IRECN Bridge Bearing-2

8/13/2019 IRECN Bridge Bearing-2

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various functions of bearings can be summarisedas given below:

(1) To allow the permitted movements.(2) To prevent the not permitted undesirable

movements.(3) To transfer the load from superstructure to

substructure.

The permitted and not permitted movements inthe bridge in relation to bearing can be betterappreciated if we analyse the degree of freedomin 3-D as shown in Fig. 1.2.

FIG. 1.2 DEGREE OF FREEDOM IN 3-D

In the era of stone and brick masonry bridges, thespans were limited and the superstructures usedto be massive, primarily developing onlycompressive stresses under the loadingconditions. Such bridges did not need specialbearings since the movements were very small.

ACROSS TRACK

ALONG TRACK

VERTICAL

PERMITTED MOVEMENT

NON-PERMITTED MOVEMENT

ACROSS TRACK

VERTICAL

ALONG TRACK


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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 usedThese are discussed below.

1.2.1 Degree of freedom : There are possible 6degrees of freedom at any support as describedearlier. These are translation in three directionsand rotation about these three axes. A bearing

may permit movement in any of these 6 degreesof freedom or in none. During the structuraldesign of the bridge girders, each support point isidealised in a specific manner by the designengineer. The bearing has to fulfill thisassumption.

Translation can be permitted by the following

modes of action :i) by sliding actionii) by rolling actioniii) by shearing strainiv) by racker and pinion devices (gears)


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Rotation can be permitted by the followingmodes:

i) by rocking/hinge actionii) by differential compression (as in

elastomeric pads)iii) by bending/ flexure (as in tall

piers, portals)

Therefore based upon degree of freedomrequirements, different degree of freedom can begiven at the support point and bearings may beclassified as:

(1) Fixed - Translation not permitted,Rotation permitted

(2) Free - Translation permitted,Rotation permitted

(3) Rocker & Roller - Roller end free, Rocker end fixed

1.2.2 Material used : A number of different materialshave been used for making bearings such assteel of various types, phosphor bronze, syntheticmaterial like rubber (elastomer) and PTFE etc.Out of these materials steel, rubber and PTFEare the most commonly used materials, today, for

bearings. 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.


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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.


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OUTERBEARINGPLATES

SLIDING SURFACE

FIG. 1.3 PLAIN SLIDING BEARING

OUTERBEARINGPLATES

ROLLERS

OUTERBEARINGPLATES

CIRCULAR ROLLER

FIG. 1.4 SINGLE ROLLER BEARING

FIG. 1.5 MULTIPLE ROLLER BEARING

OUTERBEARINGPLATES

CIRCULAR ROLLER

ROLLERS OUTERBEARINGPLATES

OUTERBEARINGPLATES

SLIDING SURFACE


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OUTER BEARINGPLATE

PIN

OUTER BEARINGPLATES

PIN

LEAVES

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


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PISTON

POT

ELASTOMERSEAL

OUTER BEARINGPLATE

OUTER BEARINGPLATES

SPHERICAL ROCKER

FIG. 1.9 POT BEARING

FIG. 1.10 SPHERICAL KNUCKLE BEARING

FIG. 1.11 POINT ROCKER BEARING

S

ELASTOMER

PISTON

POTSEAL

OUTERBEARING

PLATES

OUTERBEARINGPLATES

SPHERICAL ROCKER


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OUTER BEARINGPLATES

FIG. 1.13 CYLINDRICAL KNUCKLE BEARING

OUTERBEARINGPLATES

ELASTOMER STEEL REINFORCING PLATES

FIG. 1.12 REINFORCED ELASTOMER BEARING

ELASTOMER STEEL REINFORCING PLATES


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1.3 SELECTION OF BEARINGS

For a given bridge structure there could be anumber of different solutions for providingbearings. However, in each case there will be onemost appropriate choice of the bearing. Theselection will depend on a number of factors.These are listed and discussed below:

1.3.1 Functional Requirement : The bearing must fulfillthe functional requirement in terms of permittedmovements, load bearing and load transmission.The various functions performed by differenttypes of bearings are reproduced in Table 1.2from BS: 5400 part-IX. Table 1.3 may also bereferred for selection of bearings as this tablegives load ranges and movement capacities ofvarious types of bearings.

