Behavior of Plain Elastomeric

Pads

John Stanton & Scott Tetzlaff

University of Washington

AASHTO SCOBS T-2 Meeting

9 July 2012, Austin, TX.

Acknowledgments

• AASHTO

• MN DOT

Plain Elastomeric Pads – the

Problem.

• Plain pads are made from unreinforced

sheets of elastomer (Natural Rubber or

Neoprene.)

• MN DOT found plain pads bulging out

from the plates that they support.

• Was something wrong with the rubber?

• Are the AASHTO design values at fault?

MNDOT bearing

Girder

Steel

rocker

Elastomeric

pad

Welded

studs

Concrete

pier cap

MNDOT bearing

MNDOT bearing

Plain Pad in Compression

Plain Pad in Compression

Plain Pad in Compression

Slip No slip Slip

Comparison with Steel-Laminated

Bearing

Comparison with Steel-Laminated

Bearing

No lateral expansion –

bonded to internal plates

Plain Pad in Compression

Slip No slip Slip

Rubber volume stays constant.

Lateral expansion implies vertical deflection.

Expansion resisted by friction (m) and rubber stiffness, G.

Compression Stiffness.

Reinforced (bonded) Bearing

t

AEK cc

Kc = stiffness

A = plan area

t = rubber thickness

Ec = Effective modulus

S = Shape Factor

W

L

ti

WLt

LWS

i

224GSEc

Compression Stiffness. Plain pad

LRBcKPEPc EfE ,,

fK ≈ 0.75m (Nasty math. Depends on slip/no slip boundary).

Assumes: Incompressible material.

W = infinite (strip bearing).

Constant m (independent of contact pressure).

Plain pads:

Compression Stiffness Factor

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

m

f K

S = 2

S = 3

S = 4

S = 6

S = 8

approx

t

AEK cc

Test Plan

Compression test variables:

Elastomer material

Contact surface (may affect friction)

Shape factor

Aspect ratio

Loading type and level

Friction test variables

Contact pressure

Compression Test Matrix

Test Rubber Contact Length Width S AR Loading

Name Material Surface (inch) (inch)

1.GG.24.12 Lot 1 GG 24 12 8 2 Cyc

2.GG.24.12 Lot 2 GG 24 12 8 2 Cyc

1.CC.24.12 Lot1 CC 24 12 8 2 Cyc

1.SS.24.12 Lot1 SS 24 12 8 2 Cyc

1.CS.24.12 Lot1 CS 24 12 8 2 Cyc

1.GG.12.06 Lot1 GG 12 6 4 2 Cyc

1.GG.06.03 Lot1 GG 6 3 2 2 Cyc

1.GG.24.05 Lot1 GG 24 4.8 4 5 Cyc

1.GG.24.02 Lot1 GG 24 2.18 2 11 Cyc

1.GG.24.12 Creep Lot1 GG 24 12 8 2 Creep

1.GG.24.12.Alt Lot1 GG 24 12 8 2 Alt cyc

1.GG.24.12.Offset Lot1 GG 24 12 8 2 Offset

Test set up

Loading

beam

Galvanized

plate

Rubber pad

Concrete block on

steel plate

CROSS-SECTION

Instrumentation.

Horizontal

potentiometer

PLAN

Vertical

potentiometer

Loading Protocol.

Loading Protocols

0

100

200

300

400

500

600

700

800

900

1000

0 200 400 600 800 1000 1200

Time (sec)

Stre

ss, p

si

Normal loading

Alt loading

Test Results – Vertical Strain.

Plain Pad Compression Test: 1.GG.24.12

0

100

200

300

400

500

600

700

800

900

1000

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Axial Strain, in/in

Stre

ss, p

si

"200"

"400"

"600"

"800"

• Zero for strain difficult to define: “bedding in”.

• eel ≈ 12% at 800 psi

• Large “creep” strains

Test Results - Lateral Strain.

• Lateral strain = average slip on long sides/short dimension.

• Strain zero does not need adjustment.

• Lateral strain ≈ vertical strain (constant volume).

Plain Pad Compression Test: 1.GG.24.12

0

100

200

300

400

500

600

700

800

900

1000

0 0.05 0.1 0.15 0.2 0.25 0.3Lateral Strain, in/in

Stre

ss, p

si

Test Results - Lateral Strain.

Compression Test Results.

Marks on rubber showing slip region.

No cracks or other damage.

Compression Test Results.

Cyclic loading & creep cause additional deflection.

Total strain comparable to galvanized (1.GG.24.12).

Plain Pad Compression Test: 1.CC.24.12

0

100

200

300

400

500

600

700

800

0 0.05 0.1 0.15 0.2

Lateral Strain, in/in

Stre

ss, p

si

Test Results.

