Performance Comparison between SMA-based ...

Performance Comparison between SMA-based Natural Rubber Bearing and SMA-based High Damping Rubber Bearing Farshad Hedayati Dezfuli and M. Shahria Alam

1

ABSTRACT Shape memory alloys (SMAs) are smart and functional materials which can sense environmental excitations and respond to alterations. They can be used in a wide range of applications such as vibration control, damping, and actuation. SMAs have unique characteristics such as high superelastic effect, high energy dissipation capacity, long fatigue life, and durability. Implementing SMA bars or wires in the elastomeric base isolators can improve their performance in terms of energy dissipation capacity as well as the re-centering capability and the residual deformation. This study deals with two new kinds of SMA-based rubber bearings (SMA-RBs) in which natural and high damping rubbers are used. The effect of shear strain amplitude and the amount of pre-strain in SMA wires is investigated on the performance of the rubber bearings. It is observed that the arrangement of wires could overcome the performance limitation of superelastic range of SMA wires. Results show that, ferrous SMA wires with a high superelastic strain and a very low austenite finish temperature are the best candidate to be used in SMA-RBs subjected to high shear strain amplitudes. Keywords: shape memory alloy; superelasticity; natural rubber bearing; high damping rubber bearing; recentering; residual deformation.

INTRODUCTION The operation of civil structures such as bridges, hospitals, and fire stations during an earthquake is a crucial issue that should be considered during the design procedure and construction. Seismic isolation systems can prevent or minimize the structural damages of buildings and bridges to provide a continuous operation for such structures by regulating their seismic behavior (Ozkaya et al., 2011). The main goals of using base isolation techniques are: (i) preventing the structural collapse in severe earthquakes, (ii) avoiding or minimizing the structural damage in moderate earthquakes, and (iii) providing continuous operation in important buildings and bridges. Base isolators can considerably decrease and dissipate the earthquake energy transmitted to the structure by providing a flexible damping mechanism between the substructure and the superstructure due to their low horizontal, but high vertical and bending stiffnesses. The premise of using such devices is to have high flexibility, which shifts the natural M. Shahria Alam, Assistant Professor, School of Engineering, University of British Columbia, 1137 Alumni Avenue, EME 4225, Kelowna, BC V1V 1V7 Farshad Hedayati Dezfuli, PhD Candidate, School of Engineering, University of British Columbia, 1137 Alumni Avenue, EME 3207, Kelowna, BC V1V 1V7

1

period of the bridge structure away from the critical period range of the earthquake. Moreover, they control the displacement of the bridge piers and decks since they can dissipate the energy due to damping properties. Seismic base isolation devices are categorized into two main types: rubber bearings (RBs) and sliding bearings. Rubber bearings or elastomeric isolators are laminated structures composed of reinforced and elastomeric layers. Steel shims or fiber-reinforced plates as reinforcement provide vertical stiffness and strength while, rubber layers provide lateral flexibility and damping property. Sliding bearings in which curved or flat sliding surface is used, work based on a friction mechanism (Ozkaya et al., 2011). The most popular elastomeric isolators are natural rubber bearings (NRB), lead rubber bearings (LRB), and high damping rubber bearings (HDRB). They have been used in bridges for both new constructions and retrofit applications. However, most of these devices have some limitations related to service life and durability, temperature dependent behaviors, maintenance, and unrecoverable deformations after a strong ground motion (Dolce et al., 2000; Dion et al., 2011; Dion et al., 2012). In this regard, the use of superelastic shape memory alloy (SMA) can provide an effective solution to overcome several of these problems. Superelastic SMA is a unique material with the ability to undergo large deformation and potentially recover its inelastic deformation upon stress removal. SMA as a supplementary element in seismic base isolators can enhance the recentering capability as well as the energy dissipation capacity in order to reduce forces and relative displacements transmitted from substructure to superstructure (Attanasi et al., 2008; Ozbulut and Hurlebaus, 2011). Many studies have been carried out to develop new generations of base isolators using shape memory alloys (SMAs) (Graesser and Cozzarelli, 1991; Dong et al., 2002). Suduo and Xiongyan, 2007, introduced three types of SMA-based dampers and a base isolator incorporated with nickel-titanium SMA wires. They could find out that not only smart base isolators can efficiently mitigate the seismic response in terms of acceleration, displacement and internal forces but also, they have superior performance relative to existing rubber bearings. They concluded that, the proposed intelligent systems have many advantages such as stability, high energy dissipation capacity, good fatigue and corrosion resistance capabilities and as a result long service life. Choi et al., 2005, performed numerical study considering NiTi SMA wire wrapped around an elastomeric bearing to improve its recentering capability over the lead rubber bearing. However, at very large shear deformation (200% shear strain), this device will malfunction since wires experience axial strain beyond the NiTi’s superelastic strain range. Although Dolce et al., 2000, implemented SMA wires effectively in a base isolation device, the manufacturing of the device was quite complex. Another SMA-based isolation device was developed by Dolce et al., 2000, which showed high sensitivity and considerable variation in forces with temperature, and inefficiency in energy dissipation capacity. Liu et al., 2008, used a diagonal arrangement of large diameter SMA strands around the rubber bearing. However, this arrangement did not improve the recentering capability or the level of damping compared to the original rubber bearing. Attanasi and Aurichhio, 2001, proposed an isolation device equipped with eight SMA coil springs, which is expensive due to its complex manufacturing process and the use of large diameter SMA springs. Attanasi et al., 2008, investigated the possibility of using shape memory alloys in base isolation systems. They compared the behavior of a proposed smart isolator with that of a traditional lead rubber bearing and an equivalent linear elastic model. According to their results, the behavior of the smart isolation device with flag-shape hysteretic loops was similar to a system with elastoplastic hysteresis. They concluded that it is possible to 2

