Patent Publication Number: US-9416504-B1

Title: Bridge pier structure

Description:
TECHNICAL FIELD 
     The present invention relates to a bridge pier structure, and more particularly to a bridge pier structure for enhancing the earthquake resistance of a bridge pier of a bridge or a road, a pillar member of a civil engineering structure such as a floodgate, or a pillar member of an architectural structure such as a building. 
     BACKGROUND ART 
     Inventions have been disclosed in which, for seismic reinforcement of a pillar member of a civil engineering structure or an architectural structure, diagonal members having hysteresis damping characteristics are disposed in a brace manner between the target pillar member and a footing with the pillar member installed, to thereby support a horizontal load in an earthquake, increase a horizontal load capacity of the pillar member, and reduce horizontal displacement (see Patent Literature 1, for example). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2003-74019 (pages 3 to 4 and FIG. 1) 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to the invention disclosed in Patent Literature 1, the diagonal members extend or contract owing to deformation (tilt relative to the footing) of the pillar member in the earthquake. Therefore, the diagonal members exhibit hysteresis damping performance, thereby obtaining an effect of damping vibration of the pillar structure, reducing a seismic load, and enabling efficient seismic reinforcement. 
     According to this invention, however, the diagonal members are installed to the pillar member in a brace manner to extend laterally from the pillar member, and thus ends of the diagonal members joined to the footing project extensively from the outer circumference of the pillar member. That is, the diagonal members serving as reinforcing members occupy a large area. 
     Therefore, there is a problem in that the invention is not applicable to a case in which a bridge pier of a bridge, for example, is located close to a space-occupying structure, such as a road, a path, or an embankment. Further, there is another problem in that the invention is not applicable to a case in which the bridge pier is located in a river, a lake, a marsh, or a sea area, for example, since the reinforcing members project extensively to a water area and occupy a large area, thereby obstructing a river basin area, for example. 
     The present invention provides a bridge pier structure that solves the above-described problems and is capable of reducing the seismic load while curtailing the area occupied by the reinforcing members, without projecting extensively from the outer circumference of the pillar member. 
     Solution to Problem 
     (1) A bridge pier structure according to the present invention includes a damper having damping characteristics, a substructure joined with a lower end portion of the damper, and a pillar member provided upright on the substructure, a side surface of the pillar member being joined with an upper end portion of the damper. The damper is substantially parallel to the side surface of the pillar member. 
     (2) Further, in (1) described above, the lower end portion and the upper end portion of the damper each include a damper pin hole, and a substructure bracket including a substructure pin hole is installed to the substructure. A pillar member bracket including a pillar member pin hole is installed to the side surface of the pillar member. 
     A lower pin inserted in the damper pin hole of the lower end portion of the damper and the substructure pin hole forms a lower pin structure configured to join the damper and the substructure. 
     An upper pin inserted in the damper pin hole of the upper end portion of the damper and the pillar member pin hole forms an upper pin structure configured to join the damper and the pillar member. 
     (3) Further, in (1) described above, the pillar member has a rectangular cross section, and the side surfaces of the pillar member are flat surfaces. 
     A pair of the dampers is disposed parallel to at least one of the side surfaces of the pillar member. 
     A distance between the upper end portions of the pair of the dampers is different from a distance between the lower end portions of the pair of the dampers. 
     (4) Further, in (2) described above, the substructure includes a base having an upper surface projecting from ground, and the substructure bracket is provided on the upper surface of the base. 
     (5) Further, in (1) described above, the damper is an axial damper, a shear damper, a viscoelastic damper, a bending damper, a cylinder-piston damper, a buckling-restrained brace, an unbonded brace, a hysteresis damper, or a friction damper. 
     (6) Further, in (1) described above, the pillar member has a “cut-off” reinforced concrete structure including a full-length reinforcing bar disposed over a full length of the pillar member in a height direction and a lower reinforcing bar disposed in a lower area of the pillar member in the height direction, and the upper end portion of the damper is joined to the side surface of the pillar member at a position above an upper end of the lower reinforcing bar. 
     (7) Further, in (1) described above, the damper includes an axial force member, a stiffener stiffening the axial force member, a first connection member connected to one end portion of the axial force member and one end portion of the stiffener, and a second connection member connected to an other end portion of the axial force member. 
     The axial force member has a length equal to or shorter than a length so that a value of energy absorbed by the pillar member when the damper is not installed to the pillar member and the pillar member deforms from an allowable pillar member displacement allowed for the pillar member to a maximum design displacement determined by design energy of the pillar member is equal to energy absorbed by the damper from start of deformation of the damper to displacement to a displacement corresponding to the allowable pillar member displacement. 
     (8) Further, in (7) described above, a stopper is formed to project from an outer circumference of the second connection member, and, when the axial force member contracts, the stopper comes into contact with an other end portion of the stiffener. 
     (9) Further, in (7) described above, a stopper is formed to project from an outer circumference of the second connection member, and an other end portion of the stiffener is formed with a first reaction force portion and a second reaction force portion facing each other across the stopper. 
     The stopper comes into contact with the first reaction force portion of the stiffener when the axial force member contracts, and the stopper comes into contact with the second reaction force portion of the stiffener when the axial force member extends. 
     (10) Further, in (8) described above, the stiffener is stiffened by a second stiffener, and one end portion of the second stiffener is connected to the first connection member. 
     Advantageous Effects of Invention 
     (i) In the bridge pier structure according to the present invention, the dampers having the damping characteristics have end portions each joined to the respective side surfaces of the pillar member provided upright on the substructure (bridge pier side surfaces of a bridge pier provided upright on a footing, for example). Therefore, the dampers extend or contract owing to the deformation (tilt relative to the substructure) of the pillar member in an earthquake. Thus, a vibration damping effect is obtained, a seismic load is reduced, and efficient seismic reinforcement is provided. 
