Variable restoring force isolator

Abstract

Variable restoring force (VRF) isolators and related systems and methods adapted to protect equipment from forces such as earthquakes. One embodiment of equipment protection is seismic isolation, where individual pieces of equipment or sets of equipment are isolated from the movement of the building. Critical equipment often houses components that are susceptible to acceleration and can be damaged during an earthquake, so it is necessary to limit the inertial forces that the equipment experiences during the event. This can be achieved using a special isolator having a restoring force that can be determined according to the displacement of the isolator, i.e., a variable restoring force (VRF) isolator. An example of a VRF isolator may have a force-displacement relationship characterized by high stiffness at small and large displacements and low stiffness in between.

Description

[Technical Field]

[0001]

[0001] Exemplary embodiments of the present invention generally relate to systems, devices, and methods adapted to protect equipment while subjected to forces that may occur during an earthquake or the like. [Background technology]

[0002]

[0002] Protecting critical equipment within a building that is excited by an earthquake is essential to maintaining its functionality after the earthquake. For example, electronic equipment and computers are particularly vulnerable to damage during earthquakes or other forces. Other types of equipment may also be damaged during earthquakes and other forces. [Overview of the project] [Means for solving the problem]

[0003]

[0003] Exemplary embodiments of the variable restoring force (VRF) isolator and related systems and methods of the present invention can satisfy the need to protect equipment from forces such as earthquakes. One method of an exemplary embodiment for protecting equipment is seismic isolation, in which individual pieces of equipment or sets of equipment are isolated from the movement of the building. Critical equipment often houses acceleration-sensitive components that can be damaged during an earthquake, and therefore it is necessary to limit the inertial forces that the equipment experiences during the event. This can be achieved using a special isolator having a restoring force that can be determined according to the displacement of the isolator, i.e., a variable restoring force (VRF) isolator. The VRF isolator has a force-displacement relationship characterized by high stiffness at small and large displacements and low stiffness in between. To verify the concept of the VRF isolator, a small prototype was constructed and its force-displacement characteristics were observed with respect to repeated loads. A model was developed to predict the force-displacement relationship of the VRF isolator with good accuracy. Numerical analysis was performed on horizontal seismic isolation of equipment installed on different floors of a multi-story building excited by an earthquake. The system was input to floor acceleration records from historical ground motions, and the performance of the VRF isolator was evaluated based on the absolute acceleration response of the seismically isolated equipment.

[0004]

[0004] In addition to the novel properties and advantages mentioned above, other benefits will be readily apparent from the following description of the drawings and exemplary embodiments. [Brief explanation of the drawing]

[0005] [Figure 1]

[0005] Figure 1(a) shows an example of use of an exemplary embodiment of the VRF isolator. Figure 1(b) is a schematic diagram of the VRF isolator viewed alone. [Figure 2]

[0006] Figure 2(a) shows the ideal force-displacement curve. Figure 2(b) shows the force-displacement curve for an example of a prototype VRF isolator. [Figure 3]

[0007] It is a photograph of a part of an exemplary component of a VRF isolator. [Figure 4]

[0008] Figure 4(a) is a photograph of a part of the VRF isolator of Figure 3, with an example of the upper rail at the starting position. Figure 4(b) is a photograph of a part of the VRF isolator of Figure 3, with an example of the upper rail moving to the left. [Figure 5]

[0009] It is a schematic diagram of an example of the upper rail, cable, and pulley of an exemplary embodiment of a VRF isolator. [Figure 6]

[0010] Figure 6(a) is a schematic diagram of an example of a reel of an exemplary embodiment of a VRF isolator. Figure 6(b) is a schematic diagram of an example of a gear train of an exemplary embodiment of a VRF isolator. Figure 6(c) is a schematic diagram of an example of a VDS of an exemplary embodiment of a VRF isolator. Figure 6(d) is a schematic diagram of an example of a VPS of an exemplary embodiment of a VRF isolator. [Figure 7]

[0011] It is a schematic diagram of an example of a VDS for defining a variable radius. [Figure 8]

[0012] It is a graph of dimensionless force versus displacement for an example of a VRF isolator considering various values of β. [Figure 9]

