Seismic Isolation Technology forSeismic Isolation Technology for Highway Bridges

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Ian Buckle Foundation Professor

Department of Civil and Environmental Engineering

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University of Nevada Reno, Reno NV 89557

Topics Background Principles of Seismic Isolation Some Applications System Design Testing Requirements

Sources: FHWA/MCEER 2006, Seismic

I l ti f Hi h B id S i lIsolation of Highway Bridges, SpecialPublication MCEER-06-SP07

AASHTO 2010 Guide Specifications for

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AASHTO 2010, Guide Specifications for Seismic Isolation Design, Third Edition

Topics

BackgroundBackground Principles of Seismic Isolation

S A li ti Some Applications System Design Testing Requirements

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Conventional Seismic Design

Superstructure

Ab t tBearingsAbutment Abutment

ea gsAbutment

Footing & piles

Columns are required to support

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Columns are required to support gravity and earthquake loads, dissipate energy, and not collapse

EQ ground motion

Unacceptable Performance

Collapsed Superstructure

Ab t tBearings

Ab t t

Fractured Column

Abutmentea gs

Abutment

Piles

ColumnFooting

Piles

5

EQ ground motion

Seismic Design Objective

column strengthF t f f t 1 0Factor of safety =

earthquake force> 1.0

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Seismic Design Objective

capacityF t f f t 1 0Factor of safety = > 1.0

demand

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Conventional Design ApproachINCREASE CAPACITY

capacityF t f f t 1 0Factor of safety = > 1.0

demand

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Conventional Design

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Conventional Design

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Seismic Isolation an Alternative

capacityF t f f t 1 0Factor of safety = > 1.0

demand

REDUCE DEMAND

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Seismic Isolation an Alternative

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Seismic Isolation an Alternative

IsolationIsolation suspension system

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EQ ground motion

Basic Idea of Seismic IsolationIsolate 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 bridge such that the inertia forces in the bridgebridge such that the inertia forces in the bridge are significantly reduced.Force reduction may be sufficient to keepForce reduction may be sufficient to keep columns elastic.

Control the liveliness of the bridge (due to the flexible bearings) using energy dissipators(dampers) to limit the motion

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(dampers) to limit the motion.

Seismic Isolation: Key Point

Seismic isolation reduces the earthquakedemand on a bridge, rather than increasesgits 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

BackgroundBackground Principles of Seismic Isolation

S A li ti Some Applications 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 IsolationIn addition to flexibility and energy dissipation

most isolation systems also comprise:y p Adequate rigidity for non-seismic loads

(e g wind and braking) while(e.g. wind and braking) while accommodating thermal, creep, and other shortening effects andshortening effects, and

Self-centering capability

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Seismic Isolation: Key Point

Most seismic isolation systems comprise:1.Flexibility 2.Energy dissipation3.Rigidity for non-seismic loads g y4.Self-centering

Above criteria means all isolation systems have nonlinear properties. exceptions exist

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p p pbut are rare.

Principles of Seismic Isolation

Isolator Force, F

KdKisol

Qd

Fy Fisol

Ku

dy Ku

disol Isolator Displacement, d Ku

K = Elastic (unloading) stiffness

Kd

Qd = Characteristic strength

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Ku Elastic (unloading) stiffnessKisol = Effective stiffnessdisol = Isolator lateral displacement

d gFy = Yield strengthFisol = Isolator lateral forceKd = Post-elastic stiffness

Principles of Seismic Isolation

POLISHED STAINLESS STEEL SURFACEPOLISHED STAINLESS STEEL SURFACE

SEAL

R

SEAL

R

Lead-Rubber Isolator

STAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)

COMPOSITE LINER MATERIAL

RSTAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)

COMPOSITE LINER MATERIAL

R

Friction-Pendulum

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Friction-Pendulum Isolator

Principles of Seismic Isolation

Eradiquake Isolator

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Bridges Not Suitable for Isolation Bridges on soft sites, because lengthening

the period may increase, rather than p y ,decrease, spectral accelerations

Soft soilSoft soil spectrumRock

spectrum

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Bridges Not Suitable for Isolation

Bridges in high seismic zones on soft sites, g g ,where displacements may be large and costly expansion joints may be required to accommodate movements

Bridges with tall flexible piers, which already have long periods and little advantage is

i d ith i l tigained with isolation

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Seismic Isolation: Key Point

Bridges that are most suitable for isolation 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

BackgroundBackground Principles of Seismic Isolation

S A li ti Some Applications 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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Applications: I-680 Benecia-Martinez, CAMartinez, CA

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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(Percent of total