High Aspect Ratio (11.0), Low S ( 2.0).

High elastic strains, lower creep strains.

Plain Pad Compression Test: 1.GG.24.02

0

100

200

300

400

500

600

700

800

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Lateral Strain, in/in

Stre

ss, p

si

Test Results - Creep.

Creep increases significantly above 400 psi (= 0.5GS)

Compression Tests: 2.GG.24.12.Creep

0

100

200

300

400

500

600

700

800

900

1000

0 0.05 0.1 0.15 0.2 0.25 0.3

Lateral Strain, in/in

Stre

ss, p

si

Test Results – Creep Rate.

Compression Tests:

2.GG.24.12.Creep

0

100

200

300

400

500

600

700

800

900

0 1000 2000 3000 4000 5000 6000 7000

Time (sec)

Late

ral S

trai

n R

ate

( me

/se

c)

Compression Tests:

2.GG.24.12.Creep

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1000 2000 3000 4000 5000 6000 7000

Time (sec)

Late

ral S

trai

n (

in/i

n)

Compression Test Results -

Summary.

• Elastomer: Negligible effect.

• Contact Surface: Little effect.

• Shape Factor: High S higher elastic stiffness.

• Aspect Ratio: Little effect. (High AR less stiff).

• Loading: Large effect. Creep and cyclic load:

large increase in deflections.

Friction tests

3 steel plates (24” x 12”). (2 galvanized, 1 as-rolled).

2 rubber pads (6” x 3”).

Contact pressures: 200, 400, 600 psi.

Friction tests

Friction tests

FrictionTest: 600psi

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Deflection (in)

F/N

Elastic shear

deformation Sliding

Friction tests

Coefficient of friction:

Rubber vs. bare steel

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400 500 600 700

Compressive stress (psi)

m

mu

Previous studies have also found that rubber friction

varies with contact pressure. Similar to PTFE.

Data Analysis

• Quantify the effects of the various parameters on

compression stiffness.

• Use the measured friction coefficients in the analytical

model to compare predicted and measured

compression stiffnesses.

Data Analysis Contact Surface (Concrete, Steel, Galvanized)

Laterals strain shown includes creep.

Bare steel has lowest friction, highest lateral strain.

Differences are small.

Data Analysis Shape Factor, S.

Effect of Shape Factor

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

Lateral Strain, %

Max

Str

ess/

GS

S= 8 AR= 2

S= 4 AR= 2

S= 2 AR= 2

For 24” x 12” pad, rubber squeezed out from plates.

For smaller pads, rubber remained between plates.

Data Analysis Aspect ratio (for same S)

Effect of Aspect Ratio

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35 40

Lateral Strain, %

Max

. Str

ess/

GS

S= 4 AR= 2

S= 4 AR= 5

S= 2 AR= 2

S= 2 AR= 11

Larger AR gives slightly higher strains.

Difference is quite small

Comparison with Theory. Plain pad

W

L

t

LRBcKPEPc EfE ,,

fK ≈ 0.75m Use mave = 0.44. Then fK ≈ 0.33 Stiffness of a plain pad should be expected to be about 1/3

of that of a reinforced bearing with the same S. However,

this does not include creep deflections.

Plain pads:

Compression Stiffness Factor

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

m

f K

S = 2

S = 3

S = 4

S = 6

S = 8

approx

Tentative Design Criteria.

• Design plain pads using criteria similar to

those for steel-reinforced bearings, but

with lower values.

• No long-term damage to pads occurred,

so difficult to link design criteria to

potential damage.

• Consider a limiting compression strain,

and generate a stress criterion from it.

Tentative Design Criteria. Example:

• If strain limited to 10%, s < 0.6GS

Effect of Shape Factor

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

Lateral Strain, %

Max

Str

ess/

GS

S= 8 AR= 2

S= 4 AR= 2

S= 2 AR= 2

Governed here by bearings with S = 8.

Outcomes for Users.

• Plain pad strips under rectangular box

beams: make pad wider.

• Leveling pad (e.g. MNDOT): present pads

are at ≈ 800 psi (would need S = 16, thus

¼” thick pad). Potential problems unless

stiffer material used (70 durometer?).

• Use under prestressed girders. Problems

similar to leveling pads.

• Others?

Preliminary Conclusions

• Additional strain due to creep and cyclic

load is significant, controls behavior.

• Present AASHTO LRFD stress of 800 psi

is much too high.

• Stress limit should be related to S.

• Before recommending changes to

AASHTO LRFD Spec, need to check other

conditions (e.g. higher durometer pads).