replace existing LRBs with SMA-based bearing systems considering the amount of energy dissipation capacity. They suggested that SMA-based restrainers can be applied to rubber bearings or friction pendulum systems in order to provide recentering force and control the relative horizontal displacement and upward force transmitted to the superstructure. In this study, the performance of SMA-based natural rubber bearing (SMA-NRB) is compared with that of the SMA-based high damping rubber bearing (SMA-HDRB) through numerical simulations. In both base isolators reinforced by steel shims, SMA wires are wrapped around the device in a cross configuration. Among different types of SMAs, the most efficient one is selected based on the superelastic strain range and the compatibility with environmental thermal conditions. The effective horizontal stiffness, the residual deformation, and the energy dissipation of the proposed smart base isolators will be calculated from the lateral forcedeflection hysteresis curves. In this regard, a hyper-viscoelastic material model is used to describe the nonlinear behavior of the high damping rubber (Hedayati Dezfuli and Alam, 2012) and natural elastomer (Hedayati Dezfuli and Alam, 2013). The hysteretic shear response of SMA-RBs is evaluated using finite element method (FEM). The effect of pre-strain in SMA wires and the shear strain amplitude are explored on the performance of the device.

SHAPE MEMORY ALLOYS Shape memory alloys (SMAs) are considered as smart and functional materials that can restore their pre-determined and original shape after deformation via unloading or by applying thermal load. They have two solid phases: martensite or unstable phase in which material is at low temperature, and austenite, parent or high-temperature phase. In this regard, four characteristic temperatures are defined to determine the temperature ranges for starting and finishing the phase transformation. The martensite start temperature, Ms, and the martensite finish temperature, Mf, respectively represent the starting and finishing phase transformation from austenite to martensite. Similarly, for starting and finishing phase transformation from martensite to austenite, the austenite start temperature, As, and the austenite finish temperature, Af, are defined, respectively. Superelastic and shape memory effects are two unique characteristics of SMAs. In the superelastic effect, the generated strain due to the mechanical loading is fully recovered after unloading while in shape memory effect, the mechanical deformation should be removed by applying thermal load and increasing temperature of the alloy. The SMA materials will show the superelastic behavior if they are in the austenite phase. Shape memory alloys have a larger hysteretic deformation and a higher elastic (superelastic) strain compared to conventional alloys and metallic materials (Lagoudas, 2008). The maximum superelastic strain, εs, in such materials can even reach up to 13.5% (Tanaka et al., 2010). SMAs are excellent candidates as dampers or actuators due to their remarkable characteristics such as high damping performance, high energy dissipation capacity, significant stiffness hardening (variable stiffness), large ductility, long fatigue life, and corrosion resistance capability (Soong and Dargush, 1997). There are different types of SMAs such as Nickel-Titanium, Cu-based shape memory alloys and ferrous shape memory alloys which have the potential for smart structural applications. Some mechanical properties like the elastic modulus (EA), the austenite finish temperature (Af) and the superelastic strain (εs) under the maximum applied strain (εmax) for a number of shape memory alloys are listed in Table I. 3

