Advanced Base Isolation Systems for Light Weight Equipments

This chapter is intended to introduce the earthquake proof technology particularly in the area of base isolation systems that have been used to protect light weight structures, such as motion sensitive equipment, historic treasures, and medical instruments, etc., from earthquake damage. This chapter presents theoretical background, experimental studies, numerical analyses, and the applications of the advanced isolation systems consisting of rollingand sliding-type isolation systems for light weight structures. The efficiency of these isolators in reducing the seismic responses of light weight equipment was also investigated in this study. In addition, the results from theoretical and experimental studies for these isolators are compared and discussed.

Several techniques exist to minimize earthquake effects on structures, such as light-weight structure design, improving the ductility capacities of structures, and structural control (earthquake proof technology), etc. Structural control technology has been recognized as an effective tool in seismic mitigation, and can be classified as active, passive, hybrid and semiactive controls, which can be clarified by the following equation: where M, C, and K are the mass, the damping, and stiffness matrices, respectively, of a structure, which are the natural characteristics of a structure; .. u , . u and u denote the vectors of the relative acceleration, velocity, and displacement with respect to supports, respectively, which are structural responses during earthquakes; .. g u is the ground acceleration; B is the displacement transformation matrix; and .. . (,,,) Fuuut depicts the control force that is an external force provided by various types of power and control systems. Active control technology has a control force used to activate the control system. The control force is generated through the control signal which is based on the results calculated from the measured responses of the structure and the specified control algorithm. The structural responses can be lessened by changing its characteristic through the control force in the second term on the right hand side of Eq. (1) which could be proportional to the measured displacement, velocity and acceleration of the structure during earthquakes. From a mathematical point of view, we can then move the control force from the right hand side to the left hand side of Eq. (1) to combine with the corresponding terms depending on the values of the control fore proportional to. As a result, the mass, damping, and stiffness matrices of a structure are modified by the control force, but not by an actual device. In the passive control, there is no external control force, .. . (,,,) Fuuut , in the system, which means that there is no second term on the right hand side of Eq. (1). The mass, damping, or stiffness which are the first three terms on the left hand side are modified by adding actual devices to the structure (Soong and Dragush, 1997;Takewakin, 2009). The device used to modify the mass matrix is named the tuned mass damper. Any actual devices used to modify the second and third terms on the left hand side of Eq. (1) are called energy absorbing systems (or dampers). A device such as a fluid damper producing an internal force that is strongly dependent on the relative velocity between the two ends of the device is called a velocity dependent device (damper). On the other hand, a device such as a friction and yielding dampers producing an internal force that is strongly dependent on the relative displacement between the two ends of the device is called a displacement dependent device (damper). Usually, the velocity dependent device produces minimum internal forces at the moments of maximum displacement due to zero velocity, which means that this type device provides no damping effect to the structure while the structure deforms at critical moments of earthquakes. On the other hand, the displacement dependent device produces maximum internal forces at maximum displacements. This means that this type device can provide maximum damping effect at the moments of maximum displacement and immediately reduce the structural responses at the most critical time of structural responses. The discussions of the advantages and disadvantages of these two types of devices are out of the scope of this chapter.
www.intechopen.com A system called the base (seismic) isolation system inserts a soft layer or device (base isolator) between the structure and its foundation to isolate earthquake-induced energy trying to penetrate into the structure, thereby protecting the structure from earthquake damage (Skinner et al., 1993;Naeim and Kelly, 1999). A base isolation system is used to minimize the seismic force which is the first term on the right hand side of Eq. (1) in two ways: (i) by reflecting the seismic energy by lengthening the natural period of the entire system including the structure and the base isolator and (ii) by absorbing the seismic energy through the hysteretic loop of the isolator displacement and the force induced in the isolator. The combination of the active and passive control is called hybrid control, which also needs a large control force for controlling structural responses. By contrast, semi-active control uses substantially smaller control force in the manner of an on and off switch to improve the efficiency of the passive control system through an active control algorithm, but not massive control force. In conclusion, structural control technology protects structures through mechanisms that are used to prohibit the seismic energy from transmitting into major members such as the beams, columns and walls of a structure, which are used for supporting structural weight. In the past, there have been a lot of papers and reports concerning the use of vibration isolation technology to increase the precision of machines by isolating vibration sources resulting from the environment, such as moving vehicles, or for improving human comfort by isolating vibration sources that result from machines and moving vehicles (Rivin, 2003). Recently, the isolation technology has been acknowledged as an effective technique to promote the earthquake resistibility of the structures by controlling structural responses during earthquakes on the basis of theoretical and experimental results and earthquake events (Naeim and Kelly, 1999). Several theoretical studies have been made on the applications of the base isolation technology to critical equipment in seismic mitigation (Alhan and Gavin, 2005;Chung et al., 2008). However, little attention has been given to the experimental study of the efficiency of base isolation on the protection of vibration sensitive equipment in the events of earthquakes, especially for experimental investigations under tridirectional seismic loadings (Tsai et al., 2005bFan et al., 2008). This chapter is aimed at the seismic isolation, especially for the equipments in hospitals and facilities of emergency departments used for saving peoples' lives. These are of extreme importance and should be kept functional during and after earthquakes.