     Further, since the dampers are installed substantially parallel to the side surfaces of the pillar member, the dampers do not project extensively from the outer circumference of the pillar member, and the area occupied by the dampers serving as reinforcing members is small. Thus, the bridge pier structure according to the present invention is also applicable to a case in which the bridge pier is located close to a space-occupying structure, such as a road, a path, or an embankment, for example, and a case in which the bridge pier is located in a river, a lake, a marsh, or a sea area, for example. 
     (ii) Further, since the pillar member, the dampers, and the substructure are joined together by the pin structures, the dampers are subjected only to force acting in the axial direction thereof and not to force that bends the dampers. Therefore, the designing of the dampers is simplified, and the damping characteristics of the dampers are sufficiently exhibited. 
     (iii) Since the pair of the dampers is parallel to the flat side surface of the pillar member and arranged in a triangular or trapezoidal shape, the effect of damping earthquake vibration in a direction parallel to the side surface is obtained by the pair of the dampers. That is, since it is possible to limit the side surface of the pillar member to which the dampers are installed, the degree of freedom is increased in selecting the side surface to which the dampers are installed. It is therefore possible to improve the appearance by not installing the dampers to some of the side surfaces. 
     (iv) Further, the lower end portions of the dampers are connected to the substructure brackets provided on the upper surfaces of the bases projecting from the ground, and the dampers are separated from the ground. Therefore, the corrosion of the dampers is suppressed, and the replacement of the dampers is simplified. 
     (v) Further, the dampers have the hysteresis damping characteristics, and are commonly used. Therefore, the dampers are easily selected and procured, and make it possible to manufacture the bridge pier structure at low cost. 
     (vi) Further, the pillar member has the cut-off reinforced concrete structure, and the upper end portions of the dampers are located at positions higher than a cut-off section. Therefore, the area of the pillar member higher than the cut-off section is also reinforced and improved in earthquake resistance. In addition, the area occupied by the dampers serving as the reinforcing members is small, and thus restrictions on installation sites are reduced. 
     (vii) Further, the axial force member has the length equal to or shorter than the length so that the value of the energy absorbed by the pillar member when the pillar member deforms from the allowable pillar member displacement to the maximum design displacement is equal to the energy absorbed by the dampers until the displacement to an allowable damper displacement corresponding to the allowable pillar member displacement. Therefore, the earthquake resistance is more reliably improved. 
     (viii) Further, the stopper is provided, and the axial force member and the stiffener both support compression force. Therefore, the buckling of the axial force member is prevented, and the earthquake resistance is improved. 
     (ix) Further, since the stopper is provided and the axial force member and the stiffener both support the compression force, the buckling of the axial force member is prevented. Further, since the axial force member and the stiffener both support tensile force, a plastic deformation amount of the axial force member is reduced, and the earthquake resistance is improved. 
     (x) Further, since the second stiffener is provided, the buckling of the axial force member is more reliably suppressed, and the earthquake resistance is further improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view illustrating a bridge pier structure according to Embodiment 1 of the present invention. 
         FIG. 2  is a front view illustrating the bridge pier structure according to Embodiment 1 of the present invention. 
         FIG. 3  is a cross-sectional view of a plan view illustrating the bridge pier structure according to Embodiment 1 of the present invention (along arrow A in  FIG. 1 ). 
         FIG. 4  is a front view illustrating the bridge pier structure according to Embodiment 1 of the present invention, depicting deformation of the bridge pier structure in an earthquake. 
         FIG. 5  is a front view illustrating the bridge pier structure according to Embodiment 1 of the present invention, depicting the relationship between force acting in the earthquake and a deformation amount. 
         FIG. 6  is a correlation diagram illustrating the bridge pier structure according to Embodiment 1 of the present invention, depicting the relationship between a seismic load and a horizontal displacement amount. 
         FIG. 7A  is a front view illustrating a bridge pier structure according to Embodiment 2 of the present invention. 
         FIG. 7B  is a plan view illustrating the bridge pier structure according to Embodiment 2 of the present invention, depicting parts of the bridge pier structure. 
         FIG. 8  is a left side view illustrating a bridge pier structure according to Embodiment 3 of the present invention. 
         FIG. 9  is a right side view illustrating the bridge pier structure according to Embodiment 3 of the present invention. 
         FIG. 10  is a front view illustrating the bridge pier structure according to Embodiment 3 of the present invention. 
         FIG. 11  is a cross-sectional view of a plan view illustrating the bridge pier structure according to Embodiment 3 of the present invention (along arrow A in  FIG. 8 ). 
         FIG. 12  is a left side view illustrating the bridge pier structure according to Embodiment 3 of the present invention, depicting deformation of the bridge pier structure in an earthquake. 
         FIG. 13A  is a side view illustrating a bridge pier structure according to Embodiment 4 of the present invention, depicting a section of a part thereof. 
         FIG. 13B  is a moment diagram illustrating the bridge pier structure according to Embodiment 4 of the present invention, depicting the distribution of bending moment. 
         FIG. 13C  is a moment diagram illustrating the bridge pier structure according to Embodiment 4 of the present invention, depicting the distribution of bending moment when comparative dampers are installed. 
         FIG. 14  illustrates the bridge pier structure according to Embodiment 4 of the present invention, and (a) and (b) are side views each depicting a part (damper) of the bridge pier structure. 
         FIG. 15A  is a correlation diagram illustrating the bridge pier structure according to Embodiment 4 of the present invention, depicting the relationship between a seismic load and a horizontal displacement amount. 
