Patent Publication Number: US-2016237681-A1

Title: Column base structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a column base structure, in which a junction fitting joined to a lower end of a column member of a building structure is disposed above foundation concrete and fixed to a tip end of an anchor bolt that protrudes upward from the foundation concrete, and the anchor bolt yields before the column member. 
     2. Description of the Conventional Art 
       FIGS. 12 to 15  are figures for describing an ordinary column base structure  2 . 
     As shown in  FIG. 12 , an ordinary column base structure  2  has a column base fitting  6  having front and back surfaces each in an approximately square shape, which is provided above foundation concrete  3  through mortar  8 . The column base fitting  6  is formed of a metal, and on an upper face  6   a  (surface) thereof, a lower end surface of a steel column  4  (column member) in a rectangular tube shape having a length in the vertical direction as in the figure is joined by welding. 
     An upper end of an anchor bolt  10  protruding upward from the interior of the foundation concrete  3  is inserted into a bolt through hole  6   b  in the peripheral portion of the column base fitting  6  and a through hole of a washer  16 . An external thread formed in the upper end of the anchor bolt  10  is screwed in and engaged with an internal thread of a nut member  12 , thereby fixing the steel column  4  standing to the foundation concrete  3  through the column base fitting  6  and the mortar  8  (see, for example, PTL 1). 
     In the foundation concrete  3 , an external thread formed in the lower end of the anchor bolt  10  is screwed in and engaged with an internal thread of a nut member  14 , and a fixing plate  18  is placed on the nut member  14 . 
     In another ordinary column base structure, a column base fitting that is different from a flat plate shape is used instead of the column base fitting  6 , in which the column base fitting is constituted by a bottom plate and a pedestal that protrudes upward in the center portion of the upper surface of the bottom plate in relation to the peripheral portion thereof, and the lower end surface of the steel column is joined to the upper surface of the pedestal by welding (see, for example, PTL 2). 
     In the another column base structure according to PTL 2, an upper end of an anchor bolt protruding upward from foundation concrete is inserted to a bolt through hole penetrating a peripheral portion of the bottom plate of the column base fitting in the thickness direction, and an external thread formed in the anchor bolt is screwed in and engaged with an internal thread of a nut member, thereby fixing the steel column standing to the foundation concrete through the column base fitting and the mortar. 
     As shown in  FIG. 13 , in the case where the steel column  4  in the ordinary column base structure  2  receives a load generating a large bending moment M that is to rotate the steel column  4  in the clockwise direction around an rotation center O, which is the joint portion of the steel column  4  and the column base fitting  6 , for example, due to earthquake or the like as shown in  FIG. 13 , the bending moment M acts to float up the left end as in the figure of the column base fitting  6 . 
     On the contrary, the anchor bolt  10  fixed to the left portion of the column base fitting  6  in relation to the rotation center O as in  FIG. 13  fails to resist to the load generating the bending moment M and starts to yield, thereby absorbing the energy of the load generating the bending moment M through elongation of the length thereof upward as in the figure (i.e., plastic deformation). 
     PRIOR ART DOCUMENT 
     Patent Document 
     JP-A-2003-232078 
     JP-A-2003-336266 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the ordinary column base structure  2 , however, as shown in  FIG. 14 , the anchor bolt  10  having been plastically deformed remains in the elongated state upward as in the figure even after the load generating the bending moment M is removed, and thus a large space S remains between the lower surface of the nut member  12  and the upper surface of the washer  16 . 
     Accordingly, in the case where the load generating the bending moment M is again applied, there is a problem that the energy absorption capability of the ordinary column base structure  2  is considerably lowered as compared to the case where the load generating the bending moment M is received firstly. 
     The problem of the ordinary column base structure  2  will be described in detail with reference to  FIGS. 12 to 15 .  FIG. 15  is a hysteresis diagram (hysteresis characteristic diagram) that schematically shows the relationship between the bending moment M applied to the steel column  4  in the ordinary column base structure  2  (see  FIG. 13 ) and the rotation angle θ of the steel column  4  from the chain line passing vertical through the rotation center O, which is the joint portion to the column base fitting  6  (see  FIG. 13 ). 
