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 onEmerging 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 Bridgespublished 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 onEmerging 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 11.2 Classification of Bearings 4

1.2.1 Degree of Freedom 41.2.2 Material Used 51.2.3 Types of Bearings 6

1.3 Selection of Bearings 111.3.1 Functional Requirement 111.3.2 Expected Life 111.3.3 Maintenance Efforts 111.3.4 Cost 141.3.5 Other Factors 14

1.4 Minimizing the Requirement of Bearings 15

CHAPTER - 2 SLIDING BEARINGS2.1 General 182.2 Different Type of Sliding Bearing 182.3 Parts 20

CHAPTER - 3 ROCKER & ROLLER BEARINGS3.1 General 23

3.1.1 Parts 233.1.2 Connections 253.1.3 Segmental Rollers 263.1.4 Oil Baths 27

3.2 Design Aspects 293.3 Installation 34

ix

CHAPTER - 4 MAINTENANCE OF STEEL BEARINGS4.1 General 38

4.1.1 Lifting of Girders 384.1.2 Cleaning and Greasing of Steel Sliding

Bearings 424.1.3 Cleaning and Greasing of Rocker & Roller

Bearings of Open Web Through Girders 444.1.4 Method of Greasing 45

CHAPTER - 5 ELASTOMERIC BEARINGS5.1 General 475.2 Properties of Elastomer 485.3 Behaviour of Elastomeric Bearings 515.4 Types of Elastomeric Bearings 555.5 Design of Elastomeric Bearings 56

5.5.1 Flow Table of Design 565.5.2 Input Data Required 595.5.3 Output Expected 59

5.6 Manufacture and Quality Control 605.6.1 Properties of Elastomer 605.6.2 Dimensional Tolerances 60

5.7 Inspection and Testing 625.7.1 Lot Size 625.7.2 Level 1 Accpetance Testing 635.7.3 Level 2 Acceptance Testing 715.7.4 Inspection and Quality Control Certificate 71

5.8 Installation 745.8.1 General Guidelines 745.8.2 Process of Installation 76

5.9 Periodical Inspection and Maintenance 775.10 Elastomeric Bearings in Aid of Old Substructures 78

x5.11 Anti-Slip Devices 815.12 Sample Design Problem for Elastomeric Bearings 84

CHAPTER - 6 POT BEARINGS6.1 General 936.2 POT-PTFE Bearing vs Elastomeric Bearing 946.3 Properties of PTFE 956.4 Permissible Bearing Pressure on PTFE 1006.5 Other Recommendations for Design 101

of PTFE Sliding Bearing6.6 Design Aspects 1036.7 Installation of POT Bearings 1056.8 Design Specifications for POT-PTFE Bearings 1076.9 Design of POT- PTFE Bearings 1136.10 Design of Guides 1196.11 Design of Anchoring Arrangement 1206.12 Sample Design problem for POT-PTFE Bearings 122

CHAPTER - 7 EMERGING TRENDS IN BEARINGS7.1 General 1317.2 Shock Transmission Unit 131

7.2.1 Description 1337.2.2 Advantages 1347.2.3 Limitations 1347.2.4 STU on second Bassein Creek Road 135

Bridge, Mumbai.7.2.4.1 Type 1367.2.4.2 Cost 1367.2.4.3 Basic Requirement of Design 1367.2.4.4 Critical Factors in Design 137

xi

7.2.5 Load Testing of STU 1377.2.6 Installation of STU 138

7.3 Seismic Isolation Bearings 1397.3.1 Types of Seismic Isolation Bearing 140

xii

LIST OF ABBREVIATIONS

AASHTO American Association of State Highway andTransportation Officials

BM Bending MovementBS British StandardBSC Bridge Standards CommitteeCDA Coefficient of Dynamic AugmentDL Dead LoadEUDL Equivalent Uniformly Distributed LoadHMLS Heavy Mineral Loading StandardsIIBE Indian Institute of Bridge EngineersIRC Indian Road CongressIRHD International Rubber Hardness DegreesIRS Indian Railways SpecificationIS Indian StandardsKN Kilo NewtonLF Longitudinal ForceLL Live LoadLUD Lock Up DeviceLWR Long Welded RailMBG Modified Broad GaugeMPa Mega PascalMS Mild SteelORE Office of Research and ExperimentationPSC Pre-stressed ConcretePTFE Poly Tetra Fluoro EthyleneRBG Revised Broad GaugeRCC Reinforced Cement ConcreteRDSO Research Design and Standards OrganisationSEJ Switch Expansion Joint