TABLE I. MECHANICAL CHARACTERISTICS OF DIFFERENT SHAPE MEMORY ALLOYS εmax εs EA Af Alloy Reference (%) (%) (GPa) (°C) NiTi49.5 5.7 4.6 45.3 53.0 Strnadel et al., 1995 NiTi45 6.8 6.0 62.5 -10.0 Alam et. al., 2008 NiTi44.1 6.5 5.5 39.7 0 Alam et. al., 2008 TiNi41Cu10 4.1 3.1 91.5 50.0 Strnadel et al., 1995 TiNi25Cu25 10.0 2.5 14.3 73.0 Liu, 2003 CuAlBe 3.0 2.4 32.0 -65.0 Zhang et al., 2009 FeMnAlNi 6.1 5.5 98.4 < -50°C Omori et al., 2011 FeNiCuAlTaB 15.0 13.5 46.9 -62.0 Tanaka et al., 2010

The elastic modulus of the SMA represents the stiffness of the material in the austenite phase. The maximum strain, εmax, is defined as a strain at which the deformation in the material can be fully recovered after unloading.

SMART ELASTOMERIC ISOLATORS SMA-based smart base isolators will have many advantages such as stability, recentering capability, high energy dissipation capacity and long service life. They not only will mitigate the seismic response of structures in terms of acceleration, displacement and internal forces but also, they will have superior performance in terms of fatigue property and energy dissipation capacity compared to existing rubber bearings (Suduo and Xiongyan, 2007). Regarding the variable properties of shape memory alloys (e.g. stiffness), they are suitable candidate to be incorporated in seismic base isolators which are under various exciting forces with different magnitudes and frequencies (Wilde et al., 2000). In the present study, SMA wires are used as a supplementary element to improve the performance of steel-based natural rubber bearings in terms of energy dissipation capacity and residual deformation which happens at large shear strain amplitudes. In order to appropriately compare the performances of SMA-NRB and SMA-HDRB, identical geometrical and physical properties are considered for both natural and high damping rubber bearings as listed in Table II.

TABLE II. GEOMETRICAL PROPERTIES OF NATURAL AND HIGH DAMPING RUBBER BEARINGS Horizontal Horizontal dimensions of dimensions of tE tr ts Specimen nr ns isolator steel shims (mm) (mm) (mm) (mm × mm) (mm × mm) NRB 230 × 230 200 × 200 15 4.5 1.0 14 13 HDRB 230 × 230 200 × 200 15 4.5 1.0 14 13

tE: thickness of supporting steel plates; tr: thickness of rubber layers; ts: thickness of steel shims; nr: number of rubber layers; ns: number of steel shims.

In SMA-RBs, two SMA wires with a radius of 2.5mm are wounded around the rubber bearing diagonally as shown in Figure 1a. A steel hook is mounted at each corner on the lower and upper surfaces of the top and bottom supporting plates, respectively. The SMA wires pass

4

through these hooks. The main reason of using wires in such an arrangement is to effectively reduce the maximum strain in the wires due to large shear strain amplitudes in rubber bearing. 15 SMA Strain (%)

Supporting Steel Plate

SMA Wire 2 SMA Wire 1

10 5 0 0

50

100

150

200

Shear Strain, γ (%)

Steel Hook

(a)

(b)

Figure 1. (a) Schematic View of Smart Rubber Bearing with Cross SMA Wires, (b) Variation of Strain in SMA Wire as a Function of Shear Strain.

