Behavior and Design ofSelected Elastomeric

Bearing Pads

Leonard TulinProfessor of Civil Engineering

Department of Civil, Environmentaland Architectural Engineering

University of ColoradoBoulder. Colorado

`` Alex AswadStaff ConsultantI, Stanley Structures, Inc.Denver, Colorado

e general objective of this projectwas the determination of the be-

havior of elastomeric bearing padsunder monotonic or cyclic compression,cyclic shear and rotation, or combina-tions of these loading conditions.

Two types of bearing pads weretested, random oriented fiber (ROF)pads and Neoprene pads of AASHTOgrade. A summary of the test reportssupplied with these pads has been in-cluded in Appendix A. These pads areintended for general use under stemmedmembers or beams in the precast con-crete industry.

The scope of the project can best bedefined by describing the parameters

Note: The random oriented fiber (ROF) bearingpads, known under the trade name MASTICORD,were supplied by JVI, Inc. It should be emphasizedthat the results of the tests on ROF pads apply onlyto this product.

which were considered pertinent:1. A parameter, which has become an

index in representing pad geometry, hasbeen termed the Shape Factor. Thisquantity is related to the tendency ofthepad to bulge laterally under compres-sive loading.

Shape Factor; S =(L x W)1[2 (L + W)h

The range for S was selected as2.3 <S u 4.8. This limitation was estab-lished as being consistent with commongeometries of pads used in actual prac-tice.

2. The selection of the maximumcompressive stress to which the padswould be subjected was based upon acompromise between current practice orusage and manufacturers claims or es-tablished specification limits. The cur-rent AASHTO specifications , limitcompression stress on Neoprene pads toa maximum of 800 psi (5.52 MPa). The

16

level selected in the tests was raisedslightly to 850 psi (5.86 MPa) to examinethe practicality of the current limitation.While there are presently no publishedcode criteria limiting the compressivestress on ROF pads, current usage inparking structures employs a maximumvalue of 1200 psi (8.27 MPa) under fullcode live loads, while recent industryrecommendations-, and manufacturersallow a permissible value of 1500 psi(10.3 MPa). The value of 1300 psi (8.96MPa) for the average compression stressin the shear tests was selected as a rea-sonable compromise.

3. Three types of deformation of thepad were considered significant. Theycorrespond to shear, compression, andmoment loadings as shown in Fig. 1.Thus, the tests were designed to pro-duce such deformations either sepa-rately or in combination. Shear defor-mations of the order of fi 0.70h androtations, 8 – 0.03 radians, are recom-mended values suggested by designersand industry specialists. The probabilityof reaching all the maximum strains si-multaneously is generally quite small inan actual precast concrete structure. Innormal building structures, S is usuallymuch smaller as shown in the design ex-ample.

4. Creep, aging, or similar long-termeffects were not covered in the firstphase of the investigation. Limitedtesting on these effects was done at theend of the program (second phase). Lab-oratory temperature during the testsvaried approximately from 65° to 75°F(18° to 24°C). For simplicity, the nom-inal unloaded pad dimensions wereused in stress calculations.

CYCLIC SHEAR TESTSWITH SIMULTANEOUS

COMPRESSIONLoad actuators and reaction frames for

the cyclic direct shear apparatus areshown schematically in Fig. 2. In addi-

SynopsisRandom oriented fiber (ROF) and

Neoprene bearing pads were sub-jected to cyclic compression andshear loading in an effort to determinetheir characteristics when used asbearing pads for precast concreteparking decks and building structures.The cyclic test program included spec-imens in the thickness range from 1/a

to 1/2 in. (6.3 to 12.7 mm).These specimens were subjected to

compressive stress normal to the pad,with both parallel and nonparallelplatens, and were cycled up to 5500times to a maximum shear strain of 70percent. Apparent shear moduli andcoefficients of lateral resistance weredetermined for these specimens.

From the test results, conclusionsare drawn and design recommenda-tions are offered. A suggested designprocedure, together with a fullyworked design example of a precastprestressed parking roof structure, aregiven.

tion, Fig. 3a is an actual photograph ofthe test equipment. The vertical ac-tuator has a capacity of 220 kips (979kN), while the horizontal actuator has acapacity of 35 kips (156 kN). Both ac-tuators can be operated in load or dis-placement control by programming anMTS controller through an IBM -PCmicrocomputer.

The top platen was an 8 in. (204 mm)square concrete member, 3 in. (76 mm)thick, faced with a % in. (9.5 mm) thicksteel plate.

