Bridge Bearings

March 2006(Corrected & Reprinted : January 2014)

PREFACE TO THE SECOND EDITION

The book ‘Bearings for Railway Bridges’ was first published in1996. The book had been well taken by the field engineersas this was perhaps the first book exclusively devoted onbridge bearings.

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 compiledby Shri Ghansham Bansal, Professor/Bridges. A new chapteron ‘Emerging trends in bearings’ has been added whichincorporates the new types of bearings which are being usedin advanced 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 latestand present 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 inpublishing the first edition. Nevertheless, with numerousdevelopments taking place in this field, there was a cryingneed to bring out the 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,one sample design problem has also been included forElastomeric bearings. The critical steps have been explainedwith the help of 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 foundto be 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, theIRICEN faculty and staff have contributed immensely, notablyamong them are Mrs. Lata Sridhar, Mr. Ganesh and Mr. SunilPophale. In fact, Shri Ajit Pandit, Ex. Dean/IRICEN hadinitiated the idea of updating this book, which was taken to itslogical end. I am also thankful to Shri A.K. Yadav, SeniorProfessor Bridges for proof-checking and valuablesuggestions.

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 main-tenance of railway bridges. Trainee officers have often ex-pressed the need for a comprehensive documentation on thissubject. It is hoped that this booklet will fulfil this need and dis-seminate the knowledge and experience on this subject to thefield officials.

This book has been prepared by Professor S.M. Vaidya ofthis Institute. If there are any suggestions for improving thebook or if any error/discrepancy is noticed in its contents,kindly write to 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 bridgeswith highest standard of quality and to maintain them forsmooth movement of railway traffic. Bridge bearings play avery important role in keeping the bridge structure in goodhealth. Tradionally, Indian Railways have been using steelgirder bridges and steel bearings. With the advent of newtechnology, RCC and PSC bridges along with elastomeric andPTFE bearings are gradually replacing steel bridges andconventional bearings.

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 with bridgebearings. I hope that this publication will be very useful to allcivil engineers with the objective of maintaining the bridges ingood 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 BEARINGS2.1 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 BEARINGS6.1 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 109

6.9 Design of POT- PTFE Bearings 115

6.10 Design of Guides 121

6.11 Design of Anchoring Arrangement 122

6.12 Sample Design problem for POT-PTFE Bearings 124

6.13 Common problems with Pot-cum-PTFE bearings 1336.14 Maintenance of Pot-cum-PTFE bearings 1346.15 Elements of Inspection 1356.16 Results And Actions 136

CHAPTER - 7 EMERGING TRENDS IN BEARINGS7.1 General 137

7.2 Shock Transmission Unit 137

7.2.1 Description 139

7.2.2 Advantages 140

7.2.3 Limitations 140

7.2.4 STU on second Bassein Creek Road 141

Bridge, Mumbai.

xi

7.2.4.1 Type 142

7.2.4.2 Cost 142

7.2.4.3 Basic Requirement of Design 142

7.2.4.4 Critical Factors in Design 143

7.2.5 Load Testing of STU 143

7.2.6 Installation of STU 144

7.3 Seismic Isolation Bearings 145

7.3.1 Types of Seismic Isolation Bearing 146

List Of References 149

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, D2 = 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:

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 fromsuperstructure to the substructure of bridge. Wecan say that ‘Bearings’ assume the functionalityof a bridge by allowing translation and rotation tooccur while supporting the vertical loads. In

3

nutshell, the various functions of bearings can besummarised as given below:

(1) To allow the permitted’ movements.(longitudinal & rotational)

(2) To prevent the ‘not permitted’ undesirablemovements.