Figure 1b depicts the variation of strain in SMA wires by increasing the shear strain amplitude. Strain in the SMA wires is geometrically calculated with increasing the shear strain according to the size of the base isolator and the configuration of wires. It can be observed that for shear strain amplitudes up to 200%, the strain induced in SMA wires is lower than 10%. When the shear strain amplitude is 200%, none of the SMAs can work within the superelastic strain range except for the FeNiCuAlTaB and TiNi25Cu25 since the strain in SMA wire reaches to 7.2% which is higher than the maximum allowable strain in the most of SMAs (see Table I). Different environmental conditions such as temperature and humidity can affect the performance of elastomeric base isolators. The operational temperature range varies according to the location in which a rubber bearing operates. Since, the superelastic effect of SMA wires occurs at temperatures above the austenite finish temperature, in order to have a smart elastomeric bearing with superelastic SMA wires, the austenite finish temperature of the SMA wires should be lower than the ambient temperature. In such circumstances, since the minimum ambient temperature in countries with cold climatic conditions often gets below 0°C and in few places it can be as low as -40°C, the austenite finish temperature of the SMA wire should be lower than this minimum temperature. Therefore, NiTi45, CuAlBe, FeMnAlNi, and FeNiCuAlTaB with Af lower than zero (see Table I) can be implemented in elastomeric base isolators. When both the superelastic strain and the austenite finish temperature are considered as two important criteria for choosing the most efficient SMA, FeNiCuAlTaB with 13.5% superelastic strain and -62°C austenite finish temperature will be the best candidate to be used in SMA-NRBs. Hence, in this study, FeNiCuAlTaB SMA wire is implemented in smart rubber bearings.

FINITE ELEMENT MODELING

5

In the finite element model of the SMA-RBs, a method of superposition is implemented in order to simplify the system by decoupling the rubber bearing and SMA wires. A smooth contact is assumed between the steel hook and the SMA wire. Instead of modeling steel hooks, and the contact between the hooks and continuous SMA wires, exerted forces to the elastomeric isolator due to SMA wires are considered. Before analyzing the system, first, the strain generated in SMA wires at each pre-defined time step is calculated according to the geometry of the device. In the next step, the axial stress in SMA wires can be determined form the stress-strain relationship of shape memory alloy based on the Auricchio’s superelastic model (Auricchio, 2001) by considering the properties of SMA obtained from experimental results (Tanaka et al., 2010). Here, we assume that the stress-strain hysteresis of FeNiCuAlTaB SMA wire does not change by increasing the number of loading cycles. However, further experimental study is required in order to accurately simulate the dynamic behavior of SMA wires and take into account the strain time history. Using the amount of the axial stress and the direction of wires at each time step, the force vectors exerted from the SMA wire to the hook are computed. The idealized stress-strain curve for ferrous shape memory alloy FeNiCuAlTaB at room temperature is plotted in Figure 2 (Tanaka et al., 2010). In such a situation, the rubber bearing and the SMA wires are decoupled as two separate systems in FE simulations. Then, by measuring the force generated in SMA wires as a function of time, the effect of one system (SMA wires) is estimated on the other one (elastomeric isolator). Since SMAs have thermomechanical behavior, both thermal and mechanical loadings affect the response of SMAs. In this study, it is assumed that the environmental temperature does not change during cyclic loading. As a result, the coupling between the thermal and the mechanical loads can be neglected during an earthquake. However, the yield stress and consequently the hysteretic behavior of an SMA wire operating at 30°C may be different from the response of the SMA wire which works at temperatures below 0°C. Therefore, the temperature at which the base isolator is operating plays an important role in the behavior and performance of the device. In this study, the operational temperature is considered to be 20°C.

Stress (MPa)

1000 750 500 250 0 0

4

8 12 Strain (%)

16

Figure 2. Idealized Stress-Strain Curve of Ferrous FeNiCuAlTaB SMA at Room Temperature.

RESULTS AND DISCUSSION By computing the hysteretic behavior of SMA-NRB and SMA-HDRB subjected to a frequency of 0.2Hz, and 6MPa vertical pressure, the effect of the shear strain amplitude, γ, and the pre-strain in SMA wires have been assessed on the performance of the base isolator. In each 6

case, three operational characteristics: the effective horizontal stiffness (KH), the residual deformation (RD), and the energy dissipation capacity (energy dissipated per cycle, EDC) are calculated. The effective horizontal stiffness of the elastomeric isolator under specific shear strain amplitude, and a certain frequency of cyclic lateral displacement is obtained according to equation 1 (Naeim and Kelly, 1999). K H eff ( ) 

