Neoprene Bridge Bearings Dupont 1984

8/2/2019 Neoprene Bridge Bearings Dupont 1984

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Engineering properties o fN eoprene bridge bearings

~ ~G. us PlD.T a TM OF f ' ;


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Eng inee ring p rope rtieso f N eo pren e b rid ge b earin gsT ab le o f C on te nts

PageWhy elastomeric bridge bearings? 1Type of elastomeric bearings 1Plain pads 1Laminated pads reinforced withsteel 1Laminated pads reinforced withfabric 1

Why Neoprene in elastomeric bearings? 2Neoprene vs. natural rubber 3Relationship of hardness to stiffness 3Neoprene in compression 3Stress/strain relationships 3

Du Pont study provides new data 3Effect of shape 5Shape factor 5Beta factor 6Limitations on strain and stress 6Cffiep 6

Neoprene in shear 9Effective Rubber Thickness (ERT) 9Shear strain limitations 9Slippage 9Shear modulus 9Effect of low temperatures 9Effect of compressive stress 10Effect of dynamic loading 10

Damping characteristics of Neoprene 12Other applications that use bearingpad technology 13Important considerations forsuccessful use of Neoprenein bearing pad applications 13References 13

Elastomeric bearings have a long and successfulrecord of performance supporting bridges, rail-roads, buildings and heavy machinery. The devel-opment of elastomeric bearings in Europe datesback to the end of World War II, and bearingsbased on Neoprene synthetic rubber have been inuse in the U.S.A. since 1957. Today, millions ofbearings based on Neoprene support highway andrailroad bridges throughout the world.Sound bearing design principles have beendeveloped, based on experience accumulatedover the years, and are readily available to theengineer. But, engineering data applicable tobridge bearings are limited. Today, engineers aredesigning modern highway bridges with longerspans and ever-increasing loads. To help themaccomplish this objective, this publication providesupdated engineering data on Neoprene bearings.Specifically, it contains data on: Stress/strain in compression for compressiveloads up to 13.8 MPa [2000 psi] on bearingshaving shape factors up to 20. Shear modulus vs. compressive load Creep under compressive load Dynamic properties in compression and shear Properties of steel- and fabric-reinforcedbearings.Much of this new information was obtained onfull-size bearings, and was measured by statehighway specifying agencies or privatelaboratories.


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Whyelas tomer icb r idge bea rings?Bearings support the bridge structure and accom-modate expansion, contraction, and end rotation ofthe structure. The bearings must function properlyin all regions of the world and under constantlychanging temperature and weather conditions.Prior to the development of elastomeric bearings,bridge engineers used mechanical devices-suchas sliding plates, rollers, or rocker arms-toaccommodate expansion and contraction of bridgestructures. These devices required constantinspection and maintenance to prevent corrosionand seizing of bearings.In contrast, elastomeric bearings are economi-cal, effective, and require no maintenance. Theyare simple solid pads, with no moving parts. Thepad deflects in shear to accommodate expansion,contraction and end rotation of the bridge struc-ture. There is no need for lubrication, no need forcleaning, and no opportunity for bearings to seize.T yp es o f e la stom eric b ea rin g sThree types of elastomeric bearings are commonlyused for supporting bridges and other struc-tures-plain pads (unreinforced), laminated padsreinforced with steel, and laminated pads rein-forced with fabric. Reinforced pads can supportgreater loads than plain pads of the same size.Plain pads are the most economical to produce.They consist of blocks of elastomer compound,molded or extruded into large sheets, vulcanized,and then cut to size. Their use is generally limitedto smaller bridges where compressive stresses nohigher than 2.8-3.4 MPa [400-500 psi] areexpected.Laminated pads reinforced with steel are pro-duced by plying alternate layers of elastomer andsteel in a mold of the desired shape. The elastomeris bonded to the steel during vulcanization underheat and pressure. A thin cover layer of elastomerencapsulates the steel to prevent corrosion duringservice. The cross-section of a typical steel-rein-forced Neoprene bearing pad is shown in Figure 1.Three 12.7 mm [0.5 inch] layers of Neoprene com-pound are bonded top and bottom to steel shims; aminimum 3.2 mm [0.125 inch] cover layer of Neo-prene is used around the perimeter of the bearing.Steel-reinforced bearing pads are used on largerbridges where compressive loads of up to 5.6 MPci[800 psi] may be encountered.Laminated pads reinforced with fabric are madeby calendering the elastomeric compound intolarge sheets, plying it with alternate layers of fabric(usually fiberglass), and vulcanizing under heatand pressure. Bearings of any desired size can be

cut to order from the large pad. The edge of thefabric reinforcement may be left exposed, since itis corrosion resistant. Fabric-reinforced bearingsare not as stiff as steel-reinforced bearings, andthus will conform more readily to surface irregulari-ties in the bridge structure. Bearings of this typehave been used on bridges where compressiveloads up to 5.6 MPa [800 psi] have been encoun-tered.A typical fiberglass-reinforced Neoprene bear-ing, shown in Figure i,is made in three 12.7 mm[0.5 inch] thick "segments"; each "segment" con-sists of a 1.6 mm [0.0625 inch] cover layer of Neo-prene compound, a sheet of fiberglass, a 9.5 mm[0.375 inch] layer of Neoprene compound, anothersheet of fiberglass, and a 1.6 mm [0.0625 inch]bottom layer of Neoprene compound. When thesethree "segments" are combined and vulcanized,the resulting pad has a single layer of reinforce-ment top and bottom (with a thin outer cover ofNeoprene compound) and a double layer of rein-forcement at 12.7 mm [0.5 inch] intervals.The State of California Department of Transporta-tion has used fabric-reinforced bridge bearingpads for more than twenty years. Originally, polyes-ter fabric was used for reinforcement. But, as pad

