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Un-bonded fiber reinforced elastomeric isolator (U-FREI) is light weight and facilitates easier installation in comparison to conventional steel reinforced elastomeric isolators (SREI), in which fiber layers are used as reinforcement to replace steel shims as are normally used in conventional isolators.
Journal of Science and Technology in Civil Engineering NUCE 2018 12 (5): 10–19 EFFECT OF SHEAR MODULUS ON THE PERFORMANCE OF PROTOTYPE UN-BONDED FIBER REINFORCED ELASTOMERIC ISOLATORS Van Thuyet Ngoa,∗ a Structural Engineering Division, Department of Civil Engineering, Thuyloi University, 175 Tay Son street, Dong Da district, Hanoi, Vietnam Article history: Received 10 May 2018, Revised 10 July 2018, Accepted 30 August 2018 Abstract Un-bonded fiber reinforced elastomeric isolator (U-FREI) is light weight and facilitates easier installation in comparison to conventional steel reinforced elastomeric isolators (SREI), in which fiber layers are used as reinforcement to replace steel shims as are normally used in conventional isolators Shear modulus of elastomer has significant influence on the force-displacement relationship of U-FREI However, a few studies investigated the effect of shear modulus on the horizontal behavior of prototype U-FREI in literature In this study, effect of shear modulus on performance of prototype U-FREIs is investigated by both experiment and finite element (FE) analysis It is observed that reduction in horizontal stiffness of U-FREI with increasing horizontal displacement is due to both rollover deformation (or reduction in contact area of isolator with supports) and shear modulus of elastomer Reasonable agreement is observed between the findings from experiment and FE analysis Keywords: base isolator; prototype un-bonded fiber reinforced elastomeric isolator; rollover deformation; shear modulus; cyclic test https://doi.org/10.31814/stce.nuce2018-12(5)-02 c 2018 National University of Civil Engineering Introduction Base isolation is an efficient and viable method to reduce the vulnerability of structure in high seismic risk zone Earthquake energy transmitted to the structure can be reduced by lengthening the fundamental horizontal period of structure Base isolators are installed in between substructure and superstructure to achieve the desired horizontal period of structure Conventional steel reinforced elastomeric isolators (SREIs) consist of alternating layers of rubber bonded to intermediate steel shims with two steel end plates at top and bottom In general, SREIs are often applied for large, important buildings like hospitals and emergency centers, in developed countries such as United States, Japan, New Zealand, Italy, etc This limited use is largely due to high material, manufacturing and installation costs It is expected that the use of seismic isolators can be extended to ordinary low-rise and mediumrise buildings if the weight and cost of the isolators are reduced In view of this, fiber reinforced elastomeric isolators (FREIs) are proposed by replacing steel shims in conventional isolators by multilayer of fiber fabric as reinforcement sheets to reduce their weight and cost An un-bonded fiber reinforced elastomeric isolator (U-FREI) is a significant effort to improve FREI by removing two steel end plates and installing directly between the substructure and superstructure without any connection ∗ Corresponding author E-mail address: thuyet.kcct@tlu.edu.vn (Ngo, V T.) 10 Ngo, V T / Journal of Science and Technology in Civil Engineering to these boundaries Using U-FREI would reduce the weight and cost, easier installation, and can be made as a long strip and then easily cut to the required size It means that the U-FREIs can be used for low-rise and medium-rise buildings subjected to earthquake loading in the developing countries like Vietnam, Indonesia, Taiwan, Nepal, etc Some studies were conducted in recent time for obtaining the mechanical characteristics of FREIs leading to better understanding of their behavior Kelly and Takhirov [1] studied the mechanical properties of U-FREIs by theoretical and experimental analysis Toopchi-Nezhad et al [2] carried out experimental study to investigate the lateral behavior of U-FREI Strauss et al [3] presented experimental tests to evaluate shear modulus and damping coefficient of elastomeric bearings with various reinforcing materials and under various loadings, support conditions Dezfuli and Alam [4] prepared scaled U-FREIs with different initial shear moduli, number of elastomer layers, number of fiber layers, thickness of elastomer layers and experimentally evaluated the vertical and horizontal response of U-FREIs Ngo et al [5, 6] studied the