The present invention is intended to provide a seismic isolation device which, regardless of a load to be supported, is capable of reducing acceleration through lengthening a natural period of an upper structure, with excellent vibration energy absorbing capacity by obtaining a properly preferable frictional force as well as a returning characteristic, and others.

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to International Application No. PCT/JP2008/068949 filed on Oct. 20, 2008, which claims priority to Japanese Patent Application No. 2007-274907 filed on Oct. 23, 2007. 
     
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Technical Field of the Invention 
         [0004]    The present invention relates to a seismic isolation device for light weight structures, such as detached houses, equipments and display stands, and a seismic isolation structure with the seismic isolation device, and more particularly to a seismic isolation device, which is disposed between an upper structure such as a building and a lower structure like a foundation, for reducing transmission of a vibration of the lower structure caused by earthquake, traffic or others to the upper structure and for preventing, for instance, collapse of the upper structure or damage of displayed objects mounted on the upper structure, and so on. 
         [0005]    2. Description of the Related Art 
         [0006]    As a seismic isolation device that is mounted between an upper structure such as a building and a lower structure such as a foundation to reduce vibration of the upper structure, a device described in Patent document 1 is known. This seismic isolation device is provided with an upper plate fixed to the upper structure and having a spherically concave lower surface with a downward opening; a lower plate fixed to the lower structure and having a spherically concave upper surface with an upward opening; and a sliding body slidably disposed between the upper plate and the lower plate and having a spherically convex upper surface and a spherically convex lower surface. 
         [0007]    In the seismic isolation device described in Patent document 1, when a horizontal vibration (displacement) occurs to the lower structure due to an earthquake, sliding of the sliding body is able to prevent the horizontal vibration of the lower structure from being transmitted to the upper structure as it is, which can reduce the vibration of the upper structure. In this seismic isolation device, regardless of a load to be supported by the seismic isolation device, based on pendulum theory, a natural period of the upper structure can be lengthened, in addition, frictional resistance forces generated at contact faces between the upper plate and the sliding body, and between the lower plate and the sliding body to obtain damping action to vibration energy. 
         [0008]    Further, as a seismic isolation device floating an upper structure with gas pressure, a seismic isolation device described in Patent document 2 is provided with a skirt-like seal member on a lower face of the upper structure; a tip of the seal member hangs down in a channel formed on a lower structure to hermetically seal a lower space of the upper structure; in the hermetically sealed space is introduced gas for floatation to float the upper structure, which prevents vibration caused by an earthquake from being transmitted to the upper structure. In addition, as this kind of seismic isolation device, in Patent 3 is described a seismic isolation device in which on an accommodation box in which an air supplying means is accommodated is mounted a placing table for placing a displayed object; between the accommodation box and the placing table is arranged an air pad with plural squarely disposed air blowout ports; and from the air blowout ports are jetted air to float the placing table, which prevents the placing table from vibrating at an earthquake. 
         [0009]    Patent document 1: Japan utility model registration No. 2586794 gazette 
         [0010]    Patent document 2: Japanese patent publication No. 2000-145885 gazette 
         [0011]    Patent document 3: Japanese patent publication No. 2001-208131 gazette 
       SUMMARY OF THE INVENTION 
       [0012]    In the seismic isolation device described in Patent document 1, the frictional coefficient of the slide face is uniquely determined in accordance with the materials of the sliding body and the upper and lower structures, and a load to be supported, so that adjustable range of seismic isolation characteristic is narrow as a problem. As a result, when a small frictional force or a large frictional force is required, the material of the sliding face should be changed, and in case that the material of the sliding face is changed, it is necessary to sufficiently consider generation of an excess resistance force caused by static frictional coefficient that may be generated at the start of a slide, which provides difficulty in selection of the material. 
         [0013]    On the other hand, in the seismic isolation devices described in Patent documents 2, 3, although large effect is expected in that vibration caused by an earthquake is isolated, in order to effectively use air pressure, accurate sealing technique is required, and when the upper structure floats the friction becomes zero, unless construction accuracy to the lower structure is considerably precisely carried out, position of the upper structure at the floatation becomes unstable, and another device is necessary to return the upper structure to an original position after the vibration due to an earthquake ceased, which necessitates a large-scale facility, resulting in increased cost. Further, the seismic isolation devices described in Patent documents 2, 3 have a defect that a seismic isolation mechanism itself does not act at all when the air supplier breaks or stops due to power failure, etc. 
         [0014]    The present invention has been made in consideration of the above problems, and the object thereof is to provide a seismic isolation device, regardless of a load to be supported, capable of reducing acceleration through lengthening a natural period of the upper structure, with excellent vibration energy absorbing capacity by obtaining a properly preferable frictional force as well as a returning characteristic, and so on. 