There were two bottom platens. InSeries I, the platen was an 8 x 8 x 3 in.(203 x 203 x 76 mm) concrete block witha wood float surface finish. In Series II,the platen was an8x8x3 in. (203x203x76 mm) concrete block with a double 3

PCI JOURNAL+May-June 1987

F5 FC ^M

h 1

I ^ E E TTh (o) (b) (c)

Fig, 1, Three types of deformation corresponding to (a) shear, (b) compression and(c) rotation

L!i1NORMAL LOADREACTION FRAME

NORMAL LOADACTUATOR .-

LOAD CELL SPECIMEN

LOAD CELL

TOP SUPPORT HORIZ. LOAD ACTUATORPLATE O HORIZ. LOAD

EMRAFSPECIMEN REACTfON REACTIONFRAM E

iBOTTOM i lii ' ROLLER ."STRUCTURAL ii

SUPPORT ; FLOOR ;SYSTEM

ri

Fig. 2. Cyclic direct shear apparatus (side view schematic).

percent slope on the top surface, whichwas roughened by acid etching (see Fig.3b). The pads were not fixed by gluing orany other method to the upper or lowerplatens. All platens were cleaned aftereach testing cycle using a steel brushand/or a fluted carhorundum stone.

The pad specimens were placed be-tween the top and bottom platens andsubjected to a predetermined compres-sive load level in the vertical direction,while cyclic shear displacements equal

to 70 percent of the pad thickness wereapplied horizontally. While some pilottests were run at full cyclic strains, i.e.,displacements between +S and –b, theactual tests were operated at half-cycleshear strains (pad displacementbetween + S and 0). The rates of loadcycling varied among the various testsand are indicated on each test report.

As time progressed and more famili-arity was developed with the responseof the pads, the rate was increased to a

18

Fig. 3a. Close-up view of platens and shear box.

C

ACID ETCHED PADSURFACE

3%

M ^X X_r^1 X

MESH-ESH SHEARC DISPL,

SECTION C-C

Fig. 3b. Bottom concrete platen with sloping face.

maximum of about 1000 cycles per hourfor the % in. (9.5 mm) thickness and to1400 cycles per hour for the ' in, (6.3mm) thickness. Some of the tests lastedabout 4 hours. No noticeable tempera-ture rise was observed in any of thetests.

There were two types of specimens asshown below:

(a) EOF pads made of random fibersembedded in an elastomeric ma-trix.

(b) AASHTO grade Neoprene pads.The results of the tests are shown in

the related force-displacement curves,and the information is summarized inTables 1 and 2. Copies of the full set ofcurves may be obtained from the manu-facturer. These curves were recorded inslow motion at the beginning, half-waythrough, and at the end of the test. Thedisplacement shown on the graphs mea-sures the lower platen movement.

Two typical load-displacement curves

PCI JOURNALIMay-June 1987 19

from Test Series II are shown in Figs. 5and 6. The Iower platen had the 3 per-cent slope. Typical values for the appar-ent shear modulus, G, and the coeffi-cient of lateral resistance, (defined asthe ratio of the shear to compressivestresses on the pad), are listed in Table 3for selected sizes and shear strains. Allof the specimens, except Pad A5, weresmaller than the 8 x 8 in. (203 x 203 mm)platens.

All listed or calculated stresses arebased on the nominal dimensions. Thedifference between actual and nominalthicknesses are not significant.

MONOTONIC ANDCYCLIC COMPRESSION

The MTS servo-controlled mechani-cal-hydraulic loading frame has upperand lower smooth platens which are 8

Table 1. Summary of test information on cyclic shear of ROF pads.

Totaltest

Maxi- dura-Compressive mum tion

Pad Pad size Shape stress No. of (min-Type of tests label (in.) factor (psi) cycles utes) General remarksI. Cyclic Al 4x4x% 2.66 1300, constant 1019 60shear withcompression; Al 4x4x% 2.66 700, constant 490 60flat parallel A2 5x5x% 3.33 1300, constant 840 100platens — —

A2 5x5x% 3.33 1300, constant 338 105

A3 4x4x ya 4.00 1300, constant 570 100

A4 4x6x t/a 4.80 1300, constant 600 105

A4 4x6x¼ 4,80 700, constant 781 60

A5 8x8x a 4.00 1304, constant 183 75

A6 3Y2x4x% 2.49 1300, constant 430 105 Two pads onp laten

A6 3 6x4x% 2.49 1300, constant 580 105 Two pads onplaten

A7 3%x4x% 1300, constant 4033 245 Two pads on2.49platen

S1 6x6x% 4.00 1300, constant 470 61.

S2 5x5x5a 2.50 1300, constant 393 62

11. Cyclic EL 3Y2x4x% 2.49 1300, constant 1024 63 Two pads onshear with p latencompression;nonparallel E5 3V2x4x'/o 3.73 1300, constant 5543 240 Two pads onplatens platen

Votes: 1 psi – 0.00689 MPa; 1 in. = 25.4 mm,Top platen was flat and steel lined. Bottom platen had a wood float finish when flat, or acid etchedsurface and a double 3 percent slope otherwise.

20

Table 2. Summary of test information on AASHTO grade Neoprene pads.

