK RIVETS
COUNTERSUNKRIVETS
←
BED PLATE
PLAN
← →
23
CHAPTER 3
ROCKER & ROLLER BEARINGS
3.1 GENERAL
For railway bridges with spans in excess of30.5m, where open web through girders aregenerally provided, the amount of movementneeded and the vertical load transmitted througheach bearing is too large to be catered by thesliding bearings. It is common, on IndianRailways, to provide rocker & roller bearings atthe free end of open web through girders, androcker bearings at the fixed end.
A typical rocker and roller bearing for open webgirders of 45.7 m span is shown in Fig. 3.1 &3.2. The roller bearing consists of a base plate,two or more rollers and a top plate. Therocker & roller end is made by providing a saddleand knuckle plate on top of the rollers whereasthe same arrangement except rollers is at therocker end. The rocker & roller end of bearingpermits translation as well as rotation, whereasthe rocker end permits only rotation.
3.1.1 Parts :
(a) Roller : The rollers are made of forged steel ofClass-3, as per IS:2004 and basic raw material isas per IS:1875. The rollers may, alternatively, beturned from approved C&W axles manufacturedafter 1931. USFD test shall be conducted toensure that there are no internal flaws. Thesehave machined surface to permit smooth rollingaction.
24
44055
406
520
LINK PLATE
TOOTH BAR
SEMI CIRCULAR CUTFOR ANCHOR PIN
SADDLE PLATE
SADDLE
KNUCKLE
KNUCKLE SLAB
ROLLER
EXPANSION BASE
FIG. 3.2 ROLLER BEARING AT FREE END
KNUCKLE
SADDLE
440
SADDLE PLATE
406
680
BASE PLATEHOLE FOR
ANCHOR BOLT
LIFTING HOLES40 mm DIA
ROCKER
55
FIG. 3.1 ROCKER BEARING AT FIXED END
KNUCKLE SLAB →
25
(b) Link Bar : All the rollers are connected to eachother with a link bar to ensure that they alwaysmove together and maintain the clear gapbetween rollers.
(c) Tooth Bars : Rollers on the extreme end of agroup of rollers are provided with tooth bars. Thetooth bars rest into grooves provided in theknuckle plate on top and base plate at bottom.The purpose of tooth bars is to arrest movementof the rollers beyond a point (depending upon thedesign movement). Tooth bar in rocker-rollerbearing can be assumed to be equivalent to‘stopper plate’ in sliding bearing.
(d) Rib and Notch : To arrest transversemovement between the roller and the base orknuckle plate, a longitudinal notch can beprovided in the middle of the roller and amatching rib in the base and top plate. The ribthus guides the rollers to roll only in thelongitudinal direction and prevents any transversemovement. Rib and notch arrangement in rocker-roller bearing can be assumed to be equivalent to‘Guide strip’ in the sliding bearings.
3.1.2 Connections : The top plate or saddle plate isconnected to the end of the bottom chord. Sincethis connection is crucial for transmitting thehorizontal thrust from the bottom chord to the bedblock, it must be made tight fit. The top plate isgenerally installed to the underside of the bottomchord in-situ. However, the joint is not amenablefor riveting for want of adequate space. Thenumber of rivets required and their lengths willalso be very large. The joint is therefore madewith turned and fitted bolts. The reaming of the
26
holes in bottom chord and the saddle plate is,therefore, required to be done by assemblingthem together. The tolerances in the hole andshank diameter of turned bolts as per clause 28.6of IRS :B1 -2001 are as under :
Limit of tolerance Shank of Diameter ofbolt (mm) hole (mm)
Upper 0.000 + 0.125
Lower - 0.125 0.000
Such tolerances are prescribed to ensureadequate contact area between the hole andshank, which is presumed while allowing higherpermissible bearing stress for the design ofturned bolt connections.
The rocker-cum-roller bearings require periodicgreasing of rolling contact surfaces. This requiresthe girder to be jacked up and the contactsurfaces are cleaned to remove all entrapped dirt/dust etc. and a fresh layer of grease is applied. Afrequency of once in 3 years has been prescribedfor greasing of bearings on Indian Railways. Theoiling and greasing of roller bearings must bedone under traffic block. The maintenance detailsof rocker-roller bearing will be discussed insubsequent paras.
3.1.3 Segmental Rollers : For large span bridges (Span> 45.7 m) more than two roller are required. Thesize of the base plate required is large whennumber of rollers are more. It should be realizedthat the full periphery of a circular roller is neverutilised during the rolling action. It is, therefore,
27
prudent to cut the sides of the roller to save notonly in the weight of roller, but also to reduce thesize of the base plate. A smaller base plate willrequire smaller pier top, thus, resulting ineconomy. These cut rollers are called segmentalrollers as shown in Fig. 3.3.
FIG. 3.3 SEGMENTAL ROLLER BEARING
Size of the base plate is reduced withoutcompromising the rolling action of the rollers.
Generally, the height of the segmental rollers ismade more than its diameter so as to permit alarger effective diameter. Thus the centre of thecurved surfaces at top and bottom do notcoincide. This imparts a tendency of lifting of thegirder during the rolling action but this is negligiblysmall. IRC:83 Part-I cautions designers whileproposing use of segmental roller bearing inseismic areas as there have been instances ofthe bearing collapsing under excessivelongitudinal movements, which may occur duringan earthquake.
3.1.4 Oil Baths : Roller bearings function smoothly aslong as the contact surfaces are clean. However,
28
there is always a tendency to accumulate dirt anddebris as well as rusting of steel. Very quickly, itleads to freezing of the bearing. The smaller thesize of the rollers, more are the chances tofreeze.
There have been instances where after a fewyears, a small nest of rollers has corrodedso much that it is difficult to count the number ofrollers. The Indian Railways have rightly gone infor large size of roller in the standard designs.The minimum size of roller to be provided is102 mm as per Para 3.1.2.3 of IRS: Steel BridgeCode.
Therefore, in order to overcome this problem forspans of 76.2 m and above, an oil bath isprovided around the rollers as shown in Fig. 3.4.
FIG. 3.4 SEGMENTAL ROLLER WITHOIL BATH
The reason for providing oil bath for such largespans is that mostly these bridges are rail-cum-road bridges and lifting of the girders is verydifficult due to the very heavy dead load. Sincethe rollers are completely submerged in oil, theyare effectively protected against corrosion.
OIL INDICATORMS OUTER COVERING
DRAIN OUT LET
29
The oil bath is fabricated from MS plate and isprovided with oil seals. A gauge or an oil levelindicator is provided to enable periodical check ofoil level in the box. A drain outlet is provided atthe bottom to drain out and replace the oil withoutthe need to open out the oil bath. The necessityof such replacement may be due tocontamination of the oil, which should beperiodically sampled. Once in five years the oilbath is to be opened out after draining the oil, therollers inspected and the oil changed. The oilbaths have performed very well on IndianRailways and have contributed to the successand longevity of roller bearings. The use of oilbath should be extended to all roller bearings ofthrough spans and more so in aggressiveenvironments.
3.2 DESIGN ASPECTS
The design of rocker & roller bearing involvesselection of roller length, its diameter, radius ofthe contact surface of saddle/knuckle, thicknessand plan size of the base plate and number andsize of anchor bolts. Simple design rules areprovided in IRS: Steel Bridge Code to obtainthese values, The excerpts are given below :
a) Allowable load P for rollers
Roller on curved surface
i) For single and double roller
( )21 1/D 1/D
1 0.8 P
−= kg/mm length of roller
30
ii) For multiple rollers i.e. more than two rollers
( )21 1/D 1/D
1 0.5 P
−= kg/mm length of roller
where D1 and D2 are diameters of convex andconcave contact surface respectively as shown inFig. 3.5.
FIG. 3.5 ROLLER ON CURVED SURFACE
Roller on flat surface
For a flat surface the diameter is infinity.Substituting D2 = ∞, the above two equationsreduce to -
i) For single and double rollers-
P = 0.8 D1 kg/mm length of roller
ii) For multiple rollers
P = 0.5 D1 kg/mm. length of roller
A lower value of P for multiple rollers is taken dueto the possibility of unequal load sharing amongvarious rollers when number of rollers are morethan two. When more than two rollers are incontact with rigid plates at top and bottom, anysmall difference in diameter of rollers will resultinto unequal load distribution. This possibility isnot there where only two roller are used.
D1
P
2D
Length of roller
Diameter
Diameter
31
Therefore the tolerance in roller diameter is animportant aspect to be considered in fabricaton ofrollers. In recognition of the importance ofvariation in diameters, a very close tolerance of+0.04mm is prescribed in IRS: B1-2001,Appendix-VI.
A positive variation in diameter of one roller ismore damaging than negative variation. In a set ofrollers, a smaller diameter roller will only becomeineffective, whereas the larger diameter roller inthe same set will make many other rollersineffective depending upon its location as shownin Fig. 3.6 & 3.7.
FIG. 3.6 POSITIVE VARIATION IN DIAMETER
C - CONTACT NC - NO CONTACT
FIG. 3.7 NEGATIVE VARIATION IN DIAMETER
C NC NCC
BIGGER DIA ROLLER
C NC C C
SMALLER DIA ROLLER
32
b) Allowable load on spherical bearing
( )kg
1/D - 1/D
1
127
1 P 2
21
=
c) Size of base plate:
The length and width of the base plate will begoverned by the following three factors -
i) Total vertical load and horizontal thrust to betransmitted by the bearing to the bed block.
ii) The length and number of the rollers and thetotal movement to be accommodated. Thelength of the rollers will be governed byallowable load per roller and maximum verticalload.
iii) The permissible bearing stresses in the bed-block material.
As per clause 3.16 of IRS : Steel Bridge Code theallowable bearing pressure for different materialsis as under:
i) Stone Masonry = 36 kg/cm2
ii) PCC (1:2:4) = 31.6 kg/cm2
iii) RCC = 0.2 * fck (average pressure) = 0.3 * fck (local max. pressure)
Where fck is the characteristic strength ofconcrete.
The base plate size can be reduced by adoptingconcretes of grade higher than M-20, provided alarger size of base plate is not required fromother considerations.
33
d) Thickness of base plate :
The thickness of base plate should be adequateto withstand the bending moment (B.M.) causedby line load of rollers as shown in Fig. 3.8.
FIG. 3.8 BENDING MOMENT DIAGRAM
Thickness of base plate can be calculated frombending moment crietria as given below.
Permissible value of bending stress = M / Z
M = Max. Bending Moment
Z = bt2 / 6
where b = width,
t = thickness of base plate
A very thin plate may be adequate from max.B.M. crieteria but it has a tendency to developa curvature resulting into a non uniform loaddistribution. Therefore a minimum thickness of20 mm is recommended even if it is notrequired from B.M. point of view.
P1 2
P
LOADING DIAGRAM
BENDING MOMENT DIAGRAM
Max. B.M.
34
e) Saddle/knuckle block : The saddle plate canalso be designed as a cantilever with uniformload from top and a line load reaction from theknuckle plate. Quite often the saddle plate/knuckleblocks are made of cast steel.
In such cases the cast steel should conform tothe appropriate grade of the cast steel, thepermissible stresses are same as that for mildsteel conforming to IS:226/IS:2062 Gr. A.
f) Anchor bolt : All the longitudinal force from thegirder is transmitted to the piers through the fixedend of bearing. Any friction between the bed blockand base plate or saddle plate and bottom chordis completely ignored and the entire horizontalforce is assumed to be transmitted by theconnecting bolts/rivets. The bolts are thuschecked against shear failure and also for safebearing stresses. The permissible shear orbearing stresses for different grades of steelshould be as per IRS: Steel Bridge Code.
3.3. INSTALLATION
For proper transmission of loads, the bearingshould have uniform seating on the bed block. Itis a common practice to provide a felt packingdipped in coaltar under the base plate. This isdone with a view to provide uniform and evenseating of the bearing. It has the added advantageof damping the vibrations and impact forces andthus increasing the life of the bed block. Cementor cement epoxy grout are also used as analternative for levelling the bed block top surface.
35
Use of materials such as lead sheets whichtend to flow under loads in not recommended.
Mean position of rollers : In order that the bearingprovides the full designed movement, it should beensured that the rollers are in mean position(vertical in case of segmental rollers) at thespecified mean temperature and loadingcondition. It is common to prescribe that therollers are in mean position at the specified meantemperature and under (DL + LL + impact). Insuch a case, the rollers will take a positiontowards the inside of span at mean temperaturewhen only DL is acting. Since there is no scopefor adjusting the position of base plate, positioningof the anchor bolts must be done with a very highlevel of accuracy.
If mean position is not ensured, the bearingmovement may be limited in one directionwhereas unusable surplus movement will beavailable in other direction. The saving graceagainst an error during installation of theanchor bolts is the fact that the thermalmovements specified in most cases is quiteconservative.
A typical scheme of installation of anchor bolts isillustrated in Fig. 3.9.
If the correct position of the roller at meantemperature and DL condition is known it will be avery useful reference point for monitoring properfunctioning of the roller bearing. It is very easy tocheck the bearing position by measuring the gapbetween the top and bottom contact points of therollers in longitudinal direction. Alternatively, for
36
AN
CH
OR
TU
BE
AS
SE
MB
LYW
EL
DE
D T
O R
EIN
FO
RC
EM
EN
TO
F P
IER
CA
P
PIE
R C
AP
PIE
R
DE
TA
IL A
T 'A
'P
LAN
ELE
VA
TIO
N
10
/12
mm
TH
ICK
ST
EE
L B
ED
DIN
G
PU
NC
H M
AR
CE
NT
RE
LIN
G.I.
PIP
E W
ITH
RO
UT
ER
FA
CE
(BO
TT
OM
EN
D S
DE
TA
IL '
A'
X +
TO
LE
RA
NC
E FIG
. 3.
9 S
CH
EM
E F
OR
IN
ST
AL
LA
TIO
N O
F A
NC
HO
R B
OL
TS
NOTE
-1)
X =
Theo
retic
al d
ista
nce
from
pie
r ce
ntre
to
bear
ing
cent
reTo
lera
nce
≤
1 / 2
(D-d
)D
= I
nsid
e di
a of
anc
hor
tube
d =
Anch
or b
olt
dia
2)An
chor
tub
e to
be
seal
ed f
rom
top
dur
ing
conc
retin
g of
pie
r ca
p.3)
Anch
or b
olts
to
be g
rout
ed i
n po
sitio
n af
ter
posi
tioni
ng o
f th
e be
arin
g.4)
A th
in l
ayer
of
mor
tar
may
be
used
bet
wee
n th
e be
arin
g an
d be
ddin
gpl
ate
for
mak
ing
unifo
rm
cont
act.
DE
TA
IL A
T ‘
A’
37
full/segmental bearings, the movement may bemeasured by the position of the contact point ofthe tooth bar on top and bottom platerespectively. In case oil bath is provided, amovement gauge should be provided so as todirectly read the roller position from outsidewithout the need of opening the oil bath.
