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Highway BridgesIan Buckle
Foundation ProfessorDepartment of Civil and Environmental Engineering
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University of Nevada Reno Reno NV 89557
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Topics
Background
Principles of Seismic Isolation
Some Applications System Design
Testing Requirements
Sources: FHWA/MCEER 2006, Seismic
so a on o g way r ges, pec aPublication MCEER-06-SP07
2
,
Seismic Isolation Design, Third Edition
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Topics
Principles of Seismic Isolation
ome pp ca ons
System Design Testing Requirements
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Conventional Seismic Design
Superstructure
Bearin su men
Footing
& piles
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gravity and earthquake loads,dissipate energy, and not collapse
EQ ground motion
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Unacceptable Performance
Collapsed
Superstructure
Bearin s
Fractured
u menu men
Footing
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EQ ground motion
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Seismic Design Objective
column strength
ac or o sa e y =
earthquake force
> .
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Seismic Design Objective
capacity
ac or o sa e y = > .
demand
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Conventional Design Approach
INCREASE CAPACITY
capacity
ac or o sa e y = > .
demand
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Conventional Design
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Conventional Design
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Seismic Isolation an Alternative
capacity
ac or o sa e y = > .
demand
REDUCE DEMAND
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Seismic Isolation an Alternative
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Seismic Isolation an Alternative
suspension
system
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EQ ground motion
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Basic Idea of Seismic Isolation
Isolate the bridge from ground motion by:
Inserting a flexible support system between the
super- and sub-structure (isolation bearings).This will lengthen the natural period of the
are significantly reduced.
columns elastic.
Control the liveliness of the bridge (due to theflexible bearings) using energy dissipators
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.
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Seismic Isolation: Key Point
Seismic isolation reduces the earthquakedemand on a bridge, rather than increases
its capacity.
In many cases the reduction in demand is
such that it may be feasible to have
substructures perform elastically.
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Topics
Principles of Seismic Isolation
ome pp ca ons
System Design Testing Requirements
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Principles of Seismic Isolation
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Principles of Seismic Isolation
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Principles of Seismic Isolation
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Principles of Seismic Isolation
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Principles of Seismic Isolation
In addition to flexibility and energy dissipation
most isolation s stems also com rise:
Adequate rigidity for non-seismic loads. .
accommodating thermal, creep, and other
,
Self-centering capability
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Seismic Isolation: Key Point
Most seismic isolation systems
comprise:1.Flexibility
2.Energy dissipation
3.Ri idit for non-seismic loads4.Self-centering
Above criteria means all isolation systemshave nonlinear properties. exceptions exist
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but are rare.
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Principles of Seismic Isolation Isolator Force, F
Kd
Kisol
Qd
Fy Fisol
Ku
dyKu
disol Isolator
Displacement, dKu
=
Kd
Q = Characteristic stren th
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u
Kisol = Effective stiffness
disol = Isolator lateral displacement
Fy = Yield strength
Fisol = Isolator lateral forceKd = Post-elastic stiffness
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Principles of Seismic Isolation
POLISHED STAINLESS STEEL SURFACEPOLISHED STAINLESS STEEL SURFACE
SEALSEAL
Lead-Rubber Isolator
STAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)
COMPOSITE LINER MATERIALSTAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)
COMPOSITE LINER MATERIAL
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Isolator
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Principles of Seismic Isolation
Eradiquake Isolator
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Bridges Not Suitable for Isolation
Bridges on soft sites, because lengthening
the eriod ma increase rather than
decrease, spectral accelerations
spectrumRock
spectrum
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Bridges Not Suitable for Isolation
Brid es in hi h seismic zones on soft sites
where displacements may be large andcostly expansion joints may be required toaccommodate movements
Bridges with tall flexible piers, which alreadyhave long periods and little advantage is
ga ne w so a on
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Seismic Isolation: Key Point
Bridges that are most suitable forisolation are
(a) located on stiff and medium-stiff soil
sites,(b) have relatively stiff substructures
(e.g. short-to-medium height columns)
(c) continuous superstructures, and(d) seat-type abutments.