         [0015]    To achieve the above object, the present invention relates to a seismic isolation device, and the device is characterized by comprising: an upper plate fixed to an upper structure and having a spherically concave lower surface with a downward opening; a lower plate fixed to a lower structure and having a spherically concave upper surface with an upward opening; and a movable body movably arranged between the upper plate and the lower plate, wherein the movable body comprises a spherically convex upper surface opposing to the spherically concave lower surface of the upper plate; and a spherically convex lower surface opposing to the spherically concave upper surface of the lower plate, and the spherically convex upper surface or/and the spherically convex lower surface has plural air blowout ports, and the seismic isolation device switches whether or not air is blown out of the plural air blowout ports, or selects one of the blowing air out of the plural air blowout ports, absorbing air from the plural air blowout ports and not performing both of the blowing and absorbing air to change frictional forces between the movable body and the upper plate, and between the movable body and the lower plate, and changes, in accordance with a magnitude of a horizontal external force added to the lower plate or a magnitude of a vibration transmitted to the upper structure, a horizontal force against the external force or the vibration. 
         [0016]    With this invention, when the lower structure horizontally vibrates (displaced) due to an earthquake or the like, the movable body moves along the spherically concave lower surface of the upper plate and the spherically concave upper surface of the lower plate while supporting load between the spherically concave lower surface of the upper plate and the spherically concave upper surface of the lower plate, which can prevent the horizontal vibration of the lower structure from being transmitted to the upper structure as it is, in addition, regardless of a load to be supported by the seismic isolation device, a natural period of the upper structure can be lengthened based on pendulum theory, which allows acceleration to the upper structure to be reduced. 
         [0017]    Further, with the present invention, jetting air from the air blowout ports forms an air layer on a surface of the sliding body, or air pressure reduces load of the upper structure supported by the seismic isolation device, so that the movable body can be moved under the condition that no frictional force is generated or frictional force is small. On the other hand, stopping air from the air blowout ports and contacting the movable body to the spherically concave lower surface of the upper plate and the spherically concave upper surface of the lower plate allows the movable body can be slided by a frictional force that is determined by the materials of the upper plate and the lower plate, and a load to be supported. As a result, the frictional force can be controlled without changing the materials of the movable body and others, which makes it possible to obtain a properly preferable frictional force to secure excellent vibration energy absorbing capacity. 
         [0018]    Still further, with the present invention, after the earthquake or the like is ceased, the movable body that was moved while receiving external force gradually moves in a reverse direction along inclinations of the spherically concave lower surface of the upper plate and the spherically concave upper surface of the lower plate, and finally returns to the center, so that without another original point returning device, the movable body can be returned to the original point, that is, a returning characteristic can also be secured. 
         [0019]    In addition, the present invention relates to a seismic isolation device, and the device is characterized by comprising: an upper plate fixed to an upper structure; a lower plate fixed to a lower structure and having a spherically concave upper surface with an upward opening; a movable body movably arranged on the lower plate; and a pillar-shaped connection part arranged between the movable body and the upper plate, an upper end of the connection part pivoted on the upper plate and a lower end of the connection part fixed to the movable body, wherein the movable body comprises a spherically convex lower surface opposing to the spherically concave upper surface of the lower plate, and plural air blowout ports on the spherically convex lower surface, and the seismic isolation device switches whether or not air is blown out of the plural air blowout ports, or selects one of the blowing air out of the plural air blowout ports, absorbing air from the plural air blowout ports and not performing both of the blowing and absorbing air to change frictional forces between the movable body and the upper plate, and between the movable body and the lower plate, and changes, in accordance with a magnitude of a horizontal external force added to the lower plate or a magnitude of a vibration transmitted to the upper structure, a horizontal force against the external force or the vibration. With this invention, in the same manner as the above invention, regardless of a load to be supported, a natural period of the upper structure can be lengthened to reduce acceleration, and obtaining a properly preferable frictional force enables excellent vibration energy absorbing capacity as well as a returning characteristic. 
         [0020]    Further, the present invention relates to a seismic isolation device, and the device is characterized by comprising: an upper plate fixed to an upper structure and having a spherically concave lower surface with a downward opening; a lower plate fixed to a lower structure; a movable body movably arranged under the upper plate; and a pillar-shaped connection part arranged between the movable body and the lower plate, an upper end of the connection part fixed to the movable body and a lower end of the connection part pivoted on the lower plate, wherein the movable body comprises a spherically convex upper surface opposing to the spherically concave lower surface of the upper plate, and plural air blowout ports on the spherically convex upper surface, and the seismic isolation device switches whether or not air is blown out of the plural air blowout ports, or selects one of the blowing air out of the plural air blowout ports, absorbing air from the plural air blowout ports and not performing both of the blowing and absorbing air to change frictional forces between the movable body and the upper plate, and between the movable body and the lower plate, and changes, in accordance with a magnitude of a horizontal external force added to the lower plate or a magnitude of a vibration transmitted to the upper structure, a horizontal force against the external force or the vibration. With this invention, in the same manner as the above invention, regardless of a load to be supported, a natural period of the upper structure can be lengthened to reduce acceleration, and obtaining a properly preferable frictional force enables excellent vibration energy absorbing capacity as well as a returning characteristic. 