38
CHAPTER 4
MAINTENANCE OF STEEL BEARINGS
4.1 GENERAL
Cleaning and greasing of bearings is one of theimportant maintenance works to avoid prematurefailure of bearings and reduce recurring heavyrepair cost of bed block and masonary below bedblock. Steel bearing strip resting on steel baseplate has a tendency to stick together on accountof corrosion, and cease the movement ofbearings. This is called as Frozen Bearing.Sliding bearings of plate girders are generallydesigned keeping both ends free. When bearingsare frozen, a large amount of longitudinal force istransferred to the substructure for which thesubstructure may not have been designed. Uponintroduction of RBG, MBG and HM loadings onthe Indian Railway, longitudinal forces haveincreased considerably whereas the oldsubstructures had been designed withoutconsidering such large longitudinal forces.Sometimes, even repairs will not hold good ifcause of frozen bearings is not eliminated bygreasing. It has been laid down that the steelbearings of all girder bridges should be greasedonce in 3 years to ensure proper movement ofbearing plates. This should be done once everytwo years when track consisting of LWR iscontinued over bridge span.
4.1.1 Lifting of girders : For greasing the bearingsgirders are required to be lifted. But the gapbetween the bottom flange of plate girders and
39
the bed block generally varies from 100 mm to150 mm. The standard jacks normally availablehave a closed height of at least 300 mm. Thesejacks, therefore, can’t be used for lifting thegirders without making special jackingarrangements.
Following jacking arrangement can be adopted fordifferent types of girders:
1. For plate girders upto 6.1 m span, jacks canbe directly applied below end sleeper ensuringfirm hook bolt connection, since load to belifted is about 4 to 5 ton only.
2. Jacking arrangement for span 9.15 m plategirder requires provision of a hard wood beambelow inner top flange as shown in Fig. 4.1for lifting the girder.
3. Jacking arrangement for span 12.2, 18.3,24.4 and 30.5 m plate girder requiresprovision of a steel beam as shown in Fig 4.2.
The provision of jacking steel beam and itsremoval is difficult. It requires more man powerand also it is time-consuming on account ofheavy weight of the beam and limited workingspace on bridge piers. Field officials, many times,apply jack to the end cross frame angle(diagonal) to avoid provision of the jacking beam,to lift the girder. This may cause bending of theangle on account of its slender size, whichresults in lifting of bearing strips inside whenlowered on base plate. This improper seating ofthe bearing strip will cause hammering actionduring subsequent passage of train resulting indamage to the bed block and masonry of the
40
FIG
. 4.
1 J
AC
KIN
G A
RR
AN
GE
ME
NT
FO
R 9
.15
m S
PA
N
30 T
SC
RE
W
JAC
K
WO
OD
EN
BLO
CK
S
DIA
GO
NA
L B
RA
CIN
G
EN
D F
RA
ME
BE
AR
ING
BA
SE
PLA
TE
1850
1025
WO
OD
EN
BE
AM
175 30
0
41
FIG
. 4.
2 J
AC
KIN
G A
RR
AN
GE
ME
NT
FO
R 1
2.2
m,
18.3
m &
24.
4 m
SP
AN
S
30
T S
CR
EW
JAC
K
ST
IFF
EN
ER
WO
OD
EN
B
LOC
KS
DIA
GO
NA
L B
RA
CIN
GS
EN
D F
RA
ME
BE
AR
ING
BA
SE
PL
ATE
1830
1312
ST
IFF
NE
R
42
substructure.
Therefore the method of provision of jackingbeam to outside girder as shown in Fig 4.3 ispreferable. This requires less manpower and lesstime for lifting of the girder.
4.1.2 Cleaning and greasing of steel sliding bearings :
Following equipments are required for greasing ofsteel sliding bearings:
1. Jacks (50 ton capacity) - 2 nos.2. Hard wooden packing below and above jack3. Grease graphite Grade 3 conforming to IS:5084. Kerosene or released black oil for cleaning5. 6 mm thick steel scrapers6. Mortar pan7. Cotton waste
Greasing of sliding bearings can be undertakenunder traffic with issue of caution order and lineprotection for temporary works as per provision ofIRPWM.
Lifting of girder should be restricted to 8 to 10mm only, ensuring that the bearing strip does notget lifted over locking strip and guide strip to avoidcreep of girder in logitudinal and lateral direction.For lifting, it is not necessary to break the track.Only loosening of fish bolts and dog spikes over asmall length on both sides of the pier is sufficient.Only one end of the girder should be lifted at atime and steel scraper inserted between bearingstrip and base plate to remove old grease dustand dirt. The contact surface is cleaned with oiland grease applied. Girder is then lowered backover the base plate. Time required for all theseactivities is approximately 15 to 20 minutes.
43
BE
AR
ING
ST
IFF
EN
E
JAC
KIN
G B
EA
M (
FIX
ED
)T
O B
E D
ES
IGN
ED
FO
RE
AC
H S
PA
N
HS
FG
/ R
IVE
TT
ED
CO
NN
EC
TIO
N
FIG
. 4.
3 P
RO
PO
SE
D J
AC
KIN
G A
RR
AN
GE
ME
NT
FO
R 1
2.2
m,
18.3
m &
24.
4 m
SP
AN
S
44
4.1.3 Cleaning and greasing of Rocker & Rollerbearings of open web through girders : In case ofstandard open web through girders, no separatejacking arrangement is required as the end crossgirders are designed and provided with stiffenerand pad plate for provision of jack for lifting. Gapbetween bottom of cross girder and top of bedblock is about 600 mm, hence any type of jack of100 ton to 200 ton capacity can easily be usedfor lifting. In case of non standard spans, the endcross girder requires adequate strengthening orspecial jacking beam below the bottom boom.
The equipments required in this case are sameas for sliding bearing except that the jacks ofhigher capacity (100 ton to 200 ton) and wire ropewith turn buckle arrangement for holding the freeend are required.
Greasing of rocker and roller bearing should becarried out under traffic block under thesupervision of an official not below the rank ofADEN/ABE.
Following precautions and preliminaryarrangements are required:
1. Ensure tightness of rivets connecting endcross girder and end panel point of truss.
2. Provide hard wooden packing below the endcross girder to support the girder in case offailure of jacks. This should be done at threeplaces to prevent tilting of this girder.
3. Remove fish plates and loosen dog spikes ofrail over adjacent spans to avoid overloading
45
the jack on account of weight of adjoiningspan and stiffness of the track.
4. If trolley refuge is connected to both spans onany pier, loosen the bolted connection ofadjoining span to avoid overloading of jackand damage to the trolley refuge.
5. While lifting the fixed end, the other end beingfree, the girder is likely to creep longitudinally.To prevent this, provide hard wood packingbetween the ends of girder on pier andbetween girder and the ballast wall onabutment.
6. Jacks should be kept in working order andtested to 1.5 times the load they are expectedto lift. Keep one spare jack as stand by.
7. During lifting of girder, precaution should betaken to prevent creep of rail.
4.1.4 Method of Greasing : Greasing of fixed endrequires 20 to 25 minutes. The lifting is hardly 10mm, ensuring that the gap is created betweensaddle block and knuckle pin. Saddle is not liftedabove collar to prevent lateral creep of the girder.Steel scraper is used to remove old grease, dustand dirt. The contact area is cleaned with oil.Grease is applied and then girder is loweredback.
Greasing of free end requires 45 to 50 minutes.Knuckle plate is tied to the saddle plate with wirerope having turn buckle arrangement to releasethe load from roller when the girder is lifted.When the girder is lifted about 10 mm and rollersare free, link plate and tooth bar are removedafter opening the stud connections. All rollers
46
should be taken out and cleaned with scraper andthese are sand-papered with a fine sand-paper ofzero grade. Rollers should be examined for anypossible signs of flattening or minute cracks witha magnifying glass. Grease graphite grade 3conforming to IS 508 is applied over the baseplate evenly below the roller contact area. Therollers are then placed in position and greaseapplied at the top contact surface. Link plate andtooth bars are connected with care so that toothbar is placed in the same inclination as per thedrawing.
With the help of turn buckle of wire rope sling, theknuckle plate is lowered over the rollers. This willcreate gap between the saddle block and knuckleplate. Cleaning and greasing of this area is thencarried out similar to the fixed end and girder islowered back.
While taking out rollers for examination andgreasing, take special precautions to prevent therollers from falling-off the bed block.
47
CHAPTER 5
ELASTOMERIC BEARINGS
5.1 GENERAL
Steel bearings are good but suffer from problemsof corrosion and high level of maintenance. Dueto these problems of steel bearings, engineerswere on the lookout for a bearing which couldaccomodate large movements and at the sametime being relatively maintenance free. Elastomeras a material for making bridge bearing has beenfound to satisfy these requirements so much sothat many engineers believe that the search foran ideal material for bridge bearing has come toan end. Further developments in future mayinvolve refining the use of elastomer andenhancing its properties.
To summarise, the elastomeric bearings offernumber of advantage as listed below:
1. Requires minimum maintenance compared toall other bearings.
2. Installation is easy.3. Permits movement of the structure in all
directions, depending upon the applied forces.4. Occupies small space.5. Serves as a shock absorber due to anti-vibra-
tion properties of elastomer.6. Acts as an aid to better dispersion of
longitudinal forces to the approaches.
48
5.2 PROPERTIES OF ELASTOMER
An elastomer is a polymeric substance obtainedafter vulcanization of rubber. Vulcanization is theprocess of improving the properties of rubber byheating with sulphur. A normal rubber is notuseable as it becomes brittle at low temperatureand sticky at high temperature. Charles Goodyearhad been trying to ‘cure’ the rubber so that itcould be used in all seasons. He tried to mix allkinds of things such as ink, black pepper, cheeseand what not. But he couldn’t succeed until hedropped a piece of rubber on stove accidentally.To his surprise, he found that instead of meltingthe rubber piece hardened and remained pliable.It was found in the lab that it contained traces ofsulphur. Goodyear perfected the process andnamed it ‘vulcanization’ after the Roman God offire, ‘Vulcan’.
As a result of vulcanization, rubber molecules arecross-linked with sulphur. This cross-linkingmakes the rubber stronger. It allows the rubber tokeep its shape better even when it is stretchedover and over again. But there is a drawback ofcross-linking also. Vulcanized rubber doesn’t flowwhen it gets hot, therefore one has to mould itinto whatever shape one wants before crosslinking. Due to the same reason, it can’t berecycled – a big environment problem. The tyresof the vehicles also use the same material, andwe are not able to recycle the cross-linked rubberused in tyres.
One of the most well known natural rubber is‘Poly-isoprene’ which is harvested from the sapof ‘Hevea’ tree. Natural rubber have all the
49
excellent properties making it extremely suitableto many engineering applications, except for itsrelatively high reactivity with environmentparticularly ozone. Ozone causes surfacecracking that can rapidly penetrate even at verylow tensile stress.
To obviate this drawback many synthetic rubberswere developed, most popular among those is‘Poly-chloroprene’. Thus, we have -
1. Natural rubber - Poly-isoprene2. Synthetic rubber - Poly-chloroprene
There is often a confusion between the wordselastomer and neoprene. While elastomer refersto the generic name of the rubber, neoprenerefers to the trade name of the elastomer of oneof the leading rubber manufacturers.
Engineers are more familiar with materials whichobey Hooke’s Law i.e. behaving in a linear elasticmanner. We understand elastomers less wellthan we do concrete or steel because elastomersdo not obey Hooke’s Law. They are very flexiblein shear but very stiff in bulk compression. Thesimple theory of mechanics characterizing thebehaviour of rubber is quite different from thatused for conventional materials, and quitecomplex for the liking of the practical engineers.
It is therefore not surprising that most of thecodal provisions for design, fabrication, installationand maintenance of elastomeric bearings arebased on extensive studies and laboratory trialsconducted by ORE (Office for Research andExperiment) of UIC. These are documented inORE Report D-60. Important specifications which
50
can be referred to for elastomeric bearings arelisted below:
1. UIC 772-2R 19892. BS:5400 Part 9.13. IRC 83 Part II4. AASHTO specifications5. IS:3400 Part I to XXIV
Some of the important findings of the studiesconducted by ORE, which are relevant to thedesign of elastomeric bearings are enumeratedbelow :
1. Elastomers do not follow Hooke's Law and,therefore, the modulus of elasticity ‘E’ is notconstant.
2. The shear modulus ‘G’, however, is fairlyconstant and is more relevant for the designof elastomeric bearings than ‘E’.
3. The coefficient of friction between elastomerand the base material is unaffected by thenature of the contact surface i.e steel,concrete, painted or unpainted surfaces.
4. The coefficient of friction between theelastomer and the base material reduces withincrease in normal load on the bearing. It is
expressed by the formula N6.0
15.0 +=µ
where N = normal pressure in MPa
5. Except under extremely low temperatures(less than -150C), performance of theelastomeric bearing is not affected bytemperature variation. These have also beentested satisfactorily up to + 500C.
51
6. Under the effect of cyclic loading the bearingsbecome more flexible.
7. In some of the tests conducted onelastomeric bearings, there was a distincttendency of the elastomer to slip when theminimum normal pressure was less than2 MPa. This observation has importantramifications for use of elastomeric bearingsin railway steel bridges of smaller spanswhere normal pressure may be less than2 MPa. The elastomeric bearings in such smallbridges can be used alongwith anti creepdevices as explained in subsequent paras.
5.3 BEHAVIOUR OF ELASTOMERIC BEARINGS
In order to carry out successful design andinstallation of elastomeric bearings, it isnecessary to understand the behaviour ofelastomeric bearings against various imposedloads. The elastomer being practicallyincompressible, the total volume of the pad inloaded and unloaded conditions remainsunchanged. Therefore, under the action of acompressive load, a plain elastomeric pad withno friction on its top and bottom surfaces, flattensand expands laterally as shown in Fig. 5.1.
SLIP
FIG. 5.1 PLAIN ELASTOMERIC PAD WITHOUTFRICTION AT CONTACT PLANE
52
Since a frictionless contact surface does notexist in practice, the deformation of the pad willbe part flattening and part bulging and thebehaviour of plain elastomeric pad will be asshown in Fig. 5.2.
FIG. 5.2 PLAIN ELASTOMERIC PAD WITHFRICTION AT CONTACT PLANE
The lateral expansion of plain elastomeric pad istoo much for practical purposes and it can not beused as it is without making arrangements forreducing the lateral expansion. If the elastomer isbonded between two layers, the lateral expansionis prevented at the interfaces and bulging iscontrolled.
The compressive stiffness of the bearing,therefore, depends upon the ratio of loaded areato the area of the bearing free to bulge. This isessentially quantified by Shape Factor ‘S’ which isa dimensionless parameter defined as under:
S = Plan area loaded in compression Perimeter area free to bulge
Greater compressive stiffness is, therefore,obtained by dividing elastomer into many layersby introducing very thin, usually 1 to 3 mm, steelreinforcement plates between the elastomerlayers and bonding the plates firmly with theelastomer to prevent any relative movement. This
53
has the effect of decreasing the area free tobulge without any change in the loaded area.Hence, higher the Shape Factor, stiffer is thebearing under compressive load. Since theelastomer expands laterally, shear stresses areset up in the elastomer by the bond forces. Thesteel plate, in turn, is subjected to pure tensilestresses as shown in Fig. 5.3.
FIG. 5.3 REINFORCED ELASTOMERIC PAD
The elastomeric bearing provides horizontaltranslation by shear strains as shown in Fig. 5.4and rotation by differential compression as shownin Fig. 5.5.
FIG. 5.4 SHEAR STRAIN DUE TO SHEAR
FIG. 5.5 SHEAR STRAIN DUE TO ROTATION
54
Elastomeric bearings can accomodate horizontalmovements to an extent of 125 mm while it isclaimed that each 13 mm thickness of the padcould accomodate one degree of rotation.
