IRECN Bridge Bearing-4

8/13/2019 IRECN Bridge Bearing-4

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BEARINGS

TIFFE

N

JACKINGB

EAM(

FIXE

D)

TO

BE

DESIGNEDFO

R

EACH

SPAN

HSFG/

RIVETTED

CONNECTION

FIG.4.3

PROPOSED

JACKING

ARRANGEMENTFOR

12.2m,18.3m&

24.4m

SPANS


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4.1.3 Cleaning and greasing of Rocker & Rollerbearings of open web through girders : In case ofstandard open web through girders, no separate

jacking arrangement is required as the end cross

girders are designed and provided with stiffenerand pad plate for provision of jack for lifting. Gapbetween bottom of cross girder and top of bedblock is about 600 mm, hence any type of jack of100 ton to 200 ton capacity can easily be usedfor lifting. In case of non standard spans, the endcross girder requires adequate strengthening orspecial jacking beam below the bottom boom.

The equipments required in this case are sameas for sliding bearing except that the jacks ofhigher capacity (100 ton to 200 ton) and wire ropewith turn buckle arrangement for holding the freeend are required.

Greasing of rocker and roller bearing should becarried out under traffic block under the

supervision of an official not below the rank ofADEN/ABE.

Following precautions and preliminaryarrangements are required:

1. Ensure tightness of rivets connecting endcross girder and end panel point of truss.

2. Provide hard wooden packing below the endcross girder to support the girder in case offailure of jacks. This should be done at threeplaces to prevent tilting of this girder.

3. Remove fish plates and loosen dog spikes ofrail over adjacent spans to avoid overloading


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the jack on account of weight of adjoiningspan and stiffness of the track.

4. If trolley refuge is connected to both spans on

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 end beingfree, the girder is likely to creep longitudinally.To prevent this, provide hard wood packingbetween the ends of girder on pier andbetween girder and the ballast wall on

abutment.

6. Jacks should be kept in working order andtested to 1.5 times the load they are expectedto lift. Keep one spare jack as stand by.

7. During lifting of girder, precaution should betaken to prevent creep of rail.

4.1.4 Method of Greasing : Greasing of fixed endrequires 20 to 25 minutes. The lifting is hardly 10mm, ensuring that the gap is created betweensaddle block and knuckle pin. Saddle is not liftedabove collar to prevent lateral creep of the girder.Steel scraper is used to remove old grease, dustand dirt. The contact area is cleaned with oil.Grease is applied and then girder is lowered

back.Greasing of free end requires 45 to 50 minutes.Knuckle plate is tied to the saddle plate with wirerope having turn buckle arrangement to releasethe load from roller when the girder is lifted.When the girder is lifted about 10 mm and rollersare free, link plate and tooth bar are removedafter opening the stud connections. All rollers


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should be taken out and cleaned with scraper andthese are sand-papered with a fine sand-paper ofzero grade. Rollers should be examined for anypossible signs of flattening or minute cracks with

a magnifying glass. Grease graphite grade 3conforming to IS 508 is applied over the baseplate evenly below the roller contact area. Therollers are then placed in position and greaseapplied at the top contact surface. Link plate andtooth bars are connected with care so that toothbar is placed in the same inclination as per thedrawing.

With the help of turn buckle of wire rope sling, theknuckle plate is lowered over the rollers. This willcreate gap between the saddle block and knuckleplate. Cleaning and greasing of this area is thencarried out similar to the fixed end and girder islowered back.

While taking out rollers for examination andgreasing, take special precautions to prevent therollers from falling-off the bed block.


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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 same

time 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 the

process of improving the properties of rubber byheating with sulphur. A normal rubber is notuseable as it becomes brittle at low temperatureand sticky at high temperature. Charles Goodyearhad been trying to cure the rubber so that itcould be used in all seasons. He tried to mix allkinds of things such as ink, black pepper, cheeseand what not. But he couldnt succeed until he

dropped a piece of rubber on stove accidentally.To his surprise, he found that instead of meltingthe rubber piece hardened and remained pliable.It was found in the lab that it contained traces ofsulphur. Goodyear perfected the process andnamed it vulcanization after the Roman God offire, Vulcan.

As a result of vulcanization, rubber molecules are

cross-linked with sulphur. This cross-linkingmakes the rubber stronger. It allows the rubber tokeep its shape better even when it is stretchedover and over again. But there is a drawback ofcross-linking also. Vulcanized rubber doesnt flowwhen it gets hot, therefore one has to mould itinto whatever shape one wants before crosslinking. Due to the same reason, it cant be

recycled a big environment problem. The tyresof the vehicles also use the same material, andwe are not able to recycle the cross-linked rubberused in tyres.