Background of rolling types of bearings
To the best of the author's knowledge, the isolation system with doubled spherical concave surfaces and a rolling ball located between these two concave surfaces was first patented by Touaillon in 1870, as shown in Fig. 1(a). Several similar isolation systems with a ball located between two spherical concave surfaces were also proposed (Schär, 1910;Cummings, 1930;Bakker, 1935;Wu, 1989), as shown in Figs. 1(b)-1(e). In 1997, Kemeny propounded a ball-incone seismic isolation bearing that includes two conical concave surfaces and a ball seated between the conical surfaces, as shown in Fig. 1(f). The dynamic behavior of the ball-in-cone isolation system has been investigated (Kasalanati et al., 1997). In addition, Cummings (1930) also proposed a seismic isolation system with a rolling rod of a cylinder sandwiched between two concave surfaces, as shown in Fig. 1(c). Lin and Hone (1993), Tsai et al. (2006b) and M. H.  conducted research on the effectiveness of this type of base isolation system in seismic mitigation, as shown in Figs. 2(a) and 2(b). Kim (2004) proposed a seismic isolation system that has rollers of a bowling shape to roll in the friction channel, as shown in Fig. 2(c). Tsai (2008a, b) revealed seismic isolation systems each consisting of shafts rolling in the concave slot channels, as shown in Figs. 2(a) and 2(d). These devices are capable of resisting the uplift while the vertical force in the isolator becomes negative under severe earthquakes. Fig. 1(a). Touaillon's original patent (1870) www.intechopen.com Fig. 1(b). Schär's original patent (1910) www.intechopen.com Fig. 1(c). Cummings' original patent (1930) www.intechopen.com   The isolation system with two concave surfaces and a rolling ball (Touaillon, 1870;Wu, 1989;Kemeny, 1997) possesses some shortcomings even under small loadings like equipment and medical instruments, such as negligible damping provided by the system, a highly concentrated stress resulted from the weight of the equipment on the rolling ball and the concave surfaces due to the small contact area, and scratches and damage to the concave surfaces caused by the ball rolling motions during earthquakes. The rolling ball has a tendency to move even under environmental loadings such as human activities during regular services. In addition, the bearing size is large because of the large bearing displacements under seismic loadings due to insufficient damping provided by the rolling motion of the ball on the concave surfaces in the system. To supply more damping to the isolation system and simultaneously reduce the bearing size as a consequence of smaller bearing displacements during earthquakes, Tsai et al. (2006a) proposed a ball pendulum system (BPS). As shown in Figs. 3(a) and 3(b), this system comprises two spherical concave surfaces and a steel rolling ball covered with a special damping material to provide horizontal and vertical damping to tackle the problems mentioned above. A series of shaking table tests conducted by Tsai et al. (2006a) have proven that the BPS isolator can enhance the seismic resistibility of vibration sensitive equipment under severe earthquakes with smaller displacements compared to an isolation system with negligible damping. However, the special material covering the steel ball that supports the weight of the vibration sensitive equipment for a long period of time in its service life span might result in permanent deformation due to plastic deformation in the damping material. It may damage or flat the contacting surface of the special damping material after sustaining a certain period of service loadings and affect the isolation efficiency. Fig. 3(a). Open-up view of ball pendulum isolation system Fig. 3(b). Test set-up for ball pendulum isolation system An alternative approach for increasing damping and lessening the isolator displacement is to add a damping device to the isolation system (Fan et al., 2008). Fathali and Filiatrault (2007) presented a spring isolation system with restraint which is a rubber snubber to play a role of displacement restrainer to limit the isolator displacement. In general, the displacement restrainer will involve impact mechanisms as a result of contact made with isolated equipment, which lead to amplified acceleration responses and large dynamic forces. To increase damping for a rolling bearing and to prolong the service life of a bearing, an isolation system called the static dynamics interchangeable-ball pendulum system (SDI-BPS) shown as Figures 4(a) and 4(b) was proposed by Tsai et al. (2008a). The SDI-BPS system consists of not only two spherical concave surfaces and a steel rolling ball covered with a special damping material to provide supplemental damping and prevent any damage and scratches to the concave surfaces during the dynamic motions induced by earthquakes but also several small steel balls that are used to support the static weight to prevent any plastic deformation or damage to the damping material surrounding the steel rolling ball during the long term of service loadings. Because the concave surfaces are protected by the damping material covering the steel ball from damage and scratches, they may be designed as any desired shapes in geometry, which can be spherical, conical or concave surfaces with variable radii of curvature. The natural period of the SDI-BPS isolator depends only on the radii of curvature of the upper and lower concave surfaces, but not a function of the vertical loading (static weight). It can be designed as a function of the isolator displacement, and predictable and controllable for various purposes of engineering practice.