         FIG. 15B  is a correlation diagram illustrating the bridge pier structure according to Embodiment 4 of the present invention, depicting the relationship between force and a displacement amount for illustrating how to determine the length of a part (an axial force member of a damper) of the bridge pier structure. 
         FIG. 16  illustrates a bridge pier structure according to Embodiment 5 of the present invention, and (a), (b), and (c) are side views each depicting a part (damper) of the bridge pier structure. 
         FIG. 17  is a correlation diagram illustrating the bridge pier structure according to Embodiment 5 of the present invention, depicting the relationship between a seismic load on a bridge pier and a horizontal displacement amount of the bridge pier. 
         FIG. 18  is a side view illustrating a bridge pier structure according to Embodiment 6 of the present invention. 
         FIG. 19  is a correlation diagram illustrating the bridge pier structure according to Embodiment 6 of the present invention, depicting the relationship between a seismic load and a horizontal displacement amount. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
       FIGS. 1 to 3  illustrate a bridge pier structure according to Embodiment 1 of the present invention.  FIG. 1  is a side view,  FIG. 2  is a front view, and  FIG. 3  is a cross-sectional view of a plan view (along arrow A in  FIG. 1 ). Respective parts are schematically illustrated, and the present invention is not limited to the illustrated embodiment (in shape and relative size). 
     (Bridge Pier Structure) 
     In  FIGS. 1 to 3 , a bridge pier structure  100  has dampers  30   a ,  30   b ,  30   c , and  30   d  having damping characteristics and having respective lower end portions joined to an upper surface  11  of a footing (same as substructure)  10  and respective upper end portions joined to bridge pier side surfaces  21   a ,  21   b ,  21   c , and  21   d  of a bridge pier (same as pillar member)  20  provided upright on the footing  10 . 
     Herein, the dampers  30   a ,  30   b ,  30   c , and  30   d  are substantially parallel to the bridge pier side surfaces  21   a ,  21   b ,  21   c , and  21   d , respectively. In the following description of members having the same structure, the suffixes “a,” “b,” “c,” and “d” added to the reference signs of the members and parts may be omitted for the convenience of description. 
     Although the bridge pier structure  100  includes the bridge pier  20  as a pillar member, the present invention is not limited thereto, and may include a pillar member installed on a substructure to support a superstructure. 
     The lower end portions and the upper end portions of the dampers  30  include not-illustrated damper pin holes. Meanwhile, footing brackets  12  including not-illustrated footing pin holes are installed to the upper surface  11  of the footing  10 , and bridge pier brackets (same as pillar member brackets)  22  including not-illustrated pier pin holes are installed to the bridge pier side surfaces  21  of the bridge pier  20 . 
     Further, lower pins  31  inserted in the not-illustrated damper pin holes of the lower end portions of the dampers  30  and the not-illustrated footing pin holes form footing-side pin structures that join the dampers  30  and the footing  10 . 
     Further, upper pins  32  inserted in the not-illustrated damper pin holes of the upper end portions of the dampers  30  and the not-illustrated pier pin holes form pier-side pin structures that join the dampers  30  and the bridge pier  20 . 
     The footing  10  is buried in ground  90  with the upper surface  11  of the footing  10  located below a surface of the ground  90  (hereinafter referred to as the “ground surface  92 ”), and is supported by multiple piles  91  placed in the ground  90 . 
     Further, beams  40   a  and  40   c  are installed on the bridge pier side surfaces  21   a  and  21   c , respectively, and girders  51   a  and  52   a  and girders  51   c  and  52   c  are installed on an upper surface  41   a  of the beam  40   a  and an upper surface  41   c  of the beam  40   c , respectively, and support a floorboard (same as superstructure)  60 . Therefore, the bridge pier side surfaces  21   a  and  21   c  are parallel to a bridge axis direction (indicated by arrow). 
     (Operations) 
       FIGS. 4 to 6  illustrate the bridge pier structure according to Embodiment 1 of the present invention.  FIG. 4  is a front view depicting deformation of the bridge pier structure in an earthquake.  FIG. 5  is a front view depicting the relationship between force acting in the earthquake and a deformation amount.  FIG. 6  is a correlation diagram depicting the relationship between a seismic load and a horizontal displacement amount. Respective parts are schematically illustrated, and the same parts as the parts illustrated in  FIGS. 1 to 3  are designated by the same reference signs. 
     In  FIG. 4 , the bridge pier  20  (the bridge pier side surfaces  21   a  and  21   c ) is installed perpendicularly to the upper surface  11  of the footing  10  before the occurrence of the earthquake. Thus, the damper  30   a  (a line connecting the center of the lower pin  31   a  and the center of the upper pin  32   a ) and the damper  30   c  (a line connecting the center of the lower pin  31   c  and the center of the upper pin  32   c ) are perpendicular to the upper surface  11  (both indicated by broken lines). 
     Then, as the earthquake occurs, an area of the bridge pier  20  close to the lower end thereof is bent to tilt relative to the upper surface  11  of the footing  10 , and the bridge pier side surface  21   a  extends and the bridge pier side surface  21   c  contracts. Thus, the upper end portion of the damper  30   a  (the upper pin  32   a ) moves diagonally upward, and thus the damper  30   a  extends by a distance (hereinafter referred to as “extension amount”)  34   a . Meanwhile, the upper end portion of the damper  30   c  moves diagonally downward, and thus the damper  30   c  contracts by a distance (hereinafter referred to as “contraction amount”)  34   c.    
     In  FIG. 5 , the relationship between seismic force acting on a head portion of the bridge pier  20  (hereinafter referred to as the “damper resistance Pd”) and force acting on the dampers  30  (hereinafter referred to as the “damper axial force F”) is obtained. The height of the bridge pier  20  is represented as “H,” the displacement amount in the horizontal direction of the head portion of the bridge pier  20  (same as horizontal bridge pier displacement amount) is represented as “δ,” the height of the dampers  30  is represented as “D,” the interval between the dampers  30   a  and  30   c  is represented as “L,” and the extension amount of the damper  30   a  (same as the contraction amount of the damper  30   c ) is represented as “d.” 