     In the ordinary column base structure  2 , the value of the yield bending moment where the anchor bolt  10  starts to yield is set smaller than the value of the yield bending moment where the steel column  4  starts to yield or the value of the yield bending moment where the peripheral portion of the column base fitting  6  starts to yield due to the force received from the anchor bolt  10 , and thus the anchor bolt  10  yields before the steel column  4  and the column base fitting  6 . 
     On acting the bending moment M to the column base structure  2  shown in  FIG. 12 , accordingly, the steel column  4  and the column base fitting  6  are inclined with the left side thereof being higher in height than the right side thereof as in  FIG. 13 , and the anchor bolt  10  on the left side of the rotation center O as in the figure is elongated upward as in the figure by receiving the load generating the bending moment M from the column base fitting  6 . 
     The anchor bolt  10  on the left side as in  FIG. 13  is increased in the length dimension thereof due to elongation through elastic deformation from the state A to the state B in  FIG. 15 , and from the state B to the state C, the anchor bolt  10  starts to yield and is increased in the length dimension thereof due to elongation through plastic deformation. 
     On decreasing the bending moment M, from the state C to the state D in  FIG. 15 , the anchor bolt  10  on the left side as in  FIG. 13  is decreased in the length direction in the amount corresponding to the elongation through elastic deformation, but the amount corresponding to the elongation through plastic deformation from the state B to the state C in the figure remains, and the length thereof remains elongated from the state C to the state E. 
     On acting a bending moment in the opposite direction to the bending moment M (i.e., the anticlockwise direction as in  FIG. 13 ), the process from the state E to the state I through the states F, G and H is the same as the process from the state A to the state E except the bending moment acts in the opposite direction. 
     Accordingly, in the process from the state E to the state I through the states F, G and H in  FIG. 15 , the anchor bolt  10  on the right side as in  FIG. 13  undergoes the same deformation as the deformation of the anchor bolt  10  on the left side as in  FIG. 13  in the process from the state A to the state E in  FIG. 15 . 
     The area of the tetragon formed with the trajectory from the state A to the state E in  FIG. 15  and the area of the tetragon formed with the trajectory from the state E to the state I show the energy absorption amount that is capable of being absorbed by the ordinary column base structure  2  within the states. Accordingly, the ordinary column base structure  2  absorbs the energy without any problem from the state A to the state I. 
     However, on once reaching state  1  in  FIG. 15 , the anchor bolts  10  on both the left and right sides as in  FIG. 14  are elongated upward through plastic deformation as shown in the figure, and therefore the column base fitting  6  is not fixed to the anchor bolts  10 . As a result, a large space S is formed between the lower surface of the nut member  12  and the upper surface of the washer  16 . 
     In the case where the load generating the bending moment M is again applied in state I, the steel column  4  and the column base fitting  6  are inclined with the left side thereof being higher in height than the right side thereof from the state I to the state J, i.e., the period until the lower surface of the nut member  12  is in contact with the upper surface of the washer  16  on the left side as in  FIG. 14 . 
     From the state I to the state J, the load generating the bending moment M is not transmitted from the lower end of the steel column  4  and the column base fitting  6  to the anchor bolt  10 , and thus the ordinary column base structure  2  undergoes a slip phenomenon where no energy is absorbed from the state I to the state J. 
     In this case, since no energy is absorbed by the lower end of the steel column  4 , the column base fitting  6  and the anchor bolt  10  in the ordinary column base structure  2 , the load generating the bending moment M is not attenuated but is rapidly applied to beam members and column members constituting an upper structure above the column base structure  2 , from the state I to the state J. 
     Accordingly, there are possibilities of damaging and breaking the beam members and the column members constituting the upper structure and collapsing the building structure due to the rapid application of the load generating the bending moment M without attenuation. 
     For addressing the problem, the upper structure above the ordinary column base structure  2  is made to withstand the load generating the bending moment M by increasing the diameters and thicknesses of the beam members and the column members, but there is a problem of increasing the upper structure in size, weight and cost. 
     Furthermore, in the case where the anchor bolt  10  has an increased diameter in the ordinary column base structure  2  for increasing the energy generating the bending moment M capable of being absorbed by the anchor bolt  10 , not only the corresponding nut member  12  is increased in size, but also the thickness dimension of the column base fitting  6  is necessarily increased, thereby providing a problem of increasing the column base structure  2  in size, weight and cost. 