Background of sliding types of bearings
Most sliding types of isolation systems are suitable not only for light weight structures such as equipment and medical instruments but also for very heavy structures such as buildings and bridges. These types of bearings provide damping through frictional mechanism between sliding surfaces. As shown in Fig. 5(a), Penkuhn (1967) proposed a sliding isolation system including a concave sliding surface and a universal joint to accommodate the rotation resulted from the superstructure and the sliding motion of the universal joint, and suggested that the superstructure be supported by a rigid supporting base which was in turn supported by three proposed isolation bearings. Zayas (1987) and Zayas et al. (1987) proposed a friction pendulum system (FPS) with a concave sliding surface and an articulated slider, as shown in Figs 5(b) and 5(c). Through extensive experimental and numerical studies, the FPS isolator has proven to be an efficient device for reducing the seismic responses of structures Al-Hussaini et al., 1994).  To avoid the possibility of resonance of the isolator with long predominant periods of ground motions, Tsai et al. (2003a) presented an analytical study for a variable curvature FPS (VCFPS). In order to enhance the quakeproof efficiency and reduce the size of the FPS isolator, Tsai (2004a,b) and Tsai et al. (2003bTsai et al. ( , 2005aTsai et al. ( ,b, 2006c) proposed a sliding system called the multiple friction pendulum system (MFPS) with double concave sliding surfaces and an articulated slider located between the concave sliding surfaces, as shown in Figs. 6(a)-6(f). Based on this special design, the displacement capacity of the MFPS isolator is double of the FPS isolator that only has a single concave sliding surface, and the bending moment induced by the sliding displacement for the MFPS isolator is an half of that for the FPS isolator. Moreover, the fundamental frequency is lower than that of the FPS as a result of the series connection of the doubled sliding surfaces in the MFPS isolation system, and the bearing is a completely passive apparatus, yet exhibits adaptive stiffness and adaptive damping by using different coefficients of friction and radii of curvature on the concave sliding surfaces to change the stiffness and damping to predictable values at specifiable and controllable displacement amplitudes. Hence, the MFPS device can be given as a more effective tool to reduce the seismic responses of structures even subjected to earthquakes with long predominant periods, and be more flexible in design for engineering practice. In addition, Fenz and Constantinou (2006) conducted research and published their results on this type of base isolation system with double sliding surfaces. Kim and Yun (2007) reported the seismic response characteristics of bridges using an MFPS with double concave sliding interfaces.   Furthermore,  proposed several other types of MFPS isolators, as shown in Figs. 7(a)-7(f), each with multiple sliding interfaces, which essentially represent that each FP isolation system above and below the slider has multiple sliding interfaces connected in series (Tsai et al., , 2010a. Fenz and Constantinou (2008a, b) published their research on the characteristics of an MFPS isolator with four sliding interfaces under unidirectional loadings. Mahin (2008, 2010) investigated the efficiency of an MFPS isolator with four concave sliding interfaces on seismic mitigation of buildings. As shown in Figs. 7(g) and 7(h), Tsai et al. (2010a, b) proposed an MFPS isolator with numerous sliding interfaces (any number of sliding interfaces). As explained earlier, these types of bearings, each having N number of sliding interfaces, possess adaptive features of stiffness and damping by adopting different coefficients of friction and radii of curvature on the concave sliding surfaces to result in changeable stiffness and damping at specified displacement amplitudes. Tsai et al. (2011a) published experimental investigations on the earthquake performance of these types of friction pendulum systems. The efficiency of the MFPS isolator with multiple sliding interfaces in mitigating structural responses during earthquakes has been proven through a series of shaking table tests on a full scale steel structure.    Fig. 8(a) and called the XY-FP isolator, consisting of two orthogonal concave beams interconnected through a sliding mechanism has been published by Roussis and Constantinou (2005). The FP isolator (XY-FP) possesses the uplift-restraint property by allowing continuous transition of the bearing axial force between compression and tension, and has different frictional interface properties under compressive and tensile normal force in the isolator. A device, as shown Fig. 8(b), similar to the design concept but without the uplift-restraint property was also proposed by Tsai (2007). This device has an articulated slider seated between the FP bearings in the X and Y directions to accommodate the rotation as a result of sliding motion of the articulated slider and to maintain the isolated structure standing vertically during earthquakes. , that consists of one trench concave surface in each of two orthogonal directions, and an articulated slider situated between the trench concave surfaces to accommodate the rotation induced by the sliding motion of the slider. The TFPS possesses independent characteristics such as the natural period and damping effect in two orthogonal directions, which can be applied to a bridge, equipment or a structure with considerably different natural periods in two orthogonal directions.  