In fact, horizontal translation is being provided byelastomeric bearing without loosing the contacteither with superstructure or with substructure.Therefore, the movements are allowed withoutany relative movement of parts.
The shear deformation depends upon the heightof the elastomeric pad as shown in Fig. 5.6.
FIG. 5.6 DEFORMATION OF ELASTOMERICPAD
Shear stress = shear force plan area
b x a
H=
Shear strain = δ h
where δ = deformationh = thickness of elastomeric padH = horizontal force
a
b Hh
δ
55
a = length of elastomeric padb = width of elastomeric pad
Shear Modulus G = shear stressshear strain
G b x a
H= x δ
h
... δ = Η x h G (a x b)
Thus for a given size of bearing, the sheardeformation will depend upon thickness ofelastometric pad, value of horizontal force andvalue of ‘G’. Since horizontal force and ‘G’ cannot be altered, deformation ‘δ’ will be proportionalto thickness of elastomeric pad.
δ ∝ h
Under the influence of rotation, the compressiveloads on the inner edge is magnified and it isrelieved on the outer edge. In the design it is,therefore, ensured that, under the combined effectof normal loads and rotations, the outer edge of theelastomer does not get off-loaded completely.
5.4 TYPES OF ELASTOMERIC BEARINGS
Three basic types of elastomeric bearings areused.
1. Plain elastomeric pads2. Steel reinforced elastomeric pads3. Fibre reinforced pads
Plain pads are used for light or moderate
56
loadings. Plain pads have a tendency to bulgeunder heavy loadings. In order to reduce thetendency of bulging, the elastomer pads arereinforced with steel plates. The steel sheetsseparating the layers of elastomer are completelyencased within the elastomeric material. Forvertical load, each layer of elastomer behaves likean individual pad, while horizontal strain on eachlayer is additive. Therefore, adding steellaminations is a convient way to accommodatelarger lateral movements for the samecompressive loads. Fibre reinforced pads areusually reinforced with fibre glass.
5.5 DESIGN OF ELASTOMERIC BEARINGS
The standard drawings of bridge bearings issuedby RDSO are on the basis of UIC 772-R. Tomaintain uniformity of approach, the design ofelastomeric bearing discussed in the followingparagraphs is only as per UIC 772-R.
5.5.1. Flow Table of Design : The flow table of designgiven at Table 5.1 has been prepared to simplifythe design process and eliminate trial and errorapproach. It is expected that the number ofiterations required for successful design will beminimum if this sequence of steps is followed.
57
TABLE 5.1 FLOW TABLE OF DESIGN
SN Sequence of steps Remarks
1. Collect input data Dead load, live load, horizontalslow load, horizontal quick load,span length, rotation at ends,etc. as given in next paragraph.
2. Select width ‘b’ of Generally equal to width ofbearing girder & larger than ‘a’ due to
better rotational stability inlateral direction.
3. Calculate net plan area Depends upon max. verticalof bearing load including impact and
permissible bearing pressureon bed-block.
4. Calculate length ‘a’ of Net plan area divided by ‘b’.bearing along girder
5. Calculate Shape Factor It should be between 6 and 12.‘S’
6. Calculate min. vertical Dead load / plan area of bearing.pressure
a) If it is < 2 MPa Bearing may slip. Revise plandimensions so that verticalpressure is min. 2 MPa orprovide ‘Anti Creep Device’.
b) If it is ≥ 2 MPA O.K. Proceed further.
7. Calculate max. vertical Total vertical load includingpressure impact / plan area of bearing.
a) If it is ≤ 10 MPa O.K.
b) If it is > 10 MPA Revise plan dimension,keeping a watch on step 6 a).
8. To ensure ‘No slipcondition’
a) Calculate µ1 when only µ1 x DL should be more thanD.L. is acting slow acting horizontal force.µ1 = 0.10 + 0.6/N1
where N1 = verticalpressure due to DL only
58
b) Calculate µ2 when both µ2 x (DL + LL) should beDL + LL (including more than total horizontal force.impact) are acting.µ2 = 0.10 + 0.6/N2 whereN2 = vertical pressure
due to DL + LL
9. Calculate % distortion It should be max 70%, otherwisein shear increase ‘h’ to limit this % age.
% distortion = δ x 100 h
10. To ensure no upliftcondition
a) Under Dead Load
Permissible rotation = It should be more than actualComp. of all layers under DL rotation of span under dead
a/6 load only.
b) Under Total Load
Permissible rotation = It should be more than actualComp. of all layers under TL rotation of span under total
a/6 load including impact.
11. Total shear in elastomer Addition of all three should beincludes limited to 5 x G or 5 MPaa) Due to compression (considering G = 1 MPa).b) Due to horizontal loadc) Due to rotation
12. Thickness of steel This should be more thanlamination plate required actually provided.
= 2 (hi + hi + 1)( Pc + 1.5 Ps)
a x b x σs
59
5.5.2 Input data required : The input data required forcarrying out the design is as under:
Pc = D L or slowly applied vertical loadsPs = L L or quickly applied vertical loadsHc = Slow acting horizontal forcesHs = Quick acting horizontal forcesUC = Horizontal (shear) movement due to Hc
Us = Horizontal (shear) movement due to Hs
α C = Rotation under effect of slow acting loadsα s = Rotation under effect of quick acting loadsG = Static shear modulus of elastomerσb = Permissible bearing pressure in bed
block materialσm = Max. Permissible pressure in the
elastomer• 10 MPa (as per IRC : 83)• 5 MPa or 5G, whichever is less
(as per RDSO)• 11 MPa (as per UIC 772 - 2R)
B = width of the girder/beam
5.5.3 Output Expected : The output expected at the endof design of the elastomeric bearing is as under :
a = length of the bearing along the spanb = width of the bearing across the spann = number of layers of elastomerhi = thickness of each layer of elastomerh = total thickness of elastomer.
60
5.6 MANUFACTURE AND QUALITY CONTROL
5.6.1 Properties of Elastomer : Though elastomericbearings offer a number of advantages ascompared to steel bearings, many failures havebeen reported pertaining to these bearings. Amajority of these failures can be attributed toimproper quality of elastomer and/or faultyinstallation. These two aspects, therefore, needvery careful attention by the construction engineerin the field. The properties of the elastomer aredetailed in IRC:83 Part-II and are reproduced inTable 5.2.
The shear modulus of the elastomer dependupon the hardness of the rubber. The relationbetween shore hardness and shear modulus Gas indicated in UIC-772-2R is as under:
Shore hardness 50 60 70 80
Modulus G 0.5 0.8 1.1 1.4
Therefore with age, hardness increases, which inturn increases the value of ‘G’. For the adopteddimension of the elastomeric bearing, thehorizontal movement ‘δ’ reduces with theincrease in value of ‘G’. Therefore with age, themovement capacity reduces.
5.6.2 Dimensional Tolerance : The bearing should befabricated to the dimensional tolerances stipulatedin IRC:83 which are reproduced in Table 5.3.
61
TABLE 5.2 PROPERTIES OF ELASTOMERName of test Reference Permissible Methodology of
code for testing limits testing and remarks1) Chemical
compositioni) Poly-chloroprene ASTM- D297 Not less than Use of fillers to be
content test 60% minimized.ii) Identification ASTM- D3677 No reclaimed Test involves use
of polymer rubber or of infra rednatural rubber spectrophotometryto be used.
iii) Ash content IS 3400 Not more thanPART-XXII 5%
2) Hardness IS 3400 60+ 5 IRHD stands forPART- II International Rubber
Hardness Degree. Thehardness scale is
IRHD similar to theShore or Durometerhardness.
3) Ultimate tensile IS 3400 Min. 17 MPastrength PART-I
4) Elongation at -do- Min. 400%break
5) Accelerated IS 3400 Variation in The elastomer sampleaging test PART- IV i) Hardness - not is subjected to a
more than 5 temperature of 1000Cii) UTS - not for a period of 70 hmore than 15% and then allowed toiii) Elongation cool to room tempera-at break - not ture. The variation inmore than 30% physical properties is
subsequently measured.6) Compression IS 3400 Compression The elastomer sample
set test PART- X set not to is subjected to aexceed 35% compressive strain of
25% at a temperatureof 1000C for 24 hours.Subsequently, thesample is cooled andthe residual strain ismeasured. The resi-dual strain called the”set” should notexceed 35% of theinitial strain.
7) Ozone test IS 3400 No cracking or The sample isPART- XX disintegration subjected to a tensile
of the sample. strain of 20% in achamber where thetemperature is main-tained at 400C andOzone concentrationkept at 50 pphm(partsper hundred million)
62
TABLE 5.3 DIMENSIONAL TOLERANCES
SN ITEMS TOLERANCES
1. Overall plan dimensions -0, + 6 mm
2. Total bearing thickness -0, + 5%
3. ParallelismA. Of top surface of bearing with
repect to the bottom surfaceas datum 1 in 200
B. Of one side surface with respectto the other as datum 1 in 100
4. A. Thickness of individual internal + 20%layer of elastomer (max. of 2mm)
B. Thickness of individual outer layer -0, +1 mm
5. A. Plan dimensions of laminates -3 mm, +0B. Thickness of laminate + 10%C. Parallelism of laminate with respect 1 in 100
to bearing base as datum
5.7 INSPECTION AND TESTING
The inspection and tests on elastomer and thefinished bearings are very important aspects ofensuring a satisfactory performance of thebearing. IRC:83 has laid down detailed testingplan and acceptance criteria for elastomericbearings. The important aspects are highlighted inthe following paragraphs.
5.7.1 Lot size : Testing and acceptance of elastomericbearings should be done lot-wise. A ‘lot’ ofbearings shall comprise of all bearings of equal ornear equal size produced under identicalconditions of manufacture to be supplied for aparticular project. For the purpose of grading the
63
levels of acceptance test, the lots are classifiedas under :
1. A lot size of 24 or larger number of bearingsis defined as large lot.
2. A lot size with less than 24 bearings isdefined as small lot.
When the number of bearings for a bridge projectis large and phased production is permitted,bearings supplied in any one phase will beconsidered as a large lot. The levels ofacceptance test applicable will be as under :
Large lot - ‘Level 1’ Acceptance testingSmall lot - ‘Level 2’ Acceptance testing.
5.7.2 Level 1 Accpetance Testing : This will include thefollowing tests:
A. General inspectionB. Test on specially moulded test pieceC. Test on complete bearings.
A. General inspection
1. All bearings of the lot shall be visuallyinspected for absence of any defects insurface finish, shape or any discerniblesuperficial defects.
2. All bearings of the lot shall be checked fordimensional tolerances.
3. All bearings shall be subjected to an axial loadcorresponding to normal pressure of 15 MPaapplied in stages and held constant whilevisual examination is made for:
64
a. Misalignment of reinforcing platesb. Poor bond at interfacec. Variation in elastomer thicknessd. Surface defectse. Low stiffness.
The deflection under loads between 5 MPa and15 MPa should be measured with sufficientaccuracy. Variation in stiffness of any individualbearing from the mean of all such bearingsshould not be more than 20% of the mean value.
B. Test on specially moulded test piece
The test piece shall be moulded by themanufacturer with identical compound and underidentical vulcanizing conditions as used inmanufacture of the bearings. The test piecesshould be suitably identified and certified. The testpieces will be subjected to the following tests:
• Test for chemical composition, specific gravity,ash content etc.
• Test for physical properties such as
(i) Hardness
(ii) Ultimate tensile strength
(iii) Elongation at break
(iv) Accelerated aging test
(v) Compression set test
(vi) Ozone test
The details of these tests are given in Table 5.2.
65
C. Test on complete bearings
Two bearings should be selected at random fromthe ‘lot’. Various tests should be conducted onthese ‘test bearings’. These bearings should beexcluded from the accepted lot because all thetests given below except ‘Shear Modulus test’ aredestructive tests.
1. Shear Modulus2. Elastic Modulus3. Adhesion strength4. Ultimate compressive strength.
In addition to above tests, the ash content (%)and specific gravity of elastomer of test piecesfrom test bearing shall be compared with those ofcorresponding specially moulded test pieces andmaximum acceptable variation will be as givenbelow:
Ash content + 0.5%Specific gravity + 0.2
The test specifications and acceptance criteriashould be as per Appendix II of IRC 83 Part II.The excerpts are given below:
General about tests
1. All testing shall be done at room temperature.
2. No bearing shall be tested earlier than a weekafter vulcanization.
3. Bearing sections shall be cut from testbearings without overheating the rubber andwith smooth cut square edges.
66
4. Test for determination of ‘E’ may precede thatof ‘G’ when both tests are conducted on thesame pair of test bearings.
Test for determination of Shear Modulus
• Conditioning load : Bearings shall bepreloaded with maximum horizontal load Htest
(with Ntest or vertical load held constant) andunloaded before test loading.
• Rate of loading : Ntest corresponding toσm = 5 MPa shall be held constant during testand the horizontal loading H shall be graduallyincreased to yield a shear stress rate ofapproximately 0.05 to 0.1 MPa per minute.
• Maximum test loading Ntest : The horizontalloading H shall be increased upto a maximumHtest which corresponds to horizontaldeflection equal to ‘h’ (total elastomerthickness).
• Measurement : Load and deflectionmeasurements shall be calculated atapproximately equal intervals not less than 5.
Evaluation
A shear stress strain curve shall be plotted and thevalue of shear modulus G determined as shown inFig. 5.7.
The test result shall be deemed satisfactory if valueof G is within + 20% of 1 MPa and provided there isno evidence of instability, defect or damage detectedby close inspection during the test.
67
2 H
RIGID CONCRETE SLAB( FIXED )
RIGID CONCRETE SLAB(DEFLECTED UNDER 'H')
N test
RIGID CONCRETE SLAB(FIXED)
SH
EA
R
SHEAR STRAIN
0.0 0.2 0.4 0.6 0.8 10.0
ST
RE
SS
a) TEST ASSEMBLY
b) SHEAR STRESS STRAIN CURVE
FIG. 5.7 DETERMINATION OF SHEAR MODULUS
SHEAR STRAIN
SH
EA
R S
TR
ES
S
∆τ m
∆ tan γ
G = ∆τm / ∆ tan γ↑
↑
0.0 0.2
68
Test for determination of Elastic Modulus
• Conditioning load : Bearing shall bepreloaded upto Ntest. The load shall beretained for 10 minutes and unloaded uptoσm = 2 MPa before test loading.
• Rate of loading : The axial load ‘N’ isincreased gradually at a rate yieldingapproximately σm = 0.5 MPa to 1 MPa perminute
• Maximum test loading Ntest corresponds toσm = 20 MPa.
• Measurement : Load and deflectionmeasurements shall be made inapproximately equal load intervals not lessthan 5. Deflection shall be measured at fouredges and mean value accounted for.
Evaluation
A compressive stress strain curve shall be plottedand the value of apparent elastic modudus ‘Ea’ shallbe defermined as shown in Fig. 5.8.
Acceptance Criteria
Test result shall be deemed satisfactory if value of‘Ea’ is within + 20 percent of 1(0.2/S2 + 0.0005) andprovided, there is no evidence of any defect ordamage discerned by close visual inspection duringthe test.
