Bridge Seismic Isolation Study
on a Full Scale Bridge Test
Myrto Anagnostopoulou
SEESL Structural and Test Engineer
Ricardo Ecker Lay
Ph.D. Candidate
Andre Filiatrault
Professor, MCEER Director
Dep. Of Civil, Structural and Environmental Engineering
University at Buffalo – State University of New York
 Design of seismically isolated structures is based
on the mechanical properties of newly-fabricated
seismic isolation hardware
 Environmental effects, history of loading, aging
result in change in:
properties of isolation hardware
behavior of isolated structure
 Collect field data on the aging characteristics and
long-term service life of seismic isolation bearings
Full-Scale Isolated Bridge
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Calspan’s Ashford facility, Western NY – 50 miles from UB
Two 72-foot long adjacent single lane concrete girder bridges at a
distance of 6 feet
8 low-damping elastomeric bearings of two different elastomeric
compounds
Free vibration testing will be repeated weekly for a period of 5 years
starting end-October 2010
Remotely controlled testing from SEESL/UB facilities
Superstructure Geometry

10 girder beams: AASHTO Box cross-section (BII-36), 70 skew

8 beams weight 26 tons and 2 beams weight 32 tons

longitudinal post-tensioning at bottom plate of girder beams

transverse post-tensioning of girder beams at the support sections

9” of gravel fill equivalent to 7” concrete/asphalt deck
Gravel
Girder
Beams
Isolation
Bearings
Abutment
Spreader
Beam
ashford.wmv
Elastomeric Isolators

Target period of isolated bridge: T=2 sec

Total weight per bearing: W=100 kips

Design deformation: D=4 in

10 low-damping elastomeric bearings of
circular cross-section with two different
rubber compounds:
Group A -> G=120 psi -> k=2.7 kips/in
Group B -> G=70 psi
-> k=1.6 kips/in

Groups A and B are assigned to each of the
two adjacent bridges

Characterization testing of isolation
bearings at SEESL/UB in order to acquire
mechanical properties
20091015-MOV0DF.mpg
Bearing Characterization at SEESL
Group A
kA=2.5 kips/in
ζA=5%
Group B
kB=1.5 kips/in
ζB=3%
Full-Scale Bridge Testing
Load
Cell

Actuator spans the gap
between the two adjacent
single span bridges

Slow extension rate of
the actuator up to:
 16” to the reaction
load cell
 4” design displacement
of the bearings

Fast retraction rate of the
actuator in order to subject
the two bridges in free
vibration
Actuator
Group A
Group B
F=24kips
2.4 in
4.0 in
Forcing System Properties:
F=24kips

Max actuator stroke: 24 in

Max actuator force: 50 kips
2.5
Group A
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2
Displacement (in)
1.5
1
0.5
0
Group A
initial displacement: 2.4 in
damping: 5%
period: 2.0 sec
free vibration duration: 35 sec
number of cycles: 15
-0.5
4
-1
3
-2
2
-2.5
0


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10
20
30
40
50
60
Time (sec)
70
80
90
Group B
initial displacement: 4 in
damping: 3%
period: 2.6 sec
free vibration duration: 70 sec
number of cycles: 25
100
Displacement (in)
-1.5
Group B
1
0
-1
-2
-3
-4
0
10
20
30
40
50
60
Time (sec)
70
80
90
100
Testing Procedure
1. Collect the initial mechanical properties of the isolation bearings
(stiffness, damping)
2. Run bridge free vibration set of tests remotely from SEESL/UB
3. Collect data/info from:
 actuator, reaction load cell
 accelerometers, load cells, string potentiometers,
thermocouples (26 sensors in total)
 cameras
 weather station
4. Obtain post-testing mechanical properties of bearings and
compare to pre-testing ones
5. Visit the bridge field station in order to check condition of
bearings, actuator, instrumentation
6. Repeat the procedure weekly and for a period of five years
System Property Modification Factors

Properties of seismic isolation bearings:
 Characteristic strength, Qd
 Effective stiffness, Keff
 Post-yield stiffness, Kd
 Damping ratio, ζ

Phenomena effecting isolator properties:
 Temperature
 Aging
 Wear or Travel
 History of loading

Which are the max and min probable values of the bearing
properties within the structure’s lifetime?

Can all phenomena occur simultaneously?
λ-factors: quantify the effect of a particular phenomenon on the
nominal properties of an isolation bearing
Pn
λmax= λmax,1·λmax,2·λmax,3 ···
λmin= λmin,1·λmin,2·λmin,3 ···
Pmax = λmax·Pn
Pmin = λmin·Pn
Bounding
Analysis

Pmax controls the substructure and superstructure force response

Pmin controls the isolator displacement response

System Property Adjustment factors account for the probability
that several events occur simultaneously, depend on the
significance of the structure and their values are based on
engineering judgment

According to AASHTO (1999):
“The λ-factors listed herein are based on the available limited
data. In some cases the factors could not be established and
need to be determined by test.”
λ-factors for Elastomeric Bearings
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Temperatures for design: 700F to -220F (AASHTO, 1999)
 Low temperatures cause increase in stiffness and strength
 Duration of exposure is significant but usually neglected
λmax,t
Travel or Wear due to traffic and temperature changes:
 For a cumulative movement of 1 mile 17 sets of free
vibration tests should be conducted during one day of
testing (AASHTO, 1999)
λmax,tr
λ-factors depend significantly on the rubber compound of the
bearing
max: 800 to 1000F
min: -300 to 100F
Conclusions
 Better understanding of the effect of temperature,
environmental conditions and ware on the mechanical
properties of isolation bearings
 Realistic determination of bounding values of isolator
properties for analysis and design based on better
estimated Property Modification Factors
 Using different seismic isolation systems the bridge field
station can provide an insight to the resilience of
bridges due to naturally-occurring phenomena
Acknowledgments
 SEESL technical staff and students
 Doug Stryker and Andrew Dailey from Calspan
 H&K Services for constructing the bridge
 Hubbell Galvanizing for donating the girder beams
 Dynamic Isolation Systems for providing the bearings
Thank you! Questions?
Instrumentation/DAQ System
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actuator displacement, load cell sensors
26 sensors:
 10 accelerometers
 2 load cells (temperature range -10F – 100F)
 10 string potentiometers
 4 thermocouples
7 digital cameras
1 digital weather station
32-channel portable DAQ System compatible with existing
UB/NEES systems and software
SEESL remote
desktop controller
SEESL remote
desktop DAQ
internet Ashford host PC/
Pump controller
internet
Ashford
host PC
Actuator/Test
DAQ/Sensors
Effects on Elastomeric Bearings

Wear or Cumulative Travel

Temperature:
 low temperatures -> increase in stiffness and strength
 high temperatures -> degradation of the rubber

Coupling between wear and temperature

Duration of exposure and elastomeric compound control behavior

Lack of long-term in-situ performance data
max: 80 to 100F
min: -30 to 10F