Fmax  Fmin  max   min

(1)

where Fmax and Fmin respectively are the maximum and minimum shear forces in the direction of the horizontal cyclic loading. Δmax and Δmin are the maximum and minimum lateral displacements, respectively. The residual deformation is defined as a horizontal displacement at which the shear force in the elastomeric isolator is zero. The energy dissipated per cycle equals to the area inside the lateral force-deflection hysteresis curve in each cycle. Figure 3 shows the hysteretic shear behaviors of SMA-RBs at different shear strain amplitudes. In this Figure, solid lines represent the response of SMA-RBs and the dotted lines depict the behaviors of rubber bearings without SMA wires. Using SMA wires in NRB and HDRB causes an increase in the maximum lateral force since wires, which are under tension, apply a large force to the base isolator. Another important finding is that for both SMA-RBs, the maximum shear force goes up with the increasing shear strain. When the lateral displacement increases, the strain in SMA noticeably enhances and as a result the axial stress rises. Therefore a larger amount of force is applied to the rubber bearing. 120

a

80

Lateral Force (kN)

Lateral Force (kN)

120

40 0 -40 -80 -120 -140

-70

0

70

b

80 40 0 -40 -80 -120 -140

140

-70

0

70

140

Lateral Displacement (mm)


Figure 3. Lateral Force-Deflection Curve of (a) SMA-NRB and (b) SMA-HDRB (γ = 100%, 150%, and 200%).

The performance characteristics including the effective horizontal stiffness, the residual deformation, and the energy dissipation capacity of NRB, HDRB, SMA-NRB, and SMA-HDRB are listed in Table III. Although for both SMA-NRB and SMA-HDRB, the presence of SMA wires causes a reduction in the lateral flexibility of the elastomeric isolator, the amount of increase in the effective horizontal stiffness of SMA-NRB compared to NRB is less than that of the SMA-HDRB when it is compared to HDRB. For example at 200% shear strain, the effective lateral stiffness of SMA-HDRB is 27% higher than that of the HDRB while, the effective lateral 7

stiffness of SMA-NRB at this shear strain is 45% higher than that of the NRB (see Table III). It shows that using SMA wires in HDRB is more effective in terms of the lateral flexibility. As an advantage for smart rubber bearings, the residual deformation in NRB and HDRB can decrease by implementing the SMA wires. Results show that at γ = 200%, the maximum amount of reduction in the residual deformation of NRB and HDRB is about 25%. When the shear strain amplitude increases, the residual deformation reduction enhances since the recentering capability of SMA wires is augmented. At 100% shear strain, SMA wires reduce the residual deformation of NRB and HDRB by 14% and 4%, respectively. This finding demonstrates that at low shear strain levels, SMA-NRB works more efficiently. SMA wires considerably affect the energy dissipation capacity of the elastomeric isolator. They can improve the performance of the isolation system by dissipating the earthquake energy and as a result, the displacement of the structure can be controlled within a safe range. According to the results listed in Table III, the energy dissipated per cycle in the SMA-NRB is 1.2kJ, 2.7kJ, and 4.8kJ at 100%, 150% and 200% shear strains, respectively. The maximum enhancement in the energy dissipation capacity of SMA-NRB and SMA-HDRB are 67% and 25%, respectively. When the energies dissipated by SMA-NRB and SMA-HDRB are compared at different amplitudes of lateral displacement, it can be observed that using SMA wires in NRB is more advantageous. TABLE III. OPERATIONAL CHARACTERISTICS OF SMA-RBs COMPARED TO THOSE OF NRB AND HDRB SMA-NRB SMA-HDRB γ (%) NRB HDRB (ΔNRB)* (ΔHDRB)** 100 0.49 0.71 (44%) 0.82 1.05 (28%) KH 150 0.44 0.63 (44%) 0.73 0.94 (28%) (kN/mm) 200 0.40 0.58 (45%) 0.71 0.90 (27%) 100 8.5 7.3 (-14%) 21.9 21.1 (-4%) RD 150 11.1 9.2 (-17%) 39.6 33.9 (-14%) (mm) 200 18.1 13.7 (-24%) 69.5 51.2 (-26%) 100 0.9 1.2 (43%) 2.7 3.2 (18%) EDC 150 1.6 2.7 (67%) 5.5 6.9 (25%) (kJ) 200 3.0 4.8 (59%) 10.2 12.6 (23%) * ΔNRB: Relative difference between operational characteristics of SMA-NRB and those of the NRB ** ΔHDRB: Relative difference between operational characteristics of SMA-HDRB and those of the HDRB

Pre-strain in SMA Wires In order to efficiently use the SMA wire as a damper in the rubber bearing, the initial elastic part should be removed by pre-straining the SMA wire (Choi et al., 2005). In the prestrained SMA wire, the forward phase transformation occurs at a lower strain and as a result, the yield stress considerably decreases. Subsequently, when the SMA wire is elongated and subjected to a tension due to the cyclic displacement of the rubber bearing, a smaller force will be transferred to the superstructure. By applying a pre-strain (ε0 = 2%) to the SMA wire, the stress-strain curve shifts according to Figure 4.