One of the most well known natural rubber isPoly-isoprene which is harvested from the sapof Hevea tree. Natural rubber have all the


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excellent properties making it extremely suitableto many engineering applications, except for itsrelatively high reactivity with environmentparticularly ozone. Ozone causes surface

cracking that can rapidly penetrate even at verylow tensile stress.

To obviate this drawback many synthetic rubberswere developed, most popular among those isPoly-chloroprene. Thus, we have -

1. Natural rubber - Poly-isoprene2. Synthetic rubber - Poly-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 Hookes Law i.e. behaving in a linear elastic

manner. We understand elastomers less wellthan we do concrete or steel because elastomersdo not obey Hookes Law. They are very flexiblein shear but very stiff in bulk compression. Thesimple theory of mechanics characterizing thebehaviour of rubber is quite different from thatused for conventional materials, and quitecomplex for the liking of the practical engineers.

It is therefore not surprising that most of thecodal provisions for design, fabrication, installationand maintenance of elastomeric bearings arebased on extensive studies and laboratory trialsconducted by ORE (Office for Research andExperiment) of UIC. These are documented inORE Report D-60. Important specifications which


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can be referred to for elastomeric bearings arelisted below:

1. UIC 772-2R 1989

2. 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 N

6.015.0 +=

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 on

elastomeric 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 small

bridges 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 imposed

loads. 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 inFig. 5.1.

SLIP

FIG. 5.1 PLAIN ELASTOMERIC PAD WITHOUTFRICTION AT CONTACT PLANE


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Since a frictionless contact surface does notexist in practice, the deformation of the pad willbe part flattening and part bulging and thebehaviour of plain elastomeric pad will be 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 expansion

is prevented at the interfaces and bulging iscontrolled.

The compressive stiffness of the bearing,therefore, depends upon the ratio of loaded areato the area of the bearing free to bulge. This isessentially quantified by Shape Factor S which isa dimensionless parameter defined as under:

S =Plan area loaded in compression Perimeter area free to bulge

Greater compressive stiffness is, therefore,obtained by dividing elastomer into many layersby introducing very thin, usually 1 to 3 mm, steelreinforcement plates between the elastomerlayers and bonding the plates firmly with theelastomer to prevent any relative movement. This


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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 the

elastomer 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 forceplan area

bxaH=

Shear strain = h

where = deformationh = thickness of elastomeric padH = horizontal force

a

b Hh


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a = length of elastomeric padb = width of elastomeric pad

Shear Modulus G = shear stress

shear strain

Gbxa

H= x

!

h

... = x h

G (a x b)

Thus for a given size of bearing, the sheardeformation will depend upon thickness ofelastometric pad, value of horizontal force andvalue of G. Since horizontal force and G cannot be altered, deformation will be proportionalto thickness of elastomeric pad.

h

Under the influence of rotation, the compressiveloads on the inner edge is magnified and it isrelieved on the outer edge. In the design it is,therefore, ensured that, under the combined effectof normal loads and rotations, the outer edge of theelastomer does not get off-loaded completely.

5.4 TYPES OF ELASTOMERIC BEARINGS

Three basic types of elastomeric bearings areused.

1. Plain elastomeric pads2. Steel reinforced elastomeric pads3. Fibre reinforced pads

Plain pads are used for light or moderate


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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 sheets

separating the layers of elastomer are completelyencased within the elastomeric material. Forvertical load, each layer of elastomer behaves likean individual pad, while horizontal strain on eachlayer is additive. Therefore, adding steellaminations is a convient way to accommodatelarger lateral movements for the samecompressive loads. Fibre reinforced pads are

usually 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 5.1has 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 5.1FLOW TABLE OF DESIGN

SN Sequence of steps Remarks

1. Collect input data Dead load, live load, horizontal

slow load, horizontal quick load,span length, rotation at ends,etc. as given in next paragraph.

2. Select width b of Generally equal to width ofbearing girder & larger than a due to

better rotational stability inlateral direction.

3. Calculate net plan area Depends upon max. verticalof bearing load including impact and

permissible bearing pressureon bed-block.

4. Calculate length a of Net plan area divided by b.bearing along girder

5. Calculate Shape Factor It should be between 6 and 12.S

6. Calculate min. vertical Dead load / plan area of bearing.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 1

when only 1

x DL should be more thanD.L. is acting slow acting horizontal force.

1= 0.10 + 0.6/N

1

where N1

= verticalpressure due to DL only


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b) Calculate 2

when both 2

x (DL + LL) should beDL + LL (including more than total horizontal force.impact) are acting.