F ig ur e 1Typi ca l L am in a ted Bea ri ng Pads (C r oss -s ect io n s )

14 Gage Steel

Layers of Fberglass [0.0625 in]FIBERGLASS-REINFORCED


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Whyelas tomer icb ridge bea rings? cont inuedsizes increased in the late 1960's to accommodatelonger span bridges, these pads were found tocreep more than was predicted by available com-pressive creep data. To eliminate the creep prob-lem, studies were init iated on full-size Neoprenebearings with fiberglass-reinforcement. The results,published in 1974(1),showed that fiberglass-rein-forced pads are equivalent to steel-reinforced padsin compressive stiffness, compression/deflectionand creep under compression. Fiberglass-rein-forced pads have been specified by the State ofCalifornia and some other states since that time.Why N eoprene ine la s tome ric bea rings?Neoprene has found wide use in bridge bearingsand other load-supporting applications for twoimportant reasons. It provides the requiredmechanical properties and, when properly com-pounded, it is highly resistant to deterioration byweather and natural aging.

Neoprene was the first commercial syntheticrubber, introduced over 50 years ago. Over theyears it has established a solid reputation for relia-ble long-term performance. Due to its balancedcombination of properties, Neoprene is widelyacknowledged as the best all-around performeramong general purpose elastomeric materials.Neoprene is supplied as a raw (uncompounded)polymer in chip form, and must be combined withcarbon black, plasticizers and curatives to producea rubber compound that can be vulcanized duringthe manufacture of bearings. Throughout this bro-chure we discuss typical compounds of Neoprenethat have been especially formulated for bridgebearing use. Those compounds can be defined byperformance specifications, such as those estab-lished by the American Association of State High-way and Transportation Officials (AASHTO)-seeTable I. Updated specifications are currently beingdeveloped by AASHTO and other organizations-i.e., the American Society For Testing and Materi-als (ASTM) and the American Concrete Institute(ACI).

Table IAASHTO Sp ec if ic at io n s a n d T ypic al P ro p er tie so f N e op re ne C omp ou nd s U se d in L am in ate d B ea rin gsASTM Typical Properties OfTest Method Neoprene Compoundroperty AASHTOSpecifications

60 45-75ardness, durometer A, points D 2240-8117.92600 17.2 min.2500 min.Tensile strength, MPa D 412-80psi

Ultimate elongation, % D 412-80 500 400 min. for bearingsof 50A hardness;350 min. for bearingsof 60A hardness;300 min. for bearingsof 70A hardness

Heat resistance D 573-8170 hours at 100C [212F]Change in tensile strength, %Change in ultimate elonqation, %Change in hardness, durometer A points

-10-15+5

-15 max.-40 max. + 15 max,Compression setAfter 22 hours at 100C [212F], % D 395-78Method B 25 35 max.

Ozone resistanceOzone concentration 100 pphm(by volume), 37.7C [100F],20% strain, 100 hoursLow temperature performanceTested at - 40C [ - 40F]

D 1149-78 No Cracks No CracksD 746-79 No Failure No FailureProcedure BD 429-81Method B >7.0 [40] 7.0 [40] min.Adhesion (bond made duringvulcanization), N/ mm [Ibf/in]


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Neo pre ne v s. n atu ra l ru bb erNatural rubber is also used for bearings. Its physi-cal and mechanical properties are similar to thoseof Neoprene. However, Neoprene is far moreresistant to deterioration by weather, sunlight,ozone and heat than is natural rubber. A qualitativecomparison of Neoprene and natural rubber ismade in Table II, and more details on this subjectare provided in Elastomers Notebook No. 2 2 4 ( 2 1 .There is no long-term service background on othersynthetic elastomers in bearings, and it is unlikelythat they can be compounded to meet the physicaland environmental requirements of elastomericbridge bearings.

Table IICompar at iv e P ro p er tie s o f N e op re n ea nd N atu ra l R ub be r C omp ou nd s

NaturalNeoprene RubberPhysical properties Excellent ExcellentResistance to weathering Excellent FairHeat aging Very Good FairLow temperatureserviceability Good GoodSet resistance Good GoodOil and chemicalresistance Good Poor

Rela tio nship o fha rd ne ss to s tiffn es sBearing manufacturers generally use hardness asan indicator of stiffness, or modulus, of elastomericmaterials. Hardness is defined as the relativeresistance of a surface to indentation by anindentor of specified dimensions under a desig-nated load. It is measured by an instrument called adurometer. The durometer A scale, ranging from 0(very soft) to 100 (hard), is used for measuringmost elastomer compounds. Elastomeric bearingpads generally have a nominal durometer A hard-ness in the range of 50 to 70. For comparison, thedurometer A hardness of an inner tube is about 50,a tire tread is about 60, and a shoe heel is about70. (IRHD hardness is equivalent to durometer Ahardness.)There is a relationship between the hardness ofan elastomeric compound and its stiffness, ormodulus, in compression or shear. However, thisrelationship is not precise, nor can it be definedmathematically; it must be determined empirically.