     Herein, bending moment due to the damper resistance Pd and bending moment due to the damper axial force F are balanced, and “Pd×H=F×L” holds. Therefore, the damper resistance Pd is calculated from “Pd=F×L/H.” That is, the damper resistance Pd is increased as the installation interval L between the dampers  30  is increased (widened). 
     Further, since the extension amount d of the damper  30   a  and the horizontal bridge pier displacement amount δ of the head portion of the bridge pier  20  have a relationship “δ/H=2×d/L,” the horizontal bridge pier displacement amount δ is calculated from “δ=2×d×H/L.” 
     When the cross section area and the modulus of elasticity of the dampers  30  are represented as “A” and “E,” respectively, the extension amount d of the damper  30   a  is calculated from “d=F×D/(A×E)” based on “F=A×E×d/D.” 
     In  FIG. 6 , the vertical axis represents the seismic load acting on the bridge pier  20 , and the horizontal axis represents the horizontal bridge pier displacement amount (δ) of the bridge pier  20 . In  FIG. 6 , as to the bridge pier  20  before the dampers  30  are installed, the seismic load is elastically and gradually increased until the horizontal bridge pier displacement amount (δ) reaches a displacement amount at which the bridge pier  20  yields (hereinafter referred to as the “bridge pier yield displacement amount δy”), and the seismic load remains at a constant value after the horizontal bridge pier displacement amount (δ) reaches the bridge pier yield displacement amount δy, irrespective of the increase in the horizontal bridge pier displacement amount (δ). 
     Meanwhile, as to the dampers  30  per se, the seismic load is elastically and gradually increased until the horizontal bridge pier displacement amount (δ) reaches a displacement amount at which the dampers  30  yield (hereinafter referred to as the “damper yield displacement amount δdy”), and the seismic load remains at a constant value after the horizontal bridge pier displacement amount (δ) reaches the damper yield displacement amount δdy, irrespective of the increase in the horizontal bridge pier displacement amount (δ). 
     Herein, the damper yield displacement amount δdy is larger in value than the bridge pier yield displacement amount δy. 
     As to the bridge pier  20  to which the dampers  30  are installed, therefore, the behavior of the bridge pier  20  before the dampers  30  are installed and the behavior of the dampers  30  alone are combined to form a behavior indicated by a solid line in  FIG. 6 . 
     In this case, the following holds: 
     The extension/contraction amount (extension amount d=contraction amount d) of the dampers  30  at the time of yielding is proportional to the height D of the dampers  30 . 
     It is possible to adjust the horizontal bridge pier displacement amount δ, which is an extension/contraction amount at the time of yielding, by adjusting the height D of the dampers  30 . 
     It is possible to adjust the timing of yielding of the dampers  30  relative to the horizontal bridge pier displacement amount δ at the time of yielding of the bridge pier  20  by adjusting the height D of the damper  30 . 
     Adjustment for preventing the dampers  30  from yielding is possible by adjusting the height D of the dampers  30 , if the bridge pier  20  is in the range of elasticity. 
     As described above, the bridge pier structure  100  provides efficient seismic reinforcement of the bridge pier  20 . Further, the dampers  30  do not project extensively from the bridge pier side surfaces  21  of the bridge pier  20 , and the area occupied by the dampers  30  serving as reinforcing members is small. The bridge pier structure  100  is therefore applicable to a case in which the bridge pier  20  is located close to a space-occupying structure, such as a road, a path, or an embankment, for example, and a case in which the bridge pier  20  is located in a river, a lake, a marsh, or a sea area, for example. 
     Further, since the dampers  30  are joined by the pin structures, the dampers  30  are subjected only to the force in the axial direction thereof and not to force that bents the dampers  30 . Thus, the designing of the dampers  30  is simplified, and the damping characteristics of the dampers  30  are sufficiently exhibited. 
     Further, the dampers  30  are not limited to the one described above as long as the dampers  30  have the damping characteristics, and the dampers  30  are commonly used. Thus, the dampers  30  are easily selected and procured, and it is possible to manufacture the bridge pier structure  100  at low cost. 
     Embodiment 2 
       FIG. 7A  is a front view illustrating a bridge pier structure according to Embodiment 2 of the present invention.  FIG. 7B  is a plan view illustrating the bridge pier structure according to Embodiment 2 of the present invention, depicting parts of the bridge pier structure. Parts the same as or corresponding to those in Embodiment 1 are designated by the same reference signs, and description of parts thereof will be omitted. Further, respective parts are schematically illustrated, and the present invention is not limited to the illustrated embodiment (in shape and relative size). 
     (Bridge Pier Structure) 
     In  FIGS. 7A and 7B , a bridge pier structure  200  includes bases  13   a ,  13   b ,  13   c , and  13   d  standing on the upper surface  11  of the footing  10 , and footing brackets  12   a ,  12   b ,  12   c , and  12   d  are installed on the bases  13   a ,  13   b ,  13   c , and  13   d . The configurations other than these points are the same as those in the bridge pier structure  100  (Embodiment 1). 
     In this case, respective upper surfaces of the bases  13   a ,  13   b ,  13   c , and  13   d  project upward from the ground surface  92 , and thus the footing brackets  12   a ,  12   b ,  12   c , and  12   d  are exposed above the ground surface  92 . 
     Thus, the corrosion of the dampers  30  is prevented, and it is unnecessary to dig the ground  90  when the dampers  30  themselves or members forming the dampers  30  are to be replaced. 