[0013] Figure 9(a) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time with respect to the ground. Figure 9(b) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time with respect to the third floor. Figure 9(c) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time for Case 1. Figure 9(d) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time for Case 2. Figure 9(e) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time for Case 3. Figure 9(f) is a graph of exemplary variations of absolute acceleration and instantaneous period versus time for Case 4. [Figure 10]

[0014] Figure 10(a) is an exemplary force-displacement loop graph for an exemplary embodiment of the VRF isolator in Example 1. Figure 10(b) is an exemplary force-displacement loop graph for an exemplary embodiment of the VRF isolator in Example 2. Figure 10(c) is an exemplary force-displacement loop graph for an exemplary embodiment of the VRF isolator in Example 3. Figure 10(d) is an exemplary force-displacement loop graph for an exemplary embodiment of the VRF isolator in Example 4. [Figure 11]

[0015] Figure 11(a) is a graph of exemplary fluctuations in absolute acceleration and instantaneous period over time for a rooftop example with a 20% attenuation in the EW direction. Figure 11(b) is a graph of exemplary fluctuations in absolute acceleration and instantaneous period over time for a piece of equipment example with a 20% attenuation in the EW direction. [Figure 12]

[0016] Figure 12(a) is a graph of exemplary fluctuations in absolute acceleration and instantaneous period over time for a rooftop example with 10% attenuation in the NS direction. Figure 12(b) is a graph of exemplary fluctuations in absolute acceleration and instantaneous period over time for a piece of equipment example with 10% attenuation in the NS direction. [Figure 13]

[0017] Figure 13(a) is an exemplary force-displacement graph for an exemplary embodiment of a VRF isolator in the case of 20% damping in the EW direction. Figure 13(b) is an exemplary force-displacement graph for an exemplary embodiment of a VRF isolator in the case of 10% damping in the NS direction. [Figure 14]

[0018] This is a schematic diagram of another exemplary embodiment of a VRF isolator. [Modes for carrying out the invention]

[0006]

[0019] Exemplary embodiments of the present invention relate to devices, as well as related systems and methods for protecting equipment from undesirable forces such as earthquakes.

[0007]

[0020] Figure 1a shows a diagram of an equipment seismic isolation platform having an example of the VRF isolator 10 of the present invention installed in the seismic isolation layer. As shown in Figure 1b, the VRF isolator 10 comprises a base plate 20 on which the VRF device 30 is mounted together with a mechanism of rails 40 (i.e., a lower rail 42 and an upper rail 44) and sliders 50 (i.e., sliders 52 and sliders 54). The upper rail 44 is mounted on the slider 50 and the slider 50 is mounted on the lower rail 42 so that the upper rail 44 can move in the direction of horizontal motion. The VRF device 30 is connected between the base plate 20 and the upper rail 44, at which point the upper rail 44 is connected to the bottom of the seismic isolation platform. The VRF isolator 10 is adapted to generate a horizontal force, i.e., a restoring force, on the seismic isolation platform that always works to restore the seismic isolation platform to its original position.

[0008]

[0021] The aforementioned seismic isolation platform can be considered to be in the system together with the VRF isolator 10. This example of the seismic isolation platform is equipped with rollers. Other embodiments of the seismic isolation platform may have any suitable characteristics that facilitate the seismic isolation of the equipment and its operation with the VRF isolator.

[0009] Variable Restoring Force (VRF) Isolator Outline

[0022] The VRF isolator 10 is adapted to produce a trilinear elastic restoring force with different load-unloading paths. An exemplary force-displacement curve for the VRF isolator 10 is shown in Figure 2(a), where three different regions of the curve are identified. When the VRF isolator displacement is small (region 1), high stiffness (k1) ensures the stability of the seismic isolation system under working loads. When the VRF isolator displacement is large (region 3), high stiffness (k3) limits excessive displacement of the isolator due to extreme seismic loads. Between small and large displacements of the VRF isolator (region 2), low stiffness (k2) limits the acceleration of the seismically isolated equipment during design-level earthquakes. It is shown that the force-displacement characteristics of the VRF isolator 10 can be controlled to produce a wide range of isolator responses by appropriate selection of the geometry of its components. Other characteristics of the VRF isolator 10 in this example are as follows:

[0010]

[0023] • Restoring force is not affected by payload weight.