In order to further enhance the functionality of the TFPS isolator (Tsai et al., 2010c), a base isolation system named the multiple trench friction pendulum system (MTFPS) with numerous intermediate sliding plates was proposed by Tsai et al. (2010d). As shown in Figures 10(a) and 10(b), the MTFPS isolator has multiple concave sliding interfaces that are composed of several sliding surfaces in each of two orthogonal directions, and an articulated slider located among trench concave sliding surfaces. The MTFPS represents more than one trench friction pendulum system connected in series in each direction. The friction coefficient, displacement capacity, and radius of curvature of each trench concave sliding surfaces in each direction can be different. The natural period and damping effect for a MTFPS isolator with several sliding surfaces can change continually during earthquakes. Therefore, a large number of possibilities of combinations are available for engineering designs. Such options are dependent on the needs of engineering practicing. As shown in Figs. 11(a) and 11(b), ) developed a direction-optimized friction pendulum system (DO-FPS) which consists of a spherical concave surface, a trench concave surface, as shown in Fig. 11(a), or a trajectory concave surface, as shown in Fig. 11(b), and an articulated slider. The DO-FPS isolator possesses important characteristics such as the natural period, displacement capacity and damping effect, which are functions of the directional angle of the sliding motion of the articulated slider during earthquakes. In order to improve the contact between the spherical and trench surfaces (or the trajectory concave surface), the slider consists of circular and square contact surfaces to match the spherical and trench surfaces, respectively. To further enhance the contact, it possesses a special articulation mechanism to accommodate any rotation in the isolator and maintain the stability of the isolated structures during earthquakes. In addition, the DO-FPS isolator can continually change the natural period and adjust the capacity of the bearing displacement and damping effect as a result of the change of the angle between the articulated slider and trench concave surface during earthquakes. This isolation system exhibits adaptive stiffness and adaptive damping by using different coefficients of friction and radii of curvature on the spherical and trench (or the trajectory) concave sliding surfaces to change the stiffness and damping to predictable values at specified and controllable angle of the sliding motion of the slider in the isolator although it is a completely passive device.  Tsai et al. (2010e, 2011b proposed and studied in theory and experiment on a base isolator that features variable natural period, damping effect and displacement capacity, named the multiple direction optimized-friction pendulum system (MDO-FPS). This device is mainly composed of several spherical concave sliding surfaces, several trench sliding concave surfaces and an articulated slider located www.intechopen.com among these spherical and trench concave sliding surfaces to make the isolation period changeable with the sliding direction from only the multiple trench sliding interfaces to the combinations of the multiple trench sliding interfaces and the multiple spherical sliding interfaces.
In addition, this bearing may have N number of sliding interfaces in the trench and spherical surfaces to possess adaptive features of stiffness and damping by using different coefficients of friction and radii of curvature on the trench and spherical concave surfaces leading to changeable stiffness and damping at specified displacement amplitudes. Therefore, the MDO-FPS isolator possesses important characteristics in natural period, damping effect and displacement capacity, which are functions of the direction of the sliding motion, coefficients of friction and radii of curvature on sliding interfaces, and sliding displacements.
The advantage of the isolator is able to change its natural period, damping effect and displacement capacity continually during earthquakes to avoid possibility of resonance induced by ground motions. This base isolator has more important features and flexibility than other types of base isolation devices for engineering practice. Practicing engineers will be able to optimize the isolator at various levels of earthquakes by adopting suitable parameters of friction coefficients and radii of curvature of the sliding interfaces.