69
N test
CO
MP
RE
SS
IVE
ST
RE
SS
STRAIN
0.0 2 10 20
FIG. 5.8 DETERMINATION OF ELASTIC MODULUS
a) TEST ASSEMBLY
b) COMPRESSIVE STRESS STRAIN CURVE
RIGIDCONCRETESLAB
2
10
20
∆σm
∆ε
Ea = ∆σm / ∆ε
70
Test for determination of Stripping Strength
Two identical test pieces shall be cut from the testbearing. The plan dimensions of each test piece shallnot be less than 100 mm x 200 mm as shown inFig.5.9.
Two opposite ends of each test piece shall bevelledto an angle of 450
FIG. 5.9 DETERMINATION OF ADHESION STRENGTH
Test Procedure
• Maximum test loading: N test correspondingto σm = 4 MPa is to be held constant duringthe test.
• The horizontal loading H shall be increasedupto a maximum yielding τm = 3 MPa
Evaluation
Examine the test pieces for evidence ofcracking or peeling both in the strained andunstrained state.
Acceptance criteria
If neither test piece shows evidence ofpeeling or separation at or near the interface
2 H
LOADING PLATE
45
200
MOUNTING PLATE N test
PLATERECESSED
TO PREVENTSLIP
h ≥ 50↓
↓
71
between rubber and reinforcement layers, thebearing shall be deemed to have satisfactoryadhesion.
Test for determination of UItimate CompressiveStrength
The test pieces are to be loaded either till the failureof the steel laiminate or till the irreversible squeezingout of elastomer whichever is earlier. The testassembly and the test pieces may be identical tothose for ‘E’ test. However, a small section (not lessthan 100 x 200 mm) shall be cut from test bearingand testing to failure by placing directly between theplates of the testing machine shall also be performed.The rate of loading shall not exceed 10 MPa perminute. The result of the test shall be deemedsatisfactory if the σm at failure is not less than 69 MPa.
5.7.3 Level 2 Acceptance Testing : This will alsoinclude the same tests as in Level 1
A. General inspectionB. Test on specially moulded test pieceC. Test on complete bearings.
But here, the test ‘C’ is little different as only onetest i.e. ‘test for shear modulus’ is conducted.Since this is not a destructive test, the testbearings can be used in the bridge and theseshall become the part of the accepted ‘lot’.
5.7.4 Inspection and Quality Control Certificate : A ‘lot’under inspection should be accepted by theinspector when no defect is found in theacceptance level tests and so certified. In case ofany defect in the bearing, the whole ‘lot’ shall berejected by the inspector and certified
72
accordingly.
The bearings shall be transported to the bridgesite after final acceptance and should beaccompanied by an authenticated copy of the testcertificate. An information card giving the followingdetails should be appended to the test cerftificate:
1. Name of manufacturer2. Date of manufacture3. Grade of elastomer4. Bearing dimensions5. Production Batch No.6. Accpetance lot No.7. Date of testing8. Specific bridge location9. Explanation of markings on the bearing, if any.
A commentary on various tests conductedon specially moulded test piece
Elastomeric bearings have had an extraordinaryperformance record over the past 50 years assome of these have been used since 1950satisfactorily. A number of tests are required tobe performed on elastomers in order to ensurethe good performance. But still it is not very clearwhether all these tests are necessary or not. Inthis commentary, it has been discussed thatsome of the tests are not related to the actualperformance of an elastomeric bearing and thusmay be either scrapped or the prescribedtolerances should be diluted. These are basedupon the studies undertaken by AASHTO.
1. Ozone TestSome research and studies determined that theozone test was unnecessary as this affects only
73
the surface layer. In un-stretched rubber, ozonedegradation is confined to a thin surface layer,typically 0.5 µ. Ozone cracks develop at rightangles to the tensile strain and they degraderubber’s tensile strength and may also initiatefatigue growth that ultimately leads to failure ofrubber products. Because cracks only occur inregions where tensile stresses are induced, theyare unable to penetrate very far into objects,which are under compression. In bearings, thegrowth of cracks ceases close to the surfacebecause cracks quickly encounter compressiverather than tensile stresses. Therefore, it hasbeen contended that ozone damage is a seriousconcern in thin-walled products, but not in thosebulky products like bearings. Contrarily there is aharmful effect because the manufacturers useanti ozanant waxes to protect against ozoneattack. This viscous layer of wax is responsiblefor a significant number of serious slippingproblems where the bearings ‘walked-out’ of thesupport area.
2. Accelerated Aging
Accelerated ageing tests are conducted atelevated temperature of 100oC to know thevariation in hardness, UTS and elongation atbreak. The research showed that the size ofsample was very important in establishing thesignificance of aging. Current test methods usevery thin specimen and the results generallyshow significant changes due to aging. Theactual bearing, however, is not a thin specimenand effect of aging is not so pronounced. It wasfound that it would take hundreds of years of
74
service to change the bearing stiffness by 10%.Given that the correlation between the test andthe real situation is not known, it seems that thetest is just a quality control test, not aperformance test.
3. Hardness
Hardness has been believed to be related toshear modulus but studies have shown thathardness is a surface measurement and it onlycrudely represents the stress strain relationshipin shear.
5.8 INSTALLATION
5.8.1 General guidelines : Some of the importantprovisions of UIC-772-2R and IRC:83 Part-IIpertaining to installation of elastomeric bearingsare given below:
1. Elastomeric bearings must not be placed onebehind the other along the longitudinal axis ofthe girder on a single line of support underany circumstances.
2. Bearings of different plan size must not beinstalled next to each other (on the same pier)under the same girder/span.
Different plan area will lead to different value of ‘δ’for given values of H, h and G as under:
( )baG
h Hδ×
×=
3. The contact surface between the bearing andthe bed block must be horizontal to avoidtangential force due to gravity coming on tothe bearing (maximum tolerance 0.2%
75
perpendicular to load). In case of slopinggirders, this may be achieved by providingshims under the girder or by creating arecess with a horizontal surface on theunderside of the girder.
4. The bearings must be placed on a contactsurface which is free from local irregularities,maximum tolerance being +1 mm in height.
5. The bearings must be placed at true planposition with a maximum tolerance of +3 mm(along and across the bridge axis).
6. As a measure of ample safety againstaccidental displacement, the bearings shouldbe placed in a recess. This is speciallynecessary for bridges with low DL/(DL+LL)ratio or bridges with larger horizontaldisplacements. This is typical in railwaybridges with steel girders.
7. Where shrinkage and creep are likely toproduce excessive horizontal movements, it isadvisable to raise the girders beforecommissioning the bridge, allowing release ofthis unidirectional displacement.
8. Care should be taken in storage, handling andinstallation of the bearing to avoid anymechanical damage, contamination with oil,grease, dirt and undue exposure to sunlightand weather.
9. It is preferable to install the elastomericbearing on a pedestal to permit jacking of thegirders for future replacements and for properdrainage.
76
10. The area around elastomeric bearings shouldbe inspected and scrupulously cleaned afterthe installation is complete. Railway bridgeswith ballasted decks should be provided withsuitable expansion joints to prevent ballastfrom fouling the breathing space betweengirders, or between end span girder andballast wall.
5.8.2 Process of installation : The process ofinstallation of elastomeric bearing differs in caseof precast vis-a-vis cast-in-situ girders. Forprecast girders, the bearing may be fixed to theunderside of the girder by application of epoxyresin adhesives after specified surfacepreparation. Care should be taken to guardagainst faulty application and consequentbehaviour of the adhesive layer as a lubricant.The adhesive is considered merely as aninstallation device and is not an adequate anti-creep measure.
The process of installation of elastomeric bearingin a cast-in-situ girder is more complex. Itinvolves the following stages:
1. Placing and adjusting the bearings on the bedblock with required accuracy.
2. Preparation of a formwork sideframe allaround the bearing.
3. Filling up the space between the bearing andthe formwork frame by clean sand.
4. Placement of top forms over the side frameand sand with an opening matching the sizeand position of the bearing.
77
5. Sealing of the gaps between the opening intop form and the bearing by adhesive tape.
6. Execution of soffit formwork of the girder.
7. Concreting of the girder.
8. Removal of side forms and sand from aroundthe bearing and cleaning up the spaces.
5.9 PERIODICAL INSPECTION AND MAINTENANCE
The elastomeric bearings are considered largelyto be maintenance free. However due to possibledeficiencies in manufacture and installation, thesebearings may show signs of distress or developmalfunctioning. Before any malfunctioning of thebearings leads to distress in the girders orsubstructure, it should be detected and preventiveactions taken. It is, therefore, necessary toundertake periodical inspection of elastomericbearings. The inspecting official should look forthe following aspects:
! Correct position
! Excessive shear
! Excessive bulging
! Separation of rubber from steel lamination
! Cracking and tearing of elastomer
! Flattening out
! Off-loading of one edge due to excessiverotation
78
Under the effect of loads and inducedmovements, the elastomeric bearing will
a) compress (flatten),
b) bulge and
c) shear.
These are signs of normal functioning of thebearing and judgement regarding distress can beformed only on the basis of personal experienceof the inspecting engineer. As a general guide,however, the following movements can beconsidered to be excessive:
a) Shear deformation more than 50% of height ofelastomeric pad
b) Rotation leading to off-loading of an edge
c) Compression more than 5% of height of thepad.
Generally, malfunctioning of the elastomeric padwould result in distress either in the girder or inbed block and the area close to the bearingshould be examined for cracking or spalling ofconcrete.
5.10 ELASTOMERIC BEARINGS IN AID OF OLDSUBSTRUCTURES
There are very large number of railway bridgesconstructed before 1926. The significance of theyear 1926 is that it was in that year theIRS : Bridge Rules first incorporated provision oflongitudinal force i.e. tractive effort and brakingforce. Prior to 1926 the bridges were designedonly for the vertical effects of axle loads.
79
With the technological improvements, the bridgerules have also undergone changes primarily toreflect introduction of diesel and electric traction.RBG loading standard - 1975, MBG loadingstandard - 1987 and HM loading standard - 1995are characterised by large increase in longitudinalforce as compared to the increase in axle loads.In addition, due to the "Policy of Uni-gauge", manyMG and NG sections have been upgraded to BG.It is therefore not surprising that a large numberof railway bridge substructures are called upon tobear much larger horizontal forces than theiroriginal design values.
A check on design calculations of substructure ofsuch bridges indicates unacceptable level ofstresses in the bridge substructure. Thus manypiers are required to be strengthened or rebuilteven though they happen to be otherwisephysically in sound condition. Apart from beingcostly and time consuming, strengtheningmeasures like jacketing may not inspire fullconfidence in the load carrying capability of thesubstructure. Complete rebuilding of bridge, inmany cases is far too time consuming to fit withinthe tight time schedules laid down for gaugeconversion works.
A better and cheaper solution to the aboveproblem is possible by way of better managementof the longitudinal force on bridges. Thelongitudinal force is generated at the rail wheelcontact surface and it can either be passed on tothe substructure through the bearings or can bedispersed onto the approaches. The sharing ofthe longitudinal force depends upon the relative
80
stiffness of the "girder-bearings-pier-soil" systemor "rail - track - ballast - approach embankment"system. While the derivation of the load sharing isquite complex, it is certain that introduction of aflexible bearing under the girder can dispersemore longitudinal force on the approaches,thereby relieving the piers of the horizontal forces.A number of experiments have been conducted inIndia and abroad on this aspect and rules laiddown for dispersion of longitudinal force toapproaches. Surprisingly, there is considerablevariation in the rules adopted by various countriesin this regard. The provision of IRS : Bridge Rulesin force on Indian Railways are explained below:
1. The quantum of longitudinal force is laid downin appropriate EUDL tables in the appendix tothe IRS : Bridge Rules.
2. Subject to proper standard and condition oftrack on bridge approaches and provision ofrail free fastening on the girders, 25% of thelongitudinal force subjected to a minimum of16t for BG, can be considered to be dispersedon to the bridge approaches.
3. In case, suitably designed elastomericbearings are used, the above dispersion canbe further increased by 40%. Thus use ofelastomeric bearings results in 35% oflongitudinal force being thrown on to theapproaches.
4. In case of a span having free-fixed bearingcombination, the full longitudinal force (netafter dispersion) is transmitted to the pier withfixed bearing.
5. In case of a span having sliding bearing or
81
elastomeric bearings at both ends the netlongitudinal force is shared @ 40% at eithersupport point, the remaining being shared bythe piers beyond the loaded span.
Though the above provisions are considered tobe conservative by many, it still shows a clear cutbenefit of replacing steel sliding bearings orrocker & roller bearings by elastomeric bearings.A note of caution is however necessary that theamount of dispersion will also depend upon thecapacity of the rails, joints and approach track totransmit and absorb the longitudinal force. It ishowever beyond doubt that more studies arenecessary in this area and there is scope forretaining old substructures by use of elastomericbearings.
While replacing other bearings especially rocker& roller by elastomeric, the correction slip no. 6 toConcrete Bridge Code dated 30.7.2002 should bekept in mind, which states that “use ofelastomeric bearings in prestressed concretebridges should preferably be restricted up tomaximum clear span of 30 m.”
5.11 ANTI-SLIP DEVICESIt has been explained earlier that elastomericbearings have a tendency to slip if the minimumnormal pressure is less than 2 MPa. Such asituation is likely to occur when elastomericbearings are used in steel plate girders havingless self weight. UIC code 772-2R cautionsagainst the use of elastomeric bearings if theyare required to undergo simultaneously lightloading and large longitudinal movements.
82
For steel girders of spans ranging from 12.2 m to30.5 m the minimum bearing pressure is lessthan 2 MPa. All such bearings, therefore, requiresome anti-creep or anti-slip device.
There are number of possible ways of providingthe anti-slip device as given below:
1. By stops : In this method stopper plates orangles are welded to the underside of thegirder and embedded in the bed block. Itshould be ensured that the thickness of theelastomer butting against the stops is notconsidered in the design since this thicknesswill not take part in the shearing action. Atypical anti-slip device using stops is shown inFig. 5.10 a.
2. By embedment : A recess can be created inthe bed block in which the elastomeric padwill be placed. The functioning of the recessas an anti-slip device is same as that of thestopper plate. The bearing is usually glued tothe girder with an epoxy resin. A typical detailis shown at Fig. 5.10 b.
3. By bolting : The elastomeric bearing can bevulcanised with a stainless steel outer plate.The stainless steel plate is provided withholes through which anchor bolts can be usedfor attachment of the bearing to the girder andto the bed block. This method howeverrequires either anchor bolts to be cast alongwith the girder or dowel holes left in position inthe concrete at correct locations. A typicaldetail of this scheme is shown in Fig. 5.10 c.
83
BEARING
BED BLOCK
BEARING
STOPS
ANGLE EMBEDDED IN BED BLOCK
EXTERNAL PLATEBONDED TO BEARING
BED BLOCK
GIRDER
FIG. 5.10 ANTI-SLIP DEVICES
a) PLACEMENT IN RECESS
b) BY STOPPER PLATE
c) BY BOLTING
BEARING
BED BLOCK
ANGLE EMBEDDEDIN BED BLOCK
STOPS
BEARING
EXTERNAL PLATEBONDED TO BEARING
BED BLOCK
BEARING
BED BLOCK
84
5.12 SAMPLE DESIGN PROBLEM FORELASTOMERIC BEARINGS
Problem
Check the design of bearing as shown in Fig. 5.11, of size650 x 450 x 120 mm, for 18.3 m span PSC I - girder sectionas per HMLS 1995. Assume suitable related data and referIRS : Bridge Rules and IRC-83 Pt.- II.