(3) To transfer the load from superstructureto 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,the spans were limited and the superstructuresused to 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 pointis idealised 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 (Hinged) - 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

CIRCULARROLLER

ROLLERSOUTERBEARINGPLATES

OUTERBEARINGPLATES

SLIDING SURFACE

8

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

FIG. 1.9 POT BEARING

FIG. 1.10 SPHERICAL KNUCKLE BEARING

FIG. 1.11 POINT ROCKER BEARING

S

ELASTOMER

PISTON

POTSEAL

OUTERBEARINGPLATES

OUTERBEARINGPLATES

SPHERICALROCKER

10

FIG. 1.13 CYLINDRICAL KNUCKLE BEARING

OUTERBEARINGPLATES

FIG. 1.12 REINFORCED ELASTOMER BEARING

ELASTOMER STEEL REINFORCINGPLATES

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 beone most 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 provedto be 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 loadcarrying capacity/gauge conversion works.The bridge rule stipulates that with properlydesigned elastomeric bearings, the dispersionof the longitudinal forces to the approachescan be increased 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 thesurface condition. Generally speaking, spansshorter than 9 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 atiny part 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 forany type of bridge, but the design of substructure

17

will have to be modified to bear the entire loads.This modification 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 areresisted /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 girder mayhave a tendency to creep under the influence ofpredominantly unidirectional movement. To preventthe girders from falling off the bearings, stopper platesare provided. These plates are also known as Lockingstrips

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.

21

The base or bed plate is held in position on thebed block by anchor bolts. Since the slidingbearings do not allow free rotation, unequaldistribution of load takes place due to therotation/ deflection of the girder. This leads tostress concentration under the bearing towardsinside of the span. There have been instances offailure of the bed block material due to thisdeflection. In order to overcome this deficiency,the inside edge of the top plate of the slidingbearing is chamfered. This is commonly knownas the centralised articulated bearing. Fig. 2.1shows a typical 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 keepa tab on friction otherwise the value ofhorizontal force transmitted to sub-structure willincrease tremendously. It may so happen thatthe value of horizontal force becomes so largethat cracks may develop at the bottom of bedblock. Therefore, the frequency of lubricationhas been prescribed as once in three years. Itmay be increased to once in two years in casetrack consisting of long welded rails is providedover the 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

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 ormore rollers and a top plate. The rocker & rollerend is made by providing a saddle and knuckleplate on top of the rollers whereas the samearrangement except rollers is at the rocker end.The rocker & roller end of bearing permitstranslation as well as rotation, whereas the rockerend permits only rotation.

3.1.1 Parts :

(a) Roller : The rollers are made of forged steelof Class-3, as per IS:2004 and basic raw materialis as per IS:1875. The rollers may, alternatively,be turned from approved C&W axlesmanufactured after 1931. USFD test shall beconducted to ensure that there are no internalflaws. These have machined surface to permitsmooth rolling action.

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 toeach other with a link bar to ensure that theyalways move 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 uponthe design 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 isnegligibly small. IRC:83 Part-I cautions designerswhile proposing 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 oilwithout the need to open out the oil bath. Thenecessity of 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 takendue to the possibility of unequal load sharingamong various rollers when number of rollers aremore than 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 setof rollers, a smaller diameter roller will onlybecome ineffective, whereas the larger diameterroller in the same set will make many otherrollers ineffective depending upon its location asshown in 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 maximumvertical load.

iii) The permissible bearing stresses in the bed-block material.

As per clause 3.16 of IRS : Steel Bridge Codethe allowable bearing pressure for differentmaterials is 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

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/knuckle blocks 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 bedblock and base plate or saddle plate and bottomchord is completely ignored and the entirehorizontal force is assumed to be transmitted bythe connecting 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 addedadvantage of damping the vibrations and impactforces and thus increasing the life of the bedblock. Cement or cement epoxy grout are alsoused as an alternative for levelling the bed blocktop 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,positioning of the anchor bolts must be done witha very high level 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 bea very useful reference point for monitoringproper functioning of the roller bearing. It is veryeasy to check the bearing position by measuringthe gap between the top and bottom contact

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 =

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 p

ositi

on a

fter

posi

tioni

ng o

f th

e be

arin

g.4)

