OPTIMUM DESIGN OF RESILIENT SLIDING ISOLATION SYSTEM TO PROTECT EQUIPMENTS

Transcription

1 13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 24 Paper No OPTIMUM DESIGN OF RESILIENT SLIDING ISOLATION SYSTEM TO PROTECT EQUIPMENTS Hirokazu IEMURA 1 and Touraj TAGHIKHANY 2 SUMMARY Seismic isolation is one of the best options to protect equipments. It helps to control response acceleration transmitted to equipment under its allowable level. Among several type of isolation system, the combination of restoring spring and slider (resilient sliding system) is a very effective system for protection of equipment. In design of this type of isolation system to ensure functions of equipments, there are many suitable combinations of stiffness and friction coefficient. However at the same time it is also necessary to control relative displacement of isolation system so as to provide safety to the connections of equipment with other systems like power supply, main servers etc. This study deals with optimum design of equipments with these resilient sliders that may control acceleration under allowable level and at same time minimize the relative displacement. Optimum parameters of resilient sliding isolators are determined analytically for different levels of allowable acceleration. The validity of analytical method used, is also verified by shaking table tests. Results of this study show that 1) Optimum values of period are decreased with growth of allowable acceleration and 2) The optimum friction coefficient is increased with higher allowable acceleration. INTRODUCTION Traditional mitigation strategies like; bolting, cross bracing and structural stiffening may work to keep equipment upright but actually they provide a direct path way on which damage shock and vibration can travel. The more rigid the connection, the more likely there will be damage to fragile components like drive heads, optical lasers, and other sensitive components. Recently desired performance objective of operation or immediate occupancy of sensitive equipments has made the engineers to adopt non-conventional method for protection of these systems. Seismic isolation as a reliable and economical method can be recommended to achieve these performance objectives. So far almost all seismic isolation systems that were developed for equipment consist of coil springs to provide flexibility and energy absorbing device in the form of friction slider or oil dampers [1]. There are 1 Professor, Dept. of Civil Engg. Systems, Kyoto University, Japan, iemura@catfish.kuciv.kyoto-u.ac.jp 2 Ph.D. Student, Dept. of Civil Engg. Systems, Kyoto University, ttaghikh@catfish.kuciv.kyoto-u.ac.jp

2 many seismic isolators like Friction Pendulum Bearing (FPS) [1, 2], Resilient Friction Base Isolation (R_FBI) [3] and Hybrid Base Isolation [2] (Slider and Laminated Rubber bearing) which are using same mechanism to protect equipments. This paper focuses on dynamic behaviour and design of isolation systems that comprise of friction slider and restoring spring. First, a numerical model of a raised floor, seismically isolated with friction slider and spring is proposed. Then the model was validated by performing shaking table test. This model was used to predict dynamic behaviour of seismic isolated equipments and to reach at the optimum design of isolation system. For the purpose of this study, design of isolation system is defined as optimum if it results in minimum displacement while maintaining the maximum acceleration below allowable level. Two groups of earthquakes recommended by Transportation Ministry of Japan were considered in this paper. RESILIENT SLIDING ISOLATION (RSI) SYSTEM Sliding bearing limits the transmission of seismic force to level that is function of friction coefficient of sliding interface. This behaviour is interesting for protection of non-ductile and non-structural components against earthquake when expected acceleration is more than their strength level. However there are some negative aspects in seismic behavior of sliding bearings like lack of restoring force and transmission of high frequencies [5,6]. Transmission of high frequency excitation causes damage in sensitive equipments. To avoid these undesirable features, sliding bearings are typically used in combination with a restoring spring. When spring and slider are used in series (Fig1), sliding does not occur for seismic excitation below a certain threshold, and the isolated structure responds only in elastic part [7]. This behavior can filter direct and indirect excitation of high frequency due to stick-slip. However in strong excitation, this system may result in residual displacement. When spring and slider are in parallel combination i.e. Resilient Sliding Isolation System (Fig 2) transmission force to equipment is equal to restoring force of spring plus friction force at sliding interface. This combination can reduce both transmission of indirect high frequency excitation and residual displacement. Seismic Behavior of Resilient Sliding Isolation System Since the shear force activate the slider bearing, its horizontal stiffness drops from large value to zero and causing an unrestrained displacement. In this regards combination of slider and restoring spring provide an additional stiffness to control displacement. Thus force-deflection relationship of the combined system Fig.1 Slider and spring in series Fig.2 Slider and spring in parallel