         [0021]    The above seismic isolation device may further comprises an air supplying means for supplying air to the movable body, wherein the air supplying means controls air supply to the movable body depending on the intensity of a horizontal external force added to the lower structure or the magnitude of a vibration transmitted to the upper structure. With this construction, a frictional force suitable for the intensity of the external force and others can properly be obtained, which allows excellent vibration energy absorbing capacity to be secured. 
         [0022]    In the seismic isolation device described above, the air supplying means supplies air to the movable body when the intensity of the horizontal external force added to the lower structure or the magnitude of the vibration transmitted to the upper structure is smaller than a predetermined value, and the air supplying means stops supplying air to the movable body or absorbs air from the movable body when the intensity of the horizontal external force added to the lower structure or the magnitude of the vibration transmitted to the upper structure is more or equal to the predetermined value. 
         [0023]    With the above-mentioned construction, when the vibration is relatively small, and the intensity of the external force applied to the lower structure or the magnitude of the vibration transmitted to the upper structure is lower than the predetermined value, an air layer is formed on a surface of the movable body, or load of the upper structure supported by the seismic isolation device is reduced by air pressure, not only the movable body can slide under the condition that no or small frictional force is generated but also preferable seismic isolation effect can be obtained by decreased acceleration through lengthened period, on the other hand, in case that the vibration is relatively large, and the intensity of the external force applied to the lower structure or the magnitude of the vibration transmitted to the upper structure is more or equal to the predetermined value, in addition to the decreased acceleration through lengthened period, damping with a frictional force that is determined by the materials of the movable body and others and a load to be supported is applied, so that vibration energy absorbing capacity can be improved, which secures safety at a large earthquake, etc. Further, absorbing air from the movable body, in addition to the frictional force that is determined by the materials of the movable body and others and the load to be supported, allows vibration energy to be damped by utilizing absorption force, which provides larger damping capacity. Therefore, for example, properly using the air supply, the stoppage of the supply and the absorption of the air enables at least three-stage seismic property with a single seismic isolation device. In this connection, the intensity of the external force as the predetermined value may properly be selected from acceleration, velocity and displacement. 
         [0024]    The seismic isolation device described above can be constructed such that the air supplying means stops supplying air to the movable body or absorbs air from the movable body when the intensity of the horizontal external force added to the lower structure or the magnitude of the vibration transmitted to the upper structure is smaller than a predetermined value, and the air supplying means supplies air to the movable body when the intensity of the horizontal external force added to the lower structure or the magnitude of the vibration transmitted to the upper structure is more or equal to the predetermined value. 
         [0025]    With the above construction, in a range where a vibration is relatively small, that is, the magnitude thereof is lower than the predetermined value, until an external force corresponding to a frictional force determined by the materials of the movable body and others and a load to be supported or a frictional force of an intensity to which absorption force generated by the air absorption is added, the upper structure can immovably be maintained, on the contrary, when a vibration transmitted to the upper structure is relatively large, that is, the magnitude of an external force added to the lower structure or the magnitude of a vibration transmitted to the upper structure is more or equal to the predetermined value, it becomes possible to absorb displacement under the condition that resistance force is small with a natural period of the upper structure lengthened, which enables usage as a trigger device. 
         [0026]    In the above seismic isolation device, the spherically convex upper surface may have a curvature equal to that of the spherically concave lower surface, and the spherically convex lower surface may have a curvature equal to that of the spherically concave upper surface. With this construction, it is possible to float the movable body with predetermined clearances between the upper and lower plates or contact the movable body with the upper and lower plates through faces, which allows the upper structure to further stably be supported. 
         [0027]    In the seismic isolation device, the air blowout port has an autogenous throttle shape or an orifice shape, and the air blowout ports can be utilized as air absorption holes also, or air absorption holes may be drilled separately. 
         [0028]    The seismic isolation device may be constructed such that at least one of the upper surface and the lower surface of the movable body is formed from poly phenylene sulfide resin or reinforced synthetic resin made by mixing at least one of a glass fiber, a carbon fiber and an inorganic filler to the poly phenylene sulfide resin. This poly phenylene sulfide resin or the like has excellent sliding performance, so that both of stable load support and sliding performance can simultaneously be obtained. In addition, for example, even when an air supplier is in some trouble, preferable sliding performance is secured, which prevents excessive load from acting to the upper structure, so that a fail-safe function effect is also demonstrated. When a lubricating film with good sliding performance such as molybdenum disulfide or DLC is applied to the upper and lower surfaces of the movable body, the movable body may be formed of metallic materials. 