[0011]

[0024] • The compact design allows for the installation of isolators on upper floors while accommodating large displacements of the isolators.

[0012]

[0025] • Robust against ground motion features to allow for the installation of isolators on lower floors.

[0013]

[0026] • To minimize costs and improve reliability, basic materials and simple mechanical components are used.

[0014]

[0027] • To eliminate the need to replace components after an event, the restoring force element operates within its elastic range.

[0015]

[0028] • It has a low profile to reduce the platform height.

[0016] Description of the prototype

[0029] To present a proof of concept for the VRF isolator, 100 small prototypes were constructed and tested. A photograph of prototype 100 is shown in Figure 3, where all relevant components are identified. The components are described below. A constant force spring (CFS) 110 generates a nearly constant force while the spring deforms. A variable pitch shaft (VPS) 120 has grooves 122 of a constant diameter and spiral shape along a given length of the shaft. The distance between the grooves, i.e., the pitch, varies with respect to the length of the shaft. The remaining length of the shaft is smooth (without grooves). The variable diameter shaft (VDS) 130 has a variable diameter and shaft length, with a helical groove 132 extending from it. The groove pitch is constant, and the shaft diameter changes with respect to the shaft length. Cable 2 140 connects VPS120 to VDS130. The gear train 150 connects the VDS 130 to the storage reel 160. Cable 170 is stored on a storage reel, extends between pulleys 180, and is attached to the upper rail 190. The upper rail 190 is mounted on the slider 200, and the slider 200 is mounted on the lower rail 210 (only one rail / slider 210 is shown), which allows for unidirectional horizontal movement.

[0017] operation

[0030] The restoring force of the VRF isolator 100 is generated by winding cables 112 and 114 from a constant force spring (CFS) 110 around the smooth portion of the variable pitch shaft (VPS) 120. This creates a constant torque that resists rotation of the shaft 120 in one direction, resulting in a constant tension on cable 2 140. The constant force on cable 2 140 generates a torque on the variable diameter shaft (VDS) 130, which varies with diameter. The variable torque on VDS 130 is amplified through the gear train 150 and reaches the storage reel 160, resulting in a variable tension on cable 1 170, which is transmitted to the upper rail 190.

[0018]

[0031] The relationship between the force transmitted to the upper rail 190 and the displacement of the upper rail is determined by the position of cable 2 140 along VDS130 and VPS120. The initial position of cable 2 140 is at the closer end of VDS130 and VPS120 (the smaller end of VDS130 in Figure 3). As the upper rail 190 moves away from its starting position, VDS130 rotates, and this rotation causes VPS120 to rotate via cable 2 140. As VDS130 and VPS120 rotate together, cable 2 140 wraps around VDS130 and unwounds around VPS120, moving from the closer end of the shaft to the farther end (from the smaller end of VDS130 to the larger end in Figure 3). As cable 2 140 moves to a larger diameter of VDS130, the torque on the shaft and the force transmitted to the upper rail 190 increase. When the upper rail 190 changes direction and returns to its starting position, the cable 2 140 returns on the shafts of the VDS130 and VPS120, reducing the force on the upper rail 190.

[0019]

[0032] As cable 2 140 moves to a larger diameter than VDS 130, VPS 120 rotates faster than VDS 130. To maintain the orientation of cable 2 140 relative to the long axes of VDS 130 and VPS 120, and to ensure that cable 140 stays securely in the shaft grooves 122, 132, the pitch of VPS 120 must decrease along the length of the shaft, i.e., it is a variable pitch shaft (VPS) 120. Figure 4(a) shows a VRF isolator prototype 100 with the upper rail 190 and cable 2 140 in the starting position. Figures 4(b) and 3 show cable 2 140 moving from the near end to the far end of VDS 130 and VPS 120 as the upper rail 190 moves to the left and right of the starting position, respectively.