Totaltest

Maixi- dura-Compressive mum tion

Pact Pad size Shape stress No. of (min-Type of tests label (in.) actor (psi) cycles utes) General remarks

1. Cyclic NA2 5x5x% 3.33 850, constant 540 62 Large side bulging,shear with disappears at loadcompression; removalflat parallelplatens NA6 3 r/ax4x% 2.49 850, constant 520 65 Large side bulging,

disappears at loadremoval

1I. Cyclic NE4 3Yzx4x% 2.49 850, constant 4249 240 Top face expandedshear with permanently VA in.compression; with respect tononparallel bottom (two padsplatens on platen)

Notes: 1 psi – 0.00689 MP'; lie. = 25.4 mm.Top platen was flat and steel lined. Bottom platen had a wood float f inish when flat, or acid etchedsurface and a double 3 percent slope otherwise.

in. (203 mm) in diameter. The actuatorhas a 110 kip (489 kN) capacity and canbe operated in either force or displace-ment mode controlled by an IBM-PC.There were three types of tests con-ducted on the MTS loading frame (seesummary in Table 4 and Fig. 4):

(a) Monotonic compression on ROFand Neoprene pads: In these tests the 8x 8 x 3 in. (203 x 203 x 76 mm) concreteplaten with a flat top was placed overthe 8 in. (203 mm) diameter machineplaten and served as the lower platen forthe pad. Then a steel plate equal in sizeto the pad was placed over it beforecompressive stresses were applied, ex-cept for Pads C44 and C45, where theupper machine platen was in directcontact with the pad. The objective wasto observe the stress-strain curve forvarious stress levels.

(b) Cyclic compression on ROF Pad81: In this test the machine platenswere in direct contact with the pad. Thecompressive stress varied with a mini-mum of 1000 psi (6.9 MPa) to a maxi-

Fig. 4. MTS vertical loading framecontrolled by an IBM-PC.

PCI JOURNALMay-June 1987 21

Table 3. Selected values for apparent shear modulus, G, and lateral resistancecoefficient, µ".

Corn- shearpressive strain Shear Coeffi-

Pad stress Pad Shape (per- modulus cienttype (psi) designation factor cent) (psi)

I. Parallel ROF 1300 A3 (4x4x'/4 in,) 4 35 464 0.125platens (zero 70 390 0.210percent slope) ROF 1300 A6 (3Ys x4x % in.) 2.49 35 469 0.126

70 383 0.206

ROF 1.300 S1(6x6x%in.) 4 35 452 0.12270 385 0.207

ROF 1300 A5 (8x8x ½ in.) 4 35 402 0.10870 328 0.177

ROF 700 Al (4x4x%in.) 2.66 35 357 0.17870 321 0.321

ROF 700 A4 (4x6x' in.) 4,80 35 345 0.17365 320 0.298

U. Nonparallel ROF 1300 El (3½x4x% in.) 2.49 35 388 0.104platens (3 70 344 0.185percent slope) ROF 1300 E5 (3 1/2x4x 1/a in.) 3.73 35 423 0.114

70 362 0.195

Neo- 850 NE4 (3 x4 x% in.) 2.49 35 224 0.092prene 70 219 0.181

' The coefficient is defined as the ratio otthe shear to compressive stresses on the pad.Note: 1 psi = 0.00689 MPa; 1 in. = 25.4 mm.

mum of 1500 psi (10.3 MPa). The cyclicduration was 1 second.

(c) Cyclic concentrated bearing onROF pads: In Test D2, a5 x 5 x % (127 x127 x 9.5 mm) pad, the machine platenserved as the lower platen, and a 4 x 5 in.(102 x 127 mm) steel plate was centeredover the pad. In Test D3, a 4 x 4 x 1/4 in.(102 x 102 x 6.3 mm) pad, the 8 x 8 in.(203 x 203 mm) concrete platen wasused as the bottom platen, and a 3 x 4 in.(76 x 102 mm) steel plate was placedover the pad. Compressive stresses wereapplied in a cyclic fashion between aminimum of 1000 psi (6.9 MPa) and amaximum of 1500 psi (10.3 MPa) overthe 4 x 5 in. (102 x 127 mm) or 3 x 4 in.(76 x 102 mm) plates, respectively. Thecycle duration was 1 second.

The results of the tests are summar-ized in Table 4. The curves were re-corded in slow motion (over 6 minutesplus or minus) and, in the case of cycliccompression, were recorded before andafter the tests. Two stress-strain curvesare shown in Figs. 7 and 8. Typicalvalues for the compression strainfrom the monotonic tests are listedin Table 5.

SLOW MOTION SHEAR TESTSWITH SIMULTANEOUS

COMPRESSION OF ROF PADSThe testing equipment and setup

were the same as those described earlierfor the horizontal cyclic shear tests. The

22

Table 4. Summary of information on compression tests for FtOF and Neoprene pads.