     Accordingly, an object of the invention is to provide a column base structure that is capable of enhancing the energy absorption capability of the column base structure, thereby preventing the building structure from being damaged, broken and collapsed, and also preventing the column base structure and the building structure from being increased in size, weight and cost. 
     Means for Solving the Problem 
     For solving the problem, the column base structure according to the invention is: 
     a column base structure containing a junction fitting joined to a lower end of a column member, the junction fitting being fixed in a peripheral portion thereof with an anchor bolt that protrudes upward from foundation concrete, the anchor bolt yielding before the column member, 
     a first bending moment that initiates yield of the anchor bolt being larger than a second bending moment that initiates yield of the peripheral portion of the junction fitting, and being 1.5 times or less the second bending moment. 
     Effect of the Invention 
     According to the column base structure of the invention, which is 
     a column base structure containing a junction fitting joined to a lower end of a column member, the junction fitting being fixed in a peripheral portion thereof with an anchor bolt that protrudes upward from foundation concrete, the anchor bolt yielding before the column member, in which 
     a first bending moment that initiates yield of the anchor bolt is larger than a second bending moment that initiates yield of the peripheral portion of the junction fitting, and is 1.5 times or less the second bending moment, 
     the energy absorption capability of the column base structure can be enhanced, thereby preventing the building structure from being damaged, broken and collapsed, and also preventing the column base structure and the building structure from being increased in size, weight and cost. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIG. 1  is a partially cross sectional side view showing a column base structure  40  according to one embodiment of the invention. 
         FIG. 2  is a schematic partially cross sectional side view for describing the state where a bending moment M is applied to the column base structure  40  shown in  FIG. 1 . 
         FIG. 3  is a schematic partially cross sectional side view for describing the state where the bending moment M is applied to the column base structure  40  shown in  FIG. 1 , and thereby an anchor bolt  10  yields. 
         FIG. 4  is a hysteresis diagram schematically showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 1 . 
         FIG. 5  is a schematic illustration showing a two-layer model  50 . 
         FIG. 6  is a schematic illustration showing a four-layer model  52 . 
         FIG. 7  is a hysteresis diagram showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 12 . 
         FIG. 8  is a hysteresis diagram showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 1 . 
         FIG. 9  is a hysteresis diagram schematically showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in the case where the steel column  4  in  FIGS. 1 and 12  is joined to the foundation concrete  3  with an embedded column base. 
         FIG. 10  is a comparative diagram of column base absorption energy index in the ordinary column base structure  2  and the two-layer model  50  of the column base structure  40  of the invention. 
         FIG. 11  is a comparative diagram of column base absorption energy index in the ordinary column base structure  2  and the four-layer model  52  of the column base structure  40  of the invention. 
         FIG. 12  is a partially cross sectional side view showing the ordinary column base structure  2 . 
         FIG. 13  a schematic partially cross sectional side view for describing the state where a bending moment M is applied to the column base structure  2  shown in  FIG. 12 . 
         FIG. 14  a schematic partially cross sectional side view for describing the state where the bending moment M is applied to the column base structure  2  shown in  FIG. 12 , and thereby an anchor bolt  10  yields. 
         FIG. 15  is a hysteresis diagram schematically showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 12 . 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           2  column base structure 
           3  foundation concrete 
           4  steel column 
           6  column base fitting 
           6   a  upper surface 
           6   b  bolt through hole 
           8  mortar 
           10  anchor bolt 
           12 ,  14  nut member 
           16  washer 
           18  fixing plate 
           40  column base structure 
           42  column base fitting 
           42   a  upper surface 
           42   b  bolt through hole 
           42   c  bent portion 
           50  two-layer model 
           52  four-layer model 
           53  foundation surface 
           54  column member 
           56  beam member 
           58  joint portion 
           60  column base portion 
         A, B, C, D, E, F, G, H, I, J, K, L, N state 
         a, b, c building 
         M bending moment 
         Ma, Mb yield initiating bending moment 
         O rotation center 
         Q area 
         S, S 1  space 
         θ angle 
       
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments for carrying out the column base structure according to the invention will be described specifically with reference to the drawings below. 