horizontal stiffness and the effect of horizontal loading direction on performance of square U-FREI by both experiment and finite element (FE) analysis These studies indicate that the behavior of elastomeric isolators is affected by some factors such as material properties, sizes and shapes, loadings and directions of loading, friction between the surfaces of U-FREI and support areas, etc An effort to study the effect of shear modulus on the behavior of elastomeric isolators was conducted by Strauss et al [3] They conducted laboratory tests to study the effect of shear modulus of scaled bonded FREI as well as SREI of various dimensions, under good combinations of vertical load and cyclic horizontal displacement up to 1.0tr (tr is the total thickness of elastomer/rubber layers of isolator) Dezfuli and Alam [4] evaluated experimentally the reduction in effective shear modulus of scaled U-FREIs with increasing horizontal displacement up to 1.0tr The effect of shear modulus on horizontal response of U-FREI specimens in [4] is not clear because of different sizes and shape factors of these specimens It is necessary to study the effect of shear modulus on the response of U-FREIs with the same sizes and shape factors under the action of larger horizontal displacement Further, most of previous studies were carried out on scaled models of UFREIs with relatively low shape factor (less than 10) According to Naeim and Kelly [7], shape factor (S ) is defined as the ratio of the loaded area to load free area of an elastomer layer Range of shape factor values of typical isolators for seismic isolation is from 10 to 20 [7] Thus, the effect of shear modulus on horizontal response of prototype U-FREIs with high shape factors should be studied This will have huge significance in design and production of prototype isolators for field application This study investigates the performance of prototype U-FREIs under cyclic loading by both experiments and FE analysis In experiments, four prototype specimens with the same sizes in plan and two different values of shear modulus are produced, and then the characteristic properties including the horizontal stiffness as well as the energy dissipation capacity and the equivalent viscous damping are assessed These specimens are tested under the same constant vertical pressure and cyclic horizontal displacement up to 0.89tr In addition, the investigation of the behavior of isolators could be done up to very large applied horizontal displacements of 1.50tr using FE method Numerical results are validated with experimental findings for cyclic horizontal displacement up to 0.89tr , a limit considered from the requirement of safety of the test set up during actual experiment From experimental and numerical results, effects of shear modulus on the behavior of prototype U-FREIs are evaluated Details of test specimens Specimens are produced to use in an actual building in India with the support of METCO Pvt Ltd., Kolkata, India Four specimens are manufactured by vulcanizing elastomer layers and bi-directional 11 requirement of safety of the test set up during actual experiment From experimental and numerical results, effects of shear modulus on the behavior of prototype U-FREIs are evaluated Details of test specimens Details of testSpecimens specimens are produced to use in an actual building in India with the support of METC Ltd., Kolkata, India Four arebuilding manufactured by vulcanizing elastomer Specimens are produced to usespecimens in an actual in India with the support of METCO layers Pvt and bi-dire o o ◦ ◦ (0 /90 ) carbon fiber Twostrips longofstrips of laminated with two different values of initi Ltd.,(0Kolkata, India Four arelong manufactured by vulcanizing andvalues bi-directional /90 ) carbon fiber specimens fabric.fabric Two laminated pads elastomer withpads two layers different of initial o (0o/90 ) carbon fiber fabric Two long strips of laminated pads with two different values of initial shear modulus are made from eighteen elastomer layers interleaved with seventeen carbon shear modulus are made from eighteen elastomer layers interleaved with seventeen carbon fiber fabricfiber fabric modulus areStrip made from eighteen layers with carbon fibersimilar fabric layers Strip with initial shearelastomer modulus, G 0.78 asinterleaved 0.78 is seventeen designated aswhile type A, while similar layers with initial shear modulus, G as MPa MPa is designated as type A, such strip such str Stripwith with initial shear modulus, G as 0.78 MPa is designated as type A, while similar such strip with value of of G as 0.90 as as type B The thickness of each layer islayer mm,is mm, wh value 0.90MPa MPaisisdesignated designated type B The thickness of elastomer each