Corn- Maxi-Pad pressive mum Total test

Pad size Shape stress No, of durationType of tests label (in.) factor (psi) cycles (minutes) General remarks

T. Monotonic C11 3x4x'/a 3.43 8620 max. 1 6 Bottom: moderatecompression abrasion aton ROF pads maximum load

C33 3x4x% 2.29 8.500 max. 1 6 Bottom: moderatesplitting atmaximum load

C33 3x4x + 2.29 2500 max. 1 6 No sign of damage

C44 4x4x¼ 4.00 3300 max. 1 4 No signs ofcracking, splitting,or delamination

C45 5x5x% 3.33 3300 max. 1 4 No signs ofcracking, splittingor delamination

II. Cyclic gl 5x5x% 3.33 1000 min. 1000 17compression 1500 inax.on ROF pads

IIL Cyclic D2 5x5x% 3.33 1000 min. 3600 60 Top platen: 4x5 in.concentrated 2,96* 1500 max. steel platebearing on

D3 4x4x' 4.00 1000 min. 1000 17 Top platen: 3x4 in.ROF pads

3.43* 1504) max. steel plate

IV. Monotoniccompression NC2 5x5x% 3.33 3800 max, 1 6on Neoprene

* If portion outside of covered bearing neglected.Notes: 1 psi — 0.00689 MPa; 1 in. = 25,4 mm,

Top and bottom platens were parallel.

bottom platen had the 3 percent doubleslope with a roughened acid etched sur-face as shown in Fig. 3h.

A single, constant compressive stresslevel of 1050 psi (7.2 MPa) was used,The maximum shear displacement wasset at 0.70h. Two different pairs of padswere tested. In the first test 4 x 3½ x 1/4

in. (102 x 89 x 6.3 mm) pads were usedwhile the second test used 4 x 3 1/2 x % in.(102 x 89 x 9.5 mm) pads. Both tests wererun at a rate of 4 hours per cycle first,followed by three relatively fast cyclesat the rate of 1 minute per cycle.

The load-displacement curves fromthe second test are shown in Fig. 9. Thedashed curve is for the slow motion rate.

FAST CYCLING IN SHEARWITH SIMULTANEOUS

COMPRESSION OFPREOZONIZED ROF PADS

Testing proceeded in the same man-ner described earlier using the doublesloped platen of Fig. 3b, Before the me-chanical testing, the pads underwent ac-

PCI JOURNAL, May-June 1987 23

LOADfKIPS)

Fig. 5. Cyclic shear with compression of 1300 psi (3 percent slope on bottom platen).

LOADIKIPS)

TEST #3 4/2/25TWO NEOPRENE 3.5x4 x3/8 NE4DISP =0.262 IN. P=850 PSICYCLES 4245-4249

DISPLACEMENT (IN.)

Fig. 6. Cyclic shear with compression of 850 psi (3 percent slope on bottom platen).

24

-2U)a

U)2U)w

N

ISTRAIN IIN. /IN.)

a-

U)U)w

STRAIN IIN./IN.)

Fig. 7. Monotonic compression test for ROF pad (S = 2.29).

Fig. 8. Monotonic compression test for Neoprene pad (S = 3.33).

PCI JOURNALIMay-June 1987 25

Table 5. Selected values for compression strains.

Corn- Com-pressive pressive

Pad Shape Top Bottom stress strainPad designation type factor platen platen (psi) (percent) Remarks

I. MonotonicC11 (3x4x' in.) ROF 3.43 3x4 in., 8x8 in., 1200 16

steel concrete 2000 21C33 (3x4x% in.) ROF 2.29 3x4 in., 8x8 in., 1200 22

steel concrete 2000 29C33 (3x4x% in.) ROF 2.29 3x4 in., 8x8 in., 1200 26

steel concrete 2000 36

II. CyclicBI (5x5x% in.) ROF 3.33 8 in. dia., K in. dia.. 1000 19-20 Before and

steel steel 1200 22.5-23.5 after thecyclic test

I 11. Cyclic withconcentratedhearing ROF 3.33 4x5 in., 8 in. dia., 1000 18.5-21.5 Before and1)2 (5 x 5 x % in.) 2.96* steel steel 1200 22-25 after the

cyclic testIV. MonotonicCompressionon NeopreneNC2 (5x5x% in.) Neo- 3.33 5x5 in., 8x8 in., 600 16

prene steel concrete 800 211000 27

• if portion outside of covered bearing is neglected.Top and bottom plates were parallel.Note: I psi = 0.00689 \1Pa; 1 in. = 25.4 mm.

celerated aging in an ozone chamber atan Ohio laboratory. In this exposure, thepads were placed flat for 96 hours in thechamber at an ozone concentration of 25pphm (parts per hundred million). Thenthey were sent to the University of Colo-rado for mechanical testing.