A th

in la

yer

of m

orta

r m

ay b

e us

ed b

etw

een

the

bear

ing

and

bedd

ing

plat

efo

r m

akin

g un

iform

con

tact

.DE

TA

IL A

T ‘

A’

37

points of the rollers in longitudinal direction.Alternatively, for full/segmental bearings, themovement may be measured by the position ofthe contact point of the tooth bar on top andbottom plate respectively. In case oil bath isprovided, a movement gauge should be providedso as to directly read the roller position fromoutside without 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 adoptedfor different types of girders:

1. For plate girders upto 6.1 m span, jacks canbe directly applied below end sleeperensuring firm hook bolt connection, since loadto be lifted 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 inFig 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, which resultsin lifting of bearing strips inside when lowered onbase plate. This improper seating of the bearingstrip will cause hammering action duringsubsequent passage of train resulting in damage

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

185

0

102

5

WO

OD

EN

BE

AM

175 300

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

to the bed block and masonry of the 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 toavoid creep of girder in logitudinal and lateraldirection. For lifting, it is not necessary to breakthe track. Only loosening of fish bolts and dogspikes over a small length on both sides of thepier is sufficient. Only one end of the girdershould be lifted at a time and steel scraperinserted between bearing strip and base plate toremove old grease dust and dirt. The contactsurface is cleaned with oil and grease applied.Girder is then lowered back over the base plate.

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

Time required for all these activities isapproximately 15 to 20 minutes.

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 wirerope with turn buckle arrangement for holding thefree end 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.

45

3. Remove fish plates and loosen dog spikes ofrail over adjacent spans to avoid overloadingthe jack on account of weight of adjoiningspan and stiffness of the track.

4. If trolley refuge is connected to both spanson any 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 endbeing free, the girder is likely to creeplongitudinally. To prevent this, provide hardwood packing between the ends of girder onpier and between girder and the ballast wallon abutment.

6. Jacks should be kept in working order andtested to 1.5 times the load they areexpected to lift. Keep one spare jack asstand 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.

46

When the girder is lifted about 10 mm and rollersare free, link plate and tooth bar are removedafter opening the stud connections. All rollersshould be taken out and cleaned with scraperand these are sand-papered with a fine sand-paper of zero grade. Rollers should be examinedfor any possible signs of flattening or minutecracks with a magnifying glass. Grease graphitegrade 3 conforming to IS 508 is applied over thebase plate evenly below the roller contact area.The rollers 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,the knuckle plate is lowered over the rollers. Thiswill create gap between the saddle block andknuckle plate. Cleaning and greasing of this areais then carried out similar to the fixed end andgirder is lowered back.

While taking out rollers for examination andgreasing, take special precautions to preventthe rollers 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.

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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. CharlesGoodyear had been trying to ‘cure’ the rubber sothat it could be used in all seasons. He tried tomix all kinds of things such as ink, black pepper,cheese and what not. But he couldn’t succeeduntil he dropped a piece of rubber on stoveaccidentally. To his surprise, he found thatinstead of melting the rubber piece hardened andremained pliable. It was found in the lab that itcontained traces of sulphur. Goodyear perfectedthe process and named it ‘vulcanization’ after theRoman God of fire, ‘Vulcan’.

As a result of vulcanization, rubber molecules arecross-linked with sulphur. This cross-linkingmakes the rubber stronger. It allows the rubberto keep 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-linkedrubber used 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

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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,installation and maintenance of elastomericbearings are based on extensive studies andlaboratory trials conducted by ORE (Office forResearch and Experiment) of UIC. These aredocumented in ORE Report D-60. Important

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specifications which can be referred to forelastomeric bearings are listed 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.

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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 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

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Since a frictionless contact surface does not existin practice, the deformation of the pad will bepart flattening and part bulging and the behaviourof plain elastomeric pad will be as shown 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’ whichis a 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

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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 a

H=

Shear strain = δ h

where δ = deformationh = thickness of elastomeric padH = horizontal force

a

b Hh

δ

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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 ofthe elastomer 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

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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 behaveslike an individual pad, while horizontal strain oneach layer 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.