In this booklet are stress/strain curves in com-pression for bearings manufactured with Neoprenecompounds of 50 and 60 durometer A hardness,and shear modulus data for compounds of 50, 55and 60 durometer A hardness. These data shouldbe viewed only as approximations of the actualstress/strain relationship in a manufactured bear-ing. When an engineer specifies a Neoprene bear-ing pad of a certain hardness, he may expect itscompressive stiffness and shear modulus to beabout as predicted by the data herein, but theremay be differences between published and actualmoduli.Neop re ne in compre ss io nWhen a load is applied to any elastomer, includingNeoprene, it deforms in accordance with the forceexerted on it and within the limits provided by themass of the material itself or by the dimensions ofits container. In rubber technology this is calledcompression. Although this term is correct in theframework of normal rubber usage, it may be mis-leading to the engineer. It does not mean that theelastomer will undergo a change in volume underpressure. Rather, it means the elastomer willdeflect, or undergo a change in shape (providing itis not confined in all directions). This distinction isimportant. An elastomer is essentially an incom-pressible fluid, capable of changing its shape tothe limit of its strength under load. It will react to aload placed upon it by tending to exert force uni-formly in all directions. Even though the elastomeris changed in shape under load, it is compelled bythe characteristic of elasticity to return to its originalshape once the load is removed.Stress /s t ra in re la t ionsh ipsDu Pont study provides new data-As notedpreviously, the relationship between compressiveload, deflection and shape factor must be deter-mined empirically. There has been a gradual shiftin design to higher compressive loads and highershape factors but not many empirical data areavailable for these conditions.In 1981, Du Pont initiated a test program involv-ing full-size commercial Neoprene bearings of 50and 60 durometer A hardness with shape factorsranging from 3 to 20, and compressive loads up to13.8 MPa [2000 psi]. The bearings tested weresteel- and fiberglass-reinforced pads of typicalconstruction, made by two major suppliers ofNeoprene bearings. * The testing was done by state' Steel -reinforced bea rings were suppl ied by O il States Indus tr ies . A thens. Texas .Fiberglass -reinforced bear ings were supp lied by Ki rkhi ll Rubbe r Company , Brea,California.


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Neopre ne in compre ss io ncontinued

Figure 2Compressive Stress/Strain ofSteel-Reinforced Neoprene Bearings

(Hardness of Neoprene Compound-50 Durometer A)Shape Factor

20 16 12 9 6 51413121110c on,~ 9e n(/) 8e :'U 5 7)>'(jj

in 6)Q _E 50u 4

32

I I II~~I I

,/I I I 1 1 .....II / I 1l~I , 3 - . . !'I[Ill 'I11-I I I II II VPM V I HIN II V T~W I V V i - + l - -V Vu r f - - ' #) . . . . . . 0)V -g- ---[--,- r:;/ E O )Il~/ : : : J EE E c " '~ 8 "m1/ ~ & ! c 5 )

42,0001,8001,6001,400 '(jj

Q_e n1,200 (/)e :'U 51,000 0):0,

(/)(/)0)

800 Q _E0u600400200015o o 5 10

Figure 3Compressive Stress/Strain ofSteel-Reinforced Neoprene Bearings

(Hardness of Neoprene Compound-60 Durometer A)

Compressive Strain, %*Some speci fy ing agenc ies requi re l owe r maximum comp ress ive s t ra i n

1 200--t--t-+-t---+I5 10Compressive Strain, %

*Some speci fy i ng ag .enc ie s requi re l owe r max imum comp ress ive s tra in

400

15

Figure 4Compressive Stress/Strain ofFiberglass-Reinforced Neoprene Bearings


1612965 4 37 ~/I/ / /~~h~/'jI I : I -cI i i 'I,/ /' I / . II / u ,0)I~ u1/ cE O )

VV : : : J EI E E ~_0_c" ' " it i a s ~t-~ o : U 5

r f. 6~e n 5(/)0)c 5 ) 40)~ 3(/)

~ 2E8 1

o o 5 10Compressive Strain, %

*Some speci fy i ng agenci es require l owe r max imum comp ress ive s tra in

1,000'(jj

800 Q_e n(/)0)

600 c 5 )0)>

400 (jj(/)0)Q _200 E0u015

Figure 5Compressive Stress/Strain ofFiberglass-Reinforced Neoprene Bearings


161296 5 4 37 / )I'/II / ~ i if i r~JlI / T " '"~II. /I/ ,I/ u0)~ ucE O )

1// : : : J EE E x 8~/V c o O ) ~ i~ o : U 5 !Shape Factor

201612 9 6 5 414 rTTT17I/Ttt i""l/frllSJ/l!IIi l l 2,00013: : i ' ~ ~ ~ r T Ii::

~ 9 ' I l l / ~i-H 1,400 g ;_~ 8 f-----j--t---J-t--J-.' 1 , _ , _ / - - h 4 - - - + - - J I - / -+ 1,200 ~c 5 ) '1 '1 V I 1 - [ - - 4 U 5.~ 7 fIll I V I 1,000 .~~ : H - f -- I ff lW ' + -- j- Y -- lV , - -- > + -+ - f/ -A - + -- + -H H - -c l 800 ~o ,r;I/I~ 1 8u 4 H----j,fllr-II-1r-++-~1 /LJ__+_+__+_+_-_l____H__"'I 600

3 I - - + - l I - - 'I - / - - / - + A - V - + - - + ~ - I2 I--+H+~'-I--/ - - j , o ' / , - + - - + - - + E ~ -II' V .~~/ @~~a::

r f. 6~e n 5(/)0)c 5 ) 40)