xiii

SIB Seismic Isolation BearingSTU Shock Transmission UnitUIC International Union of Railways (Translated from

French)

xiv

LIST OF SYMBOLS

= Shear stress in elastomer due to rotationh = Shear stress in elastomer due to horizontal forcep = Shear stress in elastomer due to compressive forceb = Permissible bearing pressure in Bed block material

m= Max. Permissible pressure in elastomer

s

= Permissible tensile stress in the steel plate

min = Minimum stress in elastomer

max= Maximum stress in elastomer

c

= Rotation under effect of slow acting loads

s= Rotation under effect of quick acting loads

a = length of the bearing along the spanb = width of the bearing across the spanB = width of the girder/beamE

a= Modulus of elasticity of elastomer

ei = Compression of elastomeric layerfck = Grade of concreteG = Shear modulus of elastomerH

c= Slowly applied horizontal load

Hs

= Quickly applied horizontal loadn = number of layers of elastomerP

c= Slowly applied normal (vertical) load

Ps

= Quickly applied normal (vertical) loadS = Shape factor of elastomerhi = thickness of each layer of elastomer

xv

h = Total thickness of elastomerts

= Thickness of steel plate in the elastomeric bearingU

c= Horizontal movement due to slow acting load

Us

= Horizontal movement due to quick acting loadD1, D2 = Diameter of contact surfaces in roller bearings

1CHAPTER 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

BEARINGBED BLOCK

PIER / ABUTMENT

FOUNDATION

}}

SUPERSTRUCTURE

SUBSTRUCTURE

2a) 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 thatBearings assume the functionality of a bridge byallowing translation and rotation to occur whilesupporting the vertical loads. In nutshell, the

3various 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-DIn 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

4With 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 upona) Degree of freedomb) Material usedThese 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)

5Rotation 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.

6TABLE 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.

7OUTERBEARINGPLATES

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

8OUTER 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

9PISTON

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

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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 SPANTHEORETICAL 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 SEC

TIO

N A

T '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

SADDLEKNUCKLE

KNUCKLE SLAB

ROLLER

EXPANSION BASE

FIG. 3.2 ROLLER BEARING AT FREE END

KNUCKLESADDLE

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 tostopper 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 toGuide 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 of

bolt (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 ASPECTSThe 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 surfacei) For single and double roller

( )21 1/D 1/D1

0.8 P

=

kg/mm length of roller

30

ii) For multiple rollers i.e. more than two rollers

( )21 1/D 1/D1

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 SURFACERoller on flat surfaceFor 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 rollerii) For multiple rollers

P = 0.5 D1 kg/mm. length of rollerA 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 rolle

r

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/D1

1271

P 221

=

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 be

transmitted by the bearing to the bed block.ii) The length and number of the rollers and the

total 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/cm2ii) PCC (1:2:4) = 31.6 kg/cm2iii) 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 / ZM = Max. Bending MomentZ = bt2 / 6where b = width,

t = thickness of base plateA 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 2P

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. INSTALLATIONFor 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

ANCH

OR TU

BE AS

SEM

BLY

WEL

DED

TO R

EINF

ORCE

MEN

TO

F PI

ER CA

P

PIER

CA

P

PIER

DET

AIL

AT 'A

'

PLAN

ELEV

ATIO

N

10/1

2 m

m TH

ICK

STEE

L BE

DDIN

G

PUN

CH M

ARCE

NTRE

LI

N

G.I.

PIPE

W

ITH

RO

UTER

FAC

E(B

OTTO

M E

ND S

DET

AIL

'A

'

X +

TO

LERA

NCE F

IG.