elastomer value of Gthat as 0.90 MPafiber is designated as type B.and Thetotal thickness ofofeach elastomer layer is mm mm, while that while of each layer is 0.55 mm height each bearing is 100 Subsequently, of each fiber layer is 0.55 mm and total height of each bearing is 100 mm Subsequently, four spe of each layer is(including 0.55 mm and total specimens height of each bearing is 100A,mm Subsequently, four (1,2) specimens fourfiber specimens of two from sheet type denoted as isolator and specimens two (including of two specimens fromA,sheet type A, denoted as isolator A(1,2) Aand two fro (including of two specimens from sheet type denoted as isolator A and two specimens from sheet (1,2) specimens from sheet type B, denoted as isolator B(1,2) ) are cut to squared size of 250 × 250 × 100 B, as denoted as (1,2) isolator B(1,2) are cutsizeto ofsquared size of 250x250x100 mm typemm B, type denoted isolator ) are cut to )squared 250x250x100 view of aAoftypical vie A typical view of aBprototype isolator with layer details is shown inmm Fig A typical The shape factor prototype isolator with layer details is shown Fig The shape factor are of these isolators are S prototype isolator with details is shown in Fig higher The in shape 12.5, these isolators are Slayer = 12.5, which is significantly thanfactor those of of these modelisolators FREIs usedSin= most of which isinvestigations significantly higher those model FREIs in isolators mostinvestigations ofarethe previous investi which significantly higher than those ofthan model FREIs used inproperties most of used the previous theisprevious Geometrical details andofmaterial of the shown in Geometrical details and material properties of the isolators are shown in Table Geometrical details and material properties of the isolators are shown in Table Table Ngo, V T / Journal of Science and Technology in Civil Engineering (a) Elastomer and fiber layers in prototype U(a) Elastomer and fiber layers in prototype U-FREI (b) 3D view of a typical FREI and fiber layers in prototype (b)U3D view ofU-FREI a typical U-FREI specimen (a) Elastomer specimen FREI Fig 1 Details Figure Detailsofofprototype prototypeU-FREI U-FREI specimen specimen Experimental investigations (b) 3D view of a typical U-FREI spe Fig Details of prototype U-FREI specimen Table Geometrical details and material properties of prototype U-FREI 3.1 Experimental set-up Experimental investigations Description Isolator A Isolator B(1,2) All specimens are tested at Structural Engineering Laboratory, (1,2) Indian Institute of Technology 3.1 Experimental set-up Size of specimen, (mm) 250 ×load 250and × 100 250varying × 250 ×cyclic 100 (IIT) Guwahati, India under simultaneous action of a constant vertical horizontal NumberThe ofAll elastomer layer,test ne setup 18 displacement experimental is shown in Fig 2.Engineering A couple18 specimen is put one aboveInstitute the specimens are tested at Structural Laboratory, Indian of Tech Thickness of single elastomer te , The (mm) other and separated by a steel spacerlayer, block bearing specimens are5.0 in contact with the 5.0 upper and (IIT) Guwahati, India under simultaneous action of a constant vertical load and horizontal varying lower Total surfaces of the steel block.tr ,However, these bearings are without 90 any physical connection height of elastomer, (mm) 90 to the displacement The experimental test setup is shown in Fig A couple specimen is put one ab surfaces of theofsteel block andlayer, hence Number carbon fiber n f mimic the un-bonded condition.17A horizontally placed 17 servoother and separated a steel spacer block specimens are in0.55 contact hydraulic actuator MTSby USA) connected to theThe steelbearing block for the application of cyclic with the up Thickness of (make: single fiber layer, t f ,is(mm) 0.55 lowerfactor, of theA steel block these bearings without any physical connection displacements tosurfaces the Sassembly constant designHowever, vertical load of 350 kN (or are a constant vertical pressure Shape 12.5 12.5 surfaces of of theelastomer, steel block and hence mimic the un-bonded condition A horizontally placed Shear modulus G, (MPa) 0.78 0.90 Elastic modulus of carbon fiber laminate, E, (GPa) 40 40 hydraulic actuator (make: MTS USA) is connected to the steel block for the application of Poisson’s ratio of carbon fiber laminate, µ 0.20 0.20 displacements to the assembly A constant design vertical load of 350 kN (or a constant vertical p 3 Experimental investigations 3.1 Experimental set-up All specimens are tested at Structural Engineering Laboratory, Indian Institute of Technology (IIT) Guwahati, India under simultaneous action of a constant vertical load and horizontal varying cyclic displacement The experimental