Isolator Type

(Percent of total number of

isolated bridges in Northin North

America)

Lead rubber isolator 75%Lead-rubber isolator 75%

Eradiquake isolator 20%

Other: Friction pendulum, High damping rubber, Natural Rubber FIP isolator

5%

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Rubber, FIP isolator

Topics

BackgroundBackground Principles of Seismic Isolation

S A li ti Some Applications System Design Testing Requirements

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Design of a Bridge Isolation SystemThree step process:

1. Determine required performance criteriae e e equ ed pe o a ce c e a2. Determine properties of the isolation system

(e.g. Qd and Kd) to achieve required ( g d d) qperformance using one or more methods of analysis V Kd

3. Select isolator type and design hardware to achieve

V KdQd

required system properties (i.e.,Qd and Kd values) using

D

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d d

a rational design procedure

Performance Criteria Usually set by owner Examples include:Examples include:

o Not-to-exceed total base shear for Design Earthquake (DE)Earthquake (DE)

o Elastic columns during DEo Not-to-exceed longitudinal displacement ino Not to exceed longitudinal displacement in

superstructure during DE.o Essentially elastic behavior for the Maximumo Essentially elastic behavior for the Maximum

Considered Earthquake (MCE)o Reparable damage in MCE, but not collapse

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p g , p

Analysis Methods for Isolated BridgesBridges

Bridges with nonlinear isolators may beBridges with nonlinear isolators may be analyzed using linear methods provided equivalent properties are used such asequivalent properties are used, such as effective stiffness and

i l t i d i b d equivalent viscous damping based on the hysteretic energy dissipated by the i l tisolators.

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Analysis Methods

Simplified MethodSimplified Method Single Mode Spectral Method

M lti d S t l M th d Multimode Spectral Method Time History Method

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Simplified Method Assumptions1. Superstructure acts a rigid-diaphragm compared

to flexibility of isolators2 Single displacement describes motion of2. Single displacement describes motion of

superstructure, i.e. single degree-of-freedom systemy

3. Nonlinear properties of isolators may be represented by bilinear loops

V4. Bilinear loops can be represented by Kisol,

ff i iff d

V

effective stiffness, and energy dissipated per cycle

f l

Kisol D

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= area of loop Note Kisol & loop area are dependent on displacement, D.

Simplified Method Assumptions5. Energy dissipated per cycle may be

represented by viscous damping, i.e., work done during plastic deformation can be represented by work done moving viscous fluid through an orifice Equivalent viscousfluid through an orifice. Equivalent viscous damping ratio given by

)1(2

isol

y

isol

d

dd

FQh

6. Acceleration spectrum is inversely i l i d (S / T)

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proportional to period (SA = a / T)

Simplified Method Assumptions7. Acceleration spectra for 5% viscous

damping may be scaled for actual damping (h%) by dividing by a damping coefficient, BL

3.0

050

hBL 05.0 B is used in long period range of spectrumBL is used in long-period range of spectrum. A second factor (BS) is used in short-period range Isolated bridges fall in long period

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range. Isolated bridges fall in long-period range.

AASHTO Design Response SpectraAASHTO Spectra (SA) are for 5% damping on a rock site (Site Class B)

SA (A) Spectral Acceleration (g)5 % damping B)

For sites other than rock, the spectra are modified by Site SD1 h % damping p yFactors, Fa and FvFor damping other than 5%, the

Period T1.0s

SD1 / BL

spectra are modified by a Damping Factor, BL

SSFAS Dv 11

Period, T

SD (D) Spectral Displacement5 % damping

TBS

TBSAS

L

D

L

vA

11 TSTSFg

10SD1

5 % damping

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L

D

L

vD B

TSB

TSFgDS 112 79.94

Period, T1.0s

h % damping10SD1 / BL

Simplified MethodV

This method is alsoV

FisolQdKd

known as the Direct-Displacement

Kisol DDirect Displacement Method

d i li bl t

disol

S (D) Spectraland is applicable toa wide range of

SD (D) Spectral Displacement

10S

5 % damping

structural types - not just isolated bridges.

10SD1

h % damping10SD1 / BL

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just isolated bridges.Period, T1.0s

p g10SD1 / BL

Simplified MethodBasic steps:1. Assume value for

VFisolQd

Kdisol2. Calculate effective Kisol

D

Kd

stiffness, Kisol3. Calculate max. force,

Ddisol

Fisol4. Calculate effective

i d Tperiod, Teff

d KQK isoldKF ff WT 246

disol

isol KdK isolisolisol dKF

isoleff gK

T 2

Simplified Method Continued5. Calculate viscous

damping ratio, hV

FisolQdK6. Calculate damping

coefficient, BL KisolD

Kd

7. Calculate disol8. Compare with value

Ddisoldy

for disol in Step (1). Repeat if necessary until convergence

effL

visol TB

SFgd 124until convergence.