[0020] Experimental verification

[0033] The VRF isolator prototype 100 underwent sinusoidal displacement with constant amplitude and frequency. The force-displacement of the upper rail for 10 cycles of motion is plotted in Figure 2(b), which shows that the VRF isolator 100 works as intended. That is, the force-displacement characteristics are defined by high stiffness at small and large displacements, as well as lower stiffness in between. Figure 2(b) shows some periodic fluctuations in the force of the VRF isolator during loading and unloading. This can be attributed to defects in the VDS and VPS shafts due to the 3D printing process. Such defects can be eliminated by machining the VDS130 and VPS120 from a harder material instead of using 3D printing.

[0021]

[0034] In Figure 2(b), the difference between the load-unload curve and the unload-relief curve of the VRF isolator 100 can be attributed to friction within and between the components of the prototype. When the upper rail 190 moves away from the starting position, friction increases the force on the upper rail 190, and when it moves back towards the starting position, it decreases the force on the upper rail 190. Friction within and between the components of the prototype leads to a significant amount of damping, which tends to determine the force-displacement response shown in Figure 2(b). However, this is not expected to apply to the full-scale isolator for the following reasons: (1) the restoring force from the constant-force spring (CFS) is significantly higher, and (2) improved materials, fabrication, and component positioning reduce friction within and between the components.

[0022]

[0035] In a full-scale isolator, a decrease in component friction leads to a decrease in damping, necessitating the incorporation of an auxiliary damping element. For this purpose, a mechanical slip clutch is mounted on the base plate and connected to the end of the VPS. The slip clutch converts the rotation of the VPS into a constant torque generated by friction within the clutch. The friction added to the full-scale isolator has the same effect as the friction of the prototype components, namely increasing the force on the upper rail during loading and decreasing the force on the upper rail during unloading. Mechanical slip clutches are available in a wide range of torques to meet the damping requirements for a full-scale isolator.

[0023] Model

[0036] To derive a numerical formula describing the force-displacement relationship for the VRF isolator, we first consider only the upper rail, cable, and pulley shown in Figure 5. Note that only one pulley is shown for clarity. We provide a derivation for the displacement of the upper rail in the positive x-direction (to the right in Figure 5). Furthermore, we ignore friction and inertia between the isolator components. Cable 1 is initially in a vertical position, with one end attached to the upper rail at A and the other end in contact with a reel (not shown) at B. As the upper rail moves from point A to A', the cable wraps around the pulley, and the end in contact with the reel moves from point B to B'. The horizontal force F acting on the upper rail is the force F acting on cable 1. c1 It can be expressed as follows:

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[0024]

[0037] In Figure 5, the force F acting on the pulley is applied to cable 1. c1 The force F acts on the reel in Figure 6(a). The reel is then connected to the VDS in Figure 6(c) through the gear train in Figure 6(b), and the VDS is connected to the VPS in Figure 6(d) through cable 2. The VPS is connected to a constant force spring and a mechanical slip clutch, which generate the restoring force and damping force of the isolator, respectively. c1 Torque T generated by constant force spring and slip clutch S and T D The relationship is,

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[0025]

[0038] The force-displacement characteristics of the VRF isolator are derived from the variable torque applied to the VDS, and the variable torque changes according to the change in the diameter of the shaft along the length of the shaft. The radius r VD of the shaft varies with respect to the length of the shaft and can be defined as a function of the rotation θ VD of the shaft. Consider the diagram of the VDS shown in FIG. 7. When the VDS has a base radius

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[0026] design

[0039] The variable restoring force of the VRF isolator is mainly determined by the radius r of the pulley. p and its position d relative to the upper rail, as well as the variable diameter of the VDS, vary. In order to determine the design parameters that lead to the desired force-displacement characteristics of the VRF isolator, it is convenient to first express the force acting on the upper rail in the following dimensionless form.

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[0027]

[0040] Pulley radius r relative to the restoring force p And in order to exclude the influence of position d, gradient m over all n sections of VDS. i Set it so that it is equal to zero (C1=C2=0), and simplify the dimensionless force acting on the upper rail.

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[0028]

[0041] Figure 8 shows the initial dimensionless stiffness of the VRF isolator.