The static dynamics interchangeable -Ball pendulum system
The dynamics interchangeable-ball pendulum system (SDI-BPS) is schematized in Figs. 4(a) and 4(b) consisting of one upper concave surface (not necessary a spherical shape), one lower concave surface, several supporting steel balls to provide supports for long terms of service loadings and the frictional damping effect to the isolator at small displacements (see Case 1 of Fig. 13), several housing holes to lodge the supporting steel balls and one damped steel ball covered by damping materials to uphold the vertical loads resulting from the static and seismic loadings at large displacements (see Case 3 of Fig. 13) and supply additional damping to the bearing by deforming the damping material that could be a rubber material during earthquakes. As shown in Case 1 of Fig. 13, almost all static loadings as a result of the weight of the equipment are sustained by the supporting steel balls and negligible loadings are taken by the damped steel ball while the system is under long terms of service loadings. In the event of an earthquake, the static loadings and the dynamic loadings induced by the ground or floor accelerations are still supported by the supporting steel balls while the horizontally mobilized force is less than the total frictional force from the supporting steel balls, and the damped steel ball remains inactivated, similar to Case 1 of Fig. 13. The frictional force depends on the contact area and the coefficient of friction among the upper concave surface, the supporting steel balls and the housing holes located on the lower concave surface. This contact area and friction coefficient can be properly designed for the purpose of adjusting the frictional force and damping. When the horizontal force exceeds the frictional force, the damped steel ball is activated and starts rolling on the concave surfaces. The vertical force resulting from the static and dynamic loadings is shared by the damped steel ball and the supporting steel balls. Simultaneously, the damping effect is provided by the supporting steel balls due to the frictional force and the damped steel ball as a result of the deformation of the damping material enveloping the damped steel ball under the condition of small isolator displacement, as shown in Case 2 of Fig. 13. The natural period of the isolated system is then dominated by the radii of curvature of the concave surfaces, which is equal to Where R 1 and R 2 are the radii of curvature of the upper and lower spherical concave surfaces, respectively; and g is the gravity constant.
If the system is subjected to a large isolator displacement during an earthquake, the supporting steel balls will be detached from the upper concave surface, and the total vertical and horizontal loads will be supported by the damped steel ball only to result in more damping effect due to the larger deformation of the damping material, and no damping effect results from the frictional force caused by the supporting steel balls, as depicted in Case 3 of Fig. 13. Furthermore, the natural period of the isolated equipment is governed by the radii of curvature of the concave surfaces in this stage. The damping effect for the isolator is only provided by the deformation of the damping material covering the damped steel ball in the course of motions to reduce the size of the isolator as a result of smaller isolator displacements caused by earthquakes in comparison to a rolling isolation system with negligible damping. As shown in Case 4 of Fig. 13, because the component of the gravity force from the equipment weight tangential to the concave surface provides the restoring force, the www.intechopen.com isolator will be rolling back to the original position without a significant residual displacement after earthquakes. Therefore, the damped steel ball is subjected to temporary loadings induced by earthquakes only, and the static loadings in the life span of service won't cause any permanent deformation to the damping material enveloping the damped steel ball. In general, in the case of a service loading or a small earthquake, the static load is supported by the mechanism composed of the upper and lower concave surfaces and supporting steel balls with negligible supporting effect from the damped steel ball. On the other hand, in the events of medium and large earthquakes, the entire loads including static and dynamic loads are supported by the mechanism offered by the upper and lower concave surfaces and the damped steel ball while the isolation system is activated. These two mechanisms are interchangeable between the cases of static loading from the weight of equipment and seismic loading from the ground or floor acceleration.  1 1) ).

