A Comparative Study on the SeismicPerformance of Superelastic-Friction BaseIsolators against Near-Field Earthquakes

Osman E. Ozbulut,a) and Stefan Hurlebausb)

This paper presents a comparative seismic performance assessment of super-elastic-friction base isolator (S-FBI) systems in improving the response of bridgesunder near-field earthquakes. The S-FBI system consists of a steel-Teflon slidingbearing and a superelastic shape memory alloy (SMA) device. The other isolationsystems considered here are lead rubber bearing (LRB), friction pendulum system(FPS), and resilient-friction base isolator (R-FBI). Each isolation system isdesigned to provide the same isolation period and characteristic strength. Non-linear time-history analyses of an isolated bridge are performed to compare theperformance of various isolation systems. The results indicate that the S-FBI sys-tem shows superior performance in reducing deck displacement response andeffectively limits permanent bearing deformation, whereas residual deformationsare present for the other isolation systems in some cases. It is also observed thatthe LRB system has the largest deck drifts while the FPS system and R-FBI sys-tem produce the smallest peak deck acceleration and base shear. [DOI: 10.1193/1.4000070]

INTRODUCTION

Bridge structures play a critical role in the transportation network by providing passageover various physical obstacles. Disruption in transportation services due to the damage orcollapse of a bridge after an earthquake extensively impedes relief and rescue efforts. Inrecent years, significant damage to bridges has been observed during strong ground motions.For example, a field investigation after the 2008 Wenchuan (China) earthquake revealedthat among the 320 bridges that were examined in the earthquake region, more than50% of them were severely or moderately damaged, and some of them completely collapsed(Qiang et al. 2009).

Although numerous strategies have been proposed to improve the response of bridgestructures during earthquakes, seismic isolation has been the most commonly used methodin recent years (Ibrahim 2008, Kerber et al. 2007). Seismic isolation is essentially based onthe idea of decoupling the support of a structure from the horizontal motions of the ground byplacing flexible interfaces between the structure and its support. It reduces the lateral forcesthat act on the superstructure by shifting the fundamental period of the structure away fromthe predominant period of the ground motion and providing additional damping. A variety of

Earthquake Spectra, Volume 28, No. 3, pages 1147–1163, August 2012; © 2012, Earthquake Engineering Research Institute

a) Post Doctoral Research Associate, Texas Transportation Institute, College Station, TX 77843b) Assistant Professor, Zachry Department of Civil Engineering, Texas A&M University, College Station,TX 77843

1147

devices, including rubber isolation systems that combine laminated rubber bearings andmechanical dampers and sliding-type isolation systems that filter out earthquake forcesvia the discontinuous sliding interfaces, have been developed and used for seismic isolation.

Near-field ground motions have intense long-period velocity pulses that have detrimentaleffects on structures with long vibration periods. Several studies have investigated the seis-mic response of base-isolated structures during near-field earthquakes. Rao and Jangid(2001) evaluated the response of structures isolated by sliding systems and found thatthey may experience large isolator deformations mainly due to the normal component ofthe near-field ground motions. Liao et al. (2004) pointed out that the displacement andbase shear responses of isolated bridges increase for ground motions with high peak groundvelocity to peak ground acceleration (PGV/PGA) ratios. Shen et al. (2004) performed field-tests to evaluate the response of a bridge with lead-rubber isolation bearings subjected to anear-field earthquake. They concluded that there is considerable amplification in the responseof the bridge when the pulse period is close to the effective period of the system. Someresearchers have studied the performance of isolated bridges that have been hit by near-field earthquakes. In such a study, Park et al. (2004) explored the performance of theBolu Viaduct which was isolated by pot bearings and additional dissipative devices duringthe 1999 Düzce (Turkey) earthquake. They found that drift of the superstructure exceededthe capacity of the bearings at an early stage of the earthquake, causing major damage tothe isolation system. Jonsson et al. (2010) evaluated the response of a base-isolated bridgelocated in Iceland that had been subjected to a near-field earthquake. The isolation systemof the bridge consists of two friction bearings at the abutments and on the first piers ateach end and four lead rubber bearings, alongwith vertical steel bars between the superstructureand piers at the middle spans. They indicated that the loads applied to the structure were largerthan those prescribed by the code and the response of the bridge was dominated by the pulse.