3 Fig.3 Comparison of dynamic Behaviour of Resilient Sliding Isolator with (i) pure slider & (ii) spring. would be theoretically bilinear with infinite initial stiffness followed by finite stiffness [6]. In fact, response of SDOF model of Resilient Sliding Isolation during earthquake ranges between dynamic response of a slider bearing and a spring isolator. Figure 3 shows that the response acceleration spectra of resilient sliding isolation with friction coefficient (µ) as.2 is almost same as response spectra of spring isolation. Increasing friction coefficient makes response spectra of RSI system similar to response spectra of slider bearing. Likewise, by reducing stiffness of spring (increasing period) in this system the acceleration spectra approaches to acceleration spectra of slider. Response acceleration spectra of Resilient Sliding Isolation is function of input earthquake characteristics, period and friction coefficient. But dependency of response spectra to earthquake characteristic can be reduced by increasing the period and in such case maximum acceleration (a m ) limits to µ g (Fig 5). Constantinou et al. [8] also exhibited that maximum response acceleration of resilient sliding isolation under two different earthquakes of El Centro and Mexico City is equal when period of isolator is 4 seconds. Fig.4 shows the steady state response of this type of isolators subjected to harmonic base motion [9]. The Acc. (Cm/s^2) µ =.5, PGA=.25g Kobe E-W Pacoima Elcentro am=µ.g Period(Sec.) Fig.4 Transmissibility of FPS. [9] Fig.5 Acceleration Spectra of RSI System

4 horizontal axis represents the ratio of the period of the input motion (T) and the period of spring (T ). The vertical axis represents the ratio of the acceleration response (a m ) and the peak ground acceleration (a g ). In this figure response acceleration ratio (a m /a g ) is almost independent to variation of T/T for values less than.3. For instance, a series of isolators can be considered with period (T ) more than 3 sec and friction coefficient (µ) more than.5. Response of these isolators under sinusoidal vibration with maximum acceleration between.5g< a g <1.g is independent to input excitation if period (T) is less than 1 sec. Most of earthquakes, which are considered in seismic design of structure, are in this range of excitation period and maximum acceleration. Maximum acceleration spectra was obtained for a resilient sliding isolation system with µ=.5 under three earthquake with Peak Ground Acceleration of.25g and is shown in Fig5. This acceleration spectra for PGA/µ=2.5 (PGA=.25g, µ =.1) also clearly shows that when period of isolators is more than 2 second response acceleration for three earthquakes is the same even they have different period of content. ANALYTICAL MODELING OF RSI In numerical model of resilient sliding isolation the essential feature that need to be modeled for behavior of sliding bearing is the velocity dependence of the coefficient of friction and influence of bearing pressure in the coefficient. Although, biaxial interaction and its effect can be considered as another feature of modeling. Here, velocity dependence of friction coefficient can be modeled by the following equation [1]: µ = µ max - µ exp(-α U & ) (1) In which, µ max is the maximum value of the coefficient of friction, µ is the difference between its maximum and minimum value. Effect of bearing pressure on friction coefficient is accounted by factor α and U & is the absolute velocity. Biaxial interaction is considered as model proposed by Park and Wen [11]. In addition to friction element, laminated rubber bearings are assumed as a linear spring and frames is considered as elastic beam element. Mass of frame elements and blocks deemed as lumped mass element that have degree of freedom in horizontal direction (X-Y) and rotation about vertical axis (θ). In the present study, in addition to this numerical model, a different analytical model, termed here as SDOF model is proposed that computes maximum values of response parameters for different sets of friction coefficient and spring stiffness. In fact this model considers the bilinear model for forcedisplacement behavior of isolation system as shown Fig.3. The design parameters for this model are µ max and spring stiffness. In this analytical model equipment and raised floor is modeled as rigid block. VERIFICATION OF ANALYTICAL MODEL Provisions and codes provided procedures to verify acceptable performance of equipment for expected ground motions [12]. For example IEEE 693 [13] recommends procedures, which comprise analytical studies (Static analysis, response spectrum analysis) and experimental methods (response history testing, shaking table test). The economical impact of failure of equipment in earthquake generally makes it necessary, any seismic design method or protection strategy to be verified for its accuracy with precise numerical analysis or with experimental test. Experimental Test In order to establish the reliability of numerical model for the isolation system used in the study a series of shaking table were performed at Disaster Prevention Research Institute of Kyoto University. This shaking