Characteristic of the SDI-BPS isolator
The test setup of the SDI-BPS isolator is depicted in Figures 14(a) and 14(b). In this test, the damped steel ball consisted of a steel rolling ball of 44.55mm in diameter covered with a thickness of 6.75mm damping material which was made of natural rubber material with hardness of 60 degrees in the IRHD standard (International Rubber Hardness Degree).
The main purpose of this test was to investigate the mechanical behavior of the damped steel ball, therefore, supporting balls were removed during the component tests and all damping effect resulting from the system was provided by the damped steel ball. Figure  15 shows the relationship of the horizontal force to the horizontal displacement while the system was subjected to a vertical load of 4.56 KN and a harmonic displacement of 50mm with a frequency of 0.3Hz. The enclosed area shown in the Fig. 15 provides a damping effect into the isolation system. The test result demonstrates that the deformations of the rubber material leaded to significant damping effects in the system with negligible deformation occurring in the steel material.

Shaking table tests of motion sensitive equipment with SDI-BPS isolators
This section will investigate the performance of the SDI-BPS isolator installed in the motion sensitive equipment on seismic mitigation under tri-directional earthquakes through a series of shaking table tests of the vibration sensitive equipment isolated with the SDI-BPS isolator under tri-directional earthquakes. As shown in Figure 16, the tested vibration sensitive equipment with six inner layers was used to house the network server. The lengths in the two horizontal directions were 0.8 m and 0.6 m, respectively, and 1.98 m in height. The mass of the empty equipment was 110 kg. A mass of 108 kg at each layer from the first to the third layer and 54 kg each at the rest of the layers was added. In the case of the fixed base equipment, the natural frequency was 5.66 Hz. In the case of the isolated system, four SDI-BPS isolators with a radius of curvature of 1.0 m representing 2.84 seconds in natural period for the isolated equipment were installed beneath the equipment. The input ground motions in these tests included the 1995 Kobe earthquake (Japan) and the 1999 Chi-Chi earthquake (recorded at TCU084 station, Taiwan). Figures 17(a) and 17(b) show the comparisons of the acceleration responses at the top layer of the equipment between the fixed base and SDI-BPS-isolated systems under tri-directional earthquakes. It is observed from these figures that the SDI-BPS isolator can effectively isolate the seismic energy trying to transmit into the vibration sensitive equipment during earthquakes. Figures  18(a) and 18(b) show the hysteresis loops of the SDI-BPS isolator under the various tridirectional earthquakes. These figures illustrate that the SDI-BPS isolator can provide damping to limit the bearing displacement, and accordingly, the bearing size was decreased. It also infers from these figures that a frictional type of damping was provided by the isolation system in small displacements and other type of damping was provided by the damped steel ball of the isolation system for large displacements, referring to the hysteresis loop in Figure 15, because the upper concave surface was lifted and away from the supporting steel balls without any contact. The isolator displacement history depicted in Figure 19 demonstrates that the response of equipment had been reduced by the isolation system with acceptable displacements in the isolators and that the isolator displacement approached zero in the end of the earthquake with negligible residual displacement.  Fig. 19. Isolator displacement history in X-direction while isolated system subjected to tridirectional Chi-Chi (TCU084 Station) earthquake (PGA = X0.513g + Y0.170g +Z0.151g) www.intechopen.com

The direction-optimized friction pendulum system
A seismic isolation system, called the direction-optimized friction pendulum system (DO-FPS), consists of a concave trench, a plate with a spherically concave surface, and an articulated slider located in between, as shown in Figs. 20(a) and 20(b). The trench and the plate can usually possess different curvature radii, friction coefficients, and displacement capacities. As shown in Fig. 20(b), the concave trench is connected in series with the spherically concave plate in the X-direction, and this provides both the maximum natural period and the maximum displacement capacity. By contrast, in the Y-direction the isolator consists of just the spherically concave plate, and this provides both the minimum natural period and the minimum displacement capacity. The natural period and the displacement capacity vary with direction and are functions of the sliding angle between the articulated slider and the concave trench. In engineering practice, the DO-FPS isolator can be designed for differences in natural period and displacement capacity for the purpose of costeffectiveness. During an earthquake, the DO-FPS isolator will adjust the natural period automatically to avoid the possibility of resonance. Following the earthquake, the concave trench and the spherically concave plate can offer a centering mechanism to return the isolated structure to its original position without significant residual displacements.