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Topics
Principles of Seismic Isolation
ome pp ca ons
System Design Testing Requirements
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Applications: So. Rangitikei River, NZ
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Applications: US 101 Sierra Point, CA
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A li ti I 680 B i
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Applications: I-680 Benecia-
,
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Applications: JFK Airport Light Rail, NY
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Applications: Bolu Viaduct, Turkey
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Applications in U.S, Canada, Mexico
Applications
Isolator Type
number ofisolated bridges
n or
America)
-
Eradiquake isolator 20%
Other: Friction pendulum, Highdamping rubber, Natural 5%
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Topics
Principles of Seismic Isolation
ome pp ca ons
System Design Testing Requirements
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Design of a Bridge Isolation System
Three step process:
1. Determine re uired erformance criteria
2. Determine properties of the isolation systeme. . Q and K to achieve re uired
performance using one or more methods of
analysis V K3. Select isolator type and
design hardware to achieve
Qd
required system properties(i.e.,Qd and Kd values) using
D
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a rational design procedure
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Performance Criteria
Usually set by owner
o Not-to-exceed total base shear for Design
o Elastic columns during DE
- -superstructure during DE.
Considered Earthquake (MCE)
o Re arable dama e in MCE but not colla se
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Analysis Methods for Isolated
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Analysis Methods for Isolated
analyzed using linear methods provided,
effective stiffness and
equ va en v scous amp ng ase on
the hysteretic energy dissipated by the
so a ors.
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A l i M th d
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Analysis Methods
Single Mode Spectral Method
u mo e pec ra e o
Time History Method
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Si lifi d M th d A ti
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Simplified Method Assumptions
1. Superstructure acts a rigid-diaphragm comparedto flexibility of isolators
.
superstructure, i.e. single degree-of-freedoms stem
3. Nonlinear properties of isolators may be
represented by bilinear loops4. Bilinear loops can be
represented by Kisol,
e ect ve st ness, anenergy dissipated per cycle
K isol D
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= area o oop
Note Kisol & loop area are dependent on displacement, D.
Si lifi d M th d A ti
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Simplified Method Assumptions
5. Energy dissipated per cycle may berepresented by viscous damping, i.e., workdone during plastic deformation can berepresented by work done moving viscous
.damping ratio given by
)1(2
isol
y
isol
d
d
d
F
Qh
6. Acceleration spectrum is inversely
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proportional to period ( A = a / T)
Si lifi d M th d A ti
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Simplified Method Assumptions
7. Acceleration spectra for 5% viscousdamping may be scaled for actualdamping (h%) by dividing by a dampingcoefficient, BL
3.0
hB
L .
L - .A second factor (BS) is used in short-period
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. -
range.
AASHTO D i R S t
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AASHTO Design Response Spectra
AASHTO Spectra (SA) are for 5%damping on a rock site (Site Class
SA (A) Spectral Acceleration (g)
5 % damping
For sites other than rock, thes ectra are modified b SiteSD1
h % damping
Factors, Fa and Fv
For damping other than 5%, the1.0s
SD1 / BL
spectra are modified by a
Damping Factor, BLSSF
,
SD (D)Spectral
Displacement
TBTB LL
v
A 10SD1
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L
D
L
vD
BBDS 11
2 79.9
4
Period, T1.0s
h % damping10SD1 / BL
Simplified Method
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Simplified Method
This method is alsoFisolQd
Kd
known as the-
K isol D
Methodd isol
an s app ca e o
a wide range of
D
Displacement5 % damping
structural types - not D1
h % dam in
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.
