IRECN Bridge Bearing-5

8/13/2019 IRECN Bridge Bearing-5

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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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a. Misalignment of reinforcing platesb. Poor bond at interfacec. Variation in elastomer thicknessd. Surface defects

e. Low stiffness.

The deflection under loads between 5 MPa and15 MPa should be measured with sufficientaccuracy. Variation in stiffness of any individualbearing from the mean of all such bearingsshould not be more than 20% of the mean value.

B. Test on specially moulded test pieceThe test piece shall be moulded by themanufacturer with identical compound and underidentical vulcanizing conditions as used inmanufacture of the bearings. The test piecesshould be suitably identified and certified. The testpieces will be subjected to the following tests:

Test for chemical composition, specific gravity,ash content etc.

Test for physical properties such as(i) Hardness

(ii) Ultimate tensile strength

(iii) Elongation at break

(iv) Accelerated aging test

(v) Compression set test

(vi) Ozone test

The details of these tests are given in Table 5.2.


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C. Test on complete bearings

Two bearings should be selected at random fromthe lot. Various tests should be conducted on

these test bearings. These bearings should beexcluded from the accepted lot because all thetests given below except Shear Modulus test are

destructive tests.

1. Shear Modulus2. Elastic Modulus3. Adhesion strength4. Ultimate compressive strength.

In addition to above tests, the ash content (%)and specific gravity of elastomer of test piecesfrom test bearing shall be compared with those ofcorresponding specially moulded test pieces andmaximum acceptable variation will be as givenbelow:

Ash content + 0.5%

Specific gravity + 0.2

The test specifications and acceptance criteriashould be as per Appendix II of IRC 83 Part II.The excerpts are given below:

General about tests

1. All testing shall be done at room temperature.

2. No bearing shall be tested earlier than a weekafter vulcanization.

3. Bearing sections shall be cut from testbearings without overheating the rubber andwith smooth cut square edges.


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4. Test for determination of E may precede thatof G when both tests are conducted on thesame pair of test bearings.

Test for determination of Shear Modulus Conditioning load : Bearings shall be

preloaded with maximum horizontal load Htest

(with Ntest

or vertical load held constant) andunloaded before test loading.

Rate of loading : Ntest

corresponding to

m= 5 MPa shall be held constant during test

and the horizontal loading H shall be graduallyincreased to yield a shear stress rate ofapproximately 0.05 to 0.1 MPa per minute.

Maximum test loading Ntest

:The horizontalloading H shall be increased upto a maximumH

test which corresponds to horizontal

deflection 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)

SHEAR

SHEAR STRAIN

0.0 0.2 0.4 0.6 0.8 10.0

STRESS

a) TEST ASSEMBLY

b) SHEAR STRESS STRAIN CURVE

FIG. 5.7 DETERMINATION OF SHEAR MODULUS

SHEAR STRAIN

SHEARSTRESS

m

tan

G = m/ tan

0.0 0.2


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Test for determination of Elastic Modulus

Conditioning load :Bearing shall bepreloaded upto N

test. The load shall be

retained for 10 minutes and unloaded uptom= 2 MPa before test loading.

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

m= 0.5 MPa to 1 MPa per

minute Maximum test loading N

test corresponds to

m

= 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 plotted

and 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 ofEa is within + 20 percent of 1(0.2/S2 + 0.0005) andprovided, there is no evidence of any defect ordamage discerned by close visual inspection during

the test.


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N test

COMPRESSIVESTRESS

STRAIN

0.0 2 10 20

FIG. 5.8 DETERMINATION OF ELASTIC MODULUS

a) TEST ASSEMBLY

b) COMPRESSIVE STRESS STRAIN CURVE

RIGIDCONCRETESLAB

2

10

20

m

Ea=

m/


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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 shall

not 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

testcorresponding

to m= 4 MPa is to be held constant during

the test.

The horizontal loading H shall be increasedupto a maximum yielding

m= 3 MPa

Evaluation

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

Acceptance criteria

If neither test piece shows evidence ofpeeling or separation at or near the interface

2 H

LOADING PLATE

45

200

MOUNTING PLATE N test

PLATERECESSED

TO PREVENTSLIP

h 50


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between rubber and reinforcement layers, thebearing shall be deemed to have satisfactoryadhesion.