Properties of the direction-optimized friction pendulum system
As shown in Figs. 21(a) and 21(b), an articulated slider is located between the concave trench and the spherically concave plate. The articulated slider includes two parts: the lower part at the trench side can have a rectangular cross section, while the upper part at the plate side can have a circular cross section. These two parts are joined with a hemispherical joint to produce an articulation mechanism in the middle of the slider. Hence, the articulated slider incorporates a rotational function to accommodate the relative rotation during its movement. As shown in Fig. 21(a), the slider is coated with Teflon composite on top, bottom, and two sides to reduce the frictional forces at its interfaces with not only the concave surfaces of the trench and plate but also the side walls of the trench. Fig. 21(a). Assembly of articulated slider coated with Teflon composite Fig. 21(b). Slider located between the concave trench and the spherical concave surface.
As shown in Fig. 21(b), the width of the articulated slider almost equals that of the trench; therefore, the sides of the slider and the walls of the trench almost remain in contact. These two sides of the articulated slider can thus provide additional damping in the direction parallel to the concave trench to help dissipate seismic energy.  Figure  24 demonstrate that the coefficients of friction are functions of the vertical pressure on the contact surface and the velocity of the sliding motion. In accordance with the analytical model proposed by Tsai (2005a), the total friction force acting on the sliding interface can be expressed as:

Component tests of sliding interfaces
where A represents the contact area at the sliding interface; 1  and 2  denote the parameters associated with the quasi-static friction force at the zero velocity; P is the contact force normal to the sliding surface; and  is the amplification factor used to describe the increase of friction force with increasing the sliding velocity; and  is the parameter which controls the variation of the friction coefficient with sliding velocity; and b u  is the siding velocity of the base isolator.
The term 12 A AP    is used to describe the friction coefficient at zero velocity. According to the experimental observations, the friction coefficient depends on the sliding velocity. Therefore, the term 11 exp( ) is used to describe the amplification factor of the friction coefficient relative to that at zero velocity. The coefficient, Coef , is a decay function representing the phenomenon of degradation of the friction force with the increase of the number of cyclic reversals. The coefficient of Coef can be shown as: where 1  and 2  are parameters to describe the decay behavior of the friction coefficient at the Teflon interface associated with the energy accumulation in the history of the sliding motion; 0 t F is the friction force when the sliding velocity is equal to zero; t F is the friction force at current time t; and du b is the displacement increment of the base isolator. The effect of the coefficient, Coef, may be neglected for the purpose of engineering practice.  are the friction coefficients at zero sliding velocity and high sliding velocity, respectively. Figure 25 shows the comparison between the theoretical and experimental results. It can be concluded that the numerical result obtained from the mathematical model has good agreement with the experimental results.

Equation of motion for a rigid mass isolated with DO-FPS isolators
As shown in Fig. 26, the time history of ground accelerations can be represented by the method of interpolation of excitation by assuming that the ground motion is a linear variation between time 1 1 () s g n () where m , b c and b k are the mass, damping coefficient and horizontal stiffness of the isolated mass, respectively; b u is the displacement of the base isolator relative to the ground; () b u   is the friction coefficient of the sliding surface, which is a function of sliding velocity; and i g u  is the ground acceleration at time i t . i t  and i t can be given by: The sliding displacement of the base isolator between time 1 i t  and i t can be obtained from Eqs. (6) and (7) as: At the beginning of each time step, the sliding displacement is equal to that at the end of the previous time step, i.e.
The coefficient A of Equation (6) can be obtained as: www.intechopen.com The derivative of Eq. (8) respect to  leads to: The sliding acceleration of the base isolator relative to the ground can be given by:

Condition for non-sliding phase
The kinetic friction coefficient has been considered as the same as the static friction coefficient. Therefore, as the summation of the inertia and restoring forces imposing at the base raft is lower than the quasi-static friction force, i. e.: Then the structure will behave as a conventional fixed base structure, and the sliding displacement, sliding velocity and relative acceleration are:

Initiation of sliding phase
The base isolated rigid mass will behave as a fixed base structure unless the static friction force can be overcome. During the sliding phase, the equation given in the following should be satisfied: Because the time increment adopted in the time history analysis (e.g. 0 0005 .s e c t   ) is quite smaller than that of the sampling time of the earthquake history, it is reasonable to assume that the direction of sliding at the current time step is the same as the previous time step. It should be noted that the direction of sliding remains unchanged during a particular sliding phase. At the end of each time step, the validity of inequality Eq. (15) should be checked. If the inequality is not satisfied at a particular time step, then the structure enter a non-sliding phase and behaves as a fixed base structure.