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[0029]

[0042] The stiffness of a VRF isolator with respect to the subsequent region of the force-displacement curve can be designed by considering the asymptotic properties of the dimensionless force F* shown in Figure 8, which become approximately constant with respect to the value of β after a certain value of dimensionless displacement. For the region of the curve where the dimensionless force F* is approximately constant, the stiffness of the isolator is determined by the gradient m² with respect to the rest of the shaft to m² to achieve the desired stiffness. n By selecting this option, it is possible to design using VDS geometry. In those regions, dimensionless forces

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[0030]

[0043] Equations 18 and 21 show that the parameters of the VRF isolator can be selected to achieve the desired stiffness for different ranges of isolator displacement. Geometric parameters (i.e., d, r) p , r R , r o , r VP ,l VD Many of these are constrained by the need to make the isolator compact, but the applied torque T S and T DThe gear ratio f also has significant degrees of freedom, because its components are readily available in a wide range of capabilities.

[0031]

[0044] Equations 18 and 21 show that the stiffness of the VRF isolator over different ranges of isolator displacement is determined by the restoring torque T from the constant force spring. s and damping torque T from the mechanical slip clutch D This shows that it changes depending on the sum of the following. During the load, the damped torque T D The restored torque T s It is added to (that is, in equations 18 and 21, T D >0), which increases the stiffness of the isolator. During unloading, the damping torque T D The restored torque T s It is subtracted from (that is, in equations 18 and 21, T D <0), which reduces the stiffness of the isolator. The difference between the stiffness under load and the stiffness when unloaded indicates the amount of damping in the isolator (see Figure 2), and this amount is the damping torque T D This can be controlled by increasing or decreasing the damping. One of the advantages of a VRF isolator is that the damping can be increased or decreased without changing the stiffness under load. This can be achieved by proportionally increasing and decreasing the damping torque and restoring torque so that the total torque during the load remains constant. As a result, the stiffness and damping of the isolator under load become independent of each other. Damping torque T that can be applied to the isolator D Please note that there are practical limits to the amount of damping torque T. To ensure that cables 1 and 2 are always under tension, damping torque T D The restoring torque T s It should be smaller than (T D <T s ).

[0032]

[0045] Figure 2b shows a force-versus-displacement plot generated using the VRF isolator model, which can be compared with experimental results from a prototype VRF isolator. The model was adjusted to reflect the friction observed in the prototype during testing until a good agreement between the model and the experiment was achieved. The comparison of the model and prototype results for the VRF isolator demonstrates that the model can predict the forces of the VRF isolator with good accuracy.

[0033] Numerical analysis

[0046] Numerical analyses were performed to evaluate the effectiveness of horizontal seismic isolation of equipment in two application examples of VRF isolators. In each application example, the equipment to be seismically isolated was mounted on the floor of the building and modeled as a single-degree-of-freedom (SDOF) system, subject to only one horizontal direction of the building's movement. The governing equations for the system's motion were:

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[0034]

[0047] The design of the VRF isolators for each application was determined from parameter studies, in which the peak displacement and absolute acceleration response of the equipment were obtained for various values ​​of the initial stiffness of the isolator (i.e., restoring torque and damping torque). The input for this study was a historical record of floor acceleration corresponding to the location of the building where the seismic isolation system was assumed to be installed. The initial stiffness and damping for each VRF isolator were selected considering a balance between reducing the peak absolute acceleration of the equipment and reducing the displacement response. The values ​​selected for each application are presented in subsequent chapters, along with a description of the input for the building where the seismic isolation system was installed and the floor acceleration.

[0035] A five-story reinforced concrete building

[0048] The first numerical application example is based on a full-scale, five-story reinforced concrete (RC) building tested on an outdoor shaking table at the University of California, San Diego (UCSD). The building was equipped with numerous nonstructural components and systems (NSCS), including a computer server rigidly connected to the third floor. In particular, experimental results for a fixed-foundation building subjected to 100% of the recorded ground acceleration values ​​from the Pisco-Chincha (Ica) earthquake showed that the top of the computer server experienced a peak acceleration of 0.81g, which was more than twice the peak floor acceleration (PFA) of 0.37g measured on the third floor during the same vibration.