       FIGS. 1 to 11  are illustrations for describing a column base structure  40  according to one embodiment of the invention. In the following, the same parts as in the ordinary column base structure  2  described above are described with the same symbols attached, and duplicate descriptions for the structures that are same as the ordinary one are omitted except for some parts. 
     A column base structure  40  according to this embodiment has a column base fitting  42  (i.e., the junction fitting) in the form of a flat plate that is provided above foundation concrete  3  through mortar  8 , as shown in  FIG. 1 . The column base fitting  40  is formed of a metal and has square front and back surfaces. 
     The column base fitting  42  has on an upper surface thereof  42   a  a lower end surface of a steel column  4  (i.e., the column member) abutted thereto, which are welded to each other. The steel column  4  has a length in the vertical direction as in  FIG. 1  and is formed in a hollow rectangular tube shape. 
     An upper end of an anchor bolt  10  protruding upward from the interior of the foundation concrete  3  is inserted into a bolt through hole  42   b  formed in the column base fitting  42 . 
     An external thread formed in the upper end of the anchor bolt  10  protruding upward from the column base fitting  42  is screwed in and engaged with an internal thread of a nut member  12 , thereby fixing the steel column  4  standing to the foundation concrete  3  through the column base fitting  42  and the mortar  8 . 
     In the column base fitting  42  of this embodiment, the column base fitting  42  has a peripheral portion that has a smaller thickness than the ordinary column base fitting  6 , thereby setting in such a manner that the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42  on receiving the force from the anchor bolt  10  (i.e., the second bending moment) is smaller than the yield initiating bending moment that initiates yield of the peripheral portion of the column base fitting  6  on receiving the force from the anchor bolt  10  in the ordinary column base structure  2 . 
     In the column base structure  40  of this embodiment, the yield initiating bending moment Ma that initiates yield of the anchor bolt  10  (i.e., the first bending moment) is larger than the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42  on receiving the force from the anchor bolt  10 , and is 1.5 times or less the yield initiating bending moment Mb. 
     According to the constitution, when the load generating a bending moment is being increased, the peripheral portion of the column base fitting  42  starts to yield before the anchor bolt  10 . 
     In the case where the yield initiating bending moment Ma that initiates yield of the anchor bolt  10  is equal to or smaller than the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42 , on the contrary, when the load generating a bending moment is being increased, the anchor bolt  10  yields before the column base fitting  42 , thereby failing to solve the problem of the ordinary column base structure  2 . 
     In the case where the yield initiating bending moment Ma that initiates yield of the anchor bolt  10  is larger than  1 . 5  times the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42 , the plastic deformation of the column base fitting  42  proceeds excessively before the anchor bolt  10  yields and undergoes plastic deformation, thereby bending excessively the portion of the column base fitting  42  that is fixed with the anchor bolt  10  and the surrounding thereof, and thus the anchor bolt  10  is pressed to a direction that is not the elongation direction thereof, which may cause breakage of the anchor bolt  10 . 
     For preventing the column member  4  from yielding and undergoing plastic deformation before the anchor bolt  10  and the column base fitting  42 , the yield initiating bending moment that initiates yield of the column member  4  is set to a value that is larger than the yield initiating bending moment Ma that initiates yield of the anchor bolt  10 , the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42  on receiving the force from the anchor bolt  10 , and the yield initiating bending moment that initiates yield of the other portion of the column base fitting  42  than the peripheral portion. 
     For preventing the foundation concrete  3  or the mortar  8  from undergoing compressive yield and plastic deformation before the column base fitting  42 , the yield initiating bending moment that initiates compressive yield of the foundation concrete  3  or the mortar  8  is set to a value that is larger than the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42  on receiving the force from the anchor bolt  10 . 
     In the case where the column base structure  40  according to this embodiment receives a load generating a large bending moment M that is to rotate the steel column  4  in the clockwise direction with an rotation center O, which is the joint portion of the steel column  4  and the column base fitting  6 , as shown in  FIG. 2 , the peripheral portion of the column base fitting  42  in the vicinity of the bolt through hole  42   b  on the left side as in  FIG. 2  yields and undergoes plastic deformation on receiving the force from the nut member  12  mounted on the anchor bolt  10 , and the left end portion of the column base fitting  42  on the left side of the plastically deformed portion as in  FIG. 2  is bent in an inverted-V shape with the left end being lower in height than the plastically deformed portion, from the state B to the state C in  FIG. 4 . 