     Embodiment 3 
       FIGS. 8 to 11  illustrate a bridge pier structure according to Embodiment 3 of the present invention.  FIG. 8  is a left side view,  FIG. 9  is a right side view,  FIG. 10  is a front view, and  FIG. 11  is a cross-sectional view of a plan view (along arrow A in  FIG. 8 ). Parts the same as or corresponding to those in Embodiment 1 are designated by the same reference signs, and description of parts thereof will be omitted. Further, respective parts are schematically illustrated, and the present invention is not limited to the illustrated embodiment (in shape and relative size). 
     (Bridge Pier Structure) 
     In  FIGS. 8 to 11 , a bridge pier structure  300  includes the bases  13   a  and  13   c  along the bridge pier side surfaces  21   a  and  21   c  of the bridge pier  20  on the upper surface  11  of the footing  10 , and has no bases along the bridge pier side surfaces  21   b  and  21   d . Further, dampers  30   e  and  30   f  are disposed parallel to the bridge pier side surface  21   a , and dampers  30   g  and  30   h  are disposed parallel to the bridge pier side surface  21   c . No dampers are disposed along the bridge pier side surfaces  21   b  and  21   d.    
     The configurations other than the configuration for disposing the dampers  30   e ,  30   f ,  30   g , and  30   h  are the same as those in the bridge pier structure  200  (Embodiment 2). Further, the dampers  30   e ,  30   f ,  30   g , and  30   h  are the same as the dampers  30 . Further, in the following description of members having the same structure, the suffixes “e,” “f,” “g,” and “h” added to the reference signs of the members and parts may be omitted for the convenience of description. 
     Footing brackets  12   e  and  12   f  each including a not-illustrated footing pin hole are installed to the base  13   a  extending along the bridge pier side surface  21   a , and a bridge pier bracket  22   a  including a not-illustrated pier pin hole is installed at the center in the horizontal direction of the bridge pier side surface  21   a.    
     Further, similarly, Footing brackets  12   g  and  12   h  each including a not-illustrated footing pin hole are installed to the base  13   c  extending along the bridge pier side surface  21   c , and a bridge pier bracket  22   c  including a not-illustrated pier pin hole is installed at the center in the horizontal direction of the bridge pier side surface  21   c.    
     On the side of the bridge pier side surface  21   a , a lower pin  31   e  inserted in a damper pin hole (not illustrated) provided in the damper  30   e  and the corresponding footing pin hole forms a footing-side pin structure, and the upper pin  32   a  inserted in a damper pin hole (not illustrated) provided in the damper  30   e  and the pier pin hole forms a pier-side pin structure. Similarly, a lower pin  31   f  inserted in a damper pin hole (not illustrated) provided in the damper  30   f  and the corresponding footing pin hole forms a footing-side pin structure, and the upper pin  32   a  inserted in a damper pin hole (not illustrated) provided in the damper  30   f  and the pier pin hole forms a pier-side pin structure. 
     The dampers  30   e  and  30   f  are therefore pin-connected by the upper pin  32   a  at the upper ends thereof, forming an inverse V shape. 
     Further, similarly, on the side of the bridge pier side surface  21   c , the dampers  30   g  and  30   h  are pin-connected by the upper pin  32   c  at the upper ends thereof, forming an inverse V shape. 
     (Operations) 
       FIG. 12  is a left side view illustrating the bridge pier structure  300  according to Embodiment 3 of the present invention, depicting deformation of the bridge pier structure  300  in an earthquake. Respective parts are schematically illustrated, and the same parts as the parts illustrated in  FIGS. 8 to 10  are designated by the same reference signs. 
     In  FIG. 12 , the bridge pier  20  (the bridge pier side surfaces  21   b  and  21   d ) is installed perpendicularly to the upper surface  11  of the footing  10  before the occurrence of the earthquake. Thus, the dampers  30   e  and  30   f  form oblique sides of an isosceles triangle (indicated by broken lines). The dampers  30   g  and  30   h  similarly form oblique sides of an isosceles triangle (not illustrated). 
     Then, as the earthquake occurs, an area of the bridge pier  20  close to the lower end thereof is bent to tilt relative to the upper surface  11  of the footing  10 . Thus, respective upper end portions of the dampers  30   e  and  30   f  (both pin-connected by the upper pin  32   a ) move in a substantially horizontal direction (more accurately, slightly diagonally downward). Thus, the damper  30   e  extends by a distance represented as  34   e  (hereinafter referred to as the “extension amount”), while the damper  30   f  contracts by a distance represented as  34   f  (hereinafter referred to as the “contraction amount”). 
     Further, similarly, on the side of the bridge pier side surface  21   c , the damper  30   g  extends by the extension amount  34   e , while the damper  30   h  contracts by the contraction amount  34   f  (not illustrated). 
     The bridge pier structure  300  therefore exhibits operations and effects similar to those of the bridge pier structures  100  and  200  (Embodiments 1 and 2). 
     Although the upper end portion of the damper  30   e  and the upper end portion of the damper  30   f  overlap each other in the foregoing description, the present invention is not limited thereto. The upper end portion of the damper  30   e  and the upper end portion of the damper  30   f  may be separated from each other, as long the distance between the upper end portion of the damper  30   e  and the upper end portion of the damper  30   f  is different from the distance between the lower end portion of the damper  30   e  and the lower end portion of the damper  30   f . That is, the dampers  30   e  and  30   f  may be arranged in a trapezoidal shape. In this case, the distance between the upper end portions may be longer or shorter than the distance between the lower end portions. 
     Further, the dampers  30   e  and  30   f  may form a triangular shape, with the respective lower end portions of the dampers  30   e  and  30   f  overlapping each other. 