)1(2 yddd

FQh 3.0)

050( hBL )(79.9 1 inchesTB

SFd effvisol

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isolisol dF )05.0(L B ffL

Example: Simplified Method

The superstructure of a 2-span bridge weighs533 K It is located on a rock site where S =533 K. It is located on a rock site where SD1 = 0.55. The bridge is seismically isolated with 12 isolation bearings at the piers and12 isolation bearings at the piers and abutments.

Isolation

system

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Example

If the value of Q = 0 075W andIf the value of Qd = 0.075W and Kd = 13.0 K/in (summed over all theisolators), calculate the maximumdisplacement of the superstructure and thetotal base shear.

Neglect pier flexibility.

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Example 1Solution:1. Initialize

1.1 Qd =0.075 W = 0.075 (533) = 40 K1 2 Need initial value di l1.2 Need initial value disol

Take Teff = 1.5 sec, 5% damping (B =1 0) and calculate5% damping (BL=1.0) and calculate D = 9.79 SD1 Teff / BL

9 79 (0 55) 1 5= 9.79 (0.55) 1.5 = 8.08 in

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Take initial value for disol = D

Example 1 ContinuedSolution:1. Initialize

Qd = 40 KD = 8 08 inD 8.08 in

2 Iterate2. Iterate2.1 Set disol = D and proceed with Steps 1-7

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol2. Effective stiffness, Kisol3. Max. isolator force, Fm4. Effective period, Teff 5. Viscous damping ratio, h%6 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol3. Max. isolator force, Fm4. Effective period, Teff 5. Viscous damping ratio, h%6 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm4. Effective period, Teff 5. Viscous damping ratio, h%6 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 5. Viscous damping ratio, h%6 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 1.465. Viscous damping ratio, h%6 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 1.465. Viscous damping ratio, h% 17.66 D i ffi i t B6. Damping coefficient, BL7. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 1.465. Viscous damping ratio, h% 17.66 D i ffi i t B 1 466. Damping coefficient, BL 1.467. Isolator displacement, disol

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.00. Post-elastic stiffness, Kd 13.01. Isolator Displacement, disol 8.082. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 1.465. Viscous damping ratio, h% 17.66 D i ffi i t B 1 466. Damping coefficient, BL 1.467. Isolator displacement, disol 6.43

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.0 40.00. Post-elastic stiffness, Kd 13.0 13.01. Isolator Displacement, disol 8.08 6.432. Effective stiffness, Kisol 17.953. Max. isolator force, Fm 144.94. Effective period, Teff 1.465. Viscous damping ratio, h% 17.66 D i ffi i t B 1 466. Damping coefficient, BL 1.467. Isolator displacement, disol 6.43

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.0 40.0 40.00. Post-elastic stiffness, Kd 13.0 13.0 13.01. Isolator Displacement, disol 8.08 6.43 5.662. Effective stiffness, Kisol 17.95 20.063. Max. isolator force, Fm 144.9 113.64. Effective period, Teff 1.46 1.655. Viscous damping ratio, h% 17.6 22.46 D i ffi i t B 1 46 1 576. Damping coefficient, BL 1.46 1.577. Isolator displacement, disol 6.43 5.66

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Example 1 ContinuedStep Trial 1 Trial 2 Trial n0. Characteristic strength, Qd 40.0 40.0 40.00. Post-elastic stiffness, Kd 13.0 13.0 13.01. Isolator Displacement, disol 8.08 6.43 5.662. Effective stiffness, Kisol 17.95 20.063. Max. isolator force, Fm 144.9 113.64. Effective period, Teff 1.46 1.655. Viscous damping ratio, h% 17.6 22.46 D i ffi i t B 1 46 1 576. Damping coefficient, BL 1.46 1.577. Isolator displacement, disol 6.43 5.66

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Simplified MethodF

Basic method assumes very

Kd

Qd

F

Kisol

ystiff piers but method can

dy disol

F

Superstructure Isolator Effective Stiffness, Kisol

be modifiedto include dsub

Ksub

Substructure, Ksub

Isolator(s), Kisol

pier flexibility. sub

Fdisol dsub

Substructure Stiffness, Ksub

Keff d

63MCEER,2006.

d = disol + dsub

Combined Effective Stiffness, Keff

Multimodal Spectral Method Elastic Multimodal Method, developed for

conventional bridges, may be used for isolated bridges even though they are nonlinear systems.