FIG. 5.11 CUT SECTION OF ELASTOMERIC BEARING
Guiding considerations
1. Dimensioning(i) b < 2a where ‘b’ is across the traffic and ‘a’ is along
the traffic
The longer dimension across the girder ensuresrotational stability in lateral direction.
(ii) a/10 < h < a/5
a/10 < h so that it should ride over the irregularitiesof bed block in a better way.h < a/5 so that there is no buckling condition.
(iii) Shape Factor 6 < S < 12
6 < S so that high vertical stiffness can be givenand bulging is controlled.S < 12 not required from design considerations andthus uneconomical.
STEEL REINFORCING PLATESELASTOMER
ba
h h
85
2. Limits of vertical pressure
i) Minimum vertical pressure = 2MPa
Otherwise bearing is likely to ‘walk-out’
ii) Maximum vertical pressure =10 MPaOtherwise bearing is likely to get crushed
3. No Slip condition
a) when only DL is acting.
µ 1 X DL > slow acting horizontal force
b) when (DL + LL) both acting.
µ 2 X (DL + LL) > Total horizontal force
4. Plan dimensions of bearing should be sufficient such thatbearing pressure on bed-block should be within permissiblelimit for the material of bed block.
5. Distortion limit in shear = 70 % of height of bearing
or δmax
= 0.7 x h
6. No uplift condition
The bridge span rotates at ends due to dead and live loaddeflections. Due to this rotation, there should be no loss ofcontact of the far end of the bearing with bed block. Toensure this
i) ctanαa/6
ei>
∑when only dead load is acting.
ii) sc 1.5tanαtanαa/6
ei+>
∑when dead load and live load
both are acting.
7. Total shear stress in elastomer should be within limit of5 x G.
86
Total shear stress includes a) due to compression
b) due to horizontal load
c) due to rotation
8. Thickness of steel lamination
hs =
( )( )s x b x a1.5PPhhi2 sc1ii
σ++ +
Solution:
Thickness of top and bottom rubber pad covers = 6 mm
Steel plates = 8 nos @ 3 mm thick, total = 24 mm
Intermediate elastomer pads = 7 nos @12 mm thick
Thickness of elastomer = 7 x 12 + 2 x 6 = 96 mm
Thickness of bearing (h) = 96 + 24 = 120 mm
1. Dimensioning
(a) Considering 6 mm cover on all sides,
a = 450 - 2 x 6 = 438 mm
b = 650 - 2 x 6 = 638 mm
2a = 438 x 2 = 876
b < 2a ..................................... Hence OK
(b) a/10 = 43.8 and a/5 = 87
h should be between 43.8 mm & 87 mm
h provided = 96 mm ..........almost satisfied.
(c) S = 638)122(438
638 x 438
b)h2(a
b x a
i +=
+ = 10.82
(It is calculated for individual layer)
Since, 6 < S < 12 ........ Hence OK
87
2. Limits of vertical pressure
Data for 18.3 m span (HMLS)
Dead load of span = 226 t
Weight of ballast and track per span = 96 t (say)
Slowly applied vert. load = 226 + 96 = 322 t
Slowly applied vert. load per bearing Pc = 322/4 = 80.5 t
For a girder of 18.3 m clear spanoverall length of girder = 20 m
L (m) Total load for BM Total load for SF CDA=0.15+8/6+L———- ———————— ————————- ————-————
20 261.8 t 286.2 t 0.458
Total load for shear =286.2 t (excluding CDA)
Total load including CDA = 286.2 x 1.458
Quick acting vertical load per bearing
Ps = t 104.324
1.458 x 286.2=
σmin
(only due to DL )
= MPa 2MPa 2.88638 x 438
10 x 1000 x 80.5>= ... O.K.
…hence no slip condition
σmax
(due to both DL & LL)
= MPa 6.61638 x 438
10 x 1000 x 104.32)(80.5=
+ < 10 ... O.K.
3. No slip condition
L Tractive Effort Braking force——— ——————— ———————20.0 m 75.0 t 50.6 t
88
Horizontal load adopted = 75.0 t. (higher of TE & BF)∴ Quick acting horizontal load per bearing
HS = t 15 kg
2
0.4 x 1000 x 75=
(Considering 40% horizontal load on one support)
Assuming value of G = 1 MPa
Shear strain = shear stress/G
Shear Strain = 0.541 x 438 x 638
10 x 1000 x 15=
Movement corresponding to above strain
= 0.54 x 96 = 51.84 mm
Slow movements due to temperature effect
(Assuming temp. change of 300C with other end fixed)Temperature movement = ∝ t l
= 1.17 x 10-5 x 30 x 20 x 1000 = 7 mm
Adding 3 mm for creep and shrinkage
Total movement = 10 mm
Slow acting horizontal force Hc corresponding to
movement of 10 mm= 638 x 438 x 1 x 10/96 = 2.9
Check for no slip condition
µ = coefficient of fraction = 0.10 + 0.6/N
for DL only µ 1
= 0.10 + 0.6/2.88 = 0.31
for DL + LL µ 2
= 0.10 + 0.6/6.61 = 0.19
When only DL is acting
Slow acting horizontal force = 2.9 t
Resisting frictional force = 0.31 x 80.5 t
89
= 24.96 t > 2.9.................safe
When (DL + LL) both are acting
Total horizontal force = 2.9 + 15 = 17.9 t
Resisting force = 0.19 x (80.5+104.32)
= 35.1 > 17.9 t ..............safe
4. Bed block concrete
σmax = 6.61 MPa
use M30 grade concrete for bed block
Permissible stress in bearing = 0.25 x 30
= 7.5 MPa ............ safe
5. Limit of distortion
Shear strain = 96
1051.84 +
= 64.4% ........................safe
6. Check for no uplift condition at most lightly loaded edgeof bearing
Assumptions:
Let the vertical defection under DL + LL = 25 mm
Rotation at bearing = 25 x 2 / 20000
= 0.0025 radians
Rotation due to only DL
= radians0.0011104.3280.5
80.5x0.0025 =
+
Rotation due to LL
= radians0014.032.1045.80
32.104x0025.0 =
+
90
a) For D.L. condition
ctanαa/6
ei>
∑
3N4GS
N x hiei where 2
+=
Compression of individual layer
= mm 0.0722.88 x 3(10.82) x 1 x 4
2.88 x 122 =
+
Compression of 7 layers = 7 x 0.072 = 0.504
a/6
ei∑ = radian 0.007
438/6
0.504=
Rotation under DL = tan αc = 0.0011 radian
Hence b/6
ei∑> tan αc ................. hence safe.
b) For DL + L. L. Condition
sc 1.5tanαtanαa/6
ei+>
∑
Compression of individual layer
= mm 0.1626.61 x 3(10.82) x 1 x 4
6.61 x 122 =
+Compression of 7 layer = 7 x 0.16 = 1.14 mm
a/6
ei∑ = radians016.0
6/43814.1 =
tan αc +1.5 tan αs = 0.0011+1.5 x 0.0014
91
= 0.003 radians
Hence a/6
ei∑> tan α
c +1.5 tan α
s ............ hence safe.
7. Check for total sheer stress
a) Sheer stress due to compression load
=
+
axbs1.5PcP
S
1.5
Factor 1.5 with Ps makes allowance for severe effect
of vertical vibratory railway loads observed duringtest.
= MPa 1.1810 x 310 x 638 x 438
104.32 x 1.580.5 x
10.821.5 =
+
b) Shear stress due to horizontal load
= ( )
MPa 0.64638 x 438
10x 10 x 2.915 3
=+
c) Shear stress due to rotation
= )α 1.5tanα (tanH x hi x 2
Gasc
2
+
The factor 1.5 for tan αs makes allowance for the
fatigue effects produced by rapid variation of verticalvibrators railway loading.
= MPa 0.270.0014) x 1.5(0.001196 x 12 x 2
438 x 1 2
=+
Total shear stress = 1.18 + 0.64 + 0.27
= 2.09 MPa < 5 MPa ......... safe
92
8. Check suitability of 3 mm thick steel plates
hs > ( )( )
s
sc1ii
σ x b x a
1.5PPhh2 ++ +
σs = allowable stress in steel = 140 MPa
> 140 x 438 x 638
104.32) x 1.512)(80.52(12 ++
> 2.9 mm
Therefore, adopted size of 3 mm is safe.
----------
93
CHAPTER 6
POT BEARINGS
6.1 GENERAL
POT bearing was developed in 1959 as analternative to heavy steel sliding bearings. Itconsists of a circular non-reinforced rubber-padfully enclosed in a steel pot. The rubber isprevented from bulging by the pot walls and itacts similar to a fluid under high pressure.
From the discussion on various bearings, it hasbeen observed that most of the bearings have thelimitation of either load or movement capacity.The load range and movement capacity ofvarious types of bearings are given in Table 6.1.
TABLE 6.1 Load and Movement Ranges
Load (T) Movement
Sliding 20 – 133 ± 25 mm
Rocker & roller 60 – 266 100 mm
Elastomeric 30 – 220 60 mm
The above bearings are adequate for smallerspans having the requirement of load andmovement, within the range prescribed in theabove table. But what to do for the larger spanshaving more load and more movements? In factthe problem of load can be solved by providingmore bearing area or providing more number ofbearings so that load is shared by manybearings. However this idea of ‘sharing’ can’t beextended to movement because the number ofends can’t be more than two.
94
Therefore, it is the requirement of movementwhich is more critical than the load requirementand we require some other type of bearing wherethe horizontal movement should not be thelimiting factor. Since necessity is the mother ofinvention, a special category of bridge bearingwas developed known as ‘POT and PTFE’. PTFEis a short form of Poly Tetra Fluoro Ethylene.
6.2 POT-PTFE BEARING vs ELASTOMERICBEARING
In POT bearing, two most important syntheticmaterials i.e. Elastomer & PTFE are utilised.Elastomer has an excellent property of providingtranslation and rotation without any moving parts.In POT-PTFE bearing, the latter part is utilised bycompletely encasing the elastomer pad in steelcasing or POT. PTFE has an excellent propertyof having very low coefficient of friction and in thefree end, a sliding component is added on top ofPOT, comprising stainless steel and PTFE fortranslation. The rotation, therefore, is provided byelastomer due to differential compression andtranslation by steel and PTFE.
Elastomeric bearing, otherwise, considered to bean ideal bearing, could not be used in largerspans because of some drawbacks. Thesedrawbacks of elastomeric bearing which lead todevelopment of POT – PTFE bearing are givenbelow:
(1) The ordinary elastomeric bearing can’t beused as a fixed bearing.
(2) The translation allowed by the elastomericbearing is restricted by its thickness.
95
( )ba G
h Hδ
××
=
or δ α h
‘δ’ is generally restricted to 50-60% of totalthickeness of the elastomer.
(3) In order to have more value of ‘δ’, thethickeness of the elastomer pads will have tobe increased but the same can't be increasedbeyond a limit as thicker elastomer pads arerather unstable.
(4) There is a limit to the vertical load also whichthe elastomeric pad can safely withstand.Large vertical loads result in greater amountof compression and bulging.
(5) Large rotation create the danger of off-loadingof one edge and overstressing the other.
Fig. 6.1 to 6.4 show typical details of varioustypes of POT-PTFE bearings.
6.3 PROPERTIES OF PTFE
PTFE is a linear chain polymer of great molecularstrength, known for its chemical inertness andlow coefficient of friction. PTFE is not oxidisedeasily, it is resistant to all common solvents andremains stable at extremes of atmospherictemperatures.
It was earlier thought that lowest coefficient offriction could be obtained with PTFE slidingagainst PTFE. But on the basis of testsconducted on several frictional interfaces, it hasbeen conclusively proved that the frictionalcoefficient of PTFE sliding against groundedstainless steel surface is lower than PTFE on
96
Fre
e to
rot
ate
abou
t an
y ax
is in
th
e ho
rizo
ntal
pla
ne
Pis
ton
Har
d Fa
cing
Inte
rnal
Sea
l
Con
fine
d E
last
om
eric
Pre
ssur
e P
ad
Cyl
inde
r
Ext
erna
l Sea
l
FIG
. 6.
1 F
IXE
D T
YP
E P
OT
BE
AR
ING
97
FIG
. 6.
2 F
RE
E S
LID
ING
TY
PE
PO
T-C
UM
-PT
FE
BE
AR
ING
Fre
e to
slid
e al
on
g a
ny
dir
ecti
on
in th
e ho
rizo
ntal
pla
ne
Fre
e to
ro
tate
ab
ou
t an
y ax
is in
the
hori
zont
al p
lane
Slid
ing
Top
Wip
er S
eal
Ext
erna
l Sea
Pis
ton
Har
d Fa
cing
Inte
rnal
Sea
Co
nfi
ned
El
Pre
ssur
e P
a
Cyl
inde
r
PTF
E
Sta
inle
ss S
t
SLI
DIN
G T
OP
PLA
TE
WIP
ER
SE
AL
EX
TE
RN
AL
SE
AL
HA
RD
F
AC
ING
INT
ER
NA
L S
EA
L
CO
NF
INE
D E
LAS
TO
ME
RIC
PR
ES
SU
RE
PA
D
CY
LIN
DE
R
ST
AIN
ST
EE
L P
LAT
E
PIS
TO
N
98
FIG
. 6.
3 P
OT
-PT
FE
BE
AR
ING
WIT
H S
LID
ING
GU
IDE
S
Fre
e to
rota
te a
bou
t any
axi
s in
the
hor
izo
nta
l pla
ne
Slid
ing
To
with
sid
e
Wip
er S
e
Ext
ern
al S
Har
d F
aci
Inte
rna
l S
Con
fine
d P
ress
ure
Cyl
inde
r
Slid
ing
res
trai
ned
alon
g th
is d
irect
ion
PT
FE
PIS
TO
N
Sta
inle
s
SLI
DIN
G T
OP
PLA
TE
WIT
H S
IDE
GU
IDE
S
WIP
ER
SE
AL
EX
TE
RN
AL
SE
AL
HA
RD
F
AC
ING
INT
ER
NA
L S
EA
L
CO
NF
INE
D E
LAS
TO
ME
RIC
PR
ES
SU
RE
PA
D
CY
LIN
DE
R
ST
AIN
ST
EE
L P
LAT
E
99
Fre
e to
rota
te a
bou
t an
y ax
is in
the
horiz
ont
al p
lane
Slid
ing
rest
rain
ed a
lon
g th
is d
irect
ion
Cyl
inde
r
Slid
ing
T
Con
fine
dP
ress
ure
Inte
rna
l S
Har
d F
ac
Ext
ern
al
Wip
er S
e
PT
FE
PIS
TO
N
Sta
inle
ss
FIG
. 6.