test setup is shown in Fig A couple specimen is put one above the other and separated by a steel spacer block The bearing specimens are in contact with the 12 Thickness of single fiber layer, tf , (mm) Shape factor, S 12.5 Shape factor, S 0.55 0.55 12.5 12.5 12.5 Shear modulus of elastomer, G, (MPa) 0.78 0.90 Ngo, V T / Journal of Science and Shear modulus of elastomer, G, Technology (MPa) in Civil Engineering 0.78 0.90 upper and lower surfaces of the steellaminate, block However, these bearings are without any physical conElastic modulus of carbon fiber Elastic modulus of fibermimic laminate, and hence the un-bonded E,nection (GPa)to the surfaces of the steel blockcarbon 40 40 condition A horizontally E, (GPa) placed servo-hydraulic actuator (make: MTS USA) is connected to the 40 steel block for 40 the application of cyclic displacements to fiber the assembly A constant design Poisson's ratio of carbon laminate, µ 0.20 vertical load 0.20of 350 kN (or a constant verPoisson's of carbon fiber laminate, µ a compression 0.20 testing 0.20 tical pressure of 5.6 MPa) isratio applied using hydraulic jack from machine, where the assemblage of bearings and steel block is housed The magnitudes of vertical loads correspond to factored column loads and the values are obtained from the analysis of the actual building Thuyet, N V.2 /Schematic Journal ofrepresentation Science andand Technology in Civil Engineering Figure actual experimental set-up Schematicand representation and actualset-up experimental set-up Fig SchematicFig representation actual experimental acement of frequency f = 0.025 Hz are applied continuously for four levels of displacement Details ofofinput displacement history 3.2 Details input displacement ails of input displacement history itudes as 3.2 20 mm (0.22t 40 mm (0.44thistory r), r), 60 mm (0.67tr) and 80 mm (0.89tr) as shown in Fig The The experimental investigations are carried outcarried by subjecting the isolator cyclic displace-horizontal rimental investigation of the behavior U-FREIs is out performed up under to under the applied Theinvestigations experimental investigations areby by the isolator cyclic The experimental are carriedof out subjecting the subjecting isolator cyclic under ment, while maintaining a constant vertical pressure on the isolator Three cycles of sinusoidal disacement of 80 mm, considering the overall safety of the testpressure set-up.on All in this study are used displacement, while maintaining a constant vertical thespecimens isolator Three cycles of sinusoidal ment, while maintaining a constant vertical pressure isolator Three cycles of sinusoidal placement of frequency f = 0.025 Hz are appliedon continuously for four levels of displacement ampliactual building in mm India after being tested Thus, specimens are tested with the maximum value of tudes as 20 (0.22t r ), 40 mm (0.44tr ), 60 mm (0.67t4 r ) and 80 mm (0.89tr ) as shown in Fig The ed horizontal displacement of 80 to4keepof specimens from anyupdamage Horizontal experimental investigation of mm the behavior U-FREIs is performed to the applied horizontaldisplacement displacement of 80 mm, considering the overall safety of test set-up All specimens in this study are used corresponding horizontal forces are measured using in-built linear variable differential transformer in ancell actual in India after being tested Thus, specimens are tested with the maximum value DT) and load of building the actuator of applied horizontal displacement of 80 mm to keep specimens from any damage Horizontal displacement and corresponding horizontal forces are measured using in-built linear variable differential transformer (LVDT) and load cell of the actuator Figure Applied horizontal displacement history Fig Applied horizontal displacement history Experimental results a) Deformed shape 13 the contact surfaces without any damage The reduction in contact area due to to the reduction in effective horizontal stiffness of isolators and results nonline large displacement Ngo, V T / Journal of Science and Technology in Civil Engineering 3.3 Experimental results a Deformed shape Deformed shape of a typical specimen as obtained from experimental tests at 80 mm amplitude of horizontal displacement is shown in Fig The top and bottom surfaces of U-FREI exhibit stable roll off the contact surfaces without any damage The reduction in contact area due to rollover deformation leads to the reduction in effective horizontal stiffness of isolators and results nonlinear behavior of elastomer at large displacement Figure Deformed shape of U-FREI specimen at applied horizontal displacement of 80 mm Fig Deformed shape of U-FREI specimen at applied horizontal d b Hysteresis loops The hysteresis loop of an isolator