Test for determination of UItimate CompressiveStrength

The test pieces are to be loaded either till the failureof the steel laiminate or till the irreversible squeezingout of elastomer whichever is earlier. The testassembly and the test pieces may be identical tothose for E test. However, a small section (not lessthan 100 x 200 mm) shall be cut from test bearing

and 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

mat 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 these

shall become the part of the accepted lot.5.7.4 Inspection and Quality Control Certificate : A lot

under inspection should be accepted by theinspector when no defect is found in theacceptance level tests and so certified. In case ofany defect in the bearing, the whole lot shall berejected by the inspector and certified


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accordingly.

The bearings shall be transported to the bridgesite after final acceptance and should be

accompanied by an authenticated copy of the testcertificate. An information card giving the followingdetails should be appended to the test cerftificate:

1. Name of manufacturer2. Date of manufacture3. Grade of elastomer4. Bearing dimensions5. Production Batch No.

6. Accpetance lot No.7. Date of testing8. Specific bridge location9. Explanation of markings on the bearing, if any.

A commentary on various tests conductedon specially moulded test piece

Elastomeric bearings have had an extraordinary

performance 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 actual

performance of an elastomeric bearing and thusmay be either scrapped or the prescribedtolerances should be diluted. These are basedupon the studies undertaken by AASHTO.

1. Ozone Test

Some research and studies determined that theozone test was unnecessary as this affects only


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the surface layer. In un-stretched rubber, ozonedegradation is confined to a thin surface layer,typically 0.5 . Ozone cracks develop at rightangles to the tensile strain and they degrade

rubbers 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 of

sample was very important in establishing thesignificance of aging. Current test methods usevery thin specimen and the results generallyshow significant changes due to aging. Theactual bearing, however, is not a thin specimenand effect of aging is not so pronounced. It wasfound that it would take hundreds of years of


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service to change the bearing stiffness by 10%.Given that the correlation between the test andthe real situation is not known, it seems that thetest is just a quality control test, not a

performance 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 relationship

in 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 one

behind the other along the longitudinal axis ofthe girder on a single line of support underany circumstances.

2. Bearings of different plan size must not beinstalled next to each other (on the same pier)under the same girder/span.

Different plan area will lead to different value of for given values of H, h and G as under:

( )baG

hH!

=

3. The contact surface between the bearing andthe bed block must be horizontal to avoidtangential force due to gravity coming on tothe bearing (maximum tolerance 0.2%


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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 the

underside of the girder.

4. The bearings must be placed on a contactsurface which is free from local irregularities,maximum tolerance being +1 mm in height.

5. The bearings must be placed at true planposition with a maximum tolerance of +3 mm(along and across the bridge axis).

6. As a measure of ample safety againstaccidental displacement, the bearings shouldbe placed in a recess. This is speciallynecessary for bridges with low DL/(DL+LL)ratio or bridges with larger horizontaldisplacements. This is typical in railwaybridges with steel girders.

7. Where shrinkage and creep are likely toproduce excessive horizontal movements, it isadvisable to raise the girders beforecommissioning the bridge, allowing release ofthis unidirectional displacement.

8. Care should be taken in storage, handling andinstallation of the bearing to avoid anymechanical damage, contamination with oil,

grease, dirt and undue exposure to sunlightand weather.

9. It is preferable to install the elastomericbearing on a pedestal to permit jacking of thegirders for future replacements and for properdrainage.


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10. The area around elastomeric bearings shouldbe inspected and scrupulously cleaned afterthe installation is complete. Railway bridgeswith ballasted decks should be provided with

suitable 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. For

precast 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 all

around the bearing.

3. Filling up the space between the bearing andthe formwork frame by clean sand.

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


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5. Sealing of the gaps between the opening intop form and the bearing by adhesive tape.