3.9

SCHE

ME

FOR

INST

ALLA

TIO

N O

F AN

CHO

R BO

LTS

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 =

Ins

ide

dia

of a

ncho

r tu

bed

= An

chor

bol

t di

a2)

Anch

or t

ube

to b

e se

aled

fro

m t

op d

urin

g co

ncre

ting

of p

ier

cap.

3)An

chor

bol

ts t

o be

gro

uted

in

posi

tion

afte

r po

sitio

ning

of

the

bear

ing.

4)A

thin

lay

er o

f m

orta

r m

ay b

e us

ed b

etw

een

the

bear

ing

and

bedd

ing

plat

e fo

r m

akin

g un

iform

co

ntac

t.

DET

AIL

AT

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 GENERALCleaning 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, cant 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 can

be 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

JA

CKIN

G A

RRAN

GEM

ENT

FOR

9.15

m S

PAN

30 T

SCR

EW

JACK

WO

ODEN

BLO

CKS

DIA

GO

NAL

BRAC

ING

END

FRAM

E

BEAR

ING

BASE

PLA

TE

1850

1025

WO

OD

EN B

EAM

175

300

41

FIG

. 4.2

JA

CKIN

G A

RRAN

GEM

ENT

FOR

12.2

m, 1

8.3

m &

24.

4 m

SPA

NS

30 T

SCRE

WJA

CK

STIF

FENE

R

WO

ODE

N BL

OCK

S

DIA

GO

NAL

BRAC

ING

S

END

FRA

ME

BEAR

ING

BASE

PL

ATE

1830

1312

STIF

FNER

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 wasteGreasing 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

BEA

RIN

G ST

IFFE

NE

JACK

ING

B

EAM

(FI

XED)

TO BE

D

ESIG

NED

FOR

EAC

H SP

AN

HSF

G / R

IVET

TED

CONN

ECT

ION

FIG

. 4.3

PR

OPO

SED

JACK

ING

ARR

ANG

EMEN

T FO

R 12

.2 m

, 18.

3 m

& 2

4.4

m S

PANS

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 end

cross girder and end panel point of truss.2. Provide hard wooden packing below the end

cross 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 GENERALSteel 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 to

all 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 ELASTOMERAn 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 couldnt 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 doesnt flowwhen it gets hot, therefore one has to mould itinto whatever shape one wants before crosslinking. Due to the same reason, it cant 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 isPoly-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 isPoly-chloroprene. Thus, we have -1. Natural rubber - Poly-isoprene2. Synthetic rubber - Poly-chloropreneThere 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 Hookes Law i.e. behaving in a linear elasticmanner. We understand elastomers less wellthan we do concrete or steel because elastomersdo not obey Hookes 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 XXIVSome 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 N6.015.0 +=

where N = normal pressure in MPa5. 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.

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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 BEARINGSIn 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

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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

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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 aH

=

Shear strain =

h

where = deformationh = thickness of elastomeric padH = horizontal force

a

b Hh

55

a = length of elastomeric padb = width of elastomeric pad

Shear Modulus G = shear stressshear strain

G b x aH

= x

h

.

.

. = 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 will be proportionalto thickness of elastomeric pad.

hUnder 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 padsPlain 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 BEARINGSThe 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.

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TABLE 5.1 FLOW TABLE OF DESIGNSN Sequence of steps Remarks1. Collect input data Dead load, live load, horizontal

slow 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.pressurea) If it is < 2 MPa Bearing may slip. Revise plan

dimensions 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 including

pressure 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 slip

conditiona) Calculate 1 when only 1 x DL should be more than

D.L. is acting slow acting horizontal force.1 = 0.10 + 0.6/N1

where N1 = verticalpressure due to DL only

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b) Calculate 2 when both 2 x (DL + LL) should beDL + LL (including more than total horizontal force.impact) are acting.2 = 0.10 + 0.6/N2 whereN2 = vertical pressure

due to DL + LL9. Calculate % distortion It should be max 70%, otherwise

in shear increase h to limit this % age.