Theeffectivenessofshapememoryalloys(SMAs) incontrollingvibrationshasbeenstudiedby many researchers in recent years (DesRoches and Smith 2003, Hurlebaus and Gaul 2006,Zhang and Zhu 2007, Padgett et al. 2009). Superelastic SMAs have the ability to recover largedeformations upon unloading and they exhibit hysteretic behavior and energy dissipation cap-abilities. Due to their re-centering and energy dissipating abilities, several attempts have beenmade to use SMAs in an isolation system (Choi et al. 2006, Cardone et al. 2006, Casciati et al.2007). In such an attempt, Ozbulut and Hurlebaus (2010a) proposed a bridge structure with asuperelastic-friction base isolator (S-FBI) that combined a flat sliding bearing and an SMAdevice and investigated its seismic performance. They also explored the effects of temperaturechangesontheperformanceof thesystemandfoundthatchanges in theambient temperaturehadamodest effect.Although ithasbeenshownthatS-FBIsystemscanbefeasible isolationsystemsfor the seismic protection of bridges, further work is needed to compare the performance ofS-FBI systems with more commonly used isolation systems.

This study assesses the performance of seismically isolated bridge structures subjected tonear-field ground motions. In particular, a comparative study of the performance of variousisolation systems, including lead rubber bearings (LRB), friction pendulum systems (FPS),resilient-friction base isolators (R-FBI), and S-FBIs, for a multispan continuous bridge undernear-field ground motions is conducted. This paper is presented in the following order. First,the modeling of a three-span continuous isolated bridge is discussed. Then, each seismic

1148 O. E. OZBULUT AND S. HURLEBAUS

isolation system and its analytical model are briefly described. The ground motions used forthe analysis are introduced next. Finally, nonlinear time-history analyses are carried out tocompute the peak response quantities of the isolated bridge structure and the results for eachisolation system are compared for various excitation cases.

MODELING AN ISOLATED BRIDGE STRUCTURE

A three-span continuous bridge was selected for this comparative study (Wang et al.1998). The deck of the bridge has a mass of 771.1 × 103 kg, and the mass of each pieris 39.3 × 103 kg. The bridge has a total length of 90 m, and each pier is 8 m tall. The momentof inertia of the piers and Young’s modulus of elasticity are given as 0.64 m4 and20.67 × 109 N=m2, respectively. The fundamental period of the non-isolated bridge is0.45 s. The damping in the piers, ξp, is taken as 2% of the critical damping. The bridgedeck is assumed to be rigid. Pounding between the deck and abutment is not considered.The isolated bridge is modeled as a two-degrees-of-freedom system, as shown in Figure 1.Since the isolation systems installed at the abutment and pier have similar characteristics andthe seismic response of the bridge at the abutment and pier have the same trend, only aninternal span is considered in the analytical model.

The equations governing the motion of the isolated bridge are:

EQ-TARGET;temp:intralink-;e1;62;262

mpupðtÞ þ cp _upðtÞ þ kpupðtÞ − FISðup; _up; ud; _ud; tÞ ¼ −mpugðtÞmdudðtÞ þ FISðup; _up; ud; _ud; tÞ ¼ −mdugðtÞ (1)

where mp, md and up, ud are the masses and displacements of the pier and deck relative to theground, respectively; cp and kp represent the coefficient of damping and stiffness of the piers;ug is the ground acceleration; and FIS denotes the nonlinear restoring force of the isolationsystem.

MODELING SEISMIC ISOLATION SYSTEMS

LEAD RUBBER BEARINGS

A conventional elastomeric bearing consists of alternate layers of low damping rubberand steel bonded together and provides both horizontal flexibility and sufficient verticalrigidity. An LRB is an elastomeric bearing with a lead plug that provides hysteretic energy

md

mp

kp cp

ud

up

gu

FIS

Figure 1. The analytical models of isolated bridge.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1149

dissipation. A schematic diagram and the bilinear force-deformation behavior of an LRB areshown in Figure 2.