8

1000 Stress (MPa)

750 500 250 0 0

4

8

12

16

Strain (%)

Figure 4. Stress-Strain Curve of Ferrous SMA (FeNiCuAlTaB)

In order to investigate the effect of pre-straining on the performance of SMA-RBs, SMA wires with 2% pre-strain are used. Figure 5 depicts the hysteretic shear behavior of SMA-NRB and SMA-HDRB with 0%, and 2% pre-strained wires. All elastomeric isolators are subjected to three shear strain amplitudes (100%, 150%, and 200%) with horizontal frequency of 0.2Hz and 6MPa vertical pressures. The solid lines show the shear behavior of SMA-RBs equipped with 2% pre-strain wires and the dotted lines represents the hysteretic response of SMA-RBs with no pre-strain. 120

a Lateral Force (kN)

Lateral Force (kN)

80 40 0 -40

-80 -140

-70

0

70

b

80 40 0 -40 -80 -120 -140

140


-70 0 70 Lateral Displacement (mm)

140

Figure 5. Lateral Force-Deflection Curves of SMA-RBs with 2% and 0% Pre-strains: (a) SMA-NRB, (b) SMA-HDRB (γ = 100%, 150%, and 200%).

When 2% pre-strained SMA wires are used, the residual deformation and the energy dissipation capacity increase while, the effective horizontal stiffness decreases compared to the non-pre-strained SMA-RBs. Although the increase of residual deformation is not a desirable feature, the pre-strained SMA wires can improve the performance of the smart rubber bearing in terms of lateral flexibility and energy dissipation. The maximum shear force in SMA-NRB and SMA-HDRB is reduced when pre-strained wires are used as can be seen in Figure 5. The reason is that, due to a shift in the stress-strain curve, the stress generated in the SMA wires decreases when they are installed with a pre-strain. Consequently, a lower amount of force is exerted to the base isolator from pre-strained wires. 9

Changes in the operational characteristics of the elastomeric isolators are listed in Table IV. At 200% shear strain, the effective horizontal stiffness of the SMA-NRB and SMA-HDRB with ε0 = 2% is 9% and 6% lower than that of the SMA-NRB and SMA-HDRB with ε0 = 0%, respectively. This fact shows that the reduction in the lateral flexibility of SMA-RBs can be controlled by pre-straining SMA wires. The reason of this behavior is the lower amount of stress induced in the pre-strained SMA wires after the completion of the forward phase transformation. On the other hand, by incorporating the pre-strained SMA wires, the residual deformation of the SMA-RBs increases. When 2% pre-strained wires are elongated due to the horizontal cyclic displacement, the phase transformation in the wires starts and finishes at lower strain levels compared to unstretched SMA wires. Therefore, pre-strained wires cannot reduce the residual deformation of rubber bearings as much as unstretched wires. According to Table IV, the amount of increase in the residual deformation for both SMA-NRB and HDRB are almost the same at different shear strain amplitudes when 2% pre-strained wires are implemented. Another important finding is that at high shear strain levels (200%), the energy dissipated per cycle, EDC, will increase if pre-strained SMA wires are used in SMA-RBs. At γ = 200%, 2% pre-strained wires enhance the energy dissipation capacity of SMA-NRB and SMA-HDRB by 9% and 4%, respectively. It shows that pre-straining the SMA wire has more effect on the energy dissipation capacity of smart NRBs. By increasing the strain in SMA wires, pre-strained wires enter to the phase transformation region sooner compared to the unstretched wires. Hence, the contribution of SMA wires in the energy dissipation of the base isolator will be greater due to a larger hysteresis area of shape memory alloy. TABLE IV. OPERATIONAL CHARACTERISTICS OF SMA-NRB AND SMA-HDRB WITH 2% PRE-STRAIN WIRES SMA-NRB SMA-HDRB γ (%) ε0 = 2% ε0 = 2% ε0 = 0% ε0 = 0% (ΔNRB)* (ΔHDRB)** 100 0.71 0.63 (-11%) 1.05 0.97 (-8%) KH 150 0.63 0.57 (-10%) 0.94 0.87 (-7%) (kN/mm) 200 0.58 0.53 (-9%) 0.90 0.85 (-6%) 100 7.3 8.2 (11%) 21.1 22.8 (8%) RD 150 9.2 10.1 (10%) 33.9 37.4 (10%) (mm) 200 13.7 15.4 (13%) 46.6 61.9 (12%) 100 1.23 1.26 (3%) 3.16 3.21 (2%) EDC 150 2.65 2.60 (-2%) 6.90 6.80 (-2%) (kJ) 200 4.81 5.26 (9%) 12.56 13.10 (4%) * ΔNRB: Relative difference between operational characteristics of SMA-NRB with 2% pre-strained wires and those of the SMA-NRB without pre-strain ** ΔHDRB: Relative difference between operational characteristics of SMA-HDRB with 2% pre-strained wires and those of the SMA-HDRB without pre-strain