.~ 3(/)~ 2Eou

o o 5 10Compressive Strain, %

*Some speci fy ing agenc i es requ i re l owe r max imum comp ress ive s t ra i n

1,000'(jj

800 Q_vi(/)e :'600 U 50).2(/)400 (/)0)Q _E200 0u

0I~

agencies in New York, California, and Texas, inaccordance with ASTM Standard D 575-81,Method B, "Standard Test Method For RubberProperties in Compression", Test equipment fromthree different manufacturers was used. Details ofpad construction and test procedures are given inTable III, page 7.The compressive stress/strain results from eachtest location were averaged and then combinedinto the graphs shown in Figures 2 to 5. (Resultswere yery consistent between the three locations).These graphs provide useful engineering designdata over a range of shape factors and at highcompressive loads.


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E f f e c t o f s h a p eShape Factor-Shape factor is one of the mostimportant parameters used in designing an elasto-meric bearing, because it determines the verticalcompression/deflection characteristics of thebearing.The abilityto deform under compressive stress,and then recover, is a characteristic property ofelastomers. However, the stress/strain relationshipin compression (but not in shear or tension) in anelastomer depends upon the shape of the piece.To illustrate, consider two blocks cut from the samepiece of elastomer compound. One is a cylinder,the other a rectangular block of the same heightand loading surface area. If equal loads are placedon the two blocks, subjecting them to the samecompressive stress, the rectangular block willdeflect more than the cylinder (see Figure 6). This

is understandable if one recognizes that the blocksdid not lose any volume when the weight presseddown on them; they bulged at the sides. The rec-tangular block deflects more than the cylindricalone because the sides of the rectangular blockprovide a greater area free to bulge. (If the blockswere confined-not free to bulge-they would notcompress.)For pieces having parallel loading faces andsides normal to the loading faces, as is the case inbridge bearing pads, the influence of shape maybe numerically expressed as the "Shape Factor".This value is defined as the ratio of the area of oneloaded surface to the area free to bulge. Equationsfor calculating shape factor of rectangular and cir-cular bearings are given in Figure 6. Shape factorcan be changed by changing either the loadingsurface area or the thickness of the elastomerlayer.In laminated bearings, bulging of the elastomerlayers is restricted because they are bonded to thereinforcement. Each layer behaves in compressionlike an individual pad, with its own shape factor.

For example, the typical bearings shown in Figure 1,page 1, behave like three 12.7 mm [0.5-inch] pads,rather than as one 38.1 mm [1.5-inch] pad. Thus,the controlling shape factor for a laminated bearingis that of the thickest elastomer layer. (In shear,however, the reinforcement does not restrict lateralstrain, so the total elastomer thickness is involved-see "Effective Rubber Thickness", page 9.)The calculation of shape factor differs for steel orfiberglass-reinforced bearings due to the nature ofthe cross-sections. Shape factor for a steel-rein-forced bearing is based on the net surface area ofthe steel plate or shim, rather than finished bearingdimensions. The outer layer of elastomer in a steel-reinforced bearing is needed only to protect the

Figure 6Effect of Shape onStress/Strain Characteristicsof Elastorners in Compression

CircularPadsS = _ 1 2 _4t

RectangularPadsS =LW2t 1 unloaded surfacesfree 10 expand

whereS=ShapeFactorL = BearinqLengthW =BearingWidthD = SearingDiametert =Thicknessof ElastomerLayer

NO LOAD

225 cm2 [34.9 in2] --. SurfaceArea-- 225 e rn" [ 34.960em2 [9.3 d)-Area Free to BUlge~ 53.1 em" [ 8.23.75 ~Shape Factor- 4.24

UNDERLOAD6.9MPa[1000 psi]If - - - - - h_ - - - - - - - - - -7.4%

steel from corrosion, and is frequently neglected inshape factor calculations where its thickness isless than 3.2 mm [0.125 inch]. For fiberglass-rein-forced bearings, shape factor is based on total paddimensions.Bearing pads made from the same elastomercompound and having the same shape factor willhave essentially the same stress/strain relationshipin compression, regardless of the actual size orshape of the pad. As shape factor increases, thestress required to produce a given strain increasesor, conversely, a given stress produces less strainin the bearing. However, there is no mathematicalrelationship between shape factor and compres-sive modulus. This relationship must be determinedempirically.


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Neop re ne in compre ss io ncontinuedThe Beta Factor-Most engineers now recognizethat there is a difference in load-bearing capacitybetween the internal bonded layers of a bearingpad and the external layers, which are restrainedlaterally only by friction to the pier or bridge struc-ture. Bearings held only by friction will tend to slip,causing more vertical deflection and higher shearstresses in the outer layer than are encountered inthe internal bonded layers of reinforced pads.A growing design practice in Europe and Austra-lia to account for the differences in load-bearingcapacity of fully restrained and partially restrainedelastomeric layers is to modify the shape factorwith a correction called the beta (~) factor. Stantonand Roeder (3) have suggested use of the ~-factoras follows:~= 1.0 for internal layers of reinforced bearings1.4 for cover layers of steel-reinforced

bearings1.8 for plain (unreinforced) bearings.Application of the ~-factor is illustrated in Figure 7.

F ig ure 7( 3-Fa c to r i n De s ign o f E l as tome ri c H.,~rln,n.