To model the force-deformation characteristics of the LRB, a linear visco-elastic elementand an elastic-perfectly plastic element that represent the rubber bearing and the lead plug,respectively, are considered. Specifically, a Bouc-Wen model (Wen 1976) is used to definethe linear spring force of the rubber and hysteretic force of the lead plug, and a dashpot is usedto describe the small ability of the rubber portion of the bearing to dissipate energy. Therestoring force of the LRB is given by:

EQ-TARGET;temp:intralink-;e2;41;278FLRB ¼ cb _ub þ kbub þ ð1 − αÞFyz; (2)

where cb and kb are the viscous damping and stiffness of the rubber bearing; _ub and ub are thevelocity and deformation of the bearing; α is the ratio of the post yielding to the elastic stiff-ness; Fy is the yield strength; and z is a hysteretic dimensionless quantity governed by thefollowing differential equation:

EQ-TARGET;temp:intralink-;e3;41;196uy_zþ γz���_ub������z���n−1 þ β _ub

���z���n − A_ub ¼ 0 (3)

where A, β, γ, and n are dimensionless quantities controlling the behavior of the model. Theparameter n governs the sharpness of the transition from initial stiffness to post-yieldingstiffness and the parameters β and γ control the size and shape of the hysteretic loop.The parameter A was introduced in the original description of the model, but it has beenshown to be redundant; removing this redundancy is best achieved by fixing parameterA to 1 (Ma et al. 2004). Constantinou and Adnane (1987) suggested imposing the constraint

mb

ub

gu

Lead plug

RubberSteel schims

cb

Displacement

Forc

eFy

uy

K2Q

Ku

2

2

u

b

K

K

K k

Figure 2. Lead rubber bearing with its schematic diagram and force-deformation curve.

1150 O. E. OZBULUT AND S. HURLEBAUS

A∕β þ γ ¼ 1 to reduce the model to a formulation with well-defined properties. For theunloading path to follow the initial loading path, β ¼ γ. Hence, the parameters A, β, andγ are set to 1, 0.5, and 0.5, respectively, and the parameter n is set to 2 (Matsagar and Jangid2003). The fundamental isolation period, Tb, can be computed as:

EQ-TARGET;temp:intralink-;e4;62;591Tb ¼ 2π

ffiffiffiffiffiffiffiffiffiffiffiffiffiffimdXK2

s(4)

whereX

K2 is the total yield stiffness of the bearings.

FRICTION PENDULUM SYSTEMS

An FPS is a form of sliding bearing which consists of two curved steel plates that slide oneach other because of an articulated slider. The concave geometry of the bearing enables amechanism for restoring force, and the friction between the slider and the concave surfaceprovides damping. A schematic diagram and the force-deformation response of an FPS areshown in Figure 3.

The frictional force of the sliding systems has mainly been described by either a con-ventional friction model or a continuous hysteretic model in past studies (Constantinou et al.1990, Bozzo and Barbat 1995). In the conventional model, the frictional force of the slidingsystem is evaluated by solving different sets of equations for sliding and nonsliding phases,while the hysteretic model is based on the principles of the theory of viscoplasticity and usesthe Bouc-Wen equations to model the frictional force. Jangid (2005a) found that both modelsyield similar predictions for the seismic response of sliding isolation systems. Therefore,

mb

Displacement

Forc

e

ub

gu

µ

K2Articulated

slider

Bearing material

Concave stainlesssteel sliding surface

K2

Q2

d

d

WK

RQ W

Figure 3. Friction pendulum system with its schematic diagram and force-deformation curve.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1151

here, the continuous hysteretic model is used to define the frictional force in the FPS asfollows:

EQ-TARGET;temp:intralink-;e5;41;615FFPS ¼ μWdzþWd

Rub; (5)

where μ represents the coefficient of friction and can be approximated at sliding velocity_ub as:

EQ-TARGET;temp:intralink-;e6;41;551μ ¼ μmax − Δμexpð−aj_ubjÞ (6)

where μmax is the coefficient of friction at very high velocities, Δμ is the difference betweenthe coefficient of friction at very high and very low velocities and, a is a constant for a givenbearing pressure and condition of the sliding interface. Furthermore, Wd is the weight of thedeck, R is the radius of the concave surface, and z is a hysteretic dimensionless quantitycomputed from Equation 5. In Equation 5, the term uy represents the yield displacementof the sliding bearing and has been chosen as 0.005 cm. Also, the dimensionless parametersγ, β, and n, have the values 0.9, 0.1, and 2, respectively, as suggested by Constantinou et al.(1990) in order to describe the frictional force of sliding bearings. Finally, the parametersμmax, Δμ, and a in Equation 6 are specified to be 10.26, 7.13, and 22, respectively (Dolceet al. 2005). The isolation period, Tb, is expressed by:

EQ-TARGET;temp:intralink-;e7;41;412Tb ¼ 2π

ffiffiffiffiffiffimd

K2

r¼ 2π

ffiffiffiRg

s; (7)

where g is the acceleration due to gravity.