The compressive stress was a constant130() psi (9 MPa) and the maximumshear displacement was 0.70h. Twopairs of pads were used; 1/4 and % in, (6.3and 9.5 mm) thick, both 4 x 3 t/z in. (102 x89 mm) in plan. Each of the tests lasted 2hours, and the pads were cycled approx-imately 2900 times in shear from zero tomaximum displacement, then back tozero. Load-displacement curves fromthe second test are shown in Fig. 10,

CONCLUSIONS ANDRECOMMENDATIONS

1. Random oriented fiber (ROF) padswere subjected to fast cycles of loading.The behavior of these pads under thetabulated average stresses and uis-placements showed full rebound andno apparent stiffness deterioration at theend of the cycle sequence when theforce-displacement curves were com-pared. Some minor surface abrasion wasapparent at the end of the tests, espe-cially at the face in contact with therough concrete platens. Pads C11 andC33, however, exhibited moderate abra-sion and splitting at the lower faceswhen the maximum compressive stress

26

LOAD(KIPS)- 6-r-

SHEAR FORCE VS. SHEADISPLACEMENT 5/25/85 4

TWO PAD E9 ROF 3,5x4x3/8;P = 1050 PSIIMIN2 /CYCLE 2

4 HOUR CYCLE -- — — — 1/2 CYCLE,OT10N

^!

-4

-'61

Fig. 9. Cyclic shear with compression of 1050 psi (slow motion and fast cycling ofROF pads).

LOA DIKIPS)I_PS

SHEAR FORCE VS. SHEAR DISPLACEMENT 6/28/85TWO PAD R113-R114 3.5x4 x 3/8 PRE-OZONIZED PADS

6 HALF CYCLES COMPLETED 4CYCLES 2910-2916 ; P = 1 304 PS I

2

DISPLACEMENT IIN.)

.30 '.25 '20 '.w5 - 10 -.05

-2

4

-6

-B

Fig. 10. Cyclic shear with compression of preozonized ROF pads.

of 8500 psi (59 MPa) was reached. This and the coefficient of lateral resistance,was to be expected since the stress level p, appear to be a function of the corn-was several times greater than the nor- pressive stress, the shear strain, and themal range of 1500 psi (10.3 MPa). platen slope. However, they are not sen-

(a) The apparent shear modulus, G, sitive to the shape factor within the test

/ Dlt

PCI JOURNALrMay-June 1987 27

program range (2.49 < S -_ 4.80). For a'in. (6.3 mm) or % in. (9.5 mm) thick padsubjected to compressive stress of 1300psi (9 MPa), the average values (at 70percent shear strain and 3 percent slope)are G = 350 psi (2.4 MPa) and µ = 0.19.It is a known fact that these figures de-pend on the platen roughness and sheardisplacement rate. Because of thevisco-elastic behavior of elastomers,very slow rates can result in much lowershear moduli and lateral resistance co-efficients.

(h) The compressive strain in themonotonic or cyclic tests was highlysensitive to the shape factor and rough-ness of the lower platen. A typical valuefor this strain for a pad with a shape fac-tor, S – 3.43, tested against a concreteplaten under a compressive stress of1200 psi (8.3 MPa) was 16 percent.

2. ROF pads were also subjected toslow motion shear tests. The purpose ofthese tests was to simulate the behaviorunder a double tee leg in a parking deckroof. These roofs are subject to a dailytemperature gradient cycle which couldeasily reach a 45°F (25°C) differential ona hot day and cause a horizontal pad dis-placement of 0.12 in. (3 mm) over a 6-hour period. The actual equivalent ratewould be about 0.02 in. per hour (0.5mm per hour) or 9 to 13 times slowerthan the test rates. The tests showed thatthe coefficient, p, decreases by 47 to 60percent when the rate was reduced fromI minute per cycle to 4 hnurs per cycle.For a compression stress of 105(} psi (7.2MPa) and a shear strain of 0.70, p. equals0.095 for the ¼ in. (6.3 mm) pad, andequals 0.082 for the % in. (9.5 mm) pad.It would, therefore, be safe to assume acoefficient of lateral resistance of nomore than 10 percent for these stressand strain levels when the shear defor-mation is due to a very slow motion,such as temperature gradient effect orcreep and shrinkage. It is worthy of notethat this value is half the commonlyused figure of 20 percent, which hasbeen used by the precasting industry

and is also more in line with the findingsof Ref. 3.

3.The results of the preozonized, ran-dom oriented fiber 1/4 in. (6.3 mm) padsshowed surface abrasion only and about¼ in. (3 mm) permanent expansion onone side of the pad. The calculatedshear modulus G values are 14 percentgreater than in the nonozonized case.The - in. (9.5 mm) pads showed moder-ate abrasion, residue, and some tear atone corner of the pad (the high endduring the tests), In comparing Fig. 10with the test result on Pad El (Table 3),it may be observed that G and A. in-creased by about 16 percent. Otherwise,the general functioning of the pads didnot seem to be affected,

4. AASHTO grade Neoprene padswere also subjected to fast cycles ofloading with a steel upper platen and aconcrete lower platen. These pads gen-erally showed satisfactory behaviorunder the given stresses and displace-ments without any stiffness deteriora-tion in shear. Only insignificant surfacescratching was noticeable at the end ofthe tests with full rebound except for thetwo specimens (Pad NE4) which weresubjected to nonuniform hearing, anaverage stress of 850 psi (5.9 MPa) and4249 cycles of shear displacement. Thepermanent expansion or set of the topsurface, however, was about 4 1s in. (3mni), which is considered minor.