PLAIN ELASTOM ER IC BEAR ING PAD

1.4LAM INATED ELASTOM ERIC BEARING PAD

R ein fo rc ed w ith S te el

When using the ~-factor, the allowable compres-sive stress is determined from stress/strain chartsby using the function "shape tactor/S" in place ofthe actual shape factor. This means that the allowa-ble compressive stress for plain pads is considera-

bly less than that of a reinforced laminated bearingof the same dimensions. Also, the allowable com-pressive stress of a cover layer is less than that ofan internal bonded layer of the same laminatedbearing. The graphs in Figures 2 to 5, page 4, canbe used for plain pads or cover layers by adjustingthe shape factor with the appropriate ~-factor.L im ita tio n s o n s tra in a nd s tre ssWhen an elastomeric bearing is loaded, it deflectsvertically, producing a compressive strain in theelastomer. If this strain becomes too great, internalstresses develop in the elastomer which mayaccelerate the rate of creep and contribute toweathercracking in the bulging sides of the pad.Therefore, good design practice dictates somelimitation to the amount of compressive strain.Design limitations on compressive strain vary con-siderably among specifying agencies. Most agen-cies specify maximum strains in the range of 5 to10 per cent, but sometimes even smaller strainsare specified.Current AASHTO specifications define 5.5 MPa[800 psi] as the maximum allowable compressivestress for steel or glass fabric-reinforced elasto-meric bearings. It has been standard practice todesign bearings with a safety factor of 1.5. How-ever, very few data are available on the ultimatestrength of laminated bearings. Du Pont hasobtained more information by subjecting full-sizelaminated Neoprene bearings to failure undercompressive loads. Tests were conducted on bothsteel- and fiberglass-reinforced bearings havingshape factors of 3 to 6. Failure was defined as anydecrease in the load-bearing capacity of the pad.Average and maximum failure-causing loads arelisted in Table IV. These results suggest that steel-reinforced Neoprene bearings are able to with-stand greater compressive loads than are currentlyspecified by AASHTO. Of particular significance isthat all failures occurred by rupture of the steelreinforcement (14 gauge) in tension or tearing ofthe fiberglass reinforcement. Examination of thefailed pads showed that the Neoprene itself andthe bond between the Neoprene and the reinforce-ment remained intact.CreepWhen subjected to constant stress (compression,tension or shear), all elastomers exhibit a progres-sive increase in deformation with time, known ascreep. Creep must be considered and compen-sated. for in the design of bridge bearings.Creep characteristics in compression for a typi-cal Neoprene bearing pad compound are shown inFigure 8, page 8. These are laboratory data devel-


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Tab le I IIC omp re s siv e S tr es s /S tr ain T e st in g o f F u ll-S iz e L am in a te d N e o pr en e Be ar in g s-D e ta ils o f B e a rin g C o n st ru ct io n a n d T e st P ro c ed u re sA. Bearing Construction Steel-Reinforced Fiberglass-ReinforcedSizeShape Factor 3 to 20Effective RubberThickness, mm 3S.1in 1.5

3 to 163S.11.5

Hardness of NeopreneCompound, durometer A 50 and 60 50 and 60Reinforcement 14-gauge steel (1.9 mm [0.075 in]thickness) Wovenfrom Type E yarn withcontinuous fibersOverall Construction Steel bonded top and bottom to(See Figure 1, page 1) three 12.7 mm [0.5 in] layers ofNeoprene compound.Cover layer of Neoprene compound

3.2 mm [0.125 in] thick aroundoutside of pad-not included indesign calculations.

Composed of three 12.7 mm[0.5 in] layers with fiberglassreinforcement 1.6 mm [0.0625 in]from top and bottom of each layer.Cut edges of reinforcement leftexposed.

Supplier Oil States Industries, Athens, TX Kirkhill Rubber Co., Brea, CAB . Test ProceduresTest Method ASTM D 575-S1, Method BSpecific Test Conditions 1) Loading equipment was zeroed and calibrated using a 0.35 MPa[50 psi] load.2) Bearings were held at the specified load for 30 seconds before takingstress/strain (load/deflection) readings.3) Testing was performed in a stepped sequence-vi.e., load to 1.4 MPa[200 psi], hold for 30 seconds, take deflection readings; load to 2.SMPa [400 psi], hold for 30 seconds, take deflection readings; etc.Testing Agencies Departments of Transportation of the States of California, New Yorkand TexasTest Equipment California used MTSSystems Corporation equipment. New York usedTinius Olsen equipment. Texas used Robertson equipment.

oped approximately 25 years ago with smallunreinforced test specimens. More recently, theState of California Department of Transportationmeasured creep in compression on full-sizeNeoprene bearing pads. Measurements weremade at 6.9 MPa [1000 psi] constant load on steel-and fiberglass-reinforced pads in 1974(1)and at13.8 MPa [2000 psi] load on steel-reinforced padsonly in 1982(4).The creep curves from these testsare shown in Figures 9 and 10 on page 8. Extrapo-lation of these data indicates approximately 23%creep in compression after 10 years at 6.9 MPa[1000 psi] load for a bearing of 55 durometer Ahardness, and approximately 18% creep after 10years at 13.8 MPa [2000 psi] load for a bearing of50 durometer A hardness.