CONCLUSION Using the finite element method, the performance of SMA-NRB and SMA-HDRB were numerically assessed and compared at different shear strain amplitudes. Concluding remarks are as follows.  Using SMA wires in NRB and HDRB causes an increase in the maximum lateral force. In fact, when the lateral displacement increases, the strain in SMA noticeably enhances and 10











as a result the axial stress goes up. Therefore a larger amount of force is applied to the rubber bearing. For both SMA-NRB and SMA-HDRB, presence of SMA wires causes a reduction in the lateral flexibility of the elastomeric isolator. However, the amount of increase in the effective horizontal stiffness of NRB is less than that of the HDRB when SMA wires are used. It shows that using SMA wires in HDRB is more effective in terms of the lateral flexibility. The residual deformation in NRB and HDRB can decrease by implementing the SMA wires. When the shear strain amplitude increases, the recentering capability of SMA wires is augmented and as a result, the reduction in the residual deformation increases. Moreover, it was found out that at low shear strain levels (100%), SMA wires perform more efficiently in reducing the residual deformation of NRB compared to HDRB. SMA wires can improve the performance of the isolation system by dissipating the earthquake energy and accordingly, the displacement of the structure can be controlled. When the energy dissipation capacity of SMA-NRB and SMA-HDRB are compared at different amplitudes of lateral displacement, it was observed that using SMA wires in NRB is more effective. When 2% pre-strained SMA wires are used in both NRB and HDRB, the residual deformation and the energy dissipation capacity increase while, the effective horizontal stiffness decreases compared to the non-pre-strained SMA-RBs. The amount of increase in the residual deformation for both SMA-NRB and HDRB are almost the same at different shear strain levels when 2% pre-strained wires are implemented. However, prestraining the SMA wire has more effect on the energy dissipation capacity of smart NRBs. In general, when the operational characteristics of NRB and HDRB are compared together, it was observed that implementing SMA wires in NRBs causes the efficiency to be more significantly improved.

ACKNOWLEDGEMENT The financial contribution of Natural Sciences and Engineering Research Council of Canada (NSERC) through Discovery Grant was critical to conduct this research and is gratefully acknowledged. REFERENCES Alam, M.S., Youssef, M.A. and Nehdi, M., (2008), “Analytical prediction of the seismic behaviour of superelastic shape memory alloy reinforced concrete elements,” Engineering Structures, Vol. 30, No. 12, pp. 3399-3411. Attanasi, G., Auricchio, F., (2011), “Innovative superelastic isolation device,” Journal of Earthquake Engineering, Vol. 15(S1), pp. 72-89. Attanasi, G., Auricchio, F., Crosti, C. and Fenves, G.L., (2008), “An innovative isolation bearing with shape memory alloys,” presented at the 14th World Conference on Earthquake Engineering, Beijing, China, 12-17 October, 2008. Auricchio, F., (2001), “A Robust Integration-Algorithm for a Finite-Strain Shape-Memory-Alloy,” Plasticity, Vol. 17, No. 7, pp. 971-990. Choi, E., Nam, T.H. and Cho, B.S., (2005), “A new concept of isolation bearings for highway steel bridges using shape memory alloys,” Canadian Journal of Civil Engineering, Vol. 32, No. 5, pp. 957967.