4 P
OT
-PT
FE
BE
AR
ING
WIT
H C
EN
TR
AL
GU
IDE
SLI
DIN
G T
OP
PLA
TE
WIP
ER
SE
AL
EX
TE
RN
AL
SE
AL
HA
RD
F
AC
ING
INT
ER
NA
L S
EA
L
CO
NF
INE
D E
LAS
TO
ME
RIC
PR
ES
SU
RE
PA
D
CY
LIN
DE
R
ST
AIN
ST
EE
L P
LAT
E
100
PTFE. However PTFE has poor creep propertiesi.e. it exhibits permanent compression underloads. It also has poor bonding properties, andtherefore, always used in thin sheets (upto 3 mm)recessed in a steel plate with half the thicknessof PTFE embedded. Addition of certain fillerssuch as glass fibre, graphite or bronze to PTFEincreases the wear resistance and creepproperties but it also increases the frictionalresistance. Fillers of molybdenum sulphide orsilica though improves wear and creep propertieswithout appreciable increase in friction. In bridgebearings, pure PTFE is therefore rarely used.
Although, friction between steel and PTFE is theminimum, yet it is highly susceptible to intrusionof dust. Elaborate arrangements are, therefore,must to prevent entry of dust particles on thesliding surface. Silicon grease is generally usedas a lubricant for PTFE surfaces. Dust seals arealso recommended around PTFE bearings toprevent the ingress of dust.
6.4 PERMISSIBLE BEARING PRESSURE ONPTFE
For the purpose of design, it is important to laydown permissible bearing pressure on PTFEsliding surfaces. There is no code of practiceavailable in India for this purpose. Reference maybe made to BS:5400 Section 9.1 and AASHTOspecifications of USA for this purpose. The valuesas given in Table 6.2 obtained from varioustechnical literature give an idea about the range ofthe permissible stress in various types of PTFE.
101
TABLE 6.2 PERMISSIBLE BEARING PRESSURE
Type of PTFE Average bearing Max. edgepressure pressure(MPa) (MPa)
Filled PTFE or
unfilled recessed 24.5 35PTFE
Unfilled PTFE 14 35(not recessed)
PTFE with bronze 42 70
PTFE perforated 35 35metal composite
BS:5400 limits the average pressure on PTFEwith glass fibres to 45 MPa and PTFE in a metalmatrix to 60 MPa. For any material other than theabove, permissible values should either beestablished by tests or the values recommendedfor unfilled PTFE should be adopted.
6.5 OTHER RECOMMENDATIONS FOR DESIGNOF PTFE SLIDING BEARING
1. Expansion bearings having sliding surfaces ofPTFE shall have a provision for minimum 0.01radians of rotation. This is to preventexcessive local stresses on the PTFE slidingsurface.
2. The minimum and maximum thicknesss ofPTFE sliding surface shall be as given inTable 6.3.
102
TABLE 6.3 THICKNESS OF PTFE SURFACES
Type of PTFE Minimum Maximumthickness thickness
Filled PTFE 0.8mm 2.4mm
Unfilled PTFE 0.8mm 2.4mm
PTFE with bronze 0.8mm 3.0mm
PTFE perforatedmetal composite 1.6mm 3.0mm
3. The contact surface of PTFE shall have aminimum Brinell hardness of 125 and surfacefinish of less than 20 microns.
4. Holes or slots shall not be made in the slidingsurfaces.
5. For calculating the pressure on PTFE, thecontact surface area may be taken as thegross area of the PTFE without making anydeduction for lubricating cavities, if any.
6. The stresses in the elastomer are limited bythe effectiveness of the seal preventing it fromsqueezing out. However, they should notexceed 40 N/mm2.
7. The lateral pressure exerted on the cylinder(POT) can be considered to be that producedby the pad acting as a fluid. The stressanalysis in the pot is very complex and shouldbe verified by testing.
103
6.6 DESIGN ASPECTS
Basic elements of a POT bearing are:
- POT or a shallow cylinder
- an elastomeric pad
- a set of sealing rings
- a piston
POT bearings are fixed against all translation unlessthey are used with a PTFE sliding surface.
The POT may either be one piece constructionshaped by machining or fabricated by weldingring on to the base plate. The elastomer padinserted into the POT is restrained from beingsqueezed out of the annular gap between theside wall and the piston by means of a set of twoor three flat brass rings. The circular rings havetraditionally been brazed into a closed circle,whereas the flat ones are usually bent from astrip and the ends are not joined. Brass ringsare placed in a recess on the top of theelastomeric pad. PTFE rings have been tried, buthave been abandoned because of their poorperformance. The cover piston which fits into thePOT is placed in contact with the elastomer pad.In the POT type sliding bearing the cover/piston ismounted by a sliding assembly. The uppersurface of the piston is recessed and filled withthe PTFE disc. The upper sliding plate is providedwith a sheet of stainless steel or chrome nickelalloy steel. The PTFE disc is provided with smallcavities (lubrication pockets) containing a speciallubricant which ensure life long lubrication of thesliding surface. An all around seal is provided to
104
prevent ingress of moisture and dust on thecontact surface as well as inside the POT.Generally four holes are drilled in the top plate aswell as the plate on which POT is mounted tofacilitate attachment of the POT bearings to thegirders and bed block.
The PTFE-steel contact surface can also be inthe form of two hemispheres in which case thebearing provides for rotation as well astranslation. The following types of actions arethere in POT bearings.
a) Compression : Vertical load is carried through thepiston of the bearing and it is resisted by theelastomeric pad. Elastomer is incompressible,though deformable. Due to this property, elastomerexerts pressure on the POT wall like a fluid.Deformation of the POT wall is a concern since thisdeformation changes the clearances between thePOT and the piston. Similarly in the downwarddirection, the base plate deformation causes thePOT wall to rotate inward.
b) Rotation : POT bearings are associated with largerotations which are accomodated by the deformationof elastomeric pad. Large cyclic rotations can bevery damaging to POT bearings due to abrasion andwear of the sealing rings and elastomeric pad. Infact, during rotation, the elastomeric padcompresses on one side and expands on the other,so the elastomer is in contact with the POT wall andslips against it. This causes elastomer abrasion andsometimes contributes to elastomer leakage. Tomitigate this problem, silicon grease is generallyused.
105
c) Lateral load : Lateral load is transferred from thepiston to the POT by contact between the rim of thepiston and the wall of the POT. The contact stressesare generally very high. Lateral loads may alsocontribute to increased wear of the elastomeric padsand greater potential for wear and fracture of thesealing rings.
The damage observed in tests suggests that thelateral load should be carried through an independentmechanism wherever possible.
Design aspects will become clear in design problemat the end of chapter.
6.7 INSTALLATION OF POT BEARINGS
In a sliding POT bearing, positive fixing to themain structure may not be required if the bearingis always subjected to adequate vertical loadingwhich is the case in most of the concretebridges. In such a case, any horizontal movementwill always occur on the plane of least resistancewhich is of course the sliding surface of bearing.However, it is prudent to provide some fixing toguard against displacement during installation,impact, vibrations and accidental loading.
Malfunctioning in a bridge bearing, in majority ofcases, can be traced to faulty installation andmuch of damage usually occurs duringinstallation, handling or storage. Carelesshandling on site and ingress of dirt can easilylead to abnormally high frictional resistance. Thesliding POT bearings are, therefore, normallydelivered at site with the top and bottom partsbolted together. The bearings should not bedismantled at site. The bearings should be
106
transported and unloaded carefully and storedunder cover in clean, dry conditions.
The seating provided under the bearing should beperfectly levelled. It is common to use a mortarbedding composed of sand-cement mortar withor without epoxy resin. A cube crushing strengthof 35 N/mm2 is usually recommended for thebedding mortar. While installing the bearing, thetransit bolts must be in position and these shouldbe removed after the mortar has set and beforethe bearing is called upon to slide or rotate.
POT bearings have been used in many importantbridges on Indian Railways. It was used, inaddition to many other bridges, in the constructionof 3rd Godavari Bridge at Rajamundry having bowstring arch girders of 90 m span, on Zuari andMandovi bridges on the KRCL having 120m spanopen web steel girders. Recently these have alsobeen used on a number of bridges on Jammu-Udhampur Rail Link Project (JURL) e.g. Tansibridge (71.4+102+71.4m), Dudhar bridge(64+92+64m) and E-18 viaduct (40+29.68m) etc.POT-PTFE bearings are being used onUdhampur-Srinagar section also. POT bearings,both fixed and sliding type can be used toadvantage in all situations where there is alimitation on overall height of the bridge girdercoupled with large force/movements involved.This is so because the POT bearings aresubstantially thinner as compared to rollerbearings. PTFE sliding sheets have also beenused for the launching of superstructures ofbridges in Konkan Railway and JURL with greatadvantage.
107
6.8 DESIGN SPECIFICATIONS FOR POT-PTFEBEARINGS
General Guidelines
- Provisions apply for temperature ranges of -10oC to+50oC.
- POT bearing of dia. up to 1500 mm are within scope ofthese specifications.
- Tensile load can’t be applied to bearings.- Rotation up to 0.025 radians only considered.
Confined elastomer used in metal POT isunreinforced. It allows only rotation by virtue ofdifferential compression. The popular property ofelastomer i.e. translation due to shearing strain isnot used here in POT-PTFE bearings due toconfinement.
Specifications
1. Design horizontal force generated due to friction atsliding surface shall in no case be less than 10% andgreater than 25% of design vertical load i.e. µ = 0.10 to0.25.
Moment will be induced due to stiffness of elastomerand friction at piston-cylinder contact. This may becomputed as under:
a) Induced moment resulting from resistance to rotationdue to effect of tilting stiffness of elastomeric pressure,is given as
Me,d
= di3 x (k1 q
p + K
2 q
v)
di = dia of elastomer pad in mm
k1, k2 = constants depending upon di/he as given inTable 6.4
he = thickness of elastomer pad in mm
108
qp = rotation angle in radian due to long term effectq
v= rotation angle in radian due to short term effect
TABLE 6.4 Values of constant k1 and k2
di/he k1
k2
15.0 2.2 101.0
12.5 1.8 58.8
10.0 1.5 30.5
7.5 1.1 13.2
b) Induced moment will also result due to friction atpiston-cylinder contact which is given byM
R,d = 0.2 x C x H
C = perpendicular distance from point of action ofhorizontal force on cylinder wall to the axis of rotation(mm) as shown in Fig 6.5.H = Design Horizontal force (N)
FIG. 6.5 MOMENT ARM FOR ROTATION RESISTANCE
c
DIRECTION OF HORIZONTAL FORCE
AXIS OF ROTATION
DIAMETER
DIRECTION OFHORIZONTAL FORCE
DIAMETER
AXIS OF ROTATION
109
c) Total induced moment
MT,d
= Me,d
+ MR,d
For pin bearings Me,d
= 0
Therefore, MT,d
= MR,d
Design values of rotation and translation movement willbe multiplied by a factor of 1.3.
2. Recommendations for confined elastomeric pad
(a) Permissible limits for confined pressure on elastomerpad depends upon effectiveness of internal sealpreventing it from extruding. Therefore, it shall beverified by load testing of assembled bearing.
(b) Dimensioning of pad should be such that at designrotation, the deflection at perimeter shall not exceed15% of pad thickness (he,eff) below internal seal, asshown in Fig. 6.6.
FIG. 6.6 DEFLECTION IN ELASTOMERIC PRESSURE PAD
he
< 0.15 x he,eff Rotation 0
he
,eff
he
di
φ
110
(c) Even then average stress in pad shall not exceed 35MPa and extreme fibre stress shall not exceed 40MPa. However minimum value, under any criticalcombination of loads, shall not be less than 5 MPa.
(d) The thickness of confined elastomeric pad shall not beless than 1/5th of its diameter or 16 mm whichever ishigher. Its diameter shall not be less than 180 mm.
3. Recommendations for PTFE
(a) It can be either of the two forms given below:
(i) Dimpled large sheet
It may be circular or rectangular divided maximum intofour parts. For dimpled sheets with smallest dimensionexceeding 100 mm (dia. or smaller side of rectangle),the contact area shall be taken as gross area withoutdeduction for the area of the dimples.
(ii) Arrayed (without dimples)
Distance between individual modules shall not begreater than 10 mm. Thickness of PTFE and itsprotrusion from recess depends upon maximum plandimension as given in Table 6.5.
TABLE 6.5 Thickness of PTFE and its Protrusion
Max. dimension Minimum Max.protrusionof PTFE (mm) thickness above recess
(Diameter or (mm) (mm)diagonal)
≤ 600 4.5 2.0
> 600, ≤ 1200 5.0 2.5
> 1200, ≤ 1500 6.0 3.0
111
(b) The coefficient of friction between stainless steel anduniformly lubricated PTFE will be as given in Table 6.6.
TABLE 6.6 Coefficient of friction between PTFE andStainless steel
Average pressure on Max. value of ‘µ’ ‘µ’ for
confined PTFE (MPa) for lubricated PTFE unlubricated PTFE
5 0.08 Double the
10 0.06 values for
20 0.04 lubricated PTFE
≥ 30 0.03
For design purpose, the value of ‘µ’ corresponding tounlubricated PTFE is taken into account.
(c) Average pressure on confined PTFE shall not exceed40 MPa and extreme fibre pressure shall not exceed 45MPa.
Corresponding values for confined elastomeric padare 35 MPa and 40 MPa
5. Permissible limits in bolt and screws for Class 4.6 (RefIS : 1367).
(a) Permissible stress in axial tension ≤ 120 MPaPermissible stress in shear ≤ 80 MPaPermissible stress in bearing ≤ 250 MPaFor higher class than 4.6, the above values shall bemultiplied by factor ‘x’
yield stress or 0.2% proof stress or0.7 x UTS (whichever is greater)
Where x = —————————————————235 MPa
112
(b) For bolts and screws subjected to both shear and axialtension.
Calculated shear stress Calculated tensile stress——————————— + ———————————— ≤1.4
Permissible stress in shear Permissible stress in tension
6. Permissible stress for welds
(a) Permissible stress in fillet weld based on its throatarea shall be 110 MPa.
(b) Permissible shear stress on plug welds shall be 110MPa.
No increase in permissible stresses is allowed forseismic, wind or any other load combination forelastomer pad, PTFE, bolt and screws or welds.
113
6.9 DESIGN OF POT- PTFE BEARINGS
In absence of 3D - FEM analysis, simplified design can beadopted. For POT bearings of vertical load capacity 7500KN or higher, the analysis should always be done using3D–FEM with authentic software.
Design steps
1. Effective contact area shall be calculated with loaddistribution of confined elastomer stress as 1V : 2 H,as shown in Fig. 6.7.
FIG. 6.7 LOAD DISPERSION THROUGH BEARINGCOMPONENTS
2. The confined elastomer pad is considered to act asfluid exerting fluid pressure under vertical load.
3. Total hoop tensile stress on cross section of cylinderwall will be due to -
21
21
EFFECTIVE CONTACT AREA
EFFECTIVE CONTACT AREA
EFFECTIVE CONTACT AREA
EFFECTIVE CONTACT AREA
114
(a) Fluid pressure due to elastomer(b) Horizontal force
The sum of both shall not exceed the permissiblestress value in axial tension.
(i) The hoop tensile stress di x h
e x σ
ce due to elastomeric bearing = ——————
2 x bp x h
c
where;d
i = Dia of elastomer pad (mm)
he = thickness of elastomer pad (mm)
σce
= fluid pressure in elastomer due to vertical loadb
p = thickness of cylinder wall (mm)
hc = Height of cylinder wall (mm)
Total force on cylinder wall = projected area x σce
Projected area = di x h
e
From Fig. 6.8 cp
ceei
hx b
σ x h x d2T =
FIG. 6.8 HOOP TENSION
Therefore, Hoop tension (T) = cp
ceei
hx b x 2
σ x h x d
T
T
Di
bP
115
(ii) The hoop tensile stress due to horizontal force
H= —————— 2 x b
p x h
c
4. Shear stress at cylinder wall and base interface will bedue to
(a) Fluid pressure
(b) Horizontal force
and the total shall not exceed the permissible value inshear.