     The portion of the column base fitting  42  that is bent in an inverted-V shape is bent in the horizontal direction and returns to the approximately straight shape on receiving a load generating a bending moment in the anticlockwise direction, which is opposite to the direction of the bending moment M, from the state F to the state G in  FIG. 4 . 
     The portion on the right side as in  FIG. 2  of the column base fitting  42  receiving the load generating the bending moment M in the clockwise direction yields and undergoes plastic deformation by the force pressing onto the upper surface of the mortar  8  on the foundation concrete  3 , and is bent in a V shape with the right end portion of the column base fitting  42  as in the figure being higher in height than the plastically deformed portion. 
     The anchor bolt  10  on the left side as in  FIG. 2  yields beyond the elastic range thereof and elongated through plastic deformation, and the length dimension thereof is increased, from the state B to the state C in  FIG. 4 . 
     The process from the state E to the state I through the states F, G and H in  FIG. 4  is the same as the process from the state A to the state E except the bending moment acts in the opposite direction. 
     Accordingly, in the process from the state E to the state I through the states F, G and H in  FIG. 2 , the anchor bolt  10  on the right side as in  FIG. 2  undergoes the same deformation as the deformation of the anchor bolt  10  on the left side as in  FIG. 2  in the process from the state A to the state E through the states B, C and D in  FIG. 4 . 
     In the process from the state E to the state I through the states F, G and H in  FIG. 4 , the portion on the left side as in  FIG. 2  of the column base fitting  42  yields and undergoes plastic deformation by the force pressing onto the upper surface of the mortar  8  on the foundation concrete  3 , and is bent in a reverse dogleg shape with the left end portion of the column base fitting  42  as in the figure being higher in height than the plastically deformed portion. 
       FIG. 3  shows the column base structure  40  in the states I and J in  FIG. 4 , in which both the left and right end portions of the column base fitting  42  as in the figure each have a bent portion  42   c  bent upward as in the figure, and thereby the space S 1  between the lower surface of the nut member  12  and the upper surface of the washer  16  on the bent portion  42   c  of the column base fitting  42  is considerably smaller than the space S in the ordinary column base structure  2 . 
     Accordingly, in the case where the load generating the bending moment M is again applied in state I, the lower surface of the nut member  12  and the upper surface of the washer  16  on the bent portion  42   c  of the column base fitting  42  on the left side as in  FIG. 3  are approximately immediately in contact with each other, and thus the slip phenomenon as in the ordinary column base structure  2  is difficult to occur. 
     As described above, the load generating the bending moment M can be approximately immediately transferred from the lower end of the steel column  4  and the column base fitting  42  to the anchor bolt  10 , and therefore the column base structure  40  allows the lower end of the steel column  4 , the column base fitting  42  and the anchor bolt  10  to absorb the energy by tracing the trajectory from the state J to the state K in  FIG. 4  that is approximate to approximately elastic deformation. 
     Accordingly, the load generating the bending moment M is not applied entirely to beam members and column members constituting an upper structure above the column base structure  40 , and the upper structure above the column base structure  40  may bear only a part of the load. 
     Consequently, the column base structure  40  according to this embodiment absorbs the energy corresponding to the area Q of the triangle (hatched area) formed with the trajectory from the state J to the state N through the states K and L in  FIG. 4 , and the load generating the bending moment M can be borne by the lower end of the steel column  4 , the column base fitting  42  and the anchor bolt  10 . 
     A test performed for the ordinary column base structure  2  and the column base structure  40  according to this embodiment will be described with reference to  FIGS. 5 to 11  below. The test is performed with a two-layer model  50  shown in  FIG. 5  and a four-layer model  52  shown in  FIG. 6 , to which earthquake waves (El Centro NS) are applied in the state where a prescribed weight is loaded thereto. 
     The two-layer model  50  as shown in  FIG. 5  has three column members  54  that are disposed while being spaced from each other in the right-and-left direction as in the figure, and those column members  54  each stand on a foundation surface  53  through a column base portion  60 . 