RESILIENT-FRICTION BASE ISOLATORS

Another method of providing a restoring force to sliding isolation systems is to usethe sliding bearing in combination with a central rubber core. The R-FBI combines the resi-lience of a rubber core and the friction of Teflon-coated steel plates. A schematic diagram andthe force-deformation response of an R-FBI system are illustrated in Figure 4. The restoringforce developed in the isolator is given by:

EQ-TARGET;temp:intralink-;e8;41;258FRFBI ¼ μWdzþ cb _ub þ kbub; (8)

where cb and kb are the viscous damping and initial stiffness of the rubber bearing, respec-tively. The continuous hysteretic model described in the above section is used to model thefriction force of the sliding bearing. The isolation period of an R-FBI system can be deter-mined using Equation 4.

SUPERELASTIC-FRICTION BASE ISOLATORS

An S-FBI consists of a flat steel-Teflon sliding bearing and a superelastic SMA device.The sliding bearing decouples the superstructure from the substructure and limits the trans-mission of seismic forces to a certain level according to the friction coefficient of the slidingsurface. The SMA device mainly provides restoring force capability to the isolation system. Italso offers additional energy dissipation through hysteresis of the SMA elements, eventhough the seismic energy is essentially dissipated via friction in the sliding surface of

1152 O. E. OZBULUT AND S. HURLEBAUS

the bearings. Here, the SMA device simply consists of multiple loops of superelastic NiTiwires wrapped around low-friction wheels. Figure 5 shows a schematic diagram of an S-FBIsystem, its force-displacement curve, and its subcomponents. The restoring force of theS-FBI system is given as:

EQ-TARGET;temp:intralink-;e9;62;343FSFBI ¼ μWdzþ FSMA; (9)

where FSMA denotes the nonlinear force of the SMA device. The friction force of the flatsliding bearing is modeled using the continuous hysteretic model and the model parameters

mb

ub

gu

µ

kb

cb

Rubber core

Teflon layers

Sliding plates

Displacement

Forc

e

K2

Q2 b

d

K k

Q W

Figure 4. Resilient-friction base isolator with its schematic diagram and force-deformation curve.

gu

2 sma

d

K k

Q W

αμ

==

Displacement

DisplacementDisplacement

Forc

e

Forc

e

Forc

e

K2 Q

SMA device

Steel-Teflon

Stainless steelplate ub

mb

FSMA

μ

Figure 5. Superelastic-friction base isolator with its schematic diagram and force-deformationcurve.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1153

described above. A neuro-fuzzy model developed by Ozbulut and Hurlebaus (2010b) is usedto simulate the force-deformation behavior of the NiTi wires. This model is capable of cap-turing rate- and temperature-dependent material response, but it remains simple enough tocarry out numerical simulations. A brief description of the model is given below.

Neuro-fuzzy modeling refers creating a fuzzy inference system (FIS) that is trained by alearning algorithm based on neural networks. The basic structure of an FIS consists of threecomponents: fuzzy if-then rules, a database containing membership functions of linguisticlabels, and an inference mechanism which evaluates the rules to produce the system output.The FIS maps an input space to an output space by means of fuzzy if-then rules. The effi-ciency of the fuzzy system depends on the selection of the parameters of the if-then rules,which is usually difficult to determine by human expertise. An adaptive neuro-fuzzy infer-ence system (ANFIS) is a soft computing approach that employs neural network strategies todevelop a fuzzy system (Jang 1993). The ANFIS uses a hybrid algorithm to learn from thesample data from the system and can adapt parameters inside its network.