(i) Shear stress due to fluid pressure h
e x σ
ce= ———— for 1 mm radial slice as shown in Fig. 6.9. b
p
Radial shear on 1mm radial slice = he x 1 x σ
ce
Resisted by a section = bp x 1 at interface
FIG. 6.9 CALCULATION OF SHEAR STRESS
therefore shear stress =p
cee
b
σ x h
1mm
ShearShearSHEAR SHEAR
116
(ii) Shear stress due to horizontal force
1.5 x H= ————
bp x d
i
Factor 1.5 has been taken due to parabolic distribution
5. Bending stress at cylinder and base interface shall alsobe due to -
(a) Fluid pressure
(b) Horizontal force
The total shall not exceed the permissible value ofbending stress.
(i) Bending stress due to fluid pressure 6 x σ
ce x h
e
2
= ————————— 2 x b
p
FIG. 6.10 CALCULATION OF BENDING STRESS
M at interface = 2
ece hσ 2× as shown in Fig. 6.10
P
ce
1
b
hc he
B M DBENDING MOMENTDIAGRAM
σce
117
Bending stress at interface
f = I
yx
heceσ
2
2×
3
12pb1
I×
=
2
by
p=
Therefore,
3p
pece
b
12x
2
bx
2
.hf
2
σ=
=
2
2p
e
b x 2
hceσ x 6 ×
(ii) Bending stress due to horizontal force
= 2pi b x d
ha x H x 6 x 1.5
Where, ha = Height of application of design horizontalforce ‘H’ in N from cylinder wall above base interface inmm.Force H is acting on entire section width of d
i
Therefore, force per unit length = id
H
Max force per unit length = id
H1.5 (considering
parabolic distribution)
Therefore, M at interface = id
H1.5 x ha
118
Bending stress = I
M.y =
id
H1.5 x ha x
2
bp x 3
pb x 1
12
= 2pi b x d
ha x H x 6 x 1.5
6. Combined bending and shear shall also be checkedsuch that the equivalent stress due to co-existingbending and shear stress shall not exceed 0.9 fy.
Equivalent stress when bending is in tension
= 2)nsiontressin.ted.bendings(Calculate2
stress)ted.shear.3x(Calcula +
or
Equivalent stress when bending is in compression
= 2comp)stress.in.d.bending.(Calculate
2stress)ted.shear.3x(Calcula +
7. Minimum dimensions in POT bearing
(a) Minimum thickness of cylinder base shall not beless than 2.5% of inner diameter of POT cylinder.
(b) Minimum thickness of sliding component ofstainless steel shall not be less than 2.5% of max.dimension (dia. or diagonal) in plan.
(c) Min. thickness of any steel component shall in nocase be less than 12 mm.
8. Other minimum dimensions
(a) Minimum theoretical depth of effective contactwidth of piston at design rotation shall not be lessthan 5 mm.
(b) Minimum theoretical clearance between the topedge of cylinder and the bottom edge of piston atdesign rotation shall not be less than 5 mm.
119
(c) For sliding component, the stainless steel surfacealways overlap the PTFE even when the extrememovement occurs. The welded connection withbacking plate should be designed to withstand onlythe force generated at sliding interface due tofriction. The thickness of stainless steel plate shallbe guided by the requirement of proper welding andin no case shall be less than the thickness of weldor 3 mm (whichever is higher).
6.10 DESIGN OF GUIDES
1. Sliding surface for guides and sliding assemblies shallbe made of one of the following:
(a) Stainless steel sliding on confined PTFE(i) ‘µ’ to be taken for unlubricated PTFE
(ii) Average pressure on PTFE should not be > 40 MPa
Extreme fibre pressure should not be> 45 MPa
(b) Stainless steel sliding on stainless steel(i) ‘µ’ = 0.2
(ii) Bearing pressure should not be >150 MPaBut it may be increased by 33.33% wheneffect of wind or EQ is taken into account.
(c) Stainless sliding on composite material(i) ‘µ’ = 0.05
(ii) The pressure on composite material shallnot exceed 70 MPa.
2. Guide shall be in the form of one guide bar locatedcentrally or two guide bars attached side wise to thesliding plate. But one thing is certain that it shall bemonolithic to the component to which it is connected,thickness of which shall not be less than thickness ofguide along direction of horizontal force acting on theguide.
120
3. For central guides(a) The thickness of the recessed portion of slidingplate shall not be less than
0.3 x thickness of sliding plate or0.3 x width of recess or12 mm
(b) The vertical clearance between the guide and therecessed sliding plate shall not be less than
0.2 % of length of recess or2 mm
6.11 DESIGN OF ANCHORING ARRANGEMENT
Aim : Bearings should be replaceable with minimum lifting of the superstructure
1. The horizontal force transmission capacity of theanchorage shall be considered as the sum of frictionforce developed at the interface and the capacity of theanchorage to resist shear force. Here ‘µ’ will be takenas 0.2
2. (a) The diameter of anchor sleeves should not be < 2 x nominal diameter of bolts/screws.
(b) The length of sleeve should not be > 5 x diameter.
3. Peak stress on concrete adjacent to the anchor sleeveshall be calculated using the following expression.
L x D
δ x A x 3σ
cpu=
σcpu
= peak stress in concrete behind anchorsleeve (MPa)
A = Effective tensile area of bolt/screw (mm2)δ = permissible shear stress in bolt / screwD = Diameter of sleeve (mm)L = Length of sleeve (mm)
121
4. Specifications for bolt / screws
(i) The threaded fastening length of bolt / screw (of class4.6) should not be < 0.8 x nominal dia.For higher class, it will be proportionately adjusted.
(ii) Edge distance should not be < 1.5 x dia. of hole.
(iii) Centre to centre distance of bolt/screw should not be <1.5 x dia. of hole.
(iv) a) Anchor studs shall be made of forged steel.
For adjacent structure made of concrete of grade M35and above -- Length of stud should not be < 6 x dia. of stud- Dia of stud head should not be < 2.5 x dia. of stud- Thickness of stud head should not be < 0.4 x dia. of
stud- Stud head shall be always monolithic to the studs.
b) Centre to centre distance between studs should notbe < 3 x dia. of studs.
For manufacturing tolerances, manufacturing methods,finishing, acceptance specifications, installation andmaintenance, IRC-83 (Pt.III) may be referred.
122
6.12 SAMPLE DESIGN PROBLEM FOR POT-PTFEBEARINGS
Problem
Design the bearing for 76.2 m open web girder. Assumesuitable related data and refer IRS : Bridge Rules and IRC-83 Pt.- III. Assume seismic zone-IV.
Effective span = 78.8 mD.L. of girder + track + gangway etc = 405 tLive load for shear = 980.024 t
From Bridge Rule Appendix- XII (H M Loading standards)EUDL for shear = 980.024 t (corresponding to Span78.8 m)
Assumed data :Vertical load due to wind / bearing = 16.84 tLateral load due to wind / bearing = 24.2 tLateral load due to seismic effect = 25.18 tLongitudinal force / bearing = 67.5 tMax. horizontal movement = 63.5 mmTotal vertical load including wind only = 423.123 tTotal vertical load including seismiceffect only = 454.613 tDeflection at center = 116 mm
Rotation = 116/39400 = 0.0029 radian < 0.025, therefore allprovisions of IRC 83 - (Pt-III) can be applied.
A. Design of elastomeric pad
Guiding considerations :
••••• Minimum dia. should be 180 mm. Also the areashould be sufficient such that average verticalpermissible stress should not be > 35 N/mm2
••••• Min. average pressue should not be < 5 N/mm2
••••• Extreme fibre stress should not be > 40 N/mm2
••••• Thickness of elastomeric pad should be minimum of
(a) Dia. / 15
123
(b) 16 mm
Add 6 mm for brass ring and 25% provision for creep.
••••• Deflection at perimeter should not be > 15% of padthickness.
1. Maximum vertical load = 454.61 t (including seismiceffect)Average permissible stress = 35 N/mm2
P —————— = 35
π/4 x d2
d = 406.77 mm
Provide d = 490 mm.
2. P min = 405/4 = 101.25 t
Min. Avg. Stress = 101.25 x 10000 / π/4 x 4902
= 5.36 N/mm2 > 5 N/mm2 ............… OK
3. Max. horizontal movement = 63.5 mm at guided slidingendMax. eccentricity = 63.5/2 + 10% for unserviceableservice conditionMoment due to this eccentricity = 454.61 X 0.035= 15.91 tm
Extreme fibre stress = P/A + M/Z
454.61x 104 15.91x 107
= —————— + —————— π/4 x 4902 π/32 x 4903
= 37.86 N/mm2 < 40 N/mm2 O.K.
4. Thickness should be greater of
a. Dia of elastomer/15 = 490/15 = 32.67 say 35 mm.b. 16 mm (minimum)c. Extra 6 mm for brass ring – 3 Nos. and 25% for creep∴ Minimum thickness = (35 + 6) 1.25
= 51.25 mm Say 55 mm
124
5. Deflection at perimeter = Max. 15% of pad thickness= 0.15 x 55 = 8.25 mm
Actual rotation = 0.16869 degreesActual deflection at perimeter = 490/2 x tan 0.16869= 0.7213 < 8.25 … OK
B. Design of confined PTFE
••••• Average pressure on confined PTFE should not be >40 MPa, and
••••• Extreme fibre pressure should not be > 45 MPa
There are two alternative for shape of PTFE:
Alternative 1 Square shape of sheet
Considering square sheet of 450 x 450 mm2
Average pressure = 450450
10000454.61
×
×
= 22.45 N/mm2 < 40 MPa ………. OK
Extreme fibre stress = P/A + M/Z
454.61x 104 15.91x 107
= —————— + —————450 x 450 4503 /6
= 32.93 N/mm2 < 45 N/mm2 ....OK
For max. dimension of PTFE < 600 mm
Min. thickness = 4.5 mm Adopt 5.0 mm
Max. protrusion above recess = 2.0 mmAdopt = 2.0 mm
Alternative 2 Round shape of sheet
Consider dia. = 520 mm
Av. pressure = 454.61x 104
—————— = 21.40 N/mm2
π /4 x 5202
< 40 N/mm2 ……… OK
Extreme fibre stress = P/A + M/Z
125
= 2X520
32
π
715.91X10
2X5204
π
4454.61X10+
= 21.40 + 11.525 = 32.925 < 45 MPa … OK
Adopt 4.5 mm thick pad with max. protrusion as2.0 mm.
C. Design of POT cylinderGuiding considerations :
••••• Resultant force due to longitudinal and lateral forceshould not be less than 10% and greater than 25% ofdesign vertical load.
••••• Permissible axial tension due to hoop shall not exceed0.6 x fy.
1. Longitudinal force per bearing = 67.5 tLateral force due to wind = 24.2 tHorizontall seismic force = 25.18 t
Resultant with wind pressure = 2 267.5 24.2+= 71.71 t
Resultant with seismic force = 2 267.5 25.18+= 72.26 t
Out of wind and seismic force, only one is assumed toact at one time. (greater of above two adopted).
Design horizontal force H = 72.26 t (adopted)
72.26 should be < 25% of 454.61 = 113.65 t
and also,
72.26 should be > 10% of 454.61 = 45.46 t
Both satisfied .....OK
2. Axial tension in cross section of cylinder wall will bedue to -
a. Fluid pressureb. Horizontal force of 72.26 t
126
a. Fluid pressure = cp
ceei
h x 2b
σ x h x d
di = dia of confined elastomer pad = 490 mm
he = thickness of confined elastomer pad = 55 mm
σce
= fluid pressure in elastomer due to vertical load
= 2490 x 4 / π
410 x 454.61 = 24.12 N/mm2
bp = thickness of cylinder wall
hc = height of cylinder wall
fluid pressure = cp hb2
24.1255490
××××
……………. (I)
b. Axial or hoop tensile stress due to H = cp h x b x 2
H
=c p hx 2b
410 x 72.26 ……….…………. (II)
Total (I) + (II) = cp
4
h x b
10 x 104.76 N/mm2
This value should not exceed 0.6 fy or 0.6 x 280= 168 Mpa
Therefore cp
4
h x b
10 x 104.76= 168
∴ bp x h
c = 6235.72
168
4104.76x10=
taking bp = h
c
= 72.6235
= 78.9 mm Adopt bp = h
c = 90 mm
127
D. Design of base plate of POTAssume base plate thickness = 65 mmDispersion of stress through 65 mm plate at 1V : 2 HDia. at base resisting pressure = 490 + 2 x 2 x 65= 750 mm
Max. pressure = P/A + M/Z
Min. pressure = P/A – M/Z
Max. pressure = 332
7
24
4
750/
1091.15
750/
1061.454
x
x
x
x
ππ+
= 10.29 + 3.84 = 14.13 N/mm2
Min. pressure = 10.29 – 3.84 = 6.45 N/mm2
Moment at XX due to upward force as shown in Fig.6.11
= 30x130x2/3x12
12.8014.13
2
13012.80x130x
−
+
↑
↓
12.8
0
FIG. 6.11 MOMENT DUE TO UPWARD FORCE
128
= 108160 + 7492.33 = 115652 N-mm
Thickness of plate required:
Taking 1 mm width, 6xt1
Z2
= or t = Z6
M/Z = f = 0.66 fy (fy = 280 MPa)
= 184.8 N/mm 2
Z = M / 184.8 = 115652 / 184.8 = 625.822
t = Z6
= 61.277 mm Adopt = 65 mm
E. Checks for cylinder
(a) Check for shear stress
Shear stress at interface of cylinder wall and base willbe due to fluid pressure and horizontal force.Considering 1 mm slice of cylinder.