represents the relationship between shear forces and cyclic horizontal displacements.loops The horizontal displacements and shear forces experienced by the U-FREIs b) Hysteresis are measured by LVDT and load cells respectively, which are built-in the servo-hydraulic actuator Further, the recorded shear actually represent the applied forces on two specimens tested siThuyet, Thuyet, N V / forces Journal Science and Technology in Civil Engineering N V / of Journal of Science and Technology in Civil Engineering The hysteresis of plot anis isolator represents the relationship multaneously and hence, theloop hysteresis obtained by dividing these measured forces by two to betwe rces on oneevaluate specimen average sense Fig shows such hysteresis loops of different tested tested specimens the in shear forces on one specimen average sense Fig shows such hysteresis loopsspecimens of forces on one specimen in average sense Fig 5inshows such hysteresis loops of different horizontal displacements The horizontal displacements and shear forces expe tested specimens considered in this study onsidered indifferent this study considered in this study measured by LVDT and load cells respectively, which are built-in the servothe recorded shear forces actually represent the applied forces on two specim and hence, the hysteresis plot is obtained by dividing these measured forces b (a) Specimen A(1,2) (b) Specimen B(1,2) (a) Specimen A(1,2) A(1,2) (a) Specimen (b) Specimen B(1,2) B(1,2) (b) Specimen Figure Hysteresis loops of different specimens from experimentally observed data Fig Fig Hysteresis loops ofloops different specimens from experimentally observed data data Hysteresis of different specimens from experimentally observed c Mechanical properties of the U-FREIs Two important parameters such as effective horizontal stiffness and equivalent viscous damping (or damping factor) are obtained from the hysteresis loops The effective horizontal stiffness of an isolator at any amplitude of horizontal displacement is defined as International Building Code [8]: c) Mechanical properties of the U-FREIs c) Mechanical properties of the U-FREIs h Ke f f = Fmax − Fmin umax − umin (1) Two important parameters as effective horizontal stiffness and equivalent damping Two important parameters such assuch effective horizontal stiffness and equivalent viscousviscous damping where Fmin maximum andthe minimum values the shear force; uhorizontal umin are maximum andisolator max ,obtained max ,stiffness (or damping areare obtained from hysteresis loops The effective stiffness of an r damping factor)Ffactor) are from the hysteresis loops Theofeffective horizontal of an isolator values ofdisplacement the horizontal displacement at anyminimum amplitude of horizontal displacement is defined as International Building Code [8]: any amplitude of horizontal is defined as International Building Code [8]: ) The equivalent viscous damping of isolator (β) is computed by measuring the energy dissipated in each cycle (Wd ), which is the area enclosed byh theFhysteresis Fmax loop - Fmin The magnitude of β is computed as: maxh - Fmin K eff = K eff = umax - umin umax - umin 14 (1) , Fmin are maximum and minimum the force; shear force; uminmaximum are maximum maxare max,are here where Fmax, FFmin maximum and minimum values values of the of shear umax, uumin and and Ngo, V T / Journal of Science and Technology in Civil Engineering β= Wd 2πKehf f ∆2max (2) where ∆max is the average of the positive and negative maximum displacements, ∆max = (|umax | + |umin |) /2 Effective horizontal stiffness and equivalent viscous damping of these isolators at different horizontal displacement amplitudes are furnished in Table It can be seen from Table that the effective horizontal stiffness of an U-FREI decreases, while the equivalent viscous damping increases with the increase in horizontal displacement The decreases in effective stiffness corresponding to increase in amplitude of horizontal displacement from 20 to 80 mm are found to be 39.1%, 37.2% for specimen A(1,2) , B(1,2) , respectively These reductions are due to rollover deformation, which will result in an increase in time period of the base isolated structure leading to increase in their seismic response control efficiency Table Experimentally evaluated mechanical properties of U-FREIs Amplitude (mm) u/tr 20 40 60 80 0.22 0.44 0.67 0.89 Isolator A(1,2) Kehf f (kN/m) 464.26 403.41 324.22 282.60 Isolator B(1,2) β (%) 5.18 6.94 11.15 11.83 Kehf f (kN/m) 507.26 410.21 339.01 318.68 β (%) 5.00 9.67 12.02 10.02 FE analysis FE analyses of these isolators are also conducted under simultaneous action of constant vertical pressure and cyclic horizontal displacement by ANSYS v.14.0 FE analysis is