5 Fig.6 Experimental Setup Fig.7 Details of Test Setup table system can reproduce acceleration of 1.g in three direction with maximum stroke of.3m in horizontal and.2m in vertical direction. In this experiment a 4.15 m x 2.65 m raised steel floor, supported on four frictional sliders at the corner and two laminated-rubber bearings was considered (Fig 6 and 7). The total weight of this raised floor was 1 KN. Rubber bearings have a square plan of 25 mm x 25 mm with three different thicknesses. Modulus of elasticity of rubber is 1.2 KN/mm 2. The bearings were designed for periods of 1.1, 1.75 and 3. seconds respectively. Sinusoidal tests on sliding bearings before shaking table test showed that the minimum and maximum values of friction coefficient are.5 and.15 respectively. The system was tested using two groups of earthquakes, recommended by Transportation Ministry of Japan. These groups are T1 (offshore) and T2 (inland) and each one contain 9 records. Each group, based on soil condition of recording station further divided into three categories, records on stiff soil (soil typei), medium soil (soil type II) and soft soil (soil type III). Almost 7 runs were made with different isolators and earthquakes. Displacement, acceleration, vertical pressure on bearing and lateral force of System were Force(KN) Shear Force Time History Kobe-Soil Type I, Period=1.1 Sec Numerical Model Experiment Model Time(sec.) Displacement (Cm.) Relative Displacement Time History Kobe-Soil Type II, Period=1.75 Sec Numerical Model Experimental Model Time(sec.) (a) (b) Fig.8 Comparison of Response of Experimental and Numerical Model Maximum Shear Force Maximum Relative Displacement.2.2 Rel. Difference Soil I Soil II Soil III Rel. Difference Soil I Soil II Soil III Fig.9 Relative Difference Between Maximum Responses of Experimental and of Numerical Model

6 measured during these tests. The response of system recorded in experiment and computed by numerical model, which was compared for different design parameters of earthquake motions. Shear force of system under earthquake recorded on soil type I is shown in Fig.8a when period of spring is 1.1 second. In Fig.8b lateral relative displacement of raised floor recorded at soil type II is illustrated when period of spring is 1.75 second. These figures shows relation of results obtained from numerical model for different conditions. Fig.9 depicts relative difference of maximum response of experimental and numerical model under three earthquakes for three different periods. This figure confirms that the relative difference between maximum recorded shear force and displacement in experiment and numerical analysis is less than 2%. One of the main sources of this difference is possibly change in vertical pressure on sliders due to rocking motion. This level of accuracy for numerical model can be accepted depending on sensitivity of equipment and allowable margin of error for protecting equipment. Validation of SDOF Model The validated numerical model was then used to test reliability of proposed SDOF model. Maximum response of system is computed by using precise numerical model for different stiffness of laminated rubber bearings and friction coefficient of sliders. In other words, response spectra of this system is computed for different values friction coefficient. By comparing this spectra with response acceleration spectra of Resilient Sliding Isolation of SDOF model (Fig.3), the accuracy of SDOF model can be established for different periods and friction coefficients. In Fig 1 for maximum friction coefficient (µ max ).1, maximum acceleration spectra of model has been plotted for µ=.5. This figure show that maximum accelerations obtained by SDOF model in long periods is very close to numerical model. In Fig.11 relative difference of maximum acceleration obtained by numerical model and SDOF model has been shown for different values of friction coefficient under El Centro earthquake. This figure confirms that increase in the period reduces difference of maximum acceleration between two models. In this graph relative difference between acceleration of SDOF model and numerical model, for friction coefficient more than.8 is less than 2 percent when the period is longer than 2 seconds. DESIGN METHODOLOGY Dynamic characteristics of equipments like stiffness or damping may effect on any decision about modeling and design methodology of their isolation systems. In this regard, Almazan et al. [14] compared response parameters of a rigid block and flexible superstructure (period.5 sec.) were isolated by FPS isolator. Their result show that difference between the isolator deformation computed from both models is very small; however slightly larger discrepancies (about 1 percent) was observed in shear force. Though µ m=.1, µ= Normalized Acc SDOF Model Numerical Model Rel. Difference µ=.5 µ=.6 µ=.7 µ =.8 µ= Period(Sec.) Fig.1 Sliding Isolation Acceleration Spectra of El Centro Earthquake Fig.11 Relative Difference Between Maximum Acceleration of SDOF and Numerical Model under El Centro earthquake