(i) fluid pressure = 1 x b
1 x x h
p
cee σ
= 90
24.12 x 55
= 14.74 N/mm2
(ii) Horizontal force = pi b x d
H x 1.5
(Factor 1.5 for parabolic distribution and total H will beresisted by d
i x b
p)
= 90 x 490
10 x 72.26 x 1.5 4
= 24.58 N/mm2
129
Total shear stress = 14.74 + 24.58 = 39.32 N/mm2
< 0.45 fy or 126 N/mm2 ……. OK
(b) Check for bending stress
Bending stress at cylinder and base interfaceconsidering 1 mm radial slice of cylinder due to
(i) fluid pressure = Z
M =
/6b
2
hh x σ
2p
eece
= /690 x 2
55 x 24.122
2
= 27.02 N/mm2
(ii) Horizontal force = /6b
hx d
1.5H
2p
ai
ha = height of line of application of design horizontalforce from cylinder wall above base interface in = 75mm
= /6290 x 490
75x 410 x 72.26 x 1.5
= 122.891 N/mm2
Total bending stress = (i) + (ii) = 149.914 N/mm2
< 0.66 x fy or 184.8 N/mm2 ……………….. OK
(c) Check for combined bending & shear
= 22 ressbending.stssShear.stre +
= 22 149.9139.32 +
= 164.655 N/mm2 < 0.9 fy or 252 ……….. OK
130
F. Design of side guides for sliding of POT – PTFEbearing
Size of side guide = 350 x 50 x 30
1. Lateral load due to wind = 24.2 t
2. Horizontal seismic force = 25.18 t
Greater of the two i.e. 25.18 t is adopted
(a) Check for shear stress
Shear Stress < 0.45 fy
50350
1018.25 4
x
x < 0.45 x 230
14.39 N/mm2 < 103.5 N/mm2 ……………. OK
(b) Check for bending stress
Moment at - XX
M = 25.18 x 104 x 30
Z = 6
50 x 350 2
Z
M = 51.8 N/mm2 < 0.66 fy
< 151.8 N/mm2 …………..OK
(c) Check for combined bending and shear
Total stress = 22 ressbending.stssshear.stre x 3 +
= 22 51.814.39 x 3 +
= 57.4844 N/mm2 < 0.9 fy or 207 N/mm2
…… Hence OK
131
CHAPTER 7
EMERGING TRENDS IN BEARINGS
7.1 GENERAL
New innovations have taken place in the area ofbridge bearings, notable among them are:
(1) Shock Transmission Unit (STU) or Lock UpDevice (LUD)
(2) Seismic Isolation Bearings (SIB)
7.2 SHOCK TRANSMISSION UNIT
Ever since engineers have started designing multispan simply supported bridges, they have beendreaming of connecting all the spans together todistribute seismic loads to more than one pierduring an earthquake or other sudden loadings.Thus, the peaks due to sudden force isconsiderably reduced.
One possible solution to this requirement couldbe continuous construction of bridges so that alarge number of piers can participate in loadsharing during earthquake. But the requirementof size of expansion joint in such an arrangementwould be extraordinarily large and thus notfeasible.
The other solution has been provided by ShockTransmission Unit (STU) or Lock Up Device(LUD). It is a simple device which is designed to
132
be connected between superstructure andsubstructure of a bridge to form a temporary rigidlink as shown in Fig. 7.1. The peculiar feature ofthis device is that it forms a rigid link only underrapidly applied loads such as braking and seismicforces whereas under slowly applied loads suchas temperature, creep, shrinkage etc, it allowsthe free movement.
FIG. 7.1 CONNECTION OF STU
STU, thus, allows load sharing in case ofsuddenly applied short duration horizontal loads.The unit is connected near the bearing betweenthe superstructure and substructure. It works onprinciple that rapid passage of viscous fluidthrough a narrow gap generates considerableresistance while slow passage through the samegap only minor resistance as shown in Fig. 7.2.
133
In this figure e1 and e2 are the movementsaccomodated in STU due to superstructure andsubstructure respectively.
FIG. 7.2 PRINCIPLE OF WORKING OF STU
Use of an STU was first made by Steinman, thedesigner of Carquinez Bridge in California in 1927and thereafter it was used in many bridges in US,Europe, Netherlands and UK. In India STU has beenused for the first time in the Second Bassein CreekRoad Bridge, Mumbai. It has not yet been tried onIndian Railways.
7.2.1 Description : The STU as shown in the Fig. 7.2consists of a machined cylinder with a transmissionrod that is connected at one end to the structure andat the other end to the piston of the cylinder. Insidethe cylinder a specially formulated Siliconecompound is filled in an unpressurized condition
LUD
e1 e2→ →← ←
134
instead of normally used hydraulic oil.
The compound doesn’t adjust quickly toaccommodate the movement and therefore locksup under the action of a sudden load. Under slowacting thermal, creep or shrinkage movements,the liquefied compound gradually migrates fromone side of piston to the other and allowing thepiston and cylinder to expand or contract with thestructure.
7.2.2 Advantages :
(a) STU is simple to install with minimal trafficdisruption.
(b) STU can be used for strengthening of existingbridges also. This option of strengthening is moreeconomical and feasible than structural methods.
(c) Efficient design of new continuous bridgesdue to reduction in structural component sizes.
(d) STU can also be used advantageously forexpansion joints as shown in Fig. 7.3
FIG. 7.3 STU FOR EXPANSION JOINTS
7.2.3 Limitations : STUs are not intended for energy/shock absorption. They simply transmit load fromone part of the structure to another thus sharing
135
the load among various spans.
7.2.4 STU on second Bassein Creek Road Bridge,Mumbai : In India, the Second Bassein CreekBridge is a good example of a new multi spancontinuous bridge where installation of STUs havesaved time and money in construction of caissonfoundations.
The bridge is located 25 m upstream of existingbridge on Mumbai-Ahmedabad NH-8 nearGhodbunder at the confluence of Ulhas river andthe Arabian Sea. Being a creek area with twolow and high tides, the site is very difficult forconstruction of caissons. Average daily tidalvariation is of the order of 4.25 m with an averagevelocity of 2.4 m/s. All these conditions at sitemade the caissons construction a risky anddifficult task.
The bridge was designed for vessel collision foran impact force of 5000 KN at well cap,horizontal seismic coefficient of 0.075 andimportance factor of 1.5. The central navigationalspan being 114.7 m and earthquake shock as afunction of the deck mass, the caisson foundationwith fixed pot bearing was to be designed for9300 KN seismic horizontal loading. Duringpreliminary design it was found that for the aboveloading of 9300 KN, the size of caisson requiredwas 16.5 m for P4.
The engineers immediately realized the difficultyin constructing such a large 16.5 m caisson inthis difficult creek conditions combined with veryhigh cost. This was the situation where it wasdecided to incorporate Shock Transmission Uniton P3 and P5 to create temporary fixity at these
136
piers during seismic loading. The STU on piersP3 and P5 would lock up during an earthquakethereby distributing the seismic loading of about9300 KN among P3, P4 and P5 according to theirstiffness.
This resulted into a massive reduction of thediameter of Caisson for pier P4 from16.5 m to12.5 m This not only made the Caissonconstruction manageable but also resulted intoconsiderable cost saving.
7.2.4.1 Type : Presently two types of STUs are availablein the overseas market:
(a) Silicon putty based from UK and USA
(b) Oil based from some countries in Europe
The disadvantage of oil STU is that oil is morelikely to leak out rapidly from the cylinder thanSilicon putty, which is like dough.
7.2.4.2 Cost : There is a keen competition among thefew STU manufacturers. For the second BasseinCreek Bridge, the average cost of two STUs ofcapacity 3100 KN was US $100,000 includingsupply at site, specified tests at UK, bracketsholding down bolts airlifted from UK to the siteand assistance of a qualified supervisor for theinstallation of STUs at the site.
7.2.4.3 Basic requirement of Design : The STU mustwithstand the specified design load. The stroke ofthe piston must be bigger than the movement ofdeck due to temperature, shrinkage & creep ineach direction of the deck’s longitudinalmovement. Just to give an idea about themovement of deck to the readers, the movementof the deck was for +40 mm to –90 mm.
137
The maximum life expected for an STU is around75 years subject to appropriate maintenancebeing performed by the user.
7.2.4.4 Critical factors in Design : (a) STUs areunidirectional devices for load transmission. If outof plane forces will be acting on the STU, theangular limits of these loads must be determinedto assure that the STU can withstand them.Generally ball joints and or additional devices canbe incorporated to properly transfer these forces.
(b) For rail road bridges, the traction forces mayequal or exceed seismic forces in magnitude andapplied many times over the life of the structure.
(c) The corrosion protection for STU is veryimportant. The position rod, in particular, shouldbe protected from environment by a scaled coverable to accommodate the full stroke of the STU.
7.2.5 Load testing of STUs :
a) Seal wearb) Fatigue loadc) Drag loadd) Dynamic force transfere) Cyclic load
a) Seal wear test : Seal prevents the leakage ofsilicon putty in the cylinder. The test is conductedfor 30000 cycles assuming that the piston of STUmoved for the full design range in 24 hrs. 30,000cycles equals 82 years which is more than theexpected life of STU which is 75 years.
b) Fatigue load test : This test was carried out for100,000 cycles for maximum capacity of STU,assuming the worst case that the ‘Lock Up Load’
138
is applied to STU due to braking load four times aday for specified design life of 75 years.
c) Drag load : This test ensures that excessiveload should not be transmitted to the structure bythe STU. As per the test the STU shall have animpressed deflection of ‘0 mm – 40 mm – 0 mm– 40 mm – 0’ mm in not less than 10 hrs andnot more than 24 hrs. During the impresseddeflection cycle the STU shall develop no moredrag force than 10% of the maximum designcapacity of 3100 KN.
d) Dynamic force transfer test : This test mainlyprove the lock up capability of the device.TheSTU shall be loaded in tension from zero to thefull design load in less than 0.50 seconds and theforce sustained for 5 seconds.
Then the load shall be reversed to the full designload in less than 0.50 seconds and held again for5 seconds. The acceptance criteria for the abovetest shall be that the deflection during the loadingof the positive force and the negative force shallbe no greater than 6 mm and the deflectionduring sustained load portion shall not exceed 3mm.
e) Cyclic force transfer test : This test isspecified to prove that the STU would function asintended during a seismic event. The STU shallbe tested by applying 50 sinusoidal cycles of loadranging between the maximum design tensionand compression forces at a frequency of 1 Hz.There shall be no visible signs of distress ordegradation as a result of 50 cycles of loading.
7.2.6 Installation of STU : It is not always necessary to
139
install STUs while a structure is underconstruction but the holding down anchor boltsand the brackets are required to be fixed instructure during construction. There are twoways of connecting STUs to a structure.
a) One way is to connect the superstructureelement together normally at the expansionjoints.
b) The other way is to connect superstructureelements to substructure elements.
The first type of connection may be more helpfulfor railway bridges where excessive longitudinaltraction and braking load is applied to oneparticular viaduct and the same is required to betransmitted through STUs connected across theexpansion joints for sharing the load with adjacentunloaded viaducts.
The second type of connection is used for loaddistribution in continuous bridges.
7.3 SEISMIC ISOLATION BEARINGS
Many bridges had been built before the properseismic design specifications were known to us.These bridges require strengthening andenhancing the ductility of substructures in order tosustain the earthquake forces. Conventionalretrofitting methods are costlier (more than 3 times)than providing ‘Seismic Isolation Bearings’ (SIB).The studies have shown that:
(a) The SIB eliminated the in-plane torsional rotationof the bridge and resulted in a more uniformdistribution of seismic forces among substructuresin the transverse direction.
140
(b) The modifications in the response of the bridgeresulted as a result of use of SIB because theeffective fundamental period of the bridge getelongated due to the flexibility provided by the SIB.
(c) Average retrofitting cost using SIB is calculatedas only 30% of that using conventional retrofittingmethods.
An isolation bearing induce sufficient lateralflexibility in a bridge structure and increase theperiod of vibration, resulting in a reduced forceresponse. The increased lateral flexibility isassociated with larger horizontal displacements atthe bearing level.
In the longitudinal direction, the stiffness of a bridgewith isolation bearings is the sum of the stiffness ofall piers and abutments including the bearings. In abridge with conventional bearings. However, thelongitudinal stiffness incorporates the stiffness of afixed support alone.
The isolation bearings should also include arestoring force to return the superstructure to itspre-earthquake position.
7.3.1 Types of seismic isolation bearing : Seismicisolation is obtained by two types of isolationbearings.
a) Rubber bearings
b) Friction pendulum bearings
Rubber bearings are composed of a lead case andnatural or synthesized rubber with embedded
141
layers of steel plates as shown in Fig. 7.4.
FIG. 7.4 RUBBER BEARING
The force-displacement behaviour of this bearingis elastic under service load and inelastic during anearthquake when the lead case deforms beyond itselastic limits. In fact, the bearing distribute theinertial forces from the deck level to all supportingsubstructures on the basis of their relative stiffness.
Rubber bearings are meant to shift the vibrationalperiod of the structure so as to avoid resonancewith the excitations. These are usually combinedwith high damping material to prevent the isolatedstructures from over-displacing.
In friction pendulum bearings, the lateraldisplacement is permitted at the interface of thebearing element between the superstructure andthe substructure. Lateral movement at a slidingsurface is associated with friction forces that
F
SteelRubber
Lead
F
F(vert)
142
oppose movement as shown in FIG. 7.5
FIG. 7.5 FRICTION PENDULUM BEARING
The value of the friction force depends on variationfactors, including the vertical reaction at the slidingsurface, contact pressure, ambient temperature,speed of the movement and the condition of thesliding surface.
As shown in Fig. 7.5, the relative displacementbetween the sliding interfaces is u 1 - u 2 and thepotential energy Vg due to gravity accumulated duringthe lift-up is
−−= 2R/
2
2u
1u1 WRgV
where W is the weight supported by the bearing, Ris the radius of curvature of the sliding surface asdepicted in Fig. 7.5.
The friction force between the sliding interfaces playsthe roll of energy dissipation during the sliding motion.
u1
u2
u -u1 2
Bridge Superstructure
Pier
u -u1 2
W R
BRIDGESUBSTRUCTURE
143
LIST OF REFERENCES
1. IRS- Steel Bridge Code: Code of Practice for designof Railway Steel bridges.
2. IRS - BI :79: Code for design of steel bridges,Research Design and Standards Organization,Indian Railways, Lucknow, India.
3. Bridge Rules, Ministry of Railway (Railway Board)
4. IRC: 83 (Part I): Standard specification & code ofpractice for road bridges, Part I - 1982 Metallicbearings, Indian Roads congress, New Delhi, India.
5. IRC; 83 (Part II): Standard specification & code ofpractice for road bridges, part II- 1987 Elastomericbearings, Indian Roads Congress, New Delhi, India.
6. UIC 772 - RC: Code for the use of rubber bearingsfor rail bridges - 1969, International Union ofRailways, Paris, France.
7. BS: 5400 : Steel, concrete & composite bridges,Section 9.2 - 1983 specification for materials,manufacture and installation of bridge beariangs,British Standards Institution, London UK.
8. BS: 5400 : Steel, concrete & composite bridges,Section 9.2 - 1982 specification for materials,manufacture and installation of bridge beariangs,British Standards Institution, London UK.
9. ORE Report D - 60 Application of rubber for bridgesupporting plates
144
10. Proceedings of International Conference on Bridgesand Fly over bridge bearings - National cooperativehigh way research programme synthesis of high waypractice.
11. Concrete Bridge Practice - Analysis, design andeconomics - By V. K. Raina
12. AASHO : Standard specification for highway bridges,The American Association of State Highway Officials,Washington, U SA.
13. Retention of old MG bridges for Heavier BG loadingby use of Elastomeric bearings’’ By S.R. Agarwal andAdesh Shrama International Seminar on Failures,Rehabilitation and Retrofitting of Bridges &Aqueducts, IIBE, 17-19 Nov. 1994 Bombay.
14. Internet site : http://www.new-technologies.org/ECT/Civil/lud.htm
15. Internet site : http://ww.en.wikipedia.org
16. Design of sliding and fixed POT-PTFE bearing for76.2 m for HMLS - RDSO, Lucknow
17. American Iron and Steel Institute: Steel Bridge Bearingselection and design guide
18. Journal of Bridge Engineering: Efficiency of SeismicIsolation for Seismic retrofitting of heavysubstructured bridges. Vol.10, No.10, July/August2005
19. Bridge Inspection and Rehabilitation - A practicalGuide by Parsons Brinckerhoff.
Price Rs. 50/-
Top Related