(a) The average value for the shearmodulus, G, was approximately 200 psi(1.5 MPa) while the lateral resistancecoefficient, N., was approximately 0.18 at70 percent shear strain, 3 percent slope,and 850 psi (5.9 MPa) compressivestress.

(b) The compressive strain corre-sponding to a shape factor of 3.33 variedlinearly from 16 to 27 percent when thestress changed from 600 to 1000 psi (4.1to 6.9 MPa).

(c) The lateral bulging under com-pressive stresses, however, was signifi-cantly greater than for the ROF pads.Under compressive stresses of 1000 psi

28

(6.9 MPa), the bulging reached 0.53 in.(13 mm) each side of the specimen overa concrete platen while under 1200 psi(8.3 MPa) it was about 0.75 in. (19 mm).Its vertical deformation was also sub-stantially greater for steel platens (about50 percent more).

5. This series of tests confirms that thePCI recommendations (Section 6.5.8,Ref. 2) for allowable average compres-sive stress in random fiber reinforcedelastomeric pads are well justified. Therecommendations, however, stipulate amaximum rotation limit, 0 < 0.3t/(b orw), unless testing is done. The testsclearly demonstrated that this rotationlimit can be waived provided d _- 0.03radians.

For commonly used ROF pads with ashape factor, S, ranging from 2.66 to4.80. PCI's allowable average stresseswould he 1266 to 1480 psi (8.7 to 10.2MPa), respectively, based on 1000 + 100S. Most stemmed slabs and beams inparking or office structures would haveaverage service load stresses falling inthat range unless heavy loads are spe-cific (libraries, plaza decks with plant-ers, etc.). Occasional heavy loads, suchas parking decks whose access by firetrucks is physically possible, may stillhe accommodated using ROF pads witha spreader plate. The tests have shownthat a rare overstress of 2500 psi (17.2MPa) is acceptable and would not causepermanent damage.

6. The tests also indicate that PCI'smaximum stress recommendation forAASHTO grade, unreinforced chloro-prene (Neoprene) pads is conservativefor shape factors, S, less than 3.33. Theauthors would recommend instead anallowable average pressure of 800 psi(5.5 MPa) for all pads with a shape fac-tor of S _- 2.5. The rotation limit 8 c0.3t1(h or w) is also waived for thesame reason mentioned in Conclusion 5provided it does not exceed 0 = 0.03 ra-dians.

7. An accurate analysis of memberdeformations due to creep, shrinkage

and relaxation is warranted for non en-closed decks subject to temperaturegradients or for long span roofs usinglightweight aggregates. In such an anal-ysis, the end rotation effects on the paddisplacement should be consideredconcurrently with axial shortening ofthemember.

The design example using a 60 ft (18.3m) long parking roof made of concretewith a unit weight of 118 pcf (18.6kN/m y) shows that a typical '/a in. (6.3mm) thick ROF pad will handle the im-posed volume change deformations, in-cluding the expected temperature gra-dient.

8. If the concrete bearing surface sup-porting a stemmed member is substan-tially uneven due to air bubbles floatingto the top, a % in, (9.5 mm) thick pad isrecommended instead of the 1/4 in. (6.3mm) thick minimum for most floor orroof members.

9. The design engineer is urged tocheck with the manufacturers of ROFpads on the consistency and uniformityof the product as part of the normalquality assurance program. The tensilestrength of ROF products is relativelylow and a consistent low fiber percent-age may lead to premature tears.

FURTHER RESEARCHFurther research is needed in the fol-

lowing areas:1. Development of standard test

methods for the quality assurance ofelastomeric bearing pads used inbuilding construction.

2. Development of guidelines for post-erection rotation and displacement val-ues of pretensioned members, includingcomposite floors and noncompositeroofs over the whole range of practicalspans.

3. Development of recommended tol-erances for the supporting member'ssurface and dimensional tolerances forthe pad itself.

PCI JOURNALiiMay-June 1987 29

DESIGN PROCEDURE

The following procedure is offered asa recommended guideline in the designof bearing pads in precast concretestructures when the shape factor iswithin the range 2.5 -- S -- 4.80 and therotation change, 0, is Iess than 0.03 ra-dians:

1. Evaluate the volume changes andend rotations in the precast concretemember using a rational design methodsuch as described in Ref 5.