         [0029]    In the above seismic isolation device, the air blowout port can be formed to be a round hole in planar view, and it is also possible to arrange the plural air blowout ports such that the ports are positioned on a same circumference or the ports are drawn on several similar diagrams with different dimensions and a same center. Further, an absorption force is determined by multiplying an absorption force per unit area and an area relating to the absorption, so that the plural air blowout ports can be drilled on a bottom face of a channel portion on the spherically convex upper surface or/and the spherically convex lower surface so as to be used as air absorption ports also. 
         [0030]    Further, the present invention relates to a seismic isolation structure, and the structure is characterized by comprising at least three seismic isolation devices described above. With this invention, destruction of an upper structure, damage of a displayed object standing on the upper structure or the like can preferably be prevented. 
         [0031]    As described above, with this invention, it is possible to provide a seismic isolation device, regardless of a load to be supported, capable of reducing acceleration through lengthening a natural period of an upper structure, with excellent vibration energy absorbing capacity by obtaining properly a preferable frictional force as well as a returning characteristic. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The present invention will be more apparent from the ensuring description with reference to the drawings, wherein: 
           [0033]      FIG. 1  is a cross-sectional view showing a seismic isolation device according to the first embodiment of the present invitation; 
           [0034]      FIG. 2  is a drawing showing a movable body of the seismic isolation device shown in  FIG. 1 , in which (a) is a top view, (b) a partially fragmented cross-sectional view (the cross section taken along the line A-A of (a)); 
           [0035]      FIG. 3  is a drawing showing another air blowout spout formed on the movable body of the seismic isolation device according to the present invitation, in which (a) is a cross-sectional view, (b) a top view; 
           [0036]      FIG. 4  is a drawing showing another air blowout spout formed on the movable body of the seismic isolation device according to the present invitation, in which (a) is a cross-sectional view, (b) a top view, (c) a partially fragmented perspective view; 
           [0037]      FIG. 5  is a hysteresis curves showing relation between horizontal resistance forces and horizontal displacements (relative displacements between the upper structure  3  and the lower structure  2 ) of the seismic isolation device of the present invention, in which (a) shows the state air is supplied, (b) shows the state air is supplied without absorption, (c) shows the state air is absorbed; 
           [0038]      FIG. 6  is a diagrammatical view showing a seismic isolation structure with the seismic isolation device of the present invention; and 
           [0039]      FIG. 7  is a cross-sectional view showing a seismic isolation device according to the second embodiment of the present invitation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    Next, embodiments of the present invention will be explained in detail with reference to figures. 
         [0041]      FIG. 1  shows a seismic isolation device according to the first embodiment of the present invention, this seismic isolation device  1  is mounted between a lower structure  2  such as a foundation of a detached house and an upper structure  3 , such as a detached house, which is movable in a horizontal direction H in relation to the lower structure  2  through the seismic isolation device  1 . 
         [0042]    The seismic isolation device  1  is provided with an upper plate  11  that is fixed to the upper structure  3  and has a spherically concave lower surface  11   a  with a downward opening; a lower plate  12  that is fixed to the lower structure  2  and has a spherically concave upper surface  12   a  with an upward opening; and a movable body  13  that is movably arranged between the upper plate  11  and the lower plate  12 . The upper plate  11  is fixed to the upper structure  3  with the bolts  14 ,  15 , and the lower plate  12  is fixed to the lower structure  2  with the bolts  16 ,  17 . 
         [0043]    The movable body  13  is provided with a spherically convex upper surface  13   a  opposing to the spherically concave lower surface  11   a  of the upper plate  11 , a spherically convex lower surface  13   b  opposing to the spherically concave upper surface  12   a  of the lower plate  12 , and plural air blowout ports  13   c  on the spherically convex upper surface  13   a  and the spherically convex lower surface  13   b.  The spherically convex upper surface  13   a  of the movable body  13  has the same curvature as the spherically concave lower surface  11   a  of the upper plate  11 , and the spherically convex lower surface  13   b  of the movable body  13  has the same curvature as the spherically concave upper surface  12   a  of the lower plate  12 . 
         [0044]    As shown in  FIG. 2 , the movable body  13  is formed to be a substantially thick disk on the whole, and is provided with a metal fitting  19  for connecting a flexible hose  18  for air supply, and through the movable body  13  are drilled air passages  13   d  for introducing air taken through the metal fitting  19  to the air blowout ports  13   c.  The air blowout port  13   c  is, as clearly illustrated in  FIG. 2(   a ), formed to be a round hole in planar view, and plural air blowout ports  13   c  are arranged on the same circumference. The plural air blowout ports  13   c  can be arranged on several similar diagrams with different dimensions and the same center. 