[0036]

[0049] In the numerical analysis, a horizontal seismic isolation system with a VRF isolator was installed on the 3rd floor of a UCSD building and subjected to the same floor acceleration record value (PFA = 0.37g) from the Pisco-Chincha earthquake. Table 1 shows the results of parameter studies determining the initial stiffness and damping of the VRF isolator for four different cases. The four cases were based on the peak displacement (x max) corresponds to 20 cm (8 inches) and 25 cm (10 inches) for 10% damping, and 18 cm (7 inches) and 25 cm (10 inches) for 20% damping. The table also shows parameter x1, which is the displacement of the VRF isolator that defines the initial stiffness. The distance was chosen to be small as a strategy aimed at expanding the range of displacements over which the VRF isolator operates with zero tangential stiffness, i.e., reducing the acceleration response of the equipment.

[0037]

[0050] Table 1. Parameters and instrument peak response of the VRF isolator on the 3rd floor of the UCSD building when PFA = 0.37g. [Table 1]

[0038]

[0051] Table 1 shows the results of a numerical analysis for a five-story reinforced concrete building with a PFA of 0.37g on the third floor. These results include the peak absolute acceleration and displacement of the equipment for each case examined. The reduction rate of the peak absolute acceleration of the equipment is also presented. The reduction rate R3 is calculated for the peak absolute acceleration on the third floor, and the reduction rate R c This is calculated for the peak absolute acceleration at the top of the computer server, measured during the UCSD experiment (R c The reduction rate R3 ranged from 84% in Case 1 to 95% in Cases 2 and 4, indicating that the VRF isolator limited the transmission of floor acceleration to the equipment. c A greater reduction was observed for this, with values ​​ranging from 93% to 98% in the same cases.

[0039]

[0052] When seismic excitations propagate through a building, they generate floor acceleration, which is then transmitted to any equipment mounted on the floor. Without a seismic isolation system, the building vibrates near its fundamental frequency, amplifying the effects of the earthquake, potentially resulting in large accelerations for the equipment. As previously shown, the peak absolute acceleration measured on top of a computer server mounted on the floor during the UCSD experiment was 0.81g, which was more than twice the peak absolute acceleration measured on the third floor. However, numerical analysis results show that when the VRF isolator is properly positioned, the flexible system and damping elements absorb most of the seismic energy, reducing the amplitude of acceleration transmitted to the equipment. This is further illustrated by the time-history absolute acceleration plots for the ground, third floor, and equipment shown in Figure 9. A comparison between Figure 1(a) and Figure 1(b) shows that ground acceleration was amplified and transmitted to the third floor of the UCSD building, where the peak absolute acceleration increased from 0.20g to 0.37g. At that time, the peak absolute acceleration on the third floor was amplified and transmitted to the top of the computer server, where a peak absolute acceleration of 0.81g was measured. However, a comparison of Figure 1(cf) and Figure 1(ab) shows that the transmission of the absolute acceleration from the third floor to the seismically isolated equipment was limited by the VRF isolator, and in all cases examined in the analysis, the peak absolute acceleration received by the equipment was 0.06g or less.

[0040]

[0053] For each time history of the absolute acceleration shown in Figure 9, there is a corresponding time history of the instantaneous period of the signal, along with the mean period. Overall, these time histories demonstrate that the proposed VRF isolator deforms the seismic acceleration profile from a signal with high amplitude and frequency to a signal with quasi-constant amplitude and low frequency. The low frequencies in the absolute acceleration response of the equipment shown in Figure 9 are due to the decrease in the effective stiffness of the VRF isolator when it vibrates in the zero-stiffness region of the force-displacement curve. The larger the displacement response of the isolator, the lower the effective stiffness and the higher the effective period. Interestingly, for each VRF isolator in Figure 9, the effective period begins to increase from around 25 s, which corresponds to the time when the intensity of the floor vibration is maximum (see Figure 9(b)). During this intense vibration, the isolator displaces significantly, its effective stiffness decreases, and its effective period lengthens. Therefore, during the period of most intense floor acceleration, the VRF isolator acts as a low-pass filter, allowing low-frequency vibrations to pass through while attenuating excitations at higher frequencies. As a result, the absolute acceleration response of the equipment has amplitude and frequency components lower than the floor acceleration. The point to note is that the absolute acceleration of the isolator is proportional to its restoring force, which has a constant amplitude in the zero-stiffness region of the force-displacement curve, thus suppressing the amplitude of the VRF isolator response. This is shown in the force-displacement loops for each VRF isolator in Figure 10, where a constant amplitude is observed in the zero-stiffness region of the curve.