Period, T1.0s
Simplified Method
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Simplified Method
Basic steps:
1. Assume value for
VFisolQd
disol
2. Calculate effective K isol
d
stiffness, Kisol
3. Calculate max. force,
d isol
Fisol
4. Calculate effective
per o , eff
dQ W
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d
isol
isold isolisol isol
egK
Simplified Method Continued
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Simplified Method Continued
5. Calculate viscous
damping ratio, h
VFisolQd
6. Calculate damping
coefficient, BL K isol
d
7. Calculate disol
8. Compare with value
d isoldy
for disol in Step (1).
Repeat if necessary effL
visol T
B
SFgd 1
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.
)1(2 yd dQh 3.0
hB )(79.9 1 inchesT
SFd e
visol
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isolisol . L
Example: Simplified Method
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Example: Simplified Method
The superstructure of a 2-span bridge weighs
. D1
0.55. The bridge is seismically isolated with
abutments.
Isolation
system
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Example
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Example
d .
Kd = 13.0 K/in (summed over all the
isolators), calculate the maximum
displacement of the superstructure and thetotal base shear.
Neglect pier flexibility.
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Example 1
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Example 1
Solution:
1. Initialize
1.1 Qd =0.075 W = 0.075 (533) = 40 K. so
Take Teff= 1.5 sec,
L .
D = 9.79 SD1 Teff/ BL
= . . .= 8.08 in
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Take initial value for disol = D
Example 1 Continued
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Example 1 Continued
Solution:
1. Initialize
Qd = 40 K= .
.
2.1 Set disol = D and proceed with Steps 1-7
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol
2. Effective stiffness, Kisol
3. Max. isolator force, Fm
4. Effective period, Teff
5. Viscous damping ratio, h%
. amp ng coe c en ,L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol
3. Max. isolator force, Fm
4. Effective period, Teff
5. Viscous damping ratio, h%
. amp ng coe c en ,L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm
4. Effective period, Teff
5. Viscous damping ratio, h%
. amp ng coe c en ,L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm 144.9
4. Effective period, Teff
5. Viscous damping ratio, h%
. amp ng coe c en , L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm 144.9
4. Effective period, Teff 1.46
5. Viscous damping ratio, h%
. amp ng coe c en , L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm 144.9
4. Effective period, Teff 1.46
5. Viscous damping ratio, h% 17.6
. amp ng coe c en , L
7. Isolator displacement, disol
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Example 1 Continued
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Example 1 Continued
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm 144.9
4. Effective period, Teff 1.46
5. Viscous damping ratio, h% 17.6
. amp ng coe c en , L .
7. Isolator displacement, disol
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Example 1 Continued
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p
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0
0. Post-elastic stiffness, Kd 13.0
1. Isolator Displacement, disol 8.08
2. Effective stiffness, Kisol 17.95
3. Max. isolator force, Fm 144.9
4. Effective period, Teff 1.46
5. Viscous damping ratio, h% 17.6
. amp ng coe c en , L .
7. Isolator displacement, disol 6.43
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Example 1 Continued
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p
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0 40.0 40.0
0. Post-elastic stiffness, Kd 13.0 13.0 13.0
1. Isolator Displacement, disol 8.08 6.43 5.66
2. Effective stiffness, Kisol 17.95 20.06
3. Max. isolator force, Fm 144.9 113.6
4. Effective period, Teff 1.46 1.65
5. Viscous damping ratio, h% 17.6 22.4
. amp ng coe c en , L . .
7. Isolator displacement, disol 6.43 5.66
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Example 1 Continued
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p
Step Trial 1 Trial 2 Trial n
0. Characteristic strength, Qd 40.0 40.0 40.0
0. Post-elastic stiffness, Kd 13.0 13.0 13.0
1. Isolator Displacement, disol 8.08 6.43 5.66
2. Effective stiffness, Kisol 17.95 20.06
3. Max. isolator force, Fm 144.9 113.6
4. Effective period, Teff 1.46 1.65
5. Viscous damping ratio, h% 17.6 22.4
. amp ng coe c en , L . .