11

Dion, C., Bouaanani, N., Tremblay, R. and Lamarche, C.P., (2012), “Real-time dynamic substructuring testing of a bridge equipped with friction-based seismic isolators,” Journal of Bridge Engineering, Vol. 17, No. 1, pp. 4-14. Dion, C., Bouaanani, N., Tremblay, R., Lamarche, C.P. and Leclerc, M., (2011), “Real-time dynamic substructuring testing of viscous seismic protective devices for bridge structures,” Engineering Structures, Vol. 33, No. 12, pp. 3351-3363. Dolce, M., Cardone, D. and Marnetto, R., (2000), “Implementation and testing of passive control devices based on shape memory alloys,” Earthquake Engineering & Structural Dynamics, Vol. 29, No. 7, pp. 945-968. Dong, J.H., Xue, S.D. and Zhou, Q., (2002), “Applications of shape memory alloy in structural vibration control,” World Earthquake Engineering, Vol. 18, No. 3, pp. 123-129. Graesser, E.J. and Cozzarelli, F.A., (1991), “Shape-memory alloys as new materials for aseismic isolation,” Engineering Mechanics, Vol. 117, No. 11, pp. 2590-2608. Hedayati Dezfuli, F. and Alam, M.S., (2012), “Material modeling of high damping rubber in finite element method,” presented at the 3rd International Structural Specialty Conference, Edmonton, Alberta, Canada, 6-9 June, 2012. Hedayati Dezfuli, F. and Alam, M.S., (2013), “Shape memory alloy (SMA) wire-based smart natural rubber bearing,” accepted for Smart Materials and Structures. Lagoudas, D.C., (2008), Shape Memory Alloys. Springer Science and Business Media, LLC. Liu, Y., (2003), “Mechanical and thermomechanical properties of a Ti50Ni25Cu25 melt spun ribbon,” Materials Science and Engineering: A, Vol. 354, No. 1, pp. 286-291. Liu, H., Wang, X. and Liu, J., (2008), “The shaking table test of an SMA strands-composite bearing,” Earthquake Engineering and Engineering Vibration, Vol. 28, No. 3, pp. 152-156. Naeim, F. and Kelly, J.M., (1999), Design of seismic isolated structures: from theory to practice. John Wiley & Sons, Inc., New York. Omori, T., Ando, K., Okano, M., Xu, X., Tanaka, Y., Ohnuma, I., Kainuma, R. and Ishida, K., (2011), “Superelastic Effect in Polycrystalline Ferrous Alloys,” Science, Vol. 333, No. 6038, pp. 68-71. Ozbulut, O.E. and Hurlebaus, S., (2011), “Seismic assessment of bridge structures isolated by a shape memory alloy/rubber-based isolation system,” Smart Materials and Structures, Vol. 20, No. 1. Ozkaya, C., Akyuz, U., Caner, A., Dicleli, M. and Pınarbası, S., (2011), “Development of a new rubber seismic isolator: ‘Ball Rubber Bearing (BRB)’,” Earthquake Engineering and Structural Dynamics, Vol. 40, No. 12, pp. 1337-1352. Soong, T.T. and Dargush, G.F., (1997), Passive energy dissipation systems in structural engineering. John Wiley and Sons, Inc., Chichester, England. Strnadel, B., Ohashi, S., Ohtsuka, H., Ishihara, T. and Miyazaki, S., (1995), “Cyclic stress-strain characteristics of Ti-Ni and Ti-Ni-Cu shape memory alloys,” Materials Science and Engineering: A, Vol. 202, No. 1-2, pp. 148 156. Suduo, X. and Xiongyan, L., (2007), “Control devices incorporated wit shape memory alloy,” Earthquake Engineering and Engineering Vibration, Vol. 6, No. 2, pp. 159-169. Tanaka, Y., Himuro, Y., Kainuma, R., Sutou, Y., Omori, T. and Ishida, K., (2010), “Ferrous polycrystalline shape-memory alloy showing huge superelasticity,” Science, Vol. 327, No. 5972, pp. 1488-1490. Wilde, K., Gardoni, P. and Fujino, Y., (2000), “Base isolation system with shape memory alloy device for elevated highway bridges,” Engineering Structures, Vol. 22, No. 3, pp. 222-229. Zhang, Y., Hu, H. and Zhu, S., (2009), “Seismic performance of benchmark base-isolated bridges with superelastic Cu-Al-Be restraining damping device,” Structural Control and Health Monitoring, Vol. 16, No. 6, pp. 668-685.

12