2.Calculate the pad displacement dueto combined member shortening andend rotation. These two effects may headditive when the camber tends to growwith time. Estimate the thermal gra-

dient effects, if any, using Ref. 4, orsimilar methods.

3. Compute the maximum or dailydisplacement, fi.

4. Select the minimum pad thicknessh _- fi/0.70 but not less than r/4 in. (6.35mm) for double tee pads anda in. (9.53mm) for beams. Refer to Conclusion 8for uneven bearing surfaces.

5. Select a minimum bearing areagreater than Reaction11300 psi for ROFpads and greater than Reaction1800 psifor Neoprene pads. Round off dimen-sions to the nearest practical sizes. Use asteel shim when the pad is larger thanthe member width.

DESIGN EXAMPLE - DEFORMATION PREDICTIONOF A PARKING ROOF PAD

Assume a typical 60 ft (18.3 m) simplespan, composite double tee deck con-sisting of a 10 ft (3.05 m) wide x 24 in.(610 mm) deep precast, lightweight sec-tion with 3 in. (76 mm) thick deck asshown in Fig. 11.

The concrete unit weight is 118 pcf(18.5 kNIm 3) in the precast member and150 pcf (23.6 kN/m') in the topping.

Young's modulus of elasticity at re-lease is assumed to he equal to 2024 ksi(19,350 MPa) and the maximum shrink-

age strain is set at 0.00096.Concrete creep and aging follow the

AC! 209 recommendations.The pertinent data on section prop-

erties, loads and prestressing strands arelisted in Fig. 11(c).

Using the type of analysis suggestedin Ref. 4, the rotations and displace-ments shown below are obtained.

To estimate the temperature effects,assume that the members were erectedat 60°F (16°C) and that the temperature

Parameter Rotation DisplacementDifference in end rotation between Ht - a, = 0.0139 radianserection and final rotation (at 10 years)

Half-span shortening 8, = -0.290 in.

Displacement due to end rotation 8, = 0.0139 (21.13)change = 0.294 in.

Instantaneous end rotation under a H3 = 0.0021 radiansrealistic 18 psf passenger car loading

Fi, = 0.0021 (21.13)Displacement change due to 83 = 0.045 in.

30

C-)

0C7)ZrDDC

CD

3"C.I.P. DECK (STONE} AT ERECTIONTIME

125 °F

1

- C.G. COMPOSITE SECTION LIGHT WEIGHT

27" PRECASTY bq 21.13" MEMBER I

L.6O°F 80°F

120"

{ b) TEMPERATURE(o) ROOF CROSS SECTION PROFILE

(c) PRECAST PROPERTIES, LOADING AND STRANDS: iyA= 498IN, 2 ; 5 5 -1470IN. 3 ;I=25T17IN i1

DEAD LOAD OF DOUBLE TEE =0.408 k/FT.

TOPPING LOAD 0.375 k/FT. ; MAX. REACTION PER LEG=17745 LBS.

STRANDS: 1. 12^- 1/2 IN. DIA. LOW-RELAXATION 52PAD el ^- O.O 190

GRADE 270, JACKED TO 0•70fpu AT ERECTION S4ECCENTRICITY AT MIDSPAN = 14.24 IN.

ECCENTRICITY AT ENDS = 7.49 IN_

(d) PAD DEFORMATIONS

Fig. 11. Roof cross section, temperature profile and pad deformations.

profile on a hot summer day is as shownin Fig. 11(b). Then, using standardmethods of mechanics and Ref. 5, thefollowing values are derived for a 45°F(25°C) gradient:End rotation change over 5 hours .....

..................0, = 0.007 radiansNet displacement at leg bottom due totemperature increase and gradient ....

............. 84 = —0.095 in. (2.4 mm)In conclusion, it appears that the net

effect of creep and shrinkage (82 + 6,) isalmost zero in this case while the dailysummer temperature swing emerges asthe dominant effect with a displacementof about 0.10 in. (2.5 mm). Due to varia-tions in material properties, a value 610.12 in. (3 mm) should be considered fordaily cycling. The displacement 8;,under an 18 psf (0.86 kN/m 2) live load isopposite to rS. but much smaller in mag-n itude.

The design engineer should be awarethat the net effect of creep and shrinkagecould be much higher if the tees werehighly prestressed either because oflower or no allowable bottom tension ordue to unusually large live loads.

The service load reaction per stein iscalculated and shown in Fig. 11(e) as17,745 lbs (78.9 kN) based on 40 psf (f.9kN/m-) live load. Assuming an allowablestress of 800 psi (5.86 MPa) for anAASHTO grade Neoprene pad, the re-quired bearing area would he:

17,7451800 = 22.2 in. 2 (14323 mrn2)Use a S x 5 in. (127 x 127 mm) pad sincethe stem width is 4.75 in. (121 mm).Shape factor:

For a 1/4 in. (6.3 mm) pad, S = 5.0For a % in. (9.5 mm) pad, S = 3.3Thickness selection: The allowable

shear displacement for a 1/4 in, (6.3 mm)pad is 0.70 x 0.25 = 0.175 in. (4.5 mm),which is greater than the calculatedvalue of 0.12 in. (3 mm). Therefore, itmay be used in this case, unless the

bearing surface is significantly uneven.In the presence of extreme roughness, a% in. (10 mm) thick pad would be rec-ommended.