7. Isolator displacement, disol 6.43 5.66
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Simplified Method
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p
Basic method
assumes ver
Kd
Qd Kisol
stiff piers butmethod can
dy disol
F
SuperstructureIsolator Effective Stiffness, Kisol
be modified
to includedsub
Ksub
Substructure, Ksub
Isolator(s), Kisol
pier flexibility.Fdisoldsub
Substructure Stiffness, Ksub
Keffd
63MCEER,2006.
d = disol + dsub
Combined Effective Stiffness, Keff
Multimodal Spectral Method
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Elastic Multimodal Method, developed forconventional bridges, may be used for isolatedbridges even though they are nonlinear systems.
Modeling the nonlinear properties of the isolatorsis usually done with equivalent linearized springs
additional damping .
Recall earlier discussionof the com osite s ectrum
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Multimodal Spectral Method
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use the results from the Simplified Method of
iteration.
In this case convergence in 1 or 2 cycles is
poss e usua y
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Isolator Design
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Analysis gives required system properties
Q and K to meet desired erformance
Next step is to design an isolation system to
Isolators used in bridge design include:
-Rubber Bearing)
Flat plate slider with elastomeric spring
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Elastomeric Isolator Design (LRB)
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Elastomeric Isolator Design (LRB)
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Qd = 0.9 d2 (K)where
d = diameter of lead core (in)
d r rwhere
. . .Ar= bonded area of elastomer
=r
Period ost- ield = TWT rc 22
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d
Curved Sliding Isolators (FPS)
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R
W
Restoring force
Friction
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D
Curved Sliding Isolator Design (FPS)
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POLISHED STAINLESS STEEL SURFACEPOLISHED STAINLESS STEEL SURFACE
Qd = Wwhere
= coefficient of friction=STAINLESS STEEL
ARTICULATED SLIDERROTATIONAL PART
COMPOSITE LINER MATERIAL
RSTAINLESS STEEL
ARTICULATED SLIDERROTATIONAL PART
COMPOSITE LINER MATERIAL
R
Wd where
R
=ra us o curva ure o s er
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Period when sliding = gTd 2
Summary of LRB and FPS Designs
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as omer c
(LRB)
urve er
(FPS)
Number of isolators 12 12
9.4 in diam. 18 in diam. x 7.75 in height x 5 in (est.) height
Internal dimensions 11 x in layers radius = 41 in
Other 1.92 in diam. leadcore
coefficient offriction = 0.075
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Eradiquake Isolator
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Other Design Issues (All)
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Restoring force capability
Clearances (expansion joints, utility crossings )
Vertical load capacity and stability at high shear
strain
Uplift restrainers, tensile capacity
Non-seismic requirements (wind, braking, thermalmovements )
System Property Modification Factors (-factors) for
aging, temperature, wear and tear, andcontamination
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Topics
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Background
Principles of Seismic Isolation,
Some A lications
System Design
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Basic Testing Requirements
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Because isolators are subject to extremedeformations and loads durin lar e
earthquakes, most design codes require they
be tested to demonstrate conformance with
design expectations
extensive testing), design provisions for
conventional bearings e.g., Section 14,
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Basic Testing Requirements
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Usually three categories of tests are required:
1. Characterization Tests to confirm basic
properties such as effect of velocity, pressure,
and temperature to develop models for analysis
2. Prototype Tests for each project prior to
production to confirm mechanical properties
used in design
3. Production Tests performed on each isolator
a ong w ma er a es s or qua y
control/quality assurance.
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During this lecture we have learned:
Basic purpose of seismic isolation
Bridge types / configurations suitable for
How to calculate displacement and base
Simplified Method
About three kinds of isolators in use today
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Five questions
1. What is basic purpose of seismic isolation?. .
3. Describe bridge types and configurations that
4. Name three common types of isolators on the
market toda in the U.S.
5. Name three types of tests used to assure the