Here, the ANFIS is used to create a model of superelastic SMAs considering temperatureand loading rate effects. First, an initial FIS is created which employs strain, strain rate, andtemperature as input variables and predicts the stress as a single output. Note that this initialFIS has no information about target behavior and needs to be trained. To this end, a set oftensile tests are carried out on NiTi shape memory alloy wires with a diameter of 1.5 mm atvarious loading rates and temperatures and the data collected from experimental tests is con-catenated. After training, the developed FIS is validated using a data set that is reserved forvalidation and not used during the ANFIS training. Details in regard to the neuro-fuzzy modelof the NiTi wires can be found in Ozbulut and Hurlebaus (2010b).

The natural period of the isolated bridge can be computed as:

EQ-TARGET;temp:intralink-;e10;41;331Tb ¼ 2π

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffimdXαkSMA

s; (10)

where αkSMA denotes the secondary stiffness of the SMA device. Here, α, which representsthe ratio between the secondary stiffness and the initial stiffness of the SMA device, is takenas 0.1 and the forward transformation strain of the SMA wires is 1%; these are typical valuesfor NiTi shape memory alloy wires.

GROUND MOTIONS CONSIDERED

In this study, the program RspMatch2005 is employed to generate spectrum-compatibleground motions that are used in dynamic time-history analyses of the isolated bridge.RspMatch2005 uses wavelets to adjust the accelerograms to match the response spectraat multiple damping values, while preserving the nonstationary character of the referencetime history (Hancock et al. 2006). Here, ten historical near-field earthquake records areselected as seed accelerograms to investigate the effectiveness of the various isolation sys-tems under near-field ground motions. Table 1 gives the characteristics of the groundmotions, including magnitude, closest distance to the fault plane, peak ground acceleration,and peak ground velocity. A response spectrum constructed as per the International BuildingCode (ICC 2000) for a site in southern California, assuming firm rock conditions, is selected

1154 O. E. OZBULUT AND S. HURLEBAUS

Table 1. Description of the ground motions used in the analyses

RecordNo Earthquake Station Mw

Distance(km)

PGA(g)

PGV(cm/s)

1 Imperial Valley, CA1979/10/15

El Centro Array #6 6.5 1.0 0.44 109.8

2 N. Palm Springs, CA1986/07/08

North Palm Springs5070

6.0 8.2 0.59 73.3

3 San Fernando, CA1971/02/09

Pacoima Dam 279 6.6 6.1 0.56 94.8

4 Landers, CA1992/06/28

Lucerne 24 7.3 2.8 1.22 112.5

5 Loma Prieta, CA1989/10/18

GPC 16 6.9 1.1 0.72 97.6

6 Northridge, CA1994/01/17

Sylmar 74 6.7 6.2 0.90 102.8

7 Northridge, CA1994/01/17

Rinaldi 77 6.7 7.1 0.85 50.7

8 Northridge, CA1994/01/17

Tarzana, Cedar Hill24436

6.7 17.5 1.05 75.4

9 Kobe, Japan1995/01/16

Nishi-Akashi 6.9 11.3 0.51 37.3

10 Kocaeli, Turkey1999/08/17

Sakarya 7.4 3.1 0.38 79.5

0 1 2 3 40

1

2

3

4

5

Period (s)

Pse

udo

Spe

ctra

l Acc

eler

atio

n (g

)

0 1 2 3 40

0.5

1

1.5

2

Period (s)

TargetImperial Valley

Landers

Loma Prieta

N Palm Springs

San FernandoSylmar

Kobe

Cedar Hill

RinaldiKocaeli

5% Damping

%10 Damping

25% Damping

Figure 6. Target response spectrum compared to response spectra of selected ground motionsand the spectrally matched response spectra of all earthquakes for different damping levels.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1155

as the target spectrum (Malhotra 2003). The left subplot in Figure 6 shows the target responsespectrum used in the analysis and the response spectra of the selected ground motions for a5% damping level.

The selected seed accelerograms are adjusted using RspMatch2005 in order to simulta-neously match 5%-, 10%-, and 25%-damped response spectra. The right subplot in Figure 6shows the spectrally matched response spectra of all the earthquakes for different dampinglevels. The use of the RspMatch2005 significantly reduces the spectral misfit for all dampinglevels.

