JPH0814110B2 - Steel column base - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel column base in which a steel column of a steel building is fixed to a foundation concrete via a column base metal.

[0002]

2. Description of the Related Art The lower end of a steel column is welded or bolted to a column pedestal, and the column pedestal is fixed to an anchor bolt in basic concrete with a nut. Pillars are fixed. In this case, since a lateral force is applied to the steel column, a pulling force is applied to the anchor bolt in the foundation concrete through the column base metal. Therefore, in the conventional construction method that secures the anchoring yield strength of the anchor bolt by the yield strength of the basic concrete, the size of the basic concrete must be increased.

For example, if there are eight anchor bolts,
As shown in FIGS. 16 and 17, when a bending moment M acts on the steel frame column 1 due to an earthquake or the like, a pulling force T acts on the anchor bolt 4 at the outermost edge via the column base metal piece 2. Then, stress is applied to the basic concrete 3, and when it exceeds the proof stress of the basic concrete, the basic concrete 3 is destroyed. Specifically, as shown in FIG. 17, each anchor bolt 4
The cone breaking surface 6 of the basic concrete 3 is formed by the above, but the area 61 (A
When ccm ² ) cannot satisfy the equation (1), the basic concrete 3 is destroyed. As described above, in the case of the structure in which the bending moment M related to the steel column 1 is supported only by the anchor bolt 4, the area of the basic concrete 3 required to secure the anchoring strength of the anchor bolt 4 is large as shown in FIG. Become. Ac ≧ {σ _u /0.8(FC) ^1/2 } · Ae ・ ・ ・ (1) Ac: Horizontal projected area of cone-like fracture surface of concrete [cm ² ] σ _u : Tensile strength of anchor bolt [ Kg / cm ² ] Ae: Effective cross-sectional area of anchor bolt thread [cm ² ]

Even if the number of anchor bolts is four,
As shown in FIG. 18 and FIG. 19, in the case of the corner post, since the foundation concrete 3 is smaller than the area 61 of the cone breaking surface by the anchor bolt 4 at the outermost edge, the anchor bolt 4
Cannot secure sufficient fixing resistance. This is not limited to the corner post, and is the same for the side post.

Therefore, as shown in FIG. 21, in the steel column base having a structure in which the bending moment M applied to the steel column 1 is supported only by the anchor bolts, the size of the basic concrete 3 in each column base metal 2 becomes large. , In each column base, the concrete foundation 3 extends outside the foundation beam 32.
This not only spoils the aesthetics of the building, but also requires that the formwork be made uneven, which makes the construction itself complicated. On the other hand, if the size of the basic concrete 3 in each column base metal 2 can be reduced, as shown in FIG. 20, the basic concrete 3 does not extend to the outside of the building, and the outer circumference of the basic concrete is straight. Since it can be formed into a shape, the construction becomes easy.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a steel column base capable of ensuring anchoring resistance of anchor bolts without increasing the size of basic concrete.

[0007]

As a result of earnest research in view of the above-mentioned object, the present inventor has found that the reinforcement bolt is provided at a predetermined distance on the outer circumference of the anchor bolt in the basic concrete, and the yield stress of the entire reinforcement is determined by the anchor bolt. By setting it larger than the overall yield stress, it was discovered that the anchoring bolt anchoring proof strength can be sufficiently secured even if the dimensions of the basic concrete are reduced to such an extent that the appearance of the building is not impaired. completed.

That is, the steel column pedestal of the present invention comprises a pedestal metal member joined to the lower end of the steel column fixed on the basic concrete by anchor bolts and nuts embedded in the basic concrete. a) Reinforcing bars are embedded in the outer periphery of the anchor bolts in the basic concrete, (b) The distance between the reinforcing bars and the anchor bolts is within 12 times the diameter of each anchor bolt, and (c) the breaking of the reinforcing bars. The total area (Ar) is relative to the total cross-sectional area (Ab) of the anchor bolts arranged at the outermost edge: Ar ≧ (bF / γF) · Ab (where bF is the yield stress of the anchor bolt material) , ΓF is the yield stress of the reinforcing bar material.).

[0009]

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a sectional view showing an example of a steel column base of the present invention. For simplification of the drawings, hatching is omitted for members other than the column base metal. The steel column 1 is joined to a central projection (a rising portion) 2a of a column base metal piece 2 by welding or bolting, and the column base metal piece 2 is fixed to an anchor bolt 4 in a foundation concrete 3 by a nut 42. The anchor bolt 4 is threaded at the upper and lower ends, the anchor plate 41 for fixing is screwed on the lower end, and the nut 4 for fixing the column leg metalwork 2 is attached to the screw part at the upper end.
2 is screwed on. The central portion of the anchor bolt 4 is covered with a plastic pipe 43, which is an unrestrained portion. In order to increase the proof stress against the lateral stress applied to the steel column 1, the anchor bolt 4 is provided with an introduction tension so that a stress intensity of 0.15 to 1.2 times the yield stress intensity is generated (Patent Publication 2- 14496). In addition, in FIG. 1, 31 indicates a mortar provided on the foundation concrete 3,
The pedestal metal fitting 2 is for stationary placement.

FIG. 1 and FIG. 2 which is a sectional view taken along line AA of FIG.
The first feature of the present invention is that a reinforcing bar (rebar) 5 is provided on the outer circumference of the anchor bolt 4 located on the side of the square. Although the number of anchor bolts is eight in this embodiment, the number of anchor bolts is not limited