1.3.2 Expected life : An attempt should be made toselect a bearing whose expected life iscompatible with that of the bridge itself. Failingthis, replacement of the bearing will have to beplanned for during the life of the bridge. It shouldhowever be acknowledged that any scheme forreplacement of bearings will invariably requiresuspension of traffic, which is very costly and

troublesome.1.3.3 Maintenance efforts : The importance of proper

functioning 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


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T A B L E 1

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F U N C T I O N S P E R F O R M E D

B Y D I F F E R E N T T Y

P E S O F B R I D G E B E R A I N G S


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S N

T A B L E 1

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with moveable parts require greater maintenanceeffort as well as those made of steel due to thepossibility of corrosion, and consequent freezingof the bearing.

1.3.4. Cost : The capital cost includes cost of design,fabrication and installation of bearing. Generally,this will be a fraction of the cost of the bridge. Assuch, the initial cost alone should not be aconsideration in choice of the bearing. Manybearings which had attractive initial cost proved tobe a liability later on during maintainance.Therefore, life cycle cost should be the criteria forselection of bearing.

1.3.5 Other factors : Factors which may be relevant inour quest for the most suitable bearing are:

a) Height of the bearing : This may be critical incase of regirdering works where maintainingexisting rail / road level is the main constraint .

b) Management of horizontal force transferred to the substructure : This is an importantconsideration while upgrading the load carryingcapacity/gauge conversion works. The bridgerule stipulates that with properly designedelastomeric bearings, the dispersion of thelongitudinal forces to the approaches can beincreased from 25% to 35%.

c) Performance under seismic loads : Some-times seismic consideration may alter thechoice of bearing particularly in zone IV & V.


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Having chosen the type of bearing for a givenstructure, the following guidelines may befollowed in order to minimize the life cyclecost.

i) Choose larger size of rollers in rocker & rollerbearing, since smaller components are moreprone to accumulating dust and moisture. Alarger roller will overcome debris more easilythan smaller roller. Larger components alsofacilitate inspection and maintenance.

ii) For the material selected, specify the highestgrade of mechanical properties and thestrictest tolerance that can be practicallyattained. Maintenance efforts, thus, can begreatly reduced.

These recommendations only underscore the factthat the initial cost is not a consideration for goodbearing design and specification.

1.4 MINIMIZING THE REQUIREMENT OFBEARINGS

Bearings are unavoidable evils. In bridges of verysmall spans, however, the bearings are notrequired e.g. in slab bridges. Here, the interfacebetween the slab and the abutment-top or bedblock functions as a bearing. The coefficient offriction between concrete and concrete can betaken as 0.50 to 0.60 depending upon the surfacecondition. Generally speaking, spans shorter than9 m do not need bearings.


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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 thereby

using 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 is

accommodated at the abutments, wherebearings capable of providing large movementsare required.As per AASHTO specifications, insliding bearings up to span 50 feet, no provisionfor deflection of the spans need be made.

Excluding these special cases, all other forms ofbridges require bearings. Though bearing is a tiny

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 for anytype of bridge, but the design of substructure will


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have to be modified to bear the entire loads. Thismodification will result into high cost ofsubstructure. Therefore, provision of bearings isthe economical solution.

A bearing is a negligibly small part of a bridgeand unfortunately the attention it receives fromthe engineers is also negligibly small. In fact, theimportance of this small part should have beeninversely proportional to its size, as the entireload is transmitted through this tiny component

and 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.


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CHAPTER 2

SLIDING BEARINGS

2.1 GENERAL

A system of two plates, one sliding over the othermakes one of the simplest type of bearings.These bearings permit translation in longitudinaland transverse directions, unless specificallyrestrained in any of these directions. No rotationis permitted unless specially provided in the formof articulation and only vertical loads are resisted / transmitted by these bearings.

Common materials that have been used assliding surfaces and their coefficients of frictionare:

a) Mild steel over mild steel - 0.2 to 0.3b) Mild steel over phosphor bronze - 0.15c) PTFE over stainless steel - less than

0.08

Generally, plain sliding bearings are providedwhere span is less than 30 m, because themovement capacity of these bearings is generally

small.