used to simulate the behavior of these isolators up to very large horizontal displacement amplitude of 1.50tr , although experimental investigation is carried out for horizontal displacement amplitude up to 0.89tr because of practical constraint Loading protocol considered in FE analysis is similar to that considered in experimental investigation The comparison of results from numerically simulated model and experimental observations is performed to assess the accuracy of FE analysis 4.1 Element type for FE model In the FE model of U-FREIs, the elastomer and fiber reinforcement are modeled using SOLID185, SOLID46 respectively Two rigid horizontal plates are considered at the top and bottom of the isolator to represent the superstructure and substructure Vertical load and horizontal displacement are applied at the top plate, while all degrees of freedom of bottom plate are constrained In order to study U-FREI, contact element CONTA173 is used to define the exterior elastomer surfaces and target element TARGE170 is used to define the interior surface of top and bottom rigid plates The model is meshed with hexagonal volume sweep 15 Ngo, V T / Journal of Science and Technology in Civil Engineering Thuyet, N V / Journal of Science and Technology in Civil Enginee 4.2 Material model Thuyet, V.as / Journal ofinScience Civil Material properties ofN shown Table 1and areTechnology usedininCivil FE Elastomer isare modeled Thuyet, V.experimental /N.Journal of Science and Technology Engineering Similar toU-FREI tests, analyses ofinmodel all Engineering U-FREIs carried out with hyper-elastic and visco-elastic parameters Ogden 3-terms model has been adopted to model the displacement, while maintaining aU-FREIs constant pressure of pViscoelas=horizontal 5.6horizontal MPa dist Similar tobehavior experimental tests, analyses of all U-FREIs areiscarried out under cyclic Similar to experimental analyses arevertical carried out under cyclic hyper-elastic of tests, the elastomer andoftheall visco-elastic behavior modeled by Prony displacement, while maintaining a constant vertical pressure of p = 5.6 MPa distributed on the topdisplac steel displacement, while maintaining a constant vertical pressure of sinusoidal p = 5.6 MPacycles distributed the top steel plate of the parameter simulated model Three fully withonincreasing tic Shear Response 2 amplitudes the simulated model Three fully sinusoidal cycles with displacement amplitudes plate plate of theofsimulated model fully sinusoidal with increasing displacement Ogden µThree (N/m ); µcycles (N/m ); µincreasing = −30000 (N/m ); αplate = to up to = 1.89×106 =33600 on = 1.3; α2 up 1.50t (135 mm) as shown in Fig are applied the top steel r(3-term): as shown in 3Fig are applied thesteel top plate steel plate r5;(135 α3 =mm) −2;shown 1.50t1.50t mm) as in Fig are 3applied on theontop r (135 Prony Shear Response: a1 = 0.3333; t1 = 0.04; a2 = 0.3333; t2 = 100 4.4results FEresults analysis 4.4analysis FE analysis 4.4 FE results 4.3 Input loading a) Validation ofmodel FE model of of U-FREIs FE model of a) Validation ofa)FEValidation of U-FREIs U-FREIs Similar to experimental tests, analyses of all U-FREIs are carried out under cyclic horizontal displacement, while a constant vertical pressure of p amplitude = 5.6amplitude MPa distributed the topobtained steel Deformed shape of U-FREI under horizontal displacement 80on mm as Deformed shape ofmaintaining U-FREI under horizontal displacement of 80of mm as obtained from from Deformed shape of U-FREI under horizontal displacement amplitude of plate of the simulated model Three fully sinusoidal cycles with increasing displacement amplitudes FE analysis is shown in Fig The upper and lower faces of the U-FREI roll off the contact supports FE analysis is shown in Fig The upper and lower faces of the U-FREI roll off the contact supports The The analysis is asshown inFig Fig.3 are as The upper and lower faces of the U-FREI roll off th up of toFE 1.50t mm) shown of in applied on theduring top steel plate.test r (135 configuration pattern deformed U-FREI observed actual (Fig 4) agrees very well with pattern of deformed configuration of U-FREI as observed during actual test (Fig 4) agrees very well with pattern deformed obtained from FE analysis configuration of U-FREI as observed during actual test (Fig Fig 6Fig obtained from FEofanalysis 4.4 FE analysis results Fig obtained from FE analysis a Validation of FE model of U-FREIs Deformed shape of U-FREI under horizontal displacement amplitude of 80 mm as obtained from FE analysis is shown in Fig The upper and lower faces of the U-FREI roll off the contact supports The pattern of deformed configuration of UFREI as observed during actual test (Fig 4) agrees very well with