COMPARATIVE PERFORMANCE STUDY

In order to compare the performance of different isolation systems, the key design para-meters for each isolation system should be similar. Figure 7 shows the idealized hystereticforce-deformation curves for all the isolation systems used in this study. Here, each isolationsystem is designed so that the characteristic strength, Q, and the secondary stiffness, K2, areequal for each isolator type. Specifically, the value of the characteristic strength normalizedby the weight of the deck is 0.10 and the parameter K2 is 1,166 kN/m. For the value chosenfor K2, the period shift, which is defined as the difference between the fundamental periods ofisolated and fixed-base bridge structures (Tshif t ¼ Tb − Ts), is 2.5 sec. These values are in therange of the recommended design values for each isolation system (Jangid 2007, Jangid2005b, Iemura et al. 2007, Ozbulut and Hurlebaus 2011). Also, the yield displacementof the LRB system and the yield displacement of the SMA device for the S-FBI systemare uy ¼ 3.0 cm. For the LRB system and R-FBI system, the viscous damping ratio ofthe rubber bearing is 2%.

The seismic response of the bridge structure subjected to the ground motions describedabove was computed for various isolation systems through nonlinear time-history analyses.The peak deck drift, residual bearing deformation, peak deck acceleration, and peak baseshear normalized by the weight of the deck are presented in Figures 8–11 for each isolatortype. Also, the hysteretic force-displacement loops for each isolation system subjected todifferent ground motions are illustrated in Figures 12 and 13.

Displacement

Forc

e

K2

Q

LRBFPS or R-FBI

uy

S-FBI

Figure 7. Idealized hysteretic force-deformation curves and selected design parameters.

1156 O. E. OZBULUT AND S. HURLEBAUS

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30P

eak

deck

drif

t (cm

)

Ground motion record #

LRB FPS R-FBI S-FBI

Figure 8. Peak deck drift for the various isolation systems subjected to near-field earthquakes.

1 2 3 4 5 6 7 8 9 100

0.05

0.1

0.15

0.2

0.25

0.3

Pea

k de

ck a

ccle

ratio

n (m

/s2)

Ground motion record #

LRB FPS R-FBI S-FBI

Figure 10. Peak deck acceleration for the various isolation systems subjected to near-fieldearthquakes.

1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

2

2.5

Res

idua

l def

orm

atio

n (c

m)

Ground motion record #

LRB FPS R-FBI S-FBI

Figure 9. Peak residual isolator displacement for the various isolation systems subjected to near-field earthquakes.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1157

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

Pea

k no

rmal

ized

bas

e sh

ear

Ground motion record #

LRB FPS R-FBI S-FBI

Figure 11. Peak normalized base shear for the various isolation systems subjected to near-fieldearthquakes.

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 1

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 2

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 3

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 4

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

Isolator displacement (cm)

For

ce (

kN)

LRB

Record # 5

-20 0 20-500

0

500

Isolator displacement (cm)

FPS

-20 0 20-500

0

500

Isolator displacement (cm)

RFBI

-20 0 20-500

0

500

Isolator displacement (cm)

SFBI

Figure 12. Hysteresis curves forvarious isolation systemssubjected togroundmotion record#1–5.

1158 O. E. OZBULUT AND S. HURLEBAUS

Figure 8 shows the peak deck drift of the isolated bridge for various isolation systemssubjected to different ground motions. It can be seen that the S-FBI system consistently hasthe smallest peak deck drift for all excitation cases. In particular, the peak deck drift responseof the bridge structure isolated by the S-FBI system is reduced by up to 54% compared to theLRB system, and by up to 34% compared to the R-FBI and FPS systems for different groundmotion scenarios. Note that the LRB system produces the largest value for peak deck driftwhen the isolated bridge is subjected to different earthquakes. The higher deformationsobserved in the LRB system might be attributed to the fact that the area under the hysteresisloop of the LRB system is smaller than those of the sliding-based isolation systems for thesame displacement level, as can be seen in Figure 7. Therefore, the energy dissipation percycle of the LRB system is somewhat smaller than the other isolation systems for the selecteddesign parameters. The results for the FPS system and R-FBI system are very similar, whilethe R-FBI system has slightly smaller values for peak deck drift. This difference can be

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 6

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 7

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 8

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

For

ce (

kN)

LRB

Record # 9

-20 0 20-500

0

500

FPS

-20 0 20-500

0

500

RFBI

-20 0 20-500

0

500

SFBI

-20 0 20-500

0

500

Isolator displacement (cm)

For

ce (

kN)

LRB

Record # 10

-20 0 20-500

0

500

Isolator displacement (cm)

FPS

-20 0 20-500

0

500

Isolator displacement (cm)

RFBI

-20 0 20-500

0

500

Isolator displacement (cm)

SFBI

Figure 13. Hysteresis curves for various isolation systems subjected togroundmotion record#6–10.