     Four beam members  56  are disposed in such a manner that the members are divided vertically into two tiers with two members each being disposed in series in approximately parallel to the foundation surface  53 . Those beam members  56  are rigidly fixed to the column members  54  at joint portions  58  where both the ends in the longitudinal direction of the beam members  56  are abutted to the side of the column member  54 . 
     The four-layer model  52  as shown in  FIG. 6  has three column members  54  that are disposed while being spaced from each other in the right-and-left direction as in the figure, and the column members  54  each stand on a foundation surface  53  through a column base portion  60 , as similar to the two-layer model  50 . 
     Eight beam members  56  are disposed in such a manner that the members are divided vertically into four tiers with two members each being disposed in series in approximately parallel to the foundation surface  53 . The beam members  56  are rigidly fixed to the column members  54  at joint portions  58  where both the ends in the longitudinal direction of the beam members  56  are abutted to the side of the column member  54 . 
       FIG. 7  is a hysteresis diagram showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 13  obtained by performing the test with the ordinary column base structure  2  as the column base portion  60  of the two-layer model  50 . As shown in  FIG. 7 , the ordinary column base structure  2  undergoes the slip phenomenon as shown from the state I to the state J in  FIG. 15 . 
     On the other hand,  FIG. 8  is a hysteresis diagram showing the relationship between the bending moment M applied to the steel column  4  and the rotation angle θ thereof in  FIG. 2  obtained by performing the test with the column base structure  40  according to this embodiment as the column base portion  60  of the two-layer model  50 . As shown in  FIG. 8 , as different from the ordinary column base structure  2 , the column base structure  40  according to this embodiment does not undergo the slip phenomenon as shown from the state I to the state J in  FIG. 15 . 
       FIG. 8  shows the case where the yield initiating bending moment Ma that initiates yield of the anchor bolt  10  is set to a value that is 1.5 times the yield initiating bending moment Mb that initiates yield of the peripheral portion of the column base fitting  42 , and the cases where the yield initiating bending moment Ma is set to a range that is larger than the yield initiating bending moment Mb and is less than the yield initiating bending moment Mb show the similar tendencies as in  FIG. 8 . 
     The column base structure  40  used in the test is practiced under the same conditions as in the ordinary column base structure  2  except that the yield initiating bending moment Mb of the peripheral portion of the column base fitting  42  is set within the aforementioned range and is smaller than the yield initiating bending moment of the peripheral portion of the column base fitting  6  of the ordinary column base structure  2 . 
     The four-layer model  52  shows the similar tendencies as in the two-layer model  50  shown in  FIGS. 7 and 8 . 
       FIGS. 10 and 11  are diagrams for comparing the column base absorption energy index (W 1 /W 0 ) of the ordinary column base structure  2  and the column base structure  40  according to this embodiment.  FIG. 10  shows the comparison of the column base absorption energy index in the two-layer model  50 , and FIG.  11  shows the comparison of the column base absorption energy index in the four-layer model  52 . 
     The column base absorption energy index herein is a dimensionless value obtained by dividing the exposed-type column base absorption energy W 1  capable of being absorbed by the ordinary column base structure  2  or the column base structure  40  according to this embodiment by the embedded-type column base absorption energy WO capable of being absorbed by the column member  54  in an embedded-type column base, and a larger column base absorption energy index means a larger amount of the column base absorption energy capable of being absorbed by the column base structure. 
       FIG. 9  is a hysteresis diagram showing the relationship between the bending moment M and the rotation angle θ of the column member  54  in the embedded-type column base, i.e., the lower end of the column member  54  is embedded beneath the foundation surface  53  without the use of the column base portion  60 . 
     The column member  54  in the embedded-type column base absorbs energy corresponding to the area of the tetragon formed with the trajectory from the state B returning to the state B again through the states C, D, E and F in the figure, and this is designated as the embedded-type column base absorption energy W 0 . 
     On the contrary, the exposed-type column base absorption energy W 1  is calculated from the hysteresis diagram of the ordinary column base structure  2  or the column base structure  40  according to this embodiment as shown in  FIG. 7 or 8 . 