     Embodiment 4 
       FIGS. 13 to 15  illustrate a bridge pier structure according to Embodiment 4 of the present invention.  FIG. 13A  is a side view of the bridge pier structure depicting a section of a part thereof,  FIG. 13B  is a moment diagram depicting the distribution of bending moment, and  FIG. 13C  is a moment diagram illustrating the distribution of bending moment when comparative dampers are installed. In  FIG. 14 , (a) and (b) are side views each depicting a part (damper) of the bridge pier structure.  FIG. 15A  is a correlation diagram depicting the relationship between a seismic load and a horizontal displacement amount, and  FIG. 15B  is a correlation diagram depicting the relationship between force and a displacement amount for illustrating how to determine the length of a part (an axial force member of a damper) of the bridge pier structure. 
     Parts the same as or corresponding to those in Embodiment 1 are designated by the same reference signs, and description of parts thereof will be omitted. Respective parts are schematically illustrated, and the present invention is not limited to the illustrated embodiment (in shape and relative size). 
     (Cut-Off Section) 
     In a bridge pier structure  400  in  FIG. 13A , the bridge pier  20  serving as a pillar member in the bridge pier structure  100  is replaced by a bridge pier  420  having a “cut-off section”, and the dampers  30  is replaced by dampers  430 . 
     That is, the bridge pier  420  includes full-length reinforcing bars  421  disposed over the full length of the bridge pier  420  in the height direction, lower reinforcing bars  422  disposed in a lower portion of the bridge pier  420  in the height direction, and concrete  423 , and a cut-off section  424  is formed at a height corresponding to respective upper ends of the lower reinforcing bars  422 . Further, on the bridge pier side surfaces  21   b  and  21   d  of the bridge pier  420 , bridge pier brackets  22   b  and  22   d  are provided at positions higher than the cut-off section  424 . 
     Further, respective upper end portions of the dampers  430   b  and  430   d  (also collectively referred to as the dampers  430 ) are connected to the bridge pier brackets  22   b  and  22   d . That is, when the distance from the upper surface  11  of the footing  10  to the upper ends of the lower reinforcing bars  422  is referred to as the “minimum damper installation height K,” the dampers  430  have a length covering the “minimum damper installation height K.” 
     Although the description has been given of a case in which the dampers  430   b  and  430   d  are installed for the convenience of description, the present invention is not limited thereto. Thus, the same damper as the damper  430   b  may be each installed to four surfaces of the bridge pier  420 . 
     (Resisting Moment) 
     In  FIG. 13B , seismic bending moment acting on the bridge pier  420  (indicated by a right-downward sloping straight line) is small at an upper portion of the bridge pier  420  and increases toward the footing  10 . 
     In accordance with this, the bridge pier  420  includes the “cut-off section  424 ,” at which the amount of reinforcing bars is changed, at an intermediate position in the height direction of the bridge pier  420 . Therefore, the bending resistance (resisting moment) of the bridge pier  420  is small at the upper portion of the bridge pier  420  and large at a lower portion of the bridge pier  420 , and sharply changes at the cut-off section  424  (indicated by a dash-dotted line). 
     Further, since the dampers  430  are disposed in an area including the cut-off section  424  at which the sharp change in the resisting moment occurs, the value of the resisting moment is increased in an area not reinforced by the lower reinforcing bars  422  (same as the area between the heights K and D) (indicated by a thick solid line). 
     In  FIG. 13C , if dampers shorter than the “minimum damper installation height K” (hereinafter referred to as the “comparative dampers”) are installed, the value of the resisting moment is increased in the area lower than the cut-off section  424 , that is, the area reinforced by the full-length reinforcing bars  421  and the lower reinforcing bars  422 , but fails to be increased in an area higher than the cut-off section  424 , that is, an area not reinforced by the lower reinforcing bars  422  (same as the area between the heights D and K) (indicated by a thick solid line). 
     (Shape of Dampers) 
     In  FIG. 14A , the damper  430  (referred to as the “damper  430 L” for the convenience of description) includes an axial force member  431  having an axial force pipe length L 1 , a stiffener  432  surrounding the axial force member  431 , an upper ferrule  433  connected to respective upper end portions of the axial force member  431  and the stiffener  432 , an upper clevis  434  connected to the upper ferrule  433 , a lower ferrule/reinforcing pipe  435  connected to a lower end portion of the axial force member  431 , and a lower clevis  436  connected to the lower ferrule/reinforcing pipe  435 . 
     In (b) of  FIG. 14 , the damper  430  (referred to as the “damper  430 S” for the convenience of description) is the same in structure as the damper  430 L, but an axial force pipe length L 2  of the axial force member  431  is shorter than the axial force pipe length L 1  in the damper  430 L, and the length of the lower ferrule/reinforcing pipe  435  is longer. 
     (Seismic Load) 
     In  FIG. 15A , the vertical axis represents the seismic load on the bridge pier  420 , and the horizontal axis represents the horizontal displacement amount of the bridge pier  420 . The damper  430 L with the long axial force member  431  elastically deforms until the horizontal displacement amount of the bridge pier  420  reaches a horizontal bridge pier displacement amount δL. After the horizontal displacement amount of the bridge pier  420  reaches the horizontal bridge pier displacement amount δL, the damper  430 L plastically deforms under a constant load (indicated by a dotted line). Meanwhile, the damper  430 S with the short axial force member  431  elastically deforms until the horizontal displacement amount of the bridge pier  420  reaches a horizontal bridge pier displacement amount δS, which less than the horizontal bridge pier displacement amount δL. After the horizontal displacement amount of the bridge pier  420  reaches the horizontal bridge pier displacement amount δS, the damper  430 S plastically deforms under a constant load (indicated by a broken line). 