A COMPARATIVE STUDYON THE SEISMIC PERFORMANCEOF S-FBI AGAINST NEAR-FIELD EARTHQUAKES 1159

attributed to the small amount of viscous damping in the rubber component of the R-FBIsystem, which is not present in the FPS system.

The sufficient restoring force capacity of isolation systems is a fundamental requirementin current codes for the design of seismically isolated structures. One indication of an inade-quate restoring capability is large residual displacements after the end of the seismic event.Figure 9 shows the residual displacement of various isolation systems subjected to differentearthquakes. It is observed that the S-FBI system successfully recovers its deformations at theend of the earthquake motions. This almost perfect restoring characteristic of the S-FBI sys-tem can be explained by the re-centering ability of the NiTi shape memory alloys. For theother isolation systems, residual deformations are present for some of the excitation cases.

The peak deck acceleration responses for the various isolation systems are shown in Fig-ure 10. It should be noted that the peak deck acceleration is effectively reduced when thebridge structure is isolated by an isolator system. In general, the S-FBI produces larger peakdeck accelerations than the other isolation systems. However, note that for the ten recordsincluded in this study, the peak deck acceleration for the S-FBI system is higher by an aver-age of only 7% when compared to the acceleration for the LRB system, and an average ofabout 20% when compared to the acceleration for the R-FBI and FPS systems.

Figure 11 shows the peak base shear normalized by the weight of the deck for the variousisolation systems. The largest values for peak normalized base shear are mostly observed inthe S-FBI system and the LRB system for different excitation cases. One of the reasons forthe higher deck acceleration and base shear observed in the S-FBI system is the fact that itgenerates higher forces than the other isolation systems for the same deformation level. Thiscan be clearly seen in Figure 7 or Figures 12 and 13. In addition, since the LRB systemexperiences larger deformations, as discussed above, the force transmitted to the piers bythe deck is also larger for this system, resulting in higher base shears. For several of theearthquakes, the peak normalized base shear is similar for the FPS and R-FBI systems,but slightly larger for the FPS system.

CONCLUSIONS

This study aimed to evaluate the effectiveness of S-FBI systems for protecting bridgestructures against near-field earthquakes by performing a comparative parametric study.The other isolation systems considered here include the LRB system, the FPS system,and the R-FBI system. A three-span continuous bridge was selected for the numerical studiesand modeled as a two-degrees-of-freedom system. A total of ten historical near-field earth-quakes were modified using the RspMatch2005 to match their response spectra with a targetresponse spectrum and were used in the simulations. Each isolation system was designedsuch that the characteristic strength of the isolators and the fundamental period of the isolatedbridge were the same. Nonlinear time-history analyses of the bridge with different isolationsystems were performed. The performance of each isolation system was assessed in terms ofpeak deck drift, residual bearing deformation, peak deck acceleration, and peak base shear.The results show that seismic isolation can successfully protect bridge structures from thedamaging effects of near-field earthquakes. In particular, the S-FBI system results in thesmallest deck drift compared to the other isolation systems, while it usually experiencespeak deck acceleration and base shear demands larger than the FPS and R-FBI systems

1160 O. E. OZBULUT AND S. HURLEBAUS

but similar to the LRB system. It is also revealed that the S-FBI system has excellent re-centering ability and almost always recovers its deformations after a seismic event, whileresidual deformations are typically observed for the other isolation systems considered inthis study. That result implies that, in the event of a near-field earthquake, there wouldbe no need for the replacement or refurbishment of the isolation system; the S-FBI systemwould protect the bridge against future earthquakes.

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(Received 24 May 2010; accepted 15 July 2011)

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