     In the case of the two-layer model  50  shown in  FIG. 10 , the three buildings a, b and c having the ordinary column base structure  2  each have a value of the column base absorption energy index of 0.25 or less, but the three buildings a, b and c having the column base structure  40  according to this embodiment each have a value of the column base absorption energy index of 0.5 or more. 
     Here, the three buildings a, b and c each have column members and beam members that are different in strength from those of the other buildings. 
     In the case of the four-layer model  52  shown in  FIG. 11 , the three buildings a, b and c having the ordinary column base structure  2  each have a value of the column base absorption energy index of 0.15 or less, but the three buildings a, b and c having the column base structure  40  according to this embodiment each have a value of the column base absorption energy index of 0.3 or more. 
     As shown in  FIGS. 10 and 11 , the column base structure  40  according to this embodiment can enhance the column base absorption energy amount as compared to the ordinary column base structure  2 . 
     In the column base structure  40  according to this embodiment, the bent portions  42   c  are formed in both the left and right end portions of the column base fitting  42 , and thereby the space S 1  between the lower surface of the nut member  12  mounted on the anchor bolt  10  and the upper surface of the washer  16  on the bent portion  42   c  of the column base fitting  42  can be decreased, as shown in  FIG. 3 . 
     In the column base structure  40  according to this embodiment, furthermore, the energy amount capable of being absorbed by the column base structure  40  can be enlarged even after the anchor bolt  10  yields, as shown in  FIG. 4 . 
     Specifically, the column base structure  40  according to this embodiment does not undergo the slip phenomenon as in the ordinary column base structure  2 , and thus a part of the load generating the bending moment M is applied to the lower end of the steel column  4 , the column base fitting  42  and the anchor bolt  10 , whereby the load generating the bending moment M is not applied entirely to beam members and column members constituting the upper structure above the column base structure  40 , and the upper structure above the column base structure  40  may bear only the part of the load. 
     Consequently, the column members, beam members and the like constituting the upper structure above the column base structure  40  can be prevented from being damaged and broken, the building structure can be prevented from being collapsed, and the column members, beam members and the like constituting the upper structure can be prevented from being increased in size, weight and cost. 
     Furthermore, the column base structure  40  according to this embodiment may not necessarily use an anchor bolt  10  having a large outer diameter, and thus the anchor bolt  10  and the column base fitting  42  constituting the column base structure  40  can be prevented from being increased in size, weight and cost. 
     As described above, according to the column base structure  40  according to this embodiment, the energy absorption capability of the column base structure  40  is enhanced, thereby preventing the building structure from being damaged, broken and collapsed, and also preventing the column base structure  40  and the building structure from being increased in size, weight and cost. 
     The invention is not limited to the aforementioned embodiment, and various changes may be made in the column base structure within the range capable of achieving the objects of the invention. 
     For example, the case where the column base fitting  42  has front and back surfaces each in an approximately square shape and has the tabular shape is described for the column base structure  40  according to the aforementioned embodiment, but the front and back surfaces may be other tetragonal shapes in addition to the square shape, the tetragonal shapes having different vertical and horizontal lengths. 
     The column base fitting may be constituted by a bottom plate and a pedestal that protrudes upward in the center portion of the upper surface of the bottom plate in relation to the peripheral portion thereof as in the column base fitting according to PTL 2, and may have other constitutions than these. 
     In the column base structure  40  according to the aforementioned embodiment, the steel column  4 , the lower end surface of which is joined to the column base fitting  42 , is formed in a rectangular tube shape, but is not limited to the shape and may be, for example, in a cylinder shape. The steel column may also be in a solid core form. 
     In the column base structure  40  according to the aforementioned embodiment, the external thread formed in the upper end of the anchor bolt  10  protruding upward from the column base fitting  42  is screwed in and engaged with the internal thread of one nut member  12 , but two or more nut members  12  may be mounted (e.g., a double nut member). 
     In the column base structure  40  according to the aforementioned embodiment, the external thread formed in the lower end of the anchor bolt  10  in the foundation concrete  3  is screwed in and engaged with the internal thread of one nut member  14 , but two or more nut members  14  may be mounted, and the fixing plate  18  may be provided between the two nut members  14 .