Maximum loadwithstood,MPa [psi] 75.S [11,000] 15.S [2300]Mode of Rupture of Rupture ofFailure reinforcement reinforcementEstimated lap shear strengthat maximum compressive loadbefore rupture of steelreinforcement in tension,[MPa] psi 4.S [700]

Table IVMa xim um L oa d C a pa city o f S te el- a ndF ib e r gl as s -Re in f o rced Neop r en e Bea r in g sSteel Fiberglass

Average loadto failure,MPa [psi] 55.2 [SOOO] 11.S [1700]


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Ne op re ne in comp re ss io ncontinued

Figure 8C re ep in C ompr es sio n (T yp ic al N e op re ne Be ar in g C ompoun ds )

50Durometer A Hardness,

40c02e f t ~ 30c i 1 U~ ~ 20u.",

.S; 10

70 ~i- "~ ~i"""'" 60./'"~ I-" - 50

~ ~(J)~ - a:w> -V" 0I I I I I IIII I I

10 100 1000 3650

45%

Time, days

35%

25%

FigureSC re ep o f N eo pr en e in L am in ate d B ea rin gsU n de r 6 .S MPa (1 ,0 00 p s i) C omp re ss iv e S tr es s

50~ Hardness of Neoprene Compound: (J) l-

55 Durometer A a: a:- U'j-U'j-~ Reinforcement: > - > -

Steel or Fiberglass ~ [il-- (See Reference 1)~ . . . . . . .~_ . . . .~ ",,' . . . . . .' --~~ -0:~ 10o o 0.1 10 100 1,000 10,000 100,000 1,000,000

Time, hours

F ig ur e 1 0C re ep o f N eo pre ne in L am in ate d B ea rin gsU n de r 1 3.8 MP a (2 ,0 00 p s i) C ompr es s iv e S tr es s

50 I I I I J 1 -- Hardness of Neoprene Compound: a: a:f- 50 Durometer A U'jr- U'j-I- Reinforcement: Steel > - > -

(See Reference 4) ~ [il-l- -

. . . .,. ,I- . . . . .- -. . . . . . . . .-_ . . . . I-I- - - - - -

g 40oQl~~ 30+0:~a 20if!.0:~ 10o o

0.1 10 100 1,000 10,000 100,000 1,000.000Time, hours


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N eo pren e in shearProper functioning of a Neoprene bearing isdependent on its ability to yield laterally (i.e., inshear) without permanent distortion or reduction inload-bearing capacity. This movement is governedby the parameters discussed in this section.E f fe ct ive rubbe r thicknessThe effective rubber thickness (ERT) of a bearingdetermines the amount of lateral (horizontal) move-ment allowed. ERT is defined as the combinedthickness of all elastomeric layers in a bearing. Forsteel-reinforced bearings, ERT is the total thicknessof the bearing minus the thickness of the steelshims. For fiberglass-reinforced bearings, how-ever, the thickness of the fabric is usually consid-ered negligible, so the ERT is the total thickness ofthe bearing.

Shea r s tr ain lim it at ionsElastomeric bearings will accommodate considera-ble lateral movement with no apparent ill effects.However, under conditions causing very highshear, the bearing will distort to the extent that theeffective vertical load area is reduced. A widely-accepted design practice is to limit shear strain ina bearing to 50 percent of the ERT.SlippageWhen a beam deflects horizontally (e.g., due toexpansion and contraction), it strains the elastomerin the bearing in shear and produces shearstresses at the bearing/beam and bearing/pierinterfaces. If shear stress exceeds the force of fric-tion, the pad will slip unless it is restrained mechan-ically. If slippage occurs, loading forces maychange and the bearing will not function asdesigned.She ar modulu sShear modulus is the most important engineering.property of an elastomeric bearing because itdetermines the lateral stiffness of the elastomerlayers. Shear modulus is defined by the relation-ship:

G ., F/A~/Twhere G = shear modulus,F = the load applied to the bearing pad,A = the area of the bearing over whichthe load is applied,

~ = the maximum lateral displacement ofthe pad,and T = the thickness of the pad (ERT)

Because many laboratories are not set up to meas-ure shear modulus, bearing manufacturers gener-ally use hardness as an indicator of stiffness of theelastomer layers. (See "Relationship of Hardness toStiffness, page 3.) Data relating hardness to shearmodulus over the temperature range of +20 to-40C [+68 to -40Fj are shown in Figure 11 forbearings of nominal 50 and 60 durometer A hard-ness. Engineers will be primarily interested in therelationships at 20C [68Fj.Effect of low temperatures-As a compound ofNeoprene is cooled to about -40C [ -40Fj, itgradually becomes stiffer, but not brittle. This stif-fening must be taken into account in shear forcecalculations. (But, it can usually be neglected incompression calculations because strain limita-tions are based on the lowest compressive modu-lus). As shown in Figure 11, shear modulusincreases as the temperature decreases. Thesedata were measured at the Du Pont ElastomersLaboratory on small test specimens of typicalNeoprene bearing pad compounds, using the testprocedure outlined in ASTM D 4014-81.Neoprene can be specially compounded toreduce stiffness at temperatures of -40C [-40Fjand below. The engineer should consult with thebearing pad manufacturer if extremely low servicetemperatures are expected.