7 most of equipments have solid components, flexibility of equipment if there, do not have considerable effects on response of their isolation system. Thus in isolated equipment, response of seismic isolation system can be obtained with acceptable accuracy by assuming equipments + raised floor as a rigid mass. This SDOF model of sliding isolation system has been shown in Fig3. Determination of stiffness and friction coefficient of SDOF model based on seismic performance objective of equipments is purpose of design methodology in this part. Since seismic performance objective of equipments is qualitative term (operation during or immediately after earthquake) it should be quantified by limiting values of measurable response parameters for practical design. For example in structures, this measurable response parameter is story drift and it is limited to 2%~3% when level of performance is Life Safety [4]. In equipments and nonstructural elements, because of their rigid behaviour, performance objective is defined by limiting response acceleration or lateral force. For nonstructural components above the isolation interface, provisions and codes of seismic isolated structures [15] recommends that they shall resist the total lateral seismic force equal to the maximum dynamic response of element or component. In other words, if due to any reason nonstructural components cannot resist more than specific level of lateral seismic force, isolation system should design to control maximum lateral seismic force to less than or equal to level of resistance of equipment. Table 2 indicates suggested peak accelerations by manufacturer for some models of disk drives in computer systems. The response acceleration if exceeds this value may cause permanent damage and loss of readable data [16]. In this table, for operating and non-operating condition of different disk drives maximum seismic bearable acceleration varies between.2g-1.g. These values in practice are reduced by safety factors to Maximum Allowable Acceleration. For protecting these systems during earthquakes, stiffness of spring and friction coefficient of slider should be selected to limit horizontal input acceleration under their allowable values. Optimum Design Procedure Any combination of stiffness and friction coefficient of resilient sliding isolation, which control response acceleration under allowable level of acceleration, can be accepted as eligible design parameters for protecting equipment in earthquake. But most of equipments have connections with other systems like power, water supply or main server and safety of connections to these systems is essential to ensure the functioning of equipment during earthquake. Therefore beside safety of equipments, designer should control displacement of isolated equipments in earthquake to minimum value. In this regard, determination of stiffness and friction coefficient of isolators to control input acceleration under allowable level and to minimize lateral displacement is the optimum design of resilient sliding isolation.fig.12 clearly depicts procedure of optimum parameter recognition for specific allowable acceleration. Maximum response acceleration and displacement spectra of SDOF model of isolated equipment under El Centro earthquake is shown in Fig12-a for isolation system with friction coefficient of.3. In this figure allowable acceleration of equipment is assumed as.8g. Safety of equipment can system Table 2 Maximum seismic resistant acceleration on disk drives [16] Manufacturer's Model # Max. g / Operating Max. g / Non-Operating DEC - Alpha Server - #82.5 g.5 g 1. g SUN - Class III Drive.25 g 1. g DEC - RZ 28 Drive Unit.5 g.5 g HP - Model 2 Drive Unit.25g N / A HP - Enterprise 9.2g-.5 g.5 g 1. g

8 Accelertation Spectra Acceleration (Cm/Sec^2) *g µ=.3 Allowable Σεριεσ1 Level=.8 Accelertation Spectra Acceleration (Cm/Sec^2) Period(Sec.) *g µ=.3 µ=.5 Allowable µ=.7 Σεριεσ1 Level=.8g Period(Sec.) Displacment Spectra Displacment(Cm) Min. Allowable Disp. µ =.3 be guaranteed when maximum acceleration of its isolated model is equal or less than this level of acceleration. Safety region begins from crossing point of dashed-line with response spectra. Isolation systems with period longer than this point are eligible to ensure operation of equipment during or immediately after El Centro earthquake. Among these eligible periods just one of them has minimum displacement that is shown in displacement spectra with star symbol. This procedure is repeated for different values of friction coefficient in Fig12-b. Crossing point of dashed lines with each spectra show start point of their eligible range for allowable acceleration.8g. Period at these points for three coefficient friction µ=.3,.5,.7 are T i =2.9,3.,4.6 seconds. For periods more than T i, minimum displacement of each friction coefficient has been highlighted with star symbol. Among three highlighted points, displacement of one of them with friction coefficient.7 and period 4.6 seconds has minimum value. Between three friction coefficients for allowable acceleration.8g this point introduce property of an isolator that can protect equipment under El Centro earthquake with minimum displacement. By using this procedure for range of friction coefficients, optimum parameters of resilient sliding isolator can be determined for allowable acceleration.8g or other values of allowable acceleration. In Fig.13 optimum parameters of resilient sliding isolator for different allowable accelerations of this isolator were computed for feasible range of friction coefficient between (.3~.1) and periods between (~15sec.). These parameters were computed by using cited procedure for any allowable level of Displacment Spectra Period(Sec.) Min. Allowable Disp. µ =.5 Min. Allowable Disp. µ =.3 Min. Allowable Disp. µ = Period(Sec.) (a) (b) Fig.12 Procedure of Optimum Parameter Recognition for El Centro Earthquake Optimum Friction Coeff. Optimum Period Friction Coef. (µ ) PGA=g PGA =.75g PGA =.5g PGA =.25g PGA=g PGA=.75g PGA=.5g PGA=.25g (a) (b) Fig.13 Optimum Parameter of Resilient Sliding Isolation for El Centro Earthquake