Fig obtained from FE analysis Fig Numerically observed deformed of U-FREI displacement amplitude Fig Numerically observed deformed shapeshape of U-FREI underunder displacement amplitude of 80 of 80 Fig shows the hysteresis loops of U-FREIs mm mm under displacement up to 80 mm for data obtained Figure Numerically observed deformed shape from both FE analysis and laboratory tests ComFig Numerically observed deformed oftomm U-FREI under ofdisplacement U-FREIshape under amplitude 7ofshows the hysteresis of U-FREIs up 80 for mm for data displacem obtained Fig 7Fig shows thehysteresis hysteresis loops of U-FREIs underunder displacement up todisplacement 80 data obtained parison the loops ofloops U-FREIs as obof 80 mm mm from both FE analysis and laboratory tests Comparison of the hysteresis loops of U-FREIs as obtained from bothtained FE analysis and laboratory Comparison from experiments and FEtests analysis for each of the hysteresis loops of U-FREIs as obtained type shows discrepancy to be quite less experiments and FE analysis for each type shows the discrepancy be quite from from experiments andtheFE analysis for each type shows the discrepancy to be to quite less less Fig shows the hysteresis loops of U-FREIs under displacement up to from both FE analysis and laboratory tests Comparison of the hysteresis loops o from experiments and FE analysis for each type shows the discrepancy to be quite (a) Isolator A(1,2) (b) Isolator B(1,2) (a) Isolator A(1,2) A(1,2) (a) Isolator (b) Isolator B(1,2) B(1,2) (b) Isolator Figure Comparison of hysteresis loops of different U-FREIs obtained from experiment and FE analysis Fig Comparison of hysteresis loopsloops of different U-FREIs obtained from from experiment and FE Fig Comparison of hysteresis of different U-FREIs obtained experiment and FE analysis analysis b) Mechanical properties of U-FREIs b) Mechanical properties of U-FREIs 16 (a) Isolator A(1,2) (b) Isolator Effective horizontal stiffness and equivalent viscous damping of allofU-FREIs are calculated from from Effective horizontal stiffness and equivalent viscous damping all U-FREIs are calculated Eqs Eqs (1), (2) in Table The horizontal stiffness of U-FREIs obtained from from FE FE (1),and (2)are andpresented are presented in Table effective The effective horizontal stiffness of U-FREIs obtained Ngo, V T / Journal of Science and Technology in Civil Engineering b Mechanical properties of U-FREIs Effective horizontal stiffness and equivalent viscous damping of all U-FREIs are calculated from Eqs (1) and (2) and are presented in Table The effective horizontal stiffness of U-FREIs obtained from FE analysis decreases with the increase in horizontal displacement Specifically, the decreases in effective stiffness of U-FREIs A(1,2) , B(1,2) are found to be 57.2%, 57.0% respectively in the displacement range of 20 mm to 135 mm It can be observed from Tables and that reasonable agreement is observed in terms of mechanical properties of U-FREIs between the findings from experiment and FE analysis at displacements ranging from 20 mm to 80 mm Hence, the results obtained by FE analysis for U-FREIs at even larger displacements (from 80 mm to 135 mm) will be considered as accurate The accuracy of the FE analysis results are established for the considered problem Table Mechanical properties of U-FREIs obtained from FE analysis Amplitude (mm) u/tr 20.0 40.0 60.0 80.0 90.0 112.5 135.0 0.22 0.44 0.67 0.89 1.00 1.25 1.50 Isolator A(1,2) Isolator B(1,2) Kehf f (kN/m) β (%) Kehf f (kN/m) β (%) 457.72 385.10 321.99 272.20 251.09 219.02 195.75 7.16 9.30 11.71 13.22 13.78 14.19 14.94 515.87 426.93 357.01 301.67 281.34 247.09 222.03 7.58 9.60 12.05 13.46 14.11 14.58 15.42 Effect of shear modulus on horizontal response of prototype U-FREIs As discussed earlier, isolator A(1,2) and B(1,2) are made with same component layers and have same size of 250 × 250 × 100 mm These U-FREIs are subjected to same vertical load of 350 kN and cyclic horizontal displacement However, these isolators are having different shear moduli, where G = 0.78 MPa for isolator A and G = 0.90 MPa for isolator B The response and characteristics of Thuyet, N V / Journal of Science and Technology in Civil Engineering Figure Effective horizontal stiffness versus displacement of different prototype U-FREIs Fig Effective horizontal stiffness versus displacement of different prototype U-FREIs 17 Due to rollover deformation, the area of these specimens in contact with the support surfaces decrease with the increase in horizontal displacement, thus resulting in the reduction in effective horizontal stiffness It can be seen from Fig that at a given displacement, whereas the U-FREI types A Ngo, V T / Journal of Science and Technology in Civil