         [0045]    The upper or lower surface of the movable body  13 , or the whole of the movable body  13  may be formed from poly phenylene sulfide resin or reinforced synthetic resin made by mixing at least one of a glass fiber, a carbon fiber and an inorganic filler to the poly phenylene sulfide resin. 
         [0046]    As shown in  FIG. 1 , in order to supply air to the movable body  13 , an air pump  20  is mounted, and air is introduced to the movable body  13  via the flexible hose  18  and the metal fitting  19 . Meanwhile, the air pump  20  functions as a vacuum pump for absorbing air from the movable body  13  through the metal fitting  19  and the flexible hose  18  also. 
         [0047]    The air blowout port may be, other than a round hole with the same diameter in an axial direction as shown in  FIGS. 1 and 2 , as illustrated in  FIG. 3 , an autogenous throttle shape in which a portion of an air passage  23   a  adjacent to a blowout port  23   b  is throttled, or as shown in  FIG. 4 , the air blowout port can be a so-called orifice shape having an air blowing channel  33   c  wider than the diameter of the end portion  33   b  of the air passage  33   a.  As for arrangement of the air blowout ports, where the diameter of the movable body is φD and the diameter of a circle on which the air blowout ports are drilled isφDi, it is preferable that φDi=(0.6˜0.7)×φD. 
         [0048]    Next, the motion of the seismic isolation device  1  with the above-mentioned construction will be explained with reference to  FIGS. 1 ,  2  and  5 .  FIG. 5  shows hysteresis diagrams indicating the relation between horizontal resistance forces and horizontal displacements (relative displacements between the upper structure  3  and the lower structure  2  of the seismic isolation device  1 ), in which (a) shows a case that air is supplied to the movable body  13 ; (b) shows a case that air is not supplied to the movable body  13  and air is not absorbed from the movable body  13 ; and (c) shows a case that air is absorbed from the movable body  13 . In case of (a), that is, when air is supplied to the movable body  13 , since the movable body  13  is movable without generating frictional force, the relative displacement occurs at small horizontal resistance force; in case of (b), that is, when air is not supplied to the movable body  13  and air is not absorbed from the movable body  13 , since no air exists between the movable body  13  and the upper plate  11 , and between the movable body  13  and the lower plate  12 , the same relative displacement provides a larger horizontal resistance force; and in case of (c), that is, when air is absorbed from the movable body  13 , since stronger absorption forces act between the movable body  13  and the upper plate  11 , and between the movable body  13  and the lower plate  12 , the same relative displacement provides a much larger horizontal resistance force. 
         [0049]    In the below explanation, at first, the following motion will be explained. In the seismic isolation device  1 , when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  is smaller than a predetermined value, air is supplied from the air pump  20  to the movable body  13 , and when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  is more or equal to the predetermined value, the air pump  20  is stopped to stop supplying air to the movable body  13 . With this motion, when a vibration is relatively small and an added external force is smaller than the predetermined value, a small horizontal resistance force operates the seismic isolation device  1  to obtain preferable seismic isolation effect through reduction in acceleration due to lengthened period, on the contrary, when a vibration is relatively large and an added external force is more than the predetermined value, in addition to the reduction in acceleration due to lengthened period, damping is performed by a frictional force that is determined by the materials of the movable body and others and a load to be supported, which improves absorbing power to vibration energy, and safety at a large earthquake or the like can be secured. 
         [0050]    Normally, from the air pump  20 , through the flexible hose  18  and the metal fitting  19  is supplied air to the movable body  13 . The air supplied from the metal fitting  19  to the movable body  13  is jetted toward the spherically concave lower surface  11   a  and the spherically concave upper surface  12   a  from the air blowout ports  13   c  via the air passages  13   d.  With this, air layers are formed between the spherically convex lower surface  13   b  of the movable body  13  and the spherically concave upper surface  12   a  of the lower plate  12 , and between the spherically convex upper surface  13   a  of the movable body  13  and the spherically concave lower surface  11   a  of the upper plate  11 , so that as shown in  FIG. 5(   a ), the movable body  13  can be moved without frictional force being generated, in other words, a small horizontal resistance force will operate the seismic isolation device  1  to obtain preferable seismic insulation effect through reduction in acceleration due to lengthened period. This condition is continuously maintained as long as external forces added to the lower structure  2  are smaller than the predetermined value. 