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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

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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 shearincrease ‘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 beincludeslimited 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

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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

αC = Rotation under effect of slow acting loadsαs = 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 theend of design of the elastomeric bearing is asunder :

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 constructionengineer in the field. The properties of theelastomer are detailed in IRC:83 Part-II and arereproduced in Table 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 tolerancesstipulated in IRC:83 which are reproduced inTable 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 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)

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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 highlightedin the 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 equalor near equal size produced under identicalconditions of manufacture to be supplied for aparticular project. For the purpose of grading the

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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 axialload corresponding to normal pressure of 15MPa applied in stages and held constantwhile visual 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 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. Thetest pieces will be subjected to the followingtests:

• 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

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The details of these tests are given in Table 5.2.

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 and

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with smooth cut square edges.

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 duringtest and the horizontal loading H shall begradually increased to yield a shear stressrate of approximately 0.05 to 0.1 MPa perminute.

• 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.

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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

Δτm

Δ tan γ

G = Δτm / Δ tan γ↑

↑

0.0 0.2

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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.

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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

Δσm

Δε

Ea = Δσm / Δε

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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 of

2 H

LOADING PLATE

45

200

MOUNTING PLATE N test

PLATERECESSED

TO PREVENTSLIPh ≥ 50

↓

↓

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peeling or separation at or near the interfacebetween 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 be

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rejected by the inspector and certifiedaccordingly.

The bearings shall be transported to the bridgesite after final acceptance and should beaccompanied by an authenticated copy of thetest certificate. An information card giving thefollowing details should be appended to the testcerftificate:

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.

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1. Ozone TestSome research and studies determined that theozone test was unnecessary as this affects onlythe 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. The

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actual bearing, however, is not a thin specimenand effect of aging is not so pronounced. It wasfound that it would take hundreds of years ofservice 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 axisof the girder on a single line of support underany circumstances.

2. Bearings of different plan size must not beinstalled next to each other (on the samepier) 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 and

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the bed block must be horizontal to avoidtangential force due to gravity coming on tothe bearing (maximum tolerance 0.2%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, handlingand installation 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 the

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girders for future replacements and for properdrainage.

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.

77

4. Placement of top forms over the side frameand sand with an opening matching the sizeand position of the bearing.

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 andpreventive actions taken. It is, therefore,necessary to undertake periodical inspection ofelastomeric bearings. The inspecting officialshould look for the 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 heightof elastomeric 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",many MG and NG sections have been upgradedto BG. It is therefore not surprising that a largenumber of railway bridge substructures are calledupon to bear much larger horizontal forces thantheir original 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 bettermanagement of the longitudinal force on bridges.The longitudinal force is generated at the railwheel contact surface and it can either bepassed on to the substructure through thebearings or can be dispersed onto theapproaches. The sharing of the longitudinal force

80

depends upon the relative stiffness of the "girder-bearings-pier-soil" system or "rail - track - ballast -approach embankment" system. While thederivation of the load sharing is quite complex, itis certain that introduction of a flexible bearingunder the girder can disperse more longitudinalforce on the approaches, thereby relieving thepiers of the horizontal forces. A number ofexperiments have been conducted in India andabroad on this aspect and rules laid down fordispersion of longitudinal force to approaches.Surprisingly, there is considerable variation in therules adopted by various countries in this regard.The provision of IRS : Bridge Rules in force onIndian 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 bedispersed on 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.

81

5. In case of a span having sliding bearing orelastomeric 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. 6to Concrete Bridge Code dated 30.7.2002 shouldbe kept 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 light

82

loading and large longitudinal movements.

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 girderand to the bed block. This method howeverrequires either anchor bolts to be cast alongwith the girder or dowel holes left in positionin the 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 theirregularities of 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 considerationsand thus uneconomical.

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 tomovement 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 loadededge of 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∑

= radians016.06/438

14.1 =

tan αc +1.5 tan αs = 0.0011+1.5 x 0.0014

91

= 0.003 radians

Hence a/6

ei∑> tan αc +1.5 tan αs ............ 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 observedduring test.

= 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 thefatigue 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 havethe limitation 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

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98

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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 ON PTFE

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. Thevalues as given in Table 6.2 obtained fromvarious technical literature give an idea about therange of the permissible stress in various types ofPTFE.

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 minimum0.01 radians 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/pistonis mounted by a sliding assembly. The uppersurface of the piston is recessed and filled withthe PTFE disc. The upper sliding plate isprovided with a sheet of stainless steel or chromenickel alloy steel. The PTFE disc is provided withsmall cavities (lubrication pockets) containing aspecial lubricant which ensure life long lubricationof the sliding surface. An all around seal is

104

provided to prevent ingress of moisture and duston the contact 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 the POTwall 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 pad compresseson one side and expands on the other, so theelastomer is in contact with the POT wall and slipsagainst 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 anindependent mechanism 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 concrete bridges.In such a case, any horizontal movement willalways 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 shouldbe perfectly levelled. It is common to use amortar bedding composed of sand-cement mortarwith or without epoxy resin. A cube crushingstrength of 35 N/mm2 is usually recommended forthe bedding mortar. While installing the bearing,the transit bolts must be in position and theseshould be removed after the mortar has set andbefore the bearing is called upon to slide orrotate.

POT bearings have been used in many importantbridges on Indian Railways. It was used, inaddition to many other bridges, in theconstruction of 3rd Godavari Bridge atRajamundry having bow string arch girders of90 m span, on Zuari and Mandovi bridges on theKRCL having 120m span open web steel girders.Recently these have also been used on anumber of bridges on Jammu-Udhampur Rail LinkProject (JURL) e.g. Tansi bridge(71.4+102+71.4m), Dudhar bridge (64+92+64m)and E-18 viaduct (40+29.68m) etc. POT-PTFEbearings are being used on Udhampur-Srinagarsection also. POT bearings, both fixed and slidingtype can be used to advantage in all situationswhere there is a limitation on overall height of thebridge girder coupled with large force/movementsinvolved. This is so because the POT bearingsare substantially thinner as compared to rollerbearings. PTFE sliding sheets have also beenused for the launching of superstructures ofbridges in Konkan Railway and JURL with great

107

advantage.

6.71 Placement of Expansion End

For placement of expansion end of the bearingat a particular ambient temperature, other thanthe mean temperature, necessary correctionhas to be applied. The drawings of Pot-PTFE bearings, generally specify the valuesof the movement of the point of intersection (of bottom chord and end raker) withreference to center line of base slab undercombined effect of temperature and loading.At the mean temperature, the center line of thebearing and the point of intersection shall matchunder full live load with impact. Therefore, underthe dead load alone, the point of intersection ofgirder shall be towards the span with respect tothe centerline of the bearing. Mean temperatureis to be taken as Tmin + (Tmax-Tmin)/2, whereTmin & Tmax are the specified values of theminimum and maximum temperatures for thelocation.

6.72 Fixing of Bearings

1 To cater for vibration and accidental impact,some anchorage should be provided.Anchorage should be accurately set intorecesses cast into the structure usingtemplates and the remaining voids in therecesses should be filled with materialcapable of withstanding the loads involved.

2 Bearings that are to be installed on temporarysupports should be firmly fixed to thesubstructure by anchorage or othermeans to prevent disturbance during

108

subsequent operations. Finally, voidsbeneath the bearings should be completelyfilled with bedding material using theappropriate method.

3 Bearings may be fixed directly to metalbedding plates that may be cast in or beddedon top of the supporting structure to thecorrect level and location.

4 If the structure is of steel, the bearingsmay be bolted directly to it. Proper careshall be taken to ensure that there are nomismatch in the bolt holes of the structureand the bearing.

5 .Threaded fasteners shall be tighteneduniformly to avoid overstressing of any part ofthe bearing.

109

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 scopeof these 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 = 0.10to 0.25.

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 qp + K2 qv)

di = dia of elastomer pad in mm

k1, k

2 = constants depending upon di/he as given in

Table 6.4

he = thickness of elastomer pad in mm

110

qp = rotation angle in radian due to long term effectqv = 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 ROTATIONRESISTANCE

c

DIRECTION OF HORIZONTAL FORCE

AXIS OF ROTATION

DIAMETER

DIRECTION OFHORIZONTAL FORCE

DIAMETER

AXIS OF ROTATION

111

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 movementwill be 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

φ

112

(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 smallestdimension exceeding 100 mm (dia. or smaller side ofrectangle), the contact area shall be taken as grossarea without deduction 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

113

(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 exceed45 MPa.

Corresponding values for confined elastomericpad are 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

114

(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.

115

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

116

(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 he x σce due to elastomeric bearing = ——————

2 x bp x hc

where;di = Dia of elastomer pad (mm)he = thickness of elastomer pad (mm)σce = fluid pressure in elastomer due to vertical loadbp = thickness of cylinder wall (mm)hc = Height of cylinder wall (mm)

Total force on cylinder wall = projected area x σce

Projected area = di x he

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

117

(ii) The hoop tensile stress due to horizontal force

H= —————— 2 x bp x hc

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 he x σce= ———— for 1 mm radial slice as shown in Fig. 6.9. bp

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

118

(ii) Shear stress due to horizontal force

1.5 x H= ————

bp x di

Factor 1.5 has been taken due to parabolic distribution

5. Bending stress at cylinder and base interface shallalso be 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 bp

FIG. 6.10 CALCULATION OF BENDING STRESS

M at interface = 2

ece hσ2×

as shown in Fig. 6.10

P

ce

1

b

hc he

B M DBENDING MOMENTDIAGRAM

σce

119

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 interfacein mm.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

120

Bending stress = I

M.y =

id

H1.5 x ha x

2

b p 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.

121

(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 withstandonly the force generated at sliding interface due tofriction. The thickness of stainless steel plate shallbe guided by the requirement of proper weldingand in no case shall be less than the thickness ofweld or 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.

122

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 ofthe anchorage to resist shear force. Here ‘μ’ will betaken as 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)

123

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.

124

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 stan-dards)

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 padGuiding 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

125

(a) Dia. / 15(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

126

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

127

= 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

128

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

129

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 ⎟

⎠

⎞⎜⎝

⎛ −+

= 108160 + 7492.33 = 115652 N-mm

↑

↓

12.8

0

FIG. 6.11 MOMENT DUE TO UPWARD FORCE

130

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

Total shear stress = 14.74 + 24.58 = 39.32 N/mm2

< 0.45 fy or 126 N/mm2 ……. OK

131

(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

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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

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6.13 Common problems with Pot-cum-PTFE bearings

1. The concept of pot bearing is based on effectivelyconfining the elastomer within a metallic piston-cylinderarrangement so that the elastomer remains in a stateof uniform strain and no tension is produced whilesubjected to very large compressive stress. Unlike tradi-tional laminated elastomeric bearing, such a perfectlyconfined elastomer, while subjected to large compressivestress, behaves like a viscous fluid and therefore is ca-pable to cater for high degree of multi directionalrotational movement. Extremely tight tolerance of manu-facturing is required to be maintained together withproperly detailed effective sealing system to confine theelastomer effectively. If the steel piston is manufacturedwith high tolerance relative to the base pot, a conditionof steel-to-steel contact develops which inhibits therotational capacity of the bearing device. When thepiston is manufactured with a low tolerance relative tothe base pot, the sealing ring has difficulty in containingthe elastomer, which acts like a fluid under pressure.

2. There have been a number of problems with potbearings, due to inadequate design, incorrect selectionof raw material and improper production technique.Leakage of the elastomer has been noted, even with bear-ings, considered well manufactured. Abrasion or wearof the elastomeric pad has also been noted in somecases. A number of reasons have been postulated forthese problems including inadequate clearances or tol-erances between the piston and cylinder, excessiverotation, number of rotational cycles or lateral loads,abrasion of the sealing ring, deformation of the Pot dueto excess lateral load or gravity load, inadequate surfacefinish of the portion of piston/cylinder in contact withelastomer pad etc.

3. The other problems include inability of the steel potto withstand the transverse loadings, combined withthe pressure developed from the elastomer. Another is

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the fact that satisfactory rotational distribution is notachieved until a load of atleast 25% of the bearingcapacity, is applied. The rotational resistance of the con-fined elastomer pad, though not negligible, is often ig-nored due to lack of understanding of behavior of thepad and in some cases due to absence of data.Another problem has been experienced with the potconcept in cold climates. This deals with the rota-tional element being typically manufactured from syn-thetic rubber (chloroprene rubber) as in India. Duringextreme cold temperatures, coupled with high-design pres-sures, the rotational element has been known to crys-tallize, which in turn eliminates the rotational capacityof the device. Many cases of such types of failureshave forced the design authorities to revert back totraditional steel bearings. However, in India no such fail-ure has been reported so far.

4. All these defects/deficiencies in the confined elastomericpad element in a pot bearing cannot be detected afterinstallation and transfer of full load, unless the bearingsare removed and elastomeric pressure pad is taken outfor inspection of the elastomeric element, metalliccylinder and piston. For the same reason, the behaviourof the elastomeric rotational element cannot be checkedvisually during proof load and movement testing atmanufacturer’s works. Therefore, a lot of care has to beexercised at the manufacturing and inspection stage.

6.14 Maintenance of Pot-cum-PTFE bearings

1. The Pot-cum-PTFE bearings are designed and manufac-tured so as to make it almost maintenance free. How-ever, the surrounding area of the bearings shall alwaysbe kept clean and dry to avoid damage to thebearings.

2. Suitable easy access to the bearing shall be provided forinspection and maintenance. Provision shall be made forjacking up the superstructure so as to allow repair/replace-ment of the bearings.

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3. Inspection of bearing at site is required from time totime to ascertain the performance of the bearings.Periodic nominal maintenance of bearing shall be carriedout in order to ensure better performance and longer life.The bearings are required to be inspected at an intervalof one year for the first five years and at an intervalof two years thereafter. However, the bearings shallalso be examined carefully after unusual occurrences, likemovement of abnormal heavy traffic, earthquakes, cy-clones and battering from debris in high floods.

4. The inspection shall be preceded by careful cleaningof the bearings and its surrounding space, dependingon the actual conditions around the bearings, e.g., de-posit of salt, debris, dust or other foreign material.

6.15 Elements of Inspection

1. Measurement of movement - During inspection at site,measurements are required to be taken and documentedto compute its movement and rotation values in rela-tion to their design values to ascertain whether theperformance of the bearings are satisfactory. To ascer-tain maximum movement, measurement should be takenonce during peak winter (early morning) and once duringpeak summer (after noon) and corresponding atmospherictemperature should be recorded. The recoded value ofmovement shall be compared with the design values.

2. Measurement of dimensions - Overall dimensions of thebearings is required to be measured and compared withthe actual dimensions to ascertain any excessive stressor strain on the bearing.

3. Evidence of lock in condition- If any moveable orrotating part of a bearing is found to be in locked-in/jammed condition, necessary rectification measures shallbe taken immediately.

4. Evidence of corrosion - If corrosion of any part ofexterior exposed steel surface of the bearing is detectedthe following measures may be taken.

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i. Detect affected part

ii. Wire brush the affected portion to clean the rust.

iii. Apply protective coating as per manufacturer’s speci-fications.

5. Condition of the adjacent bridge structure - The adjacentstructure of the bearings are also required to be in-spected for any damage and necessary actions torepair the same, should be initiated. The root causeof defect should be searched and remedial actionsshould be planned to avoid recurrence of the problem.

6.16 RESULTS AND ACTIONS

The results of every inspection to be recorded in the in-spection report and shall be classified in each case intothe following types of action:

i) No action

ii) Further measures/long-term monitoring or design analy-sis needed (e.g., considering extreme temperatures/exposures, variation of loads, etc.).

iii) Minor repair works e.g. cleaning, repainting etc.

iv) Repair or replacement of entire bearing or parts of thebearing.

In case of defects where the cause of necessaryactions cannot be determined by the inspecting personor the responsible bridge engineer, the design engineers/

manufacturer should be consulted.

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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

138

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.

139

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 hasbeen used for the first time in the Second BasseinCreek Road Bridge, Mumbai. It has not yet beentried on Indian 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→ →← ←

140

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

141

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 STUshave saved time and money in construction ofcaisson foundations.

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 anaverage velocity of 2.4 m/s. All these conditionsat site made the caissons construction a riskyand difficult 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

142

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 totheir stiffness.

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.

143

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. Ifout of 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’

144

is applied to STU due to braking load four timesa day 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 is specifiedto prove that the STU would function as intendedduring a seismic event. The STU shall be testedby applying 50 sinusoidal cycles of load rangingbetween the maximum design tension andcompression 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

145

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 withadjacent unloaded 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 torsionalrotation of the bridge and resulted in a moreuniform distribution of seismic forces amongsubstructures in the transverse direction.

146

(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

147

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)

148

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 accumulatedduring the 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 slidingmotion.

u1

u2

u -u1 2

Bridge Superstructure

Pier

u -u1 2

W R

BRIDGESUBSTRUCTURE

149

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. IRC; 83 (Part III) : Standard specificaions and codeof pracive for road bridges part III-Section IX,Bearings POT, POT Cum PTEF, PIN and metalicguide bearings Indian road congress, New Delhi,India.

7. UIC 772 - RC: Code for the use of rubber bearingsfor rail bridges - 1969, International Union ofRailways, Paris, France.

8. BS: 5400 : Steel, concrete & composite bridges,Section 9.2 - 1983 specification for materials,manufacture and installation of bridge beariangs,British Standards Institution, London UK.

150

9. BS: 5400 : Steel, concrete & composite bridges,Section 9.2 - 1982 specification for materials,manufacture and installation of bridge beariangs,British Standards Institution, London UK.

10. ORE Report D - 60 Application of rubber for bridgesupporting plates

11. Proceedings of International Conference on Bridgesand Fly over bridge bearings - National cooperativehigh way research programme synthesis of high waypractice.

12. Concrete Bridge Practice - Analysis, design andeconomics - By V. K. Raina

13. AASHO : Standard specification for highway bridges,The American Association of State Highway Officials,Washington, U SA.

14. Retention of old MG bridges for Heavier BG loadingby use of Elastomeric bearings’’ By S.R. Agarwal

and Adesh Shrama International Seminaron Failures, Rehabilitation and Retrofittingof Bridges & Aqueducts, IIBE, 17-19 Nov. 1994 Bombay.

15. Internet site : http://www.new-technologies.org/ECT/Civil/lud.htm

16. Internet site : http://ww.en.wikipedia.org

17. Design of sliding and fixed POT-PTFE bearing for76.2 m for HMLS - RDSO, Lucknow

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18. American Iron and Steel Institute: Steel BridgeBearing selection and design guide

19. Journal of Bridge Engineering: Efficiency of SeismicIsolation for Seismic retrofitting of heavysubstructured bridges. Vol.10, No.10, July/August2005

20. Bridge Inspection and Rehabilitation - A practicalGuide by Parsons Brinckerhoff.

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