Engineering these isolators are compared to infer on the influence of shear modulus on isolators Reduction in effective horizontal stiffness of U-FREI types A and B with increasing horizontal displacement as obtained from both experiments and FE analyses is shown in Fig Due to rollover deformation, the area of these specimens in contact with the support surfaces decrease with the increase in horizontal displacement, thus resulting in the reduction in effective horizontal stiffness It can be seen from Fig that at a given displacement, whereas the U-FREI types A and B are likely to have same area in contact with the supports, the horizontal stiffness of U-FREI decreases with the decrease in shear modulus Thus, the decrease in horizontal stiffness of U-FREI with increasing horizontal displacement is not only due to rollover deformation through the decrease in area in contact with the supports, but also due shear modulus of isolator Conclusions This paper presents experimental as well as numerical analysis of prototype U-FREIs under cyclic load Experimental investigations are done up to a displacement limit and finding from FE analysis are validated Evaluation of influence of shear modulus on the behavior of U-FREIs are studied The concluding remarks are as follows: - Due to rollover deformation, the effective horizontal stiffness of U-FREIs decreases with the increase in horizontal displacement, while the equivalent viscous damping increases The decreases in effective horizontal stiffness of U-FREIs A(1,2) , B(1,2) as obtained from experimental study are found to be 39.1%, 37.2% respectively in the displacement range of 20 mm to 80 mm; while those of U-FREIs A(1,2) , B(1,2) as obtained from FE analysis are found to be 57.2%, 57.0% respectively in the displacement range of 20 mm to 135 mm - Reasonable agreement is found between the findings from experimental and FE analysis at displacements ranging from 20 mm to 80 mm FE analysis can be adopted effectively to a very large range of displacement (135 mm), which may be otherwise difficult in experimental study - Reduction in horizontal stiffness of U-FREI with increasing horizontal displacement is due to both shear modulus and the contact area of the isolator with support surfaces Acknowledgements Author (a former research scholar in IIT Guwahati, Assam, India) would like to acknowledge the contribution of METCO Pvt Ltd., Kolkata, India, for manufacturing U-FREI and staffs of Structural Engineering Laboratory, Department of Civil Engineering, IIT Guwahati, India for their help during experimental investigation References [1] Kelly, J M., Takhirov, S M (2001) Analytical and experimental study of fiber-reinforced elastometric isolators PEER Report, 2001/11, Pacific Earthquake Engineering Research Center, University of California, Berkeley, USA [2] Toopchi-Nezhad, H., Tait, M J., Drysdale, R G (2008) Lateral response evaluation of fiber-reinforced neoprene seismic isolators utilized in an unbonded application Journal of Structural Engineering, 134 (10):1627–1637 [3] Strauss, A., Apostolidi, E., Zimmermann, T., Gerhaher, U., Dritsos, S (2014) Experimental investigations of fiber and steel reinforced elastomeric bearings: Shear modulus and damping coefficient Engineering Structures, 75:402–413 18 Ngo, V T / Journal of Science and Technology in Civil Engineering [4] Dezfuli, F H., Alam, M S (2014) Performance of carbon fiber-reinforced elastomeric isolators manufactured in a simplified process: experimental investigations Structural Control and Health Monitoring, 21 (11):1347–1359 [5] Ngo, V T., Dutta, A., Deb, S K (2017) Evaluation of horizontal stiffness of fibre-reinforced elastomeric isolators Earthquake Engineering & Structural Dynamics, 46(11):1747–1767 [6] Ngo, V T., Deb, S K., Dutta, A (2018) Effect of horizontal loading direction on performance of prototype square unbonded fibre reinforced elastomeric isolator Structural Control and Health Monitoring, 25(3): 1–18 [7] Naeim, F., Kelly, J M (1999) Design of seismic isolated structures: from theory to practice John Wiley & Sons [8] IBC-2000 (2000) International building code USA 19 ... physical conElastic modulus of carbon fiber Elastic modulus of fibermimic laminate, and hence the un- bonded E,nection (GPa)to the surfaces of the steel blockcarbon 40 40 condition A horizontally E, (GPa)... effect of shear modulus on the response of U-FREIs with the same sizes and shape factors under the action of larger horizontal displacement Further, most of previous studies were carried out on scaled... between the surfaces of U-FREI and support areas, etc An effort to study the effect of shear modulus on the behavior of elastomeric isolators was conducted by Strauss et al [3] They conducted