         [0051]    On the other hand, at a large earthquake or the like by which an external force more than the predetermined value is added, the operation of the air pump  20  is stopped to stop feeding air to the movable body  13 . With this, the spherically convex lower surface  13   b  of the movable body  13  and the spherically concave upper surface  12   a  of the lower plate  12 , and the spherically convex upper surface  13   a  of the movable body  13  and the spherically concave lower surface  11   a  of the upper plate  11  respectively directly contact with each other to allow the movable body  13  to slide. As a result, as shown in  FIG. 5(   b ), in addition to the reduction in acceleration due to lengthened period, damping is performed by a frictional force that is determined by the materials of the movable body  13  and others and a load to be supported, which improves absorbing power to vibration energy to secure safety. 
         [0052]    Meanwhile, in the above embodiment, when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  becomes more or equal to a predetermined value, the air pump  20  is stopped, at this moment, the air pump  20  is caused to function as a vacuum pump to absorb air from the movable body  13  also. In such a case, as shown in  FIG. 5(   c ), in addition to a frictional force determined by the materials of the movable body  13  and others and a load to be supported, vibration energy can be damped by utilizing absorption power, which enables larger damping capacity. 
         [0053]    Next, the following motion will be explained. In the seismic isolation device  1 , when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  is smaller than a predetermined value, the air pump  20  is stopped to stop supplying air to the movable body  13 , and when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  is more or equal to the predetermined value, the air pump  20  is operated to feed air to the movable body  13 . With this motion, in a range where a vibration is relatively small, until an external force corresponding to a frictional force determined by the materials of the movable body  13  and others and a load to be supported is inputted, the upper structure  3  can immovably be maintained, on the contrary, when a vibration is large and the intensity of an external force becomes more or equal to the predetermined value, it is possible to absorb displacement under the condition that a resistance force is small with a natural period of the upper structure  3  lengthened, which allows the seismic isolation device  1  to function as a trigger device. 
         [0054]    Meanwhile, in the above embodiment, when the intensity of an external force added to the lower structure  2 , or the magnitude of a vibration transmitted to the upper structure  3  is smaller than the predetermined value, the air pump  20  is stopped, at this moment, the air pump  20  is caused to function as a vacuum pump to absorb air from the movable body  13  also. In such a case, in addition to a frictional force determined by the materials of the movable body  13  and others and a load to be supported, the upper structure  3  can immovably be maintained through a larger force by utilizing an absorption force. 
         [0055]    Further, although the air blowout ports with autogenous throttle shape shown in  FIG. 3  have the same motion as described above, as shown in  FIG. 4 , the air blowout ports with orifice shape with the air blowout channels  33   c  are able to supply or absorb air to and from the overall area of the sliding body, which secures stabler motion of the sliding body. In this connection, an absorption force is determined by multiplying an absorption force per unit area and an area relating to the absorption, so that more absorption channels can be formed to increase the absorption force. 
         [0056]    Next, a seismic isolation structure to which the above seismic isolation device  1  is applied will be explained with reference to  FIG. 6 . 
         [0057]    This seismic isolation structure  35  is provided with at least three seismic isolation devices  1  that are mounted between a foundation  36  and a detached house  37 , and each seismic isolation device  1  transmits the load of the detached house  37  to the foundation  36  as well as permits a relative horizontal displacement between the foundation  36  and the detached house  37 . Then, at a horizontal displacement of the foundation  36  due to an earthquake or the like, the seismic isolation devices  1  function as described above, which prevents a horizontal vibration of the foundation  36  from transmitting to the detached house  37  as it is and lengthens a natural period of the detached house  37 , resulting in reduced acceleration to the detached house  37 , etc. 
         [0058]    Next, a seismic isolation device according to the second embodiment of the present invention will be explained with reference to  FIG. 7 . This seismic isolation device  41  is also mounted between a lower structure  42  such as a foundation of a detached house and an upper structure  43 , such as a detached house, which is movable in a horizontal direction H in relation to the lower structure  42  via the seismic isolation device  41 . 
         [0059]    The seismic isolation device  41  is provided with an upper plate  51  that is fixed to the upper structure  43 ; a lower plate  52  that is fixed to the lower structure  42  and has a spherically concave upper surface  52   a  with an upward opening; a movable body  53  that is movably arranged between the upper plate  51  and the lower plate  52 ; and a pillar-shaped connection part  44  arranged between the movable body  53  and the upper plate  51 , in which a spherical member  44   a  of the pillar-shaped connection part  44  is pivoted on the upper plate  51  through retaining members  45 ,  46 , and a lower end of a body portion  44   b  of the pillar-shaped connection part  44  is fixed to the movable body  53 . The upper plate  51  is fixed to the upper structure  43  with the bolts  54 ,  55 , and the lower plate  52  is fixed to the lower structure  42  with the bolts  56 ,  57 . 