Simplified mathematical model for DO-FPS isolator
The simplified model based on the equilibrium at the slider of the DO-FPS isolator can be shown in the following. As shown in Fig. 28, horizontal forces 1 F and 2 F imposing at the concave trench and spherical sliding surfaces, respectively, can be expressed as: and where W is the vertical load resulting from the superstructure ; 1 R and 2 R represent the radii of curvature of the concave trench and spherical sliding surfaces, respectively; 1 u and 2 u depict the horizontal sliding displacements of the slider relative to the centers of the concave trench and spherical sliding surfaces, respectively;  represents the friction coefficient for the Teflon composite interface which depends on the sliding velocity; and 1 u  and 2 u  are the sliding velocities of the articulated slider. Rearrangement of Eqs. (16) and (17) where b K represents the horizontal stiffness of the DO-FPS isolator and can be expressed as: Hence, the isolation period of the DO-FPS isolated structure in the direction of the concave trench surface is as follows: www.intechopen.com

Shaking table tests of equipment isolated with DO-FPS isolators
In order to examine the seismic behavior of motion sensitive equipment isolated with a direction-optimized friction pendulum system, a series of shaking table tests were carried out in the Department of Civil Engineering at Feng Chia University, as shown Fig. 31. In this full-scale experiment, a modem rack was adopted to simulate high-technology equipment such as server computers and workstations. The dimensions of the critical equipment were 0.8 m × 0.6 m × 1.98 m (length × width × height). Within the critical equipment were six levels, and lumped masses in the range from 50 kg to 100 kg were placed on these in 10 kg increments from top to bottom. The fundamental period of the critical equipment without isolators was measured in the shaking table test as 0.18 s. In order to prove the benefit provided by the DO-FPS isolator, we used two extreme conditions with angles of 0° and 90°, respectively. The input ground motions included those of the earthquakes at El Centro (USA, 1940), Kobe (Japan, 1995), and Chi-Chi (station TCU084, Taiwan, 1999). Accelerometers and LVDTs were installed to measure the accelerations and displacements of the critical equipment plus DO-FPS isolators when subjected to the various ground motions.   Figures 34(a) and 34(b) show those for the Chi-Chi earthquake (PGA of 0.3g). These results illustrate that the direction-optimized friction pendulum system effectively reduced the responses of the critical equipment by lengthening its fundamental period.  Fig. 40(b). Base isolator displacement of DO-FPS isolator at angle of 0° during 0.3 g Chi-Chi ground motion As shown in Tables 1 and 2, the maximum roof accelerations of critical equipment have been reduced remarkably under various types of earthquakes. Therefore, the DO-FPS isolator can be recognized as an effective tool for upgrading the seismic resistance of high-technology facilities by isolating earthquake induced energy trying to impart into the equipment. This device also supplies significant damping for the isolation system through the frictional force resulting from the sliding motion of the slider on the sliding surface to reduce the isolator displacement and the bearing size.

Discussions
Earthquake proof technologies such as energy absorbing systems and base isolation systems have been accepted as powerful tools to protect structures and equipments from earthquake damage. At the same time, what we should bear in mind is that the design earthquakes utilized for estimating the performance of such technologies in safeguarding structures and equipments still leave room for future research. More recently, a new methodology based on damages indices to obtain design earthquake loads for seismic-resistant design of traditional building structures was proposed by Moustafa (2011). The prerequisite for examining the efficiencies of the earthquake proof technologies in protecting building, bridge, and lifeline structures and equipments is having rational design earthquake loads associated with the occurrence and their characteristics (e.g. time, location, magnitude, frequency content and duration, etc.). More research efforts in this subject are needed to make this possible.

Max. Response
Roof Acceleration (

Conclusions
The isolators presented in this chapter, which provide damping as a result of the deformed material or the frictional force between the sliding interfaces, have rectified the drawbacks of the rolling ball isolation system, such as little damping provided by the system, highly concentrated stress produced by the rolling ball or cylindrical rod due to the small contact area between the rolling ball (or cylindrical rod) and the concave surfaces, and scratches and damage to the concave surfaces caused by the ball or cylindrical rod motions during earthquakes. The presented isolators not only effectively lengthen the natural period of the vibration sensitive equipment but also provide significant damping to reduce the bearing displacement and size and the protection to the contact area between the damped steel ball (or articulated slider) and the concave surface to prevent any damage or scratch on the concave surfaces. Further, the advanced isolators possess a stable mechanical behavior during the life span of service. In addition, the isolators can isolate energy induced by earthquakes to ensure the safety and functionality of the vibration sensitive equipment located in a building. It can be concluded from these studies that the presented isolators in this chapter, including the rolling and sliding types of isolators, exhibit excellent features for preventing vibration sensitive equipment from earthquake damage.