6. Execution of soffit formwork of the girder.

7. Concreting of the girder.

8. Removal of side forms and sand from aroundthe bearing and cleaning up the spaces.

5.9 PERIODICAL INSPECTION AND MAINTENANCE

The elastomeric bearings are considered largelyto be maintenance free. However due to possibledeficiencies in manufacture and installation, thesebearings may show signs of distress or developmalfunctioning. Before any malfunctioning of thebearings leads to distress in the girders orsubstructure, it should be detected and preventiveactions taken. It is, therefore, necessary toundertake periodical inspection of elastomericbearings. The inspecting official should look forthe following aspects:

! Correct position

! Excessive shear

! Excessive bulging

! Separation of rubber from steel lamination

! Cracking and tearing of elastomer

! Flattening out

! Off-loading of one edge due to excessiverotation


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With the technological improvements, the bridgerules have also undergone changes primarily toreflect introduction of diesel and electric traction.RBG loading standard - 1975, MBG loading

standard - 1987 and HM loading standard - 1995are characterised by large increase in longitudinalforce as compared to the increase in axle loads.In addition, due to the "Policy of Uni-gauge", manyMG and NG sections have been upgraded to BG.It is therefore not surprising that a large numberof railway bridge substructures are called upon tobear much larger horizontal forces than theiroriginal design values.

A check on design calculations of substructure ofsuch bridges indicates unacceptable level ofstresses in the bridge substructure. Thus manypiers are required to be strengthened or rebuilteven though they happen to be otherwisephysically in sound condition. Apart from being

costly 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 above

problem is possible by way of better managementof the longitudinal force on bridges. Thelongitudinal force is generated at the rail wheelcontact surface and it can either be passed on tothe substructure through the bearings or can bedispersed onto the approaches. The sharing ofthe longitudinal force depends upon the relative


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stiffness of the "girder-bearings-pier-soil" systemor "rail - track - ballast - approach embankment"system. While the derivation of the load sharing isquite complex, it is certain that introduction of a

flexible bearing under the girder can dispersemore longitudinal force on the approaches,thereby relieving the piers of the horizontal forces.A number of experiments have been conducted inIndia and abroad on this aspect and rules laiddown for dispersion of longitudinal force toapproaches. Surprisingly, there is considerablevariation in the rules adopted by various countriesin this regard. The provision of IRS : Bridge Rulesin force on Indian Railways are explained below:

1. The quantum of longitudinal force is laid downin appropriate EUDL tables in the appendix tothe IRS : Bridge Rules.

2. Subject to proper standard and condition oftrack on bridge approaches and provision of

rail free fastening on the girders, 25% of thelongitudinal force subjected to a minimum of16t for BG, can be considered to be dispersedon to the bridge approaches.

3. In case, suitably designed elastomericbearings are used, the above dispersion canbe further increased by 40%. Thus use ofelastomeric bearings results in 35% of

longitudinal force being thrown on to theapproaches.

4. In case of a span having free-fixed bearingcombination, the full longitudinal force (netafter dispersion) is transmitted to the pier withfixed bearing.

5. In case of a span having sliding bearing or


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elastomeric bearings at both ends the netlongitudinal force is shared @ 40% at eithersupport point, the remaining being shared bythe piers beyond the loaded span.

Though the above provisions are considered tobe conservative by many, it still shows a clear cutbenefit of replacing steel sliding bearings orrocker & roller bearings by elastomeric bearings.A note of caution is however necessary that theamount of dispersion will also depend upon thecapacity of the rails, joints and approach track to

transmit and absorb the longitudinal force. It ishowever beyond doubt that more studies arenecessary in this area and there is scope forretaining old substructures by use of elastomericbearings.

While replacing other bearings especially rocker& roller by elastomeric, the correction slip no. 6 toConcrete Bridge Code dated 30.7.2002 should be

kept in mind, which states that use ofelastomeric bearings in prestressed concretebridges should preferably be restricted up to

maximum clear span of 30 m.

5.11 ANTI-SLIP DEVICES

It has been explained earlier that elastomericbearings have a tendency to slip if the minimum

normal pressure is less than 2 MPa. Such asituation is likely to occur when elastomericbearings are used in steel plate girders havingless self weight. UIC code 772-2R cautionsagainst the use of elastomeric bearings if theyare required to undergo simultaneously lightloading and large longitudinal movements.


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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 not

considered 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 recess

as 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 used

for attachment of the bearing to the girder andto the bed block. This method howeverrequires either anchor bolts to be cast alongwith the girder or dowel holes left in position inthe concrete at correct locations. A typicaldetail of this scheme is shown in Fig. 5.10 c.


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BEARING

BED BLOCK

BEARING

STOPS

ANGLE EMBEDDED

IN BED BLOCK

EXTERNAL PLATE

BONDED 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

Documents

IRECN Bridge Bearing-5