         [0060]    The movable body  53  is provided with a flat upper surface  53   b  and a spherically convex face at a lower portion thereof, and is formed to be a substantially thick disk on the whole. To the upper surface  53   b  is fixed the lower end of the pillar-shaped connection part  44 ; a spherically convex lower surface  53   a  opposing to the spherically concave upper surface  52   a  of the lower plate  52  is mounted; and the spherically convex lower surface  53   a  is provided with plural air blowout ports  53   c.  The spherically convex lower surface  53   a  of the movable body  53  has the same curvature as the spherically concave upper surface  52   a  of the lower plate  52 . 
         [0061]    The movable body  53  is provided with a metal fitting  59  for connecting a flexible hose  58  for air supply, and through the movable body  53  are drilled air passages  53   d  for introducing air taken through the metal fitting  59  to the air blowout ports  53   c.  The air blowout port  53   c  is, although drawing thereof is omitted, in the same manner as the movable body  13  shown in  FIG. 2 , formed to be a round hole in planar view, and plural air blowout ports  53   c  are arranged on the same circumference. 
         [0062]    The lower surface or the whole of the movable body  53  may be formed from poly phenylene sulfide resin or reinforced synthetic resin made by mixing at least one of a glass fiber, a carbon fiber and an inorganic filler to the poly phenylene sulfide resin. When a lubricant with good sliding performance such as molybdenum disulfide or DLC is applied to the lower surface, the movable body may be formed of metallic materials. 
         [0063]    In order to supply air to the movable body  53 , an air pump  60  is mounted, and air is introduced to the movable body  53  via the flexible hose  58  and the metal fitting  59 . Meanwhile, the air pump  60  functions as a vacuum pump for absorbing air from the movable body  53  through the metal fitting  59  and the flexible hose  58  also. 
         [0064]    The pillar-shaped connection part  44  is provided with the spherical portion  44   a  at an upper portion thereof, and the truncated-cone-shaped and columnar body portion  44   b  at a lower portion thereof, and as described above, the spherical portion  44   a  is pivoted on the upper plate  51  through the retaining members  45 ,  46 , and a lower end of the body portion  44   b  is fixed to the movable body  53 . With this, the pillar-shaped connection part  44  is movable together with the movable body  53 , and is rotatably connected with the upper plate  51  as well. 
         [0065]    Next, the motion of the seismic isolation device  41  with the above-mentioned construction will be explained with reference to  FIG. 7 . 
         [0066]    In the below explanation also, in the same manner as the first embodiment, at first, the following motion will be explained. In the seismic isolation device  41 , when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  is smaller than a predetermined value, air is supplied from the air pump  60  to the movable body  53 , and when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  is more or equal to the predetermined value, the air pump  60  is stopped to stop supplying air to the movable body  53 . With this motion, when a vibration is relatively small and an added external force is smaller than the predetermined value, a small horizontal resistance force operates the seismic isolation device  41  to obtain preferable seismic isolation effect through reduction in acceleration due to lengthened period, on the contrary, when a vibration is relatively large and an added external force is larger than the predetermined value, in addition to the reduction in acceleration due to lengthened period, damping is performed by a frictional force that is determined by the materials of the movable body and others and a load to be supported, which improves absorbing power to vibration energy, and safety at a large earthquake or the like can be secured. 
         [0067]    Normally, from the air pump  60  and through the flexible hose  58  and the metal fitting  59  is supplied air to the movable body  53 . The air supplied from the metal fitting  59  to the movable body  53  is jetted toward the spherically concave upper surface  52   a  from the air blowout ports  53   c  via the air passages  53   d.  With this, an air layer is formed between the spherically concave lower surface  53   a  of the movable body  53  and the spherically concave upper surface  52   a  of the lower plate  52 , so that while rotating the spherical portion  44   a  of the pillar-shaped connection part  44  in relation to the retaining members  45 ,  46 , the movable body  53  can be moved without frictional force being generated, in other words, a small horizontal resistance force will operate the seismic isolation device  41  to obtain preferable seismic insulation effect through reduction in acceleration due to lengthened period. This condition is continuously maintained as long as external forces added to the lower structure  42  are smaller than the predetermined value. 
         [0068]    On the other hand, at a large earthquake or the like, the operation of the air pump  60  is stopped to stop feeding air to the movable body  53 . With this, the spherically convex lower surface  53   a  of the movable body  53  and the spherically concave upper surface  52   a  of the lower plate  52  directly contact with each other to allow the movable body  53  to slide on the spherically concave upper surface  52   a  of the lower plate  52  through the spherically convex lower surface  53   a  while rotating the spherical portion  44   a  of the pillar-shaped connection part  44  in relation to the retaining members  45 ,  46 . As a result, in addition to the reduction in acceleration due to lengthened period, damping is performed by a frictional force that is determined by the materials of the movable body  53  and the lower plate  52  and a load to be supported, which improves absorbing power to vibration energy to secure safety. 