2= 0.10 + 0.6/N

2where

N2 = vertical pressuredue to DL + LL

9. Calculate % distortion It should be max 70%, otherwisein shear increase h to limit this % age.

% distortion =

x 100 h

10. To ensure no upliftcondition

a) Under Dead LoadPermissible rotation = It should be more than actualComp. of all layers under DL rotation of span under dead

a/6 load only.

b) Under Total Load

Permissible rotation = It should be more than actualComp. of all layers under TL rotation of span under total

a/6 load including impact.

11. Total shear in elastomer Addition of all three should beincludes limited to 5 x G or 5 MPaa) Due to compression (considering G = 1 MPa).b) Due to horizontal loadc) Due to rotation

12. Thickness of steel This should be more thanlamination plate required actually provided.

= 2 (h

i + h

i + 1)( P

c+ 1.5 P

s)

a x b x s


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

Pc

= D L or slowly applied vertical loads

Ps = L L or quickly applied vertical loadsH

c= Slow acting horizontal forces

Hs

= Quick acting horizontal forcesU

C= Horizontal (shear) movement due to H

c

Us

= Horizontal (shear) movement due to Hs

C

= Rotation under effect of slow acting loads

s= Rotation under effect of quick acting loads

G = Static shear modulus of elastomer

b = Permissible bearing pressure in bedblock material

m

= Max. Permissible pressure in theelastomer 10 MPa (as per IRC : 83) 5 MPa or 5G, whichever is less

(as per RDSO) 11 MPa (as per UIC 772 - 2R)

B = width of the girder/beam

5.5.3 Output Expected :The output expected at the endof design of the elastomeric bearing is as under :

a = length of the bearing along the spanb = width of the bearing across the spann = number of layers of elastomerh

i= thickness of each layer of elastomer

h = total thickness of elastomer.


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5.6 MANUFACTURE AND QUALITY CONTROL

5.6.1 Properties of Elastomer : Though elastomericbearings offer a number of advantages ascompared to steel bearings, many failures havebeen reported pertaining to these bearings. Amajority of these failures can be attributed toimproper quality of elastomer and/or faultyinstallation. These two aspects, therefore, needvery careful attention by the construction engineerin the field. The properties of the elastomer aredetailed in IRC:83 Part-II and are reproduced inTable 5.2.

The shear modulus of the elastomer dependupon the hardness of the rubber. The relationbetween shore hardness and shear modulus Gas indicated in UIC-772-2R is as under:

Shore hardness 50 60 70 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 reduces with the

increase in value of G. Therefore with age, themovement capacity reduces.

5.6.2 Dimensional Tolerance : The bearing should befabricated to the dimensional tolerances stipulatedin IRC:83 which are reproduced in Table 5.3.


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TABLE 5.2 PROPERTIES OF ELASTOMER

Name of test Reference Permissible Methodology of

code for testing limits testing and remarks

1) Chemicalcomposition

i) Poly-chloroprene ASTM- D297 Not less than Use of fillers to becontent test 60% minimized.

ii) Identification ASTM- D3677 No reclaimed Test involves useof polymer rubber or of infra red

natural 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 the

Shore 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 issubsequently measured.

6) Compression IS 3400 Compression The elastomer sampleset test PART- X set not to is subjected to a

exceed 35% compressive strain of25% at a temperatureof 1000C for 24 hours.Subsequently, thesample is cooled andthe residual strain ismeasured. The resi-dual strain called theset should notexceed 35% of theinitial strain.

7) Ozone test IS 3400 No cracking or The sample isPART- XX disintegration subjected to a tensile

of the sample. strain of 20% in achamber where thetemperature is main-tained at 400C andOzone concentrationkept at 50 pphm(parts

per 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 TESTINGThe inspection and tests on elastomer and thefinished bearings are very important aspects ofensuring a satisfactory performance of thebearing. IRC:83 has laid down detailed testingplan and acceptance criteria for elastomericbearings. The important aspects are highlighted inthe following paragraphs.

5.7.1 Lot size : Testing and acceptance of elastomericbearings should be done lot-wise. A lot ofbearings shall comprise of all bearings of equal ornear equal size produced under identicalconditions of manufacture to be supplied for aparticular project. For the purpose of grading the


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levels of acceptance test, the lots are classifiedas under :

1. A lot size of 24 or larger number of bearings

is 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 of

acceptance 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 inspection

B. Test on specially moulded test pieceC. Test on complete bearings.

A. General inspection

1. All bearings of the lot shall be visuallyinspected for absence of any defects insurface finish, shape or any discerniblesuperficial defects.

2. All bearings of the lot shall be checked fordimensional tolerances.

3. All bearings shall be subjected to an axial loadcorresponding to normal pressure of 15 MPaapplied in stages and held constant whilevisual examination is made for:

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