% distortion =

x 100 h

10. To ensure no upliftconditiona) Under Dead LoadPermissible 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 be

includes 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

s

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5.5.2 Input data required : The input data required forcarrying out the design is as under:P

c= D L or slowly applied vertical loads

Ps

= L L or quickly applied vertical loadsH

c= Slow acting horizontal forces

Hs

= Quick acting horizontal forcesUC = Horizontal (shear) movement due to HcU

s= Horizontal (shear) movement due to H

s

C = Rotation under effect of slow acting loads

s= Rotation under effect of quick acting loads

G = Static shear modulus of elastomerb = Permissible bearing pressure in bed

block material

m= 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/beam5.5.3 Output Expected : The output expected at the end

of 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.

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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 80Modulus 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 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.

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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 theset 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)

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TABLE 5.3 DIMENSIONAL TOLERANCES

SN ITEMS TOLERANCES1. Overall plan dimensions -0, + 6 mm2. Total bearing thickness -0, + 5%3. Parallelism

A. Of top surface of bearing withrepect 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 mm5. A. Plan dimensions of laminates -3 mm, +0

B. Thickness of laminate + 10%C. Parallelism of laminate with respect 1 in 100

to bearing base as datum

5.7 INSPECTION AND TESTINGThe 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 inspection1. All bearings of the lot shall be visually

inspected 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:

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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 pieceThe 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 testThe details of these tests are given in Table 5.2.

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C. Test on complete bearingsTwo 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 tests1. All testing shall be done at room temperature.2. No bearing shall be tested earlier than a week

after vulcanization.3. Bearing sections shall be cut from test

bearings without overheating the rubber andwith smooth cut square edges.

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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 be

preloaded with maximum horizontal load Htest(with Ntest or vertical load held constant) andunloaded before test loading.

Rate of loading : Ntest corresponding to

m = 5 MPa shall be held constant during test

and 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.

EvaluationA 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.

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2 H

RIGID CONCRETE SLAB( FIXED )

RIGID CONCRETE SLAB(DEFLECTED UNDER 'H')

N test

RIGID CONCRETE SLAB(FIXED)

SHEA

R

SHEAR STRAIN

0.0 0.2 0.4 0.6 0.8 10.0

STR

ESS

a) TEST ASSEMBLY

b) SHEAR STRESS STRAIN CURVE

FIG. 5.7 DETERMINATION OF SHEAR MODULUS

SHEAR STRAIN

SHEA

R ST

RESS

m

tan

G = m

/ tan

0.0 0.2

68

Test for determination of Elastic Modulus Conditioning load : Bearing shall be

preloaded upto Ntest. The load shall beretained for 10 minutes and unloaded upto

m = 2 MPa before test loading.

Rate of loading : The axial load N isincreased gradually at a rate yieldingapproximately

m = 0.5 MPa to 1 MPa per

minute Maximum test loading Ntest corresponds to

m

= 20 MPa. Measurement : Load and deflection

measurements shall be made inapproximately equal load intervals not lessthan 5. Deflection shall be measured at fouredges and mean value accounted for.

EvaluationA compressive stress strain curve shall be plottedand the value of apparent elastic modudus Ea shallbe defermined as shown in Fig. 5.8.Acceptance CriteriaTest result shall be deemed satisfactory if value ofEa 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.

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N test

COM

PRES

SIVE

STR

ESS

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

m

Ea =

m/

70

Test for determination of Stripping StrengthTwo 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 STRENGTHTest Procedure Maximum test loading: N

test correspondingto

m = 4 MPa is to be held constant during

the test. The horizontal loading H shall be increased

upto a maximum yielding m = 3 MPa

EvaluationExamine the test pieces for evidence ofcracking or peeling both in the strained andunstrained state.

Acceptance criteriaIf 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 CompressiveStrengthThe 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

m at failure is not less than 69 MPa.

5.7.3 Level 2 Acceptance Testing : This will alsoinclude the same tests as in Level 1A. 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 lotunder 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

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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 elastom