         [0069]    Meanwhile, in the above embodiment, when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  becomes more or equal to the predetermined value, the air pump  60  is stopped, at this moment, the air pump  60  is caused to function as a vacuum pump to absorb air from the movable body  53  also. In such a case, in addition to the frictional force determined by the materials of the movable body  53  and others and a load to be supported, vibration energy can be damped by utilizing absorption power, which enables larger damping capacity. 
         [0070]    Next, the following motion will be explained. In the seismic isolation device  41 , when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  is smaller than a predetermined value, the air pump  60  is stopped to stop supplying air to the movable body  53 , and when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  is more or equal to the predetermined value, the air pump  60  is operated to feed air to the movable body  53 . With this motion, in a range where a vibration is relatively small, until an external force corresponding to a frictional force determined by the materials of the movable body  53  and others and a load to be supported, the upper structure  43  can immovably be maintained, on the contrary, when a vibration is large and the intensity of an external force becomes more or equal to the predetermined value, it is possible to absorb displacement under the condition that a resistance force is small with a natural period of the upper structure  43  lengthened, which allows the seismic isolation device  1  to function as a trigger device. 
         [0071]    Meanwhile, in the above embodiment, when the intensity of an external force added to the lower structure  42 , or the magnitude of a vibration transmitted to the upper structure  43  is smaller than the predetermined value, the air pump  60  is stopped, at this moment, the air pump  60  is caused to function as a vacuum pump to absorb air from the movable body  53  also. In such a case, in addition to the frictional force determined by the materials of the movable body  53  and others and a load to be supported, the upper structure  43  can immovably be maintained through a larger force by utilizing an absorption force. 
         [0072]    Further, in the above embodiment, although explanation was made when on the upper face of the movable body  53  is fixed the pillar-shaped connection part  44 , and through the spherically convex lower surface  53   a,  on the spherically concave upper surface  52   a  of the lower plate  52  moves the movable body  53 , the constituents may vertically be reversed in  FIG. 7 , and it is possible to construct such that on the lower face of the movable body is fixed the pillar-shaped connection part, and through the spherically convex upper surface, on the spherically concave lower surface of the upper plate moves the movable body. In addition, although in the seismic isolation device  41  shown in  FIG. 7 , the spherical portion  44   a  is pivoted on the upper plate  51  via the retaining members  45 ,  46 , and the body portion  44   b  is fixed to the movable body  53 , it is possible to allow the spherical portion  44   a  to be pivoted on the movable body  53  via the retaining members  45 ,  46 , and the body portion  44   b  to be fixed to the upper plate  51 . 
         [0073]    The seismic isolation device  41  illustrated in  FIG. 7  is also, in place of the seismic isolation device  1 , applicable to the seismic isolation structure  35  shown in  FIG. 6 , which functions in the same manner as the seismic isolation device  1 . 
       EXPLANATION OF REFERENCE NUMBERS 
       [0074]      1  seismic isolation device 
         [0075]      2  lower structure 
         [0076]      3  upper structure 
         [0077]      11  upper plate 
         [0078]      11   a  spherically concave lower surface 
         [0079]      12  lower plate 
         [0080]      12   a  spherically concave upper surface 
         [0081]      13  movable body 
         [0082]      13   a  spherically convex upper surface 
         [0083]      13   b  spherically convex lower surface 
         [0084]      13   c  air blowout port 
         [0085]      13   d  air passage 
         [0086]      14 - 17  bolts 
         [0087]      18  flexible hose 
         [0088]      19  metal fitting 
         [0089]      20  air pump 
         [0090]      23   a  air passage 
         [0091]      23   b  blowout port 
         [0092]      33   a  air passage 
         [0093]      33   b  end portion 
         [0094]      33   c  air blowing channel 
         [0095]      35  seismic isolation structure 
         [0096]      36  foundation 
         [0097]      37  detached house 
         [0098]      41  seismic isolation device 
         [0099]      42  lower structure 
         [0100]      43  upper structure 
         [0101]      44  pillar-shaped connection part 
         [0102]      44   a  spherical portion 
         [0103]      44   b  body portion 
         [0104]      45  retaining member 
         [0105]      46  retaining member 
         [0106]      51  upper plate 
         [0107]      52  lower plate 
         [0108]      52   a  spherically concave upper surface 
         [0109]      53  movable body 
         [0110]      53   a  spherically convex lower surface 
         [0111]      53   b  upper surface 
         [0112]      53   c  air blowout port 
         [0113]      53   d  air passage 
         [0114]      54 - 57  bolts 
         [0115]      58  flexible hose 
         [0116]      59  metal fitting 
         [0117]      60  air pump