[0041] 10-story reinforced concrete building

[0054] The second numerical application example is based on a 10-story reinforced concrete commercial building located in San Jose, California, equipped with a measurement substrate under the Strong Motion Instrumentation Program (CSMIP, Station 57355). The building was determined to have a basic period of 0.97s in the north-south (NS) direction and 0.85s in the east-west (EW) direction, with torsional effects minimized. Three VRF isolators are defined in each direction to protect equipment on the building's roof, fifth floor, and basement.

[0042]

[0055] Table 2 shows the results of a parameter study determining the parameters of the VRF isolator on each floor and in each direction of a 10-story reinforced concrete building. The table also includes the peak absolute acceleration and displacement of the equipment, the peak restoring force of the isolator, and the reduction rate between the peak absolute acceleration of the equipment and the floor. The parameters of the VRF isolator were selected considering the balance between the reduction of the peak absolute acceleration of the equipment and the reduction of displacement. Interestingly, the value of the initial stiffness k1 of the isolator was found to be higher in the NS direction than in the EW direction on the 5th floor (5) and rooftop (R), and the opposite was true in the basement (B). Furthermore, with 20% damping, the peak absolute acceleration of the equipment was lower on the upper floors in the EW direction than with 10% damping, but in the NS direction, the peak absolute acceleration of the equipment was higher on the same floor than with 10% damping on the same floor. On the other hand, with 20% damping, the peak absolute acceleration of the equipment was lower in the basement in both ways than with 10% damping.

[0043]

[0056] According to the reduction rates of the peak absolute acceleration response of the equipment shown in Table 2, the VRF isolator limits the transmission of floor acceleration to the equipment. The greatest reductions were observed on the 5th floor and rooftop in the EW direction of the building, with reduction rates of 75% and 71%, respectively. In the NS direction, the reduction rates were lower on the upper floors, ranging from 26% to 46%. In the basement, the reduction rates were similar in the EW and NS directions, with a reduction rate of approximately 25% in the case of 10% attenuation and a reduction rate of 54% in the case of 20% attenuation.

[0044]

[0057] Table 2. Parameters and peak response of the VRF isolator for each floor and direction in a 10-story reinforced concrete building. [Table 2]

[0045]

[0058] Figures 11 and 12 show the absolute acceleration and instantaneous period of the rooftop and equipment in the EW and NS directions. Since this layer experienced the highest intensity vibration, and the VRF isolators achieved a significant reduction in the peak acceleration response, the time history at the rooftop is shown. Furthermore, only the VRF isolators that achieved the greatest reduction in each direction—that is, isolators with 20% damping in the EW direction and 10% damping in the NS direction—are shown. The force-displacement plots for these VRF isolators are also presented in Figure 13. A comparison of the absolute acceleration responses of the rooftop and equipment in both directions again shows that the VRF isolators transformed high-amplitude and high-frequency floor accelerations into lower-amplitude and low-frequency equipment accelerations. As explained earlier, the suppressed amplitude is due to a constant VRF isolator force (see Figure 13), and the lower frequency is due to a reduction in the effective stiffness of the isolator in the zero-stiffness region of the force-displacement curve. From the time history of the instantaneous period of equipment acceleration, it was observed here again that the period shift corresponds to the floor vibration of the most severe earthquake.