9 Friction Coeff. (µ ) Friction Coeff. (µ ) acceleration between (.4g-.11g). Input earthquake was scaled to.25g,.5g,.75g and 1.g to evaluate the effect of Peak Ground Acceleration (PGA) on optimum parameters. Fig.13-a illustrates, optimum friction coefficient has ascending trend with increasing of allowable level of acceleration but different values of peak ground acceleration have not clear effect on optimum value of this parameter. Optimum period in Fig.13-b has descending variation with increase of allowable acceleration. In this figure optimum period of isolation system under higher level of peak ground acceleration of earthquake is longer. EVALUATION OF OPTIMUM PARAMETERS To determine variation of optimum parameters of resilient sliding isolators under several earthquakes gives an evaluation about optimum design of these isolators based on seismic performance objective of equipments. In this part, optimum parameters of resilient sliding isolators are obtained analytically for T1 (offshore) and T2 (in land) groups of motions. In order to have proper comparison all earthquakes are scaled to site specific Peak Ground Acceleration (PGA) equal.25g (Moderate seismic zone) and.5g (High seismic zone)[17]. Fig.14 shows optimum parameters of resilient sliding isolators under records of T1 that were scaled to.25g. In this figure earthquakes recorded on stiff, medium and soft soil are scripted with T1-I, T1-II and T1-III. Variation of optimum period and friction coefficient with allowable level of acceleration has same trend with variation of these parameters in El Centro earthquake. For design purposes Mean and, Mean ± Standard deviation of optimum values for all earthquakes show that optimum frictions are almost in the same line for all earthquakes in T1 while optimum period can be selected from a band of period for any allowable acceleration of equipments. In Fig 15 optimum parameters are computed for T2 motions that were also scaled for same peak ground acceleration of.25g. These earthquakes depending on soil type of recording station were divided in three Optimum Friction Earthquake Type 2 (PGA=.25g) T2-I-1 T2-I-2 T2-II-1 T2-II-2 T2-II-3 T2-III-1 T2-III-2 T2-III-3 mean mean+st mean-st Optimum Friction Earthquakes Type 1 (PGA=.25g) T1-I-1 T1-I-2 T1-I-3 T1-II-1 T1-II-2 T1-II-3 T1-III-1 T1-III-2 T1-III-3 mean mean+st mean-st Optimum Period Earthquakes Type 1 (PGA=.25g) Optimum Period Earthquake Type 2 (PGA=.25g) T2-I-1 T2-I-2 T2-II-1 T2-II-2 T2-II-3 T2-III-1 T2-III-2 T2-III-3 mean mean+st mean-st Fig.15 Optimum Friction and Period of Resilient Sliding Isolation System under records of T T1-I-1 T1-I-2 T1-I-3 T1-II-1 T1-II-2 T1-II-3 T1-III-1 T1-III-2 T1-III-3 mean mean+st mean-st Fig.14 Optimum Friction and Period of Resilient Sliding Isolation System under records of T1