F ig u re 1 1R ela tio n sh ip o f S he ar M od ulu s to H ard n es s o fN e op re n e C ompounds a t V a ri ou s T empe ra tu re sTemperature, O F

00 ~ ~ 0 -~ -~2.82.62 . 4

2.22.0

400_ j_LL'f~/

Hardness, s o Durometer 'lv /V ~VV V_",.~ l/'. . , .V"~ ,..... Hardness, 50 Durometer A

300

&:1.8: : ; ; :(j1.6e n- 1 . 4u~1.2O Jl'10(f)

'(ijQ_(je n200 _2:::JUo: : ; ; :O JQ)s:(f)0.8

0.60 . 4

0.2o2 0

100

o -10 -20 -30 -40Ternperature=C

o10


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N eo prene in shearcontinuedSome Neoprene compounds, if cooled to amoderately low temperature-e.g., somewherebetween 0 and -18C [32 and OF]-and heldthere for a period of time, gradually undergo a

phase change, or molecular realignment, whichcauses them to become stiffer and tougher. Thischange is called "crystallization". Although itincreases in stiffness, "crystall ized" Neoprenedoes not become brittle. Crystallization is a reversi-ble process; it disappears when the Neoprenewarms up. Also, crystallization is removed bymechanical work, such as vibration due to traffic ormovement due to thermal expansion and contrac-tion of the bridge structure.During 25 years of experience with properlycompounded Neoprene bearing pads, cold tem-peratures have not caused any problems. In serv-ice, the bearing is strained slowly, so the Neoprenehas time to accommodate. Also, most bridge bear-ings are subject to almost constant vibration anddimensional changes which will tend to inhibit crys-tallization.Effect of compressive stress-Static shearmeasurements on fullsized steel- and fiberglass-reinforced Neoprene bearings under compressiveloads were made by Engineering Computer Corpo-ration of Sacramento, California in cooperation withthe University of California-Davis. The compres-sive stresses applied were up to 13.8 MPa [2000psi] for steel-reinforced pads and up to 10.4 MPa[1500 psi] for fiberglass-reinforced pads. Thesedata, reported in Figures 12 and 13, show little

F ig ur e 1 2E ff ec t o f C omp re ss iv e S tr es s o n She ar Mo du lu so f N eo pre ne C ompo un ds in L am in ate d B ea rin gs

Compressive Stress. psi500 1000 1500 2000

100o0.7

0.6 . ~-Shear Strain g~ """.,.."..

Hardness of Neoprene Compound:55Durometer A

- Reinforcement: SteelI(see Rrrencr 5)

90 0.680 'iii coCLCL :2'cj cj 0.5ui70 .2 ui:::J .2'0 :::J0 '060 :2' 0:2' 04(;j (;jQ).L Q)50 (f) .L(f)

0.340

r l " .:2 '~ 05


13/16

Tab le VDescription of Neoprene Bearings used in Cyclic Shear Deformation Studies

Length Bearing Pad Number of ShapeWidth, Thickness (T b) , Neoprene Laminates, Factor,mm [in] mm [in] N S

Type ofBearing PadHardness,Durometer A

150 [6] 44 [1.75] 2.57Fiberglass-Reinforced 3 5538 [1.5] 3 3.33 55Steel-Reinforced

T~~~~l~~~~Figure 14

Effect of Maximum Shear Strain on DynamicShear Modulus of Neoprene Compoundsin Laminated Bearings

Maximum Shear Displacement Amplitude, Ino 02 0.4 0.6 0.8 10 1.2

Hardness of Neoprene Compound:55 Durometer A

Reinforcement: SteelCompressive Stress 5.5 MPa [800 psi](See Reference 5 and Table V)

225200 '8_

10075

0.4 0~~1'::-0-~20~--::'30::---4~0::--~5::':0~~~""7~0-~80Maximum Shear Strain, %

Schematic Diagram of ApparatusTesting with Dynamic Shear Loading

Figure15Effect of Maximum Shear Strain on DynamicShear Modulus of Neoprene Compoundsin Laminated Bearings

Maximum Shear Displacement Amplitude, Ino 02 0.4 0.6 0.8 10 12Maximum Shear Displacement Amplitude mm

5 10 15 20 25 30250

1.7Har~ness Jf NeoJrene 6ompo~nd- - 55 Durometer AReinforcement: Flbergla~S J

'\- Compressive Stress 5.5 MPa [800 psi ) -=

(See Reference 5 and Table V)

1 \ \\ "- " - o . . . 18Hz


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Damping characte ris ticsNeoprene bearings can provide damping of noiseand vibration in buildings when used over or underrailway installations, or when used to support heavymachinery. Railway vibration frequencies occur inthe range of 10 to 60 Hz, with the maximum vibra-tional energy transmitted at about 40 Hz.Laboratory tests of damping characteristics havebeen run by Du Pont using an MTS dynamic testingmachine and small, unreinforced Neoprene testspecimens. The test pellets were preloaded in

compression to induce a 15% static compressivestrain. Then, a cyclic vertical load was applied toinduce a 5% strain at a frequency of 10Hz. Dataon tan 0, damping coefficient and spring constantat temperatures of 21"C [70F] and -18e [OaF]arelisted in Table VI. Because sample size has aninfluence on damping characteristics of elasto-meric materials, these data should be used only asapproximations for large bearings.