[0046] Alternative Embodiments of VRF Isolators

[0059] In an alternative embodiment of the VRF isolator 300, as shown in Figure 14, the VPS, VDS, cable 2, gear train, reel, and cable 1 are removed from prototype 100, leaving only the base plate 310, constant force springs (CFS) 320 (each with a CFS drum 322), mechanical slip clutch (MSC) 330, pulley 340, lower rail 350, slider 360, and upper rail 370. A second set of pulleys 342 is also added, and a total of four pulleys 340, 342 are mounted directly to the base plate 310 such that the faces of the pulleys 340, 342 are parallel to the faces of the base plate 310 (these faces are perpendicular in Figure 3). One CFS 320 and one MSC 330 are located on either side of the pulleys 340, 342, respectively. The CFS320 is mounted on the base plate 310 and the MSC330 is mounted on the CFS320 so that the rotation of the CFS drum 322 causes the same rotation within the MSC330. The cables 380 and 390 of the CFS320 extend between the pulleys 340 and 342, respectively, and are attached to the column 400 that extends downward from the upper rail 370.

[0047]

[0060] When the upper rail 370 is displaced in the seismic isolation direction, the cables 380 and 390 wrap around the pulleys 340 and 342 (see Figure 5) from the constant force spring 320, causing the CFS drum 322 to rotate. As the CFS drum 322 rotates, the MSC 330 rotates in response, and the resulting force on the CFS cables has a restoring component due to the spring 320 and a damping component due to the friction of the clutch 330. The force from the MSC 330 increases the restoring force when the upper rail 370 moves away from the starting position and decreases the restoring force when the upper rail 370 moves toward the starting position.

[0048]

[0061] In this embodiment of the VRF isolator 300, the horizontal force acting on the upper rail 370 is due to the geometry of the pulleys 340 and 342, that is, the radius r of each of the pulleys 340 and 342. pAnd it changes with respect to the displacement of the upper rail based on the respective distances d (shown in Figure 14) from the centers of pulleys 340 and 342 to the CFS cable mounting point on column 400. This relationship is explained by equations 1 and 2, where force F c1 This represents the force acting on cables 380 and 390 of the CFS320 due to the deformation of springs 320 and MSC330. The force-displacement curve of the alternative embodiment of the VRF isolator 300 is characterized by the isolator force being large in the range where the isolator displacement is small, and then the force becoming constant. These characteristics are the same as those of the VRF isolator model examined in the numerical analysis described earlier (see Figures 10 and 13). Therefore, the same seismic isolation performance as observed in the numerical analysis can be expected even when using the alternative embodiment.

[0049]

[0062] Any embodiment of the present invention may include any optional or preferred characteristics of other embodiments of the present invention. The exemplary embodiments disclosed herein are neither exhaustive nor unnecessarily limiting the scope of the present invention. Exemplary embodiments have been selected and described to illustrate some of the principles of the present invention so that those skilled in the art can carry out the present invention. While exemplary embodiments of the present invention have been shown and described, those skilled in the art will understand that many variations and modifications can be made to the described invention. Many of these variations and modifications lead to the same results and are included in the spirit of the claimed invention. Therefore, it is not intended to limit the present invention only as indicated by the claims.

Claims

1. A system adapted to provide seismic isolation with variable restoring force, A set of lower rails comprising a first lower rail and a second lower rail, A set of sliders comprising a first slider and a second slider, wherein the first slider is fitted to slide on the first lower rail and the second slider is fitted to slide on the second lower rail, An upper rail, the upper rail being associated with the set of sliders such that it extends between the first slider and the second slider, A variable restoring force device, wherein the variable restoring force device is associated with the upper rail such that the upper rail is adapted to slide against the set of lower rails, and the variable restoring force device is adapted to provide force to the upper rail; A system equipped with these features.

2. The system according to claim 1, further comprising a seismic isolation platform, wherein the upper rail is associated with the seismic isolation platform so as to be adapted to move together with the upper rail.

3. The variable restoring force device is A spring adapted to produce a virtually constant force, A first shaft having a variable pitch and associated with the spring, A second shaft having a variable diameter and associated with the first shaft and The system according to claim 1, comprising:

4. The variable restoring force device is A spring adapted to produce a virtually constant force, A mechanical slip clutch associated with the aforementioned spring and The system according to claim 1, comprising:

5. The system according to claim 1, further comprising at least one additional upper rail adapted to operate similarly to the upper rail.

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