10 categories T2-I, T2-II and T2-III. Even the variation of optimum parameters is the same as earthquakes in T1 but Standard deviation of optimum values is further. Comparison between mean values of optimum parameters under records of T1 and T2 and their Standard deviation is shown in Fig 16. In this figure mean values of optimum friction coefficient for two types of earthquakes are nearly same and have linear variation with increasing of allowable level of acceleration. But it can be seen that the difference between mean values of optimum period in two types of earthquakes is around 2 seconds and both have descending variation by increasing allowable level of acceleration. For these earthquakes that scaled to PGA=.25g, optimum parameter selected among range of friction coefficient between (.3~.1) and period between (~15sec.) and computed for equipments that their allowable level of acceleration varies between (.4g-.11g). Fig17 shows optimum parameters of resilient sliding isolators under records T2 with PGA.5g. In this figure optimum parameters were selected among range of friction coefficient between (.7~.14) and allowable level of acceleration between (.8g-.15g). Standard deviation of optimum parameters for PGA=.5g is observed to be more than this variable in Fig.15. Also, such as variation of optimum parameters of isolation system under El Centro earthquake, mean values of optimum periods for higher level of PGA is more than lower levels. Increasing of standard deviation of optimum parameters for higher value of PGA can be explained with behaviour of resilient sliding isolators (FPS isolation) under harmonic loading. It was shown for range of relative periods (T/T ) and maximum acceleration ratios (a g /µ), maximum response acceleration is constant and independent to frequency of input motion. For an isolator with period T and friction coefficient µ, Fig.4 shows for lower values of a g /µ, range of constant response of isolator is longer than that for higher values. In other words when maximum acceleration ratio Optimum Friction Optimum Period.12 Earthquakes Type1 & 2 (PGA=.25g) 18 Earthquakes Type1 & 2 (PGA=.25g) Coef. Friction (µ ) Type2 Type2+st Type1 Type1-st Type 2 Type2+st Type1 Type1-st Fig.16 Optimum Friction and Period of Resilient Sliding Isolation under records of T2 Friction Coeff. (µ ) Optimum Friction Coef. Earthquake Type2 (PGA=.5g) T2-I-1 T2-I-2 T2-I-3 T2-II-1 T2-II-2 T2-II-3 T2-III-1 T2-III-2 T2-III-3 Mean Mean+s Mean-s Optimum Period Earthquake Type2 (PGA=.5g) T2-I-1 T2-I-2 T2-I-3 T2-II-1 T2-II-2 T2-II-3 T2-III-1 T2-III-2 T2-III-3 Mean Mean+s Mean-s Fig.17 Optimum Friction and Period of Resilient Sliding Isolation under records of T2

11 of input motion (a g /µ) reduces, response of isolator below certain value of excitation period (T) will be constant. Though earthquake is comprised several harmonic excitation with different period but its maximum response acceleration is dominated by two or three harmonic excitation. Therefore for higher value of PGA, optimum design parameter also depends on frequency content of input motion and thereby increases the Standard deviation. CONCLUSIONS The paper discusses the evaluation of design parameters of the Resilient Sliding Isolation system to achieve performance objective of equipments. Analytical method based on single degree of freedom is proposed to obtain these parameters. In addition the design parameters obtained by this method also lead minimum relative displacement. The accuracy of the method is validated by shaking table test of raised floor isolated by resilient sliders. Optimum design parameters of these resilient sliding systems subjected to two type of Japan standard earthquakes are obtained for different values of allowable level of acceleration for the equipments. Results of analysis show: 1. For higher values of peak ground acceleration of earthquake, optimum period of resilient sliding isolation is longer. 2. Optimum friction coefficient of isolation system under earthquakes T1 and T2 in moderate seismic zone has almost linear relation with increasing level of allowable acceleration. 3. Optimum period of isolation system under earthquakes T1 and T2 in moderate seismic zone becomes shorter when allowable level of acceleration increases. 4. In high seismic zone, standard deviation of optimum parameters is larger than moderate seismic zone. Mean of optimum parameters in high seismic zone has same trend of variation with moderate seismic zones. ACKNOWLEDGEMENT Authors acknowledge the constructive comments given by Dr. Sarvesh K. Jain that helped in improvement of the manuscript. REFERENCES 1. Morikawa Y, Fujita S. Development of Seismic Isolation System for Light Equipment using Friction Pendulum Bearing Proceeding of Tenth World Conference on Earthquake Engineering, Madrid, Spain, 1992, pp Saadeghvaziri AM, Feng MQ, Experimental and Analytical Study of Base-Isolation for Electric Power Equipment, Report on Research Progress and Accomplishments: 2-21, MCEER Publication, State University of New York, Buffalo. 3. Lei KM, Hemried AG. Seismic Response of Equipment in Resilient-Friction Base Isolated Structures Proceeding of Tenth World Conference on Earthquake Engineering, Madrid, Spain, 1992, pp Federal Emergency Management Agency, NEHRP guidelines for the Seismic Rehabilitation of Building, FEMA-356,Chapter 9,Washington,D.C., Skinner RI, Robinson WH, McVerry GH., An Introduction to Seismic Isolation,John Wiley & Sons,U.K., Kelly JM, Chalhoub MS. Earthquake Simulator Testing of a Combined Sliding Bearing and Rubber Bearing Isolation System,UCB/EERC-87/4, 199

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