Table VIDampin g Cha ra ct er is tic s o f T yp ic al N e op re n e C ompoun d sDamping SpringCoefficient, C Constant, k

Test N'sec [lbf-sec] N [Ibf]Temperature Tan 1) mm [in] mm [in]21C [70F] 0.07 0.42 [2.4] 385 [2200]-18C [OaF] 0.34 2.57 [14.7] 473 [2700]Ratio -18.oC/21C [0F/70F] 4.9 6.1 1.2321C [70F] 0.11 0.85 [4.8] 508 [2900]-18C [OaF] 0.37 4.0 [22.7] 683 [3900]Ratio -18C/21C [0F/70F] 3.4 4.7 1.34

Hardness50 durometer A

60 durometer A

Test Conditions:MTS Dynamic Test MachinePreload 15% compressive strainDynamic Load 5% strainFrequency 10HzSample Size Round pellet, 2.8 em [1.1 in] diameter, 1.3 em [0.5 in] thiek (Compression Set Pellet, TypeI, ASTM D 395-78)


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O the r ap plicatio ns that useb ea rin g p ad te chnolo gyThe technology related to Neoprene bridge bear-ings can be utilized in many other applications,some of which are listed below. Regardless of theend use, Neoprene bearings can be manufacturedeconomically, are mechanically effective, andrequire no maintenance. Long term durability ofcompounds of Neoprene under variable weather-ing condit ions and natural agjng has been demon-strated repeatedly in a variety of applications forover 50 years.Other applications where Neoprene is (or canbe) used for bearings and pads are:Building construction Isolating columns from footing Absorbing movement of long-span concretebeams Absorbing movement of "floating" roofs Acoustical insulation between floors Vibration isolation of laboratory and testingfacilitiesRail transportation Reducing noise and vibration transmissionbetween trains and track supportsMounting of heavy machinery Reducing noise and vibration transmission Reducing effects of physical movement onsurrounding areas.

This prestressed concrete highway bridge in Victoria County,Texas, built in 1957, is reported to be the first bridge in theUnited States to use Neoprene bearing pads. The originalpads have yet to require any service or maintenance and arethe best testimonial to the performance of Neoprene structuralbearings.

Im po rta nt co nsid eratio ns fo rsuccessful use o f N eo pren ein b ea ring app lic atio nsAlthough elastomeric bearings made with Neo-prene have demonstrated years of maintenance-free service, simply specifying Du Pont Neoprenedoes not automatically guarantee satisfactory per-formance of a bearing. The performance of thebearing depends not only on the materials used,but also on the quality of its design and the work-manship with which it is manufactured andinstalled.First, the bearing must be properly designed inaccordance with known design principles. Theengineering data on Neoprene bearings providedin this brochure should be helpful in this regard.Second, the Neoprene must be properly com-pounded for bearing pad applications. Third, thebearing must be manufactured by a knowledgea-ble, quality-conscious supplier. Fourth, samplebearings should be tested under simulated serviceconditions to verify design and fabrication. And,finally, the bearing must be installed by competentworkmen in compliance with the intended design.Du Pont does not make or install finished bear-ings. However, we can provide, on request, namesof reputable suppliers who use dependableDu Pont Neoprene. For more information, call ourElastomers Inquiry Center at 800-441-7111.References1. E.F. Nordlin, J.R. Stoker, et ai, "A Laboratory Evaluat ion of

Full-Size Elastomeric Bridge Bearing Pads", State of Califor-nia, Department ofTransportat ion Highway Research Report ,June 1974

2. J.C. Bryan, "Neoprene Bearings Support Highway Bridges",Elastomers Notebook 224, Winter 19823. J.F. Stanton and C.w. Roeder, "Elastomeric Bearings-

Design, Construction and Materials", NCHRP Report No.248, University of Washington, Seatt le, Washington, August1982

4. R.J. Spring, J.R. Stoker and E.F. Nordl in, "An Evaluation ofFiberglass- and Steel-Reinforced Elastomeric Bridge BearingPads", State of California, Department of Transportation,Research Report, January 1982

5. R.A. Imbsen, "Earthquake-Resistant Bridge Bearings",Vol. I, CONCEPT, FHWAlRD-82/165

Special Note-Except as otherwise provided by law outside the USA,the following information should be noted:The information set forth herein is furnished free of charge and IS basedon technical data that Du Pont believes to be reliable. It IS intended foruse by persons having technical skill, at their own discretion and risk. Thehandling precaution information contained herein is given with the under-standing that those using it will satisfy themselves that their particularconditions of use present no health or safety hazards. Since conditionsof product use are outside our control, we make no warranties, expressor implied, and assume no liability in connection with any use of thisinformation. As with any material, evaluation of any compound underend-use conditions prior to specification is essential. Nothing herein is tobe taken as a l icense to opera te under or a recommendat ion to inf ringeany patents.


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DU PONT COMPANYPOLYMER PRODUCTS DEPARTMENTWILMINGTON, DELAWARE 19898

Elastomers Inquiry Center800-441-7111

CanadaDu Pont Canada, Inc.TORONTOP.O. Box 26, Toronto-Dominion CentreToronto, Ontario M5K 1B6(416) 362-5621MONTREALP.O. Box 660Montreal 3, Quebec 113C 2V1(514) 861-3861

United KingdomDu Pont (U.K.) LimitedMaylands Ave., Hemel HempsteadHertfordshire, EnglandHemel Hempstead 61251

Europe, Africa and Near EastDu Pont de Nemours International SA50 52 route des AcaciasCH-1211 Geneva 24, Switzerland(022) 37-81-11AustraliaDu Pont (Australia) LimitedNorthside Gardens168 Walker StreetP.O. Box 930North Sydney, N.SW. 2060, Australia(02) 929-8455

Latin America and Far EastE. I. du Pont de Nemours & Co. (Inc.)Polymer Products DepartmentInternational Marketing Services DivisionWilmington, Delaware 19898, U.SA(302) 774-3784

om>.~G. us PAT&TMOF"

Documents

Neoprene Bridge Bearings Dupont 1984