     Further, the body of the bridge pier  420  elastically deforms until the horizontal displacement amount of the bridge pier  420  reaches the horizontal bridge pier displacement amount δ. After the horizontal displacement amount of the bridge pier  420  reaches the horizontal bridge pier displacement amount δ, the body of the bridge pier  420  plastically deforms under a constant load (indicated by a dash-dotted line). 
     Thus, the seismic load supported by the bridge pier  420  equipped with the damper  430 L changes at the horizontal bridge pier displacement amounts  6  and δL (indicated by a thin solid line). Further, the seismic load supported by the bridge pier  420  equipped with the damper  430 S changes at the horizontal bridge pier displacement amounts δ and δS (indicated by a thick solid line). 
     That is, the yield extension/contraction amount of the axial force member  431  is proportional to the length of the axial force member  431 . Further, it is possible to adjust the yield extension/contraction amount of the damper  430  by changing the length of the axial force member  431 , even if the full length of the damper  430  is fixed. 
     In this case, the full length of the damper  430  needs to cover the “minimum damper installation height K.” In this case, however, it is possible to provide a structure with good energy absorption performance by reducing the length of the axial force member  431  and increasing the length of the lower ferrule/reinforcing pipe  435 . 
     In  FIG. 15B , the seismic load supported by the body of the bridge pier  420  (to which the dampers  430  are not installed) linearly declines after the bridge pier  420  plastically deforms to an allowable displacement (same as allowable pillar member displacement) amount δu. Thereafter, the bridge pier  420  deforms with a constant value until a design displacement amount δ 0  determined by the design energy of the bridge pier  420 . That is, the bridge pier  420  absorbs energy E 420  corresponding to the area indicated by left-downward sloping lines even after the bridge pier  420  is displaced to the allowable displacement amount δu. 
     Meanwhile, the damper  430 S absorbs energy E 430  corresponding to the area indicated by right-downward sloping lines during the displacement to the allowable displacement amount δu. Therefore, the length of the axial force member  431  of the damper  430 S is determined so that the energy E 430  equals or exceeds in value the energy E 420 . 
     Embodiment 5 
       FIGS. 16 and 17  illustrate a bridge pier structure according to Embodiment 5 of the present invention. In  FIG. 16 , (a), (b), and (c) are side views each depicting a part (damper) of the bridge pier structure.  FIG. 17  is a correlation diagram illustrating the relationship between a seismic load on a bridge pier and a horizontal displacement amount of the bridge pier. Parts the same as or corresponding to those in Embodiment 4 are designated by the same reference signs, and description of parts thereof will be omitted. 
     In a not-illustrated bridge pier structure  500 , the dampers  430  in the bridge pier structure  400  (Embodiment 4) are replaced by dampers  530 T,  530 V, or  530 W described below. The parts other than this point are the same as those in the bridge pier structure  400 . The changed parts will be described below. 
     (Stopper) 
     In (a) of  FIG. 16 , a stopper  531  projecting from the outer circumference of the lower ferrule/reinforcing pipe  435  of the damper  430 S is installed to the damper  530 T, and a gap Δ is formed between an upper surface of the stopper  531  and a lower end of the stiffener  432 . After the axial force member  431  contracts and the lower end of the stiffener  432  comes into contact with the stopper  531 , therefore, the axial force member  431  and the stiffener  432  both support compressive force. 
     (Reaction Force Member) 
     In (b) of  FIG. 16 , the stopper  531  projecting from the outer circumference of the lower ferrule/reinforcing pipe  435  of the damper  430 S is installed to the damper  530 V, and a reaction force member  535  is provided to the lower end of the stiffener  432 . The reaction force member  535  includes an upper reaction force plate (same as upper reaction force portion)  532  forming the gap A from the upper surface of the stopper  531 , a lower reaction force plate (same as lower reaction force portion)  534  forming the gap A from a lower surface of the stopper  531 , and a reaction force sleeve  533  connecting the upper reaction force plate  532  and the lower reaction force plate  534  and housing the stopper  531 . 
     After the axial force member  431  contracts and a lower surface of the upper reaction force plate  532  comes into contact with the upper surface of the stopper  531 , therefore, the axial force member  431  and the stiffener  432  both support the compressive force. By contrast, after the axial force member  431  extends and an upper surface of the lower reaction force plate  534  comes into contact with the lower surface of the stopper  531 , the axial force member  431  and the stiffener  432  both support tensile force. 
     (Second Stiffener) 
     In (c) of  FIG. 16 , the damper  530 W has a second stiffener  536  surrounding the stiffener  432  of the damper  530 V and installed to the upper ferrule  433 . 
     Therefore, a gap ▴ is provided between a lower end of the second stiffener  536  and an upper surface of the upper reaction force plate  532 . Thus, the axial force member  431  and the stiffener  432  are stiffened by the second stiffener  536 , and the occurrence of buckling of the axial force member  431  and the stiffener  432  is suppressed. Further, after the axial force member  431  contracts and the lower surface of the upper reaction force plate  532  comes into contact with the upper surface of the stopper  531 , the axial force member  431  and the stiffener  432  both support the compressive force. Further, if the compression is increased, the upper surface of the upper reaction force plate  532  comes into contact with the lower end of the second stiffener  536 , and three members of the axial force member  431 , the second stiffener  536 , and the stiffener  432  support the compressive force. 
     Since the number of members sharing the compressive force is increased in the damper  530 W, as described above, the compressive force acting on each of the members is reduced, and thereby the occurrence of bucking is suppressed. 
     In  FIG. 17 , the value of the gap Δ between the upper surface of the stopper  531  and the lower end of the stiffener  432  satisfies “δu=2·Δ·H/L” in the damper  530 T. Herein, δu represents the allowable displacement (same as allowable pillar member displacement) amount, H represents the height of the bridge pier  420 , and L represents the interval between the dampers  530 T facing each other (see  FIG. 5 ). 