Rotation check: The calculations indi-cate a rotation B less than 0.03 radians atall times.

Conclusion: Use a 5 x 5 x'/a in. (127 x127 x 6.3 mm) pad.

An ROF type pad may also be used inthe place of Neoprene since its allow-able compression stress is 1300 psi (9MPa) from the test, or: 1000 + 10051500 psi (I0 MPa) by Ref. 2. Note that ifthe precast tee were made of normalweight concrete and had a narrow stem,an ROF pad may be the only feasiblesolution.

ACKNOWLEDGMENT

The tests described in this paper wereperformed in 1985 at the University ofColorado in Boulder for the ColoradoPrestressers Association under an origi-nal agreement with Stanley Structures,Inc. The principal investigator wasProfessor Leonard G. Tulin, Ph.D., P.E.The laboratory technician was EricStauffer.

Special thanks are due to F. J. Jacquesof Stanley Structures and Paul Mack ofRocky Mountain Prestress for theirhelpful comments.

The authors gratefully acknowledgethe financial support by JVI, Inc. andmember companies of the Colorado Pre-stressers Association, namely StanleyStructures, Inc., Rocky Mountain Pre-stress and Stresscon Corporation.

A report containing detailed descrip-tions and log sheets is available to prin-cipal investigators as Appendix B fromJVI, Inc., 7315 North Monticello,Skokie, Illinois 60076. Telephone: 312/675-1560.

32

REFERENCES

1. Standard Specifications for Highway Illinois, 1985, I11 pp.Bridges, Thirteenth Edition, American 4, Aswad, A., "Rational Deformation Predic-

Association of State Highway and Trans- tion of Prestressed Members," Deflectionsportation Officials, Washington, D.C., of Concrete Members, Special Publication

1983. SP-86, American Concrete Institute, De-2. PCI Design Handbook — Precast and Pre- troit, Michigan, 1985, pp. 263-282.

stressed Concrete, Third Edition, 1985, 5. ACI Committee 435, "State-of-the-Art Re-

Prestressed Concrete Institute, Chicago, port on Temperature-Induced DeflectionsIllinois. of Reinforced Concrete Members," De-

3. Iverson, James K., and Pfeifer, Donald W., flections of Concrete Members, Special

"Criteria for Design of Bearing Pads,"

Publication SP-86, American Concrete In-

Prestressed Concrete Institute, Chicago, stitute, Detroit, Michigan, 1985, pp. 1-14.

NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by February 1, 1988.

PCI JOURNAL/May-June 1987 33

APPENDIX A - SUMMARY OF MATERIALTEST REPORTS

1. Random oriented fiber (ROF)pads (supplied by JVI Inc.)Hardness, Shore A: 76 to 78Maximum compression: 10,200 to

11,200 psiTensile strength, ASTM D412 (Die C):

1053 to 1297 psiElongation: 64 to 139 percentTear strength, ASTM D624 (Die B): 385

to 462 lbs/in.Heat aging, ASTM D573:

a. Change in tensile strength: –3 to–18 percent

b. Change in elongation: –4.5 to –17percent

c. Change in hardness: +1 to 2 ptsOil swell, ASTM D471: 37 to 56 percentShear modulus: Constant in all direc-

tions parallel to the bearing plane (fora given shear strain).

2. AASHTO grade Neoprene pads(supplied by Scougal Rubber)Hardness: 62Compression set, ASTM D395 (Method

B): 19Tensile strength, ASTM D412: 3281 psiElongation at break, ASTM D412: 460

percentTear strength, ASTM D624 (Die C): 321

lbs/in.Heat aging, ASTM D573, 70 hrs/212°F:

a. Change in tensile strength: 1.08percent

b. Change in elongation: –5.4 percentc. Change in hardness: +3 pts.

Ozone test, ASTM D-1149 (100 hrs at100 pphm, 20 percent strain): pass

Note: I psi = O.()689 4MPa; 110. = 25.4 mm; 1 lb —4.448 N.

34

APPENDIX B -- NOTATION

G = apparent secant shear modulush = pad nominal thicknessL = length of padW = width of padS = shape factor, ratio of the bearing

area to lateral (unconfined) surfaceof the pad

8 = horizontal pad displacement8 C. = uniform vertical pad displacementN = pad rotationE.c = coefficient of lateral resistance,

defined as the ratio of the shear tothe compressive stresses on thepact

PCI JOURNALYMay-June 1987 35