Modeling the nonlinear properties of the isolators is usually done with equivalent linearized springs and the response spectrum is modified for theand the response spectrum is modified for the additional damping in the isolated modesin the isolated modes .Recall earlier discussionof the composite spectrum

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p p

Multimodal Spectral Method

Method is iterative and a good strategy is toMethod is iterative and a good strategy is to use the results from the Simplified Method of Analysis to obtain starting values for theAnalysis to obtain starting values for the iteration.

In this case convergence in 1 or 2 cycles is ibl llpossible usually

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Isolator Design Analysis gives required system properties

(Qd and Kd) to meet desired performance( d d) p Next step is to design an isolation system to

have these propertieshave these properties Isolators used in bridge design include:

Elastomeric bearings with lead cores (Lead Elastomeric bearings with lead cores (Lead-Rubber Bearing)

Curved sliders (Friction Pendulum System) Curved sliders (Friction Pendulum System) Flat plate slider with elastomeric spring

dampers (Eradiquake System)

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dampers (Eradiquake System)

Elastomeric Isolator Design (LRB)

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Elastomeric Isolator Design (LRB) Qd = 0.9 d2 (K)

where d = diameter of lead core (in)

K = G A / T Kd = G Ar / Trwhere G = shear modulus of elastomer (e g 0 1 Ksi)G = shear modulus of elastomer (e.g. 0.1 Ksi)Ar = bonded area of elastomerT = total thickness of elastomerTr total thickness of elastomer

Period (post-yield) =

gT

GgKWT rcd

2)(2

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Period (post yield) gGgKdd )(

Curved Sliding Isolators (FPS)

R

W

Restoring force

Friction

g

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D(Displacement)

Curved Sliding Isolator Design (FPS)POLISHED STAINLESS STEEL SURFACEPOLISHED STAINLESS STEEL SURFACE

Qd = Wwhere SEALSEAL = coefficient of frictionW = weight per isolator

STAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)

COMPOSITE LINER MATERIAL

SEAL

RSTAINLESS STEELARTICULATED SLIDER(ROTATIONAL PART)

COMPOSITE LINER MATERIAL

SEAL

R

W weight per isolator

K =

( )( )

W Kd = whereR di f t f lid

RW

R=radius of curvature of slider

R270

Period when sliding = gRTd 2

Summary of LRB and FPS Designs

El t i C d SlidElastomeric (LRB)

Curved Slider(FPS)

Number of isolators 12 12

External dimensions9.4 in diam. 18 in diam.

External dimensionsx 7.75 in height x 5 in (est.) height

Internal dimensions 11 x in layers radius = 41 in

Other 1.92 in diam. lead corecoefficient of

friction = 0.075

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Eradiquake Isolator

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Other Design Issues (All) Restoring force capability Clearances (expansion joints, utility crossings )( p j y g ) Vertical load capacity and stability at high shear

strain Uplift restrainers, tensile capacity Non-seismic requirements (wind, braking, thermal q ( g

movements ) System Property Modification Factors (-factors) for

aging, temperature, wear and tear, and contamination

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Topics

Background Principles of Seismic Isolation, Some Applicationspp System Design Testing Requirements Testing Requirements

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Basic Testing Requirements Because isolators are subject to extreme

deformations and loads during large g gearthquakes, most design codes require they be tested to demonstrate conformance with design expectations

For both reasons (extreme loads andFor both reasons (extreme loads and extensive testing), design provisions for isolation bearings may differ from that forisolation bearings may differ from that for conventional bearings e.g., Section 14, AASHTO LRFD Design Specifications

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AASHTO LRFD Design Specifications

Basic Testing RequirementsUsually three categories of tests are required:

1. Characterization Tests to confirm basic C a acte at o ests o co bas cproperties 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 ( l ith t i l t t ) f lit(along with material tests) for quality control/quality assurance.

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During this lecture we have learned: Basic purpose of seismic isolation Four components of an isolation systemFour components of an isolation system Bridge types / configurations suitable for

seismic isolationseismic isolation How to calculate displacement and base

shear in an isolated bridge using theshear in an isolated bridge using the Simplified Method

About three kinds of isolators in use today

Five questions

1. What is basic purpose of seismic isolation? 2 List the four components of an isolation system2. List the four components of an isolation system.3. Describe bridge types and configurations that

are suitable for seismic isolation.are suitable for seismic isolation.4. Name three common types of isolators on the

market today in the U.S. y5. Name three types of tests used to assure the

quality of seismic isolatorsquality of seismic isolators