     When the compressive force acts on the damper  530 T and the contraction amount reaches Δ, therefore, the lower end of the stiffener  432  comes into contact with the stopper  531 . Thus, the compressive force acting thereafter is supported by both the axial force member  431  and the stiffener  432 , and the seismic load is increased. Then, the displacement reaches δv, and the stiffener  432  starts to plastically deform (indicated by a broken line). 
     Therefore, the reduction of the seismic load after the displacement of the bridge pier  420  to the allowable displacement amount δu is less in the bridge pier  420  to which the dampers  530 T are installed (indicated by a thick solid line) than in the bridge pier  420  to which the dampers  430 S (indicated by a thin solid line) are installed. 
     Embodiment 6 
       FIGS. 18 and 19  illustrate a bridge pier structure according to Embodiment 6 of the present invention.  FIG. 18  is a side view of the bridge pier structure, and  FIG. 19  is a correlation diagram depicting the relationship between a seismic load and a horizontal displacement amount. Parts the same as or corresponding to those in Embodiment 1 are designated by the same reference signs, and description of parts thereof will be omitted. Respective parts are schematically illustrated, and the present invention is not limited to the illustrated embodiment (in shape and relative size). 
     (Preload) 
     In  FIG. 18 , a bridge pier structure  600  is the same as the bridge pier structure  100 , but the dampers  30   b  and  30   d  are preloaded with force acting in a direction of lifting the floorboard  60 . That is, a portion of the bridge pier  20  between the bridge pier brackets  22  and the upper surface  11  of the footing  10  is constantly (except in an earthquake) stretched by the dampers  30   b  and  30   d.    
     Although the dampers  30   b  and  30   d  installed to the bridge pier  20  are previously contracted, a mechanism for preloading the dampers  30   b  and  30   d  is not limited. Further, although the configuration that preloads the dampers  30   b  and  30   d  is illustrated, the present invention is not limited thereto, and the dampers  30   a  and  30   c  may also be preloaded. Further, preloading may similarly be performed in the bridge pier structures  200  to  500  (Embodiments 2 to 5). 
     (Seismic Load) 
     In  FIG. 19 , the vertical axis represents a seismic load acting on the bridge pier  20 , and the horizontal axis represents a horizontal displacement amount on the bridge pier  20 . In  FIG. 19 , the resistance of the body of the bridge pier  20  is linearly reduced after becoming constant at resistance R 20 . Herein, if a vertical load acting on the bridge pier  20  is large, the range of the resistance R 20  is small, and the resistance is reduced in a relatively small range of the horizontal displacement (indicated by a dotted line). By contrast, if the vertical load acting on the bridge pier  20  is small, the range of the resistance R 20  is increased, and the resistance is reduced in a relatively large range of the horizontal displacement (indicated by a dash-dotted line). 
     Further, the resistance of the dampers  30   b  and  30   d  is linearly increased and thereafter maintained constant at resistance R 30  (indicated by a broken line). 
     Therefore, if the dampers  30   b  and  30   d  not preloaded are installed to the bridge pier  20 , that is, if the vertical load acting on the bridge pier  20  is large, the resistance is reduced at a relatively small value of the horizontal displacement amount (indicated by a thin solid line). 
     Meanwhile, if the preloaded dampers  30   b  and  30   d  are installed to the bridge pier  20 , that is, if the vertical load acting on the bridge pier  20  is small, the resistance is reduced at a relatively large value of the horizontal displacement amount (indicated by a thick solid line). Herein, if the vertical load due to the own weight of the floorboard  60  and other factors and the preload provided to each of the dampers  30   b  and  30   d  are represented as “N 2 ” and “ND,” respectively, a vertical load N 1  acting on a lower portion of the bridge pier  20  (an area lower than the bridge pier brackets  22   b  and  22   d ) is expressed as “N 1 =N 2 −2·ND.” 
     INDUSTRIAL APPLICABILITY 
     The present invention reliably obtains a large plastic deformation amount (seismic energy absorption amount) and allows installation in a relatively small space, and thus is widely applicable as a vibration-damping, earthquake-resistant member of a civil engineering structure or an architectural structure, not limited to the bridge substructure or the like. 
     REFERENCE SIGNS LIST 
       10  footing  11  upper surface  12   a  to  12   h  footing bracket  13   a  to  13   d  base  20  bridge pier  21   a  to  21   d  bridge pier side surface  22   a  to  22   d  bridge pier bracket  30   a  to  30   h  damper  31   a  to  31   h  lower pin  32   a  to  32   d  upper pin  34   a  extension amount  34   c  contraction amount  34   e  extension amount  34   f  contraction amount  40   a ,  40   c  beam  41   a ,  41   c  upper surface  51   a ,  51   c  girder  52   a ,  52   c  girder  60  floorboard  90  ground  91  pile  92  ground surface  100  bridge pier structure (Embodiment 1)  200  bridge pier structure (Embodiment 2)  300  bridge pier structure (Embodiment 3)  400  bridge pier structure (Embodiment 4)  420  bridge pier  421  full-length reinforcing bar  422  lower reinforcing bar  423  concrete  424  cut-off section  430  damper  430 L damper 
       430 S damper  430   b  damper  430   d  damper  431  axial force member  432  stiffener  433  upper ferrule  434  upper clevis  434   d  upper clevis  435  lower ferrule/reinforcing pipe  436  lower clevis  500  bridge pier structure (Embodiment 5)  530 T damper  530 V damper 
       530 W damper  531  stopper  532  upper reaction force plate  533  reaction force sleeve  534  lower reaction force plate  535  reaction force member  536  second stiffener  600  bridge pier structure (Embodiment 6)