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


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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


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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

may have a tendency to creep under theinfluence of predominantly unidirectionalmovement. To prevent the girders from falling offthe bearings, stopper plates are provided.

Guide strip : To regulate the movement of thegirder in the correct alignment, guide strips areprovided parallel to the span.

Size of bearing plate : The size of the bearingplate is governed by the total vertical load (DL +LL + CDA) on the bearing and the allowablebearing stress in the bed block material. The soleplate of the bearing is directly connected to thegirder by welding or countersunk rivets / bolts.The base or bed plate is held in position on the


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bed block by anchor bolts. Since the slidingbearings do not allow free rotation, unequaldistribution of load takes place due to the rotation/ deflection of the girder. This leads to stressconcentration under the bearing towards inside ofthe span. There have been instances of failure ofthe bed block material due to this deflection. Inorder to overcome this deficiency, the inside edgeof the top plate of the sliding bearing ischamfered. This is commonly known as thecentralised articulated bearing. Fig. 2.1 shows atypical sketch of the standard centralisedarticulated bearing adopted on Indian Railways,These bearings are used in steel plate girders,composite girders and underslung girders.

On a reference made by RDSO to BritishRailways it was learnt that the PTFE bearings arein use on British Railways since middle ofseventies without any maintenance problems.

The British Railways have provided thesebearings in through type girder bridges of spansupto 40 m and concrete bridges of various typesupto 90 m spans.

Sliding bearings are the simplest type ofbearings, used up to 30.5 m span girders. Theirregular maintenance is very important, to keep atab on friction otherwise the value of horizontalforce transmitted to sub-structure will increasetremendously. It may so happen that the value ofhorizontal force becomes so large that cracksmay develop at the bottom of bed block.Therefore, the frequency of lubrication has beenprescribed as once in three years. It may beincreased to once in two years in case trackconsisting of long welded rails is provided overthe bridge.


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CLEAR SPANTHEORETICAL SPAN

OVERALL LENGTH

PLATE GIRDER

GENERAL ARRANGEMENT

A - EXPANSION GAP 12 TO 20 mm B - INSTALLATION GAP 1.5 TO 2 mm

NOTE: FOR ENSURING THESE GAPS, THE ANCHOR BOLTS SHOULDBE INSTALLED ACCURATELY IN PROPER POSITION.

FIG. 2.1 CENTRALISED ARTICULATED BEARING

LOCKINGSTRIP

C.L . OF GIRDER

BEARINGPLATE

ANCHORE BOLT

GUIDE STRIP(ON OPPOSITE SIDE ON OTHEREND OF GIRDER)

PLAN

A

B S E C T I O N A T

' X X '

X

X

BEARINGPLATESCSK RIVETSBEARING FLAT

BEARING

PLATE

ELEVATION

CSK RIVETS

COUNTERSUNKRIVETS

BED PLATE

PLAN


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CHAPTER 3

ROCKER & ROLLER BEARINGS

3.1 GENERAL

For railway bridges with spans in excess of30.5m, where open web through girders aregenerally provided, the amount of movementneeded and the vertical load transmitted througheach bearing is too large to be catered by thesliding bearings. It is common, on IndianRailways, to provide rocker & roller bearings atthe free end of open web through girders, androcker bearings at the fixed end.

A typical rocker and roller bearing for open webgirders of 45.7 m span is shown in Fig. 3.1 &3.2 . The roller bearing consists of a base plate,two or more rollers and a top plate. The

rocker & roller end is made by providing a saddleand knuckle plate on top of the rollers whereasthe same arrangement except rollers is at therocker end. The rocker & roller end of bearingpermits translation as well as rotation, whereasthe rocker end permits only rotation.

3.1.1 Parts :

(a) Roller : The rollers are made of forged steel ofClass-3, as per IS:2004 and basic raw material isas per IS:1875. The rollers may, alternatively, beturned from approved C&W axles manufacturedafter 1931. USFD test shall be conducted toensure that there are no internal flaws. Thesehave machined surface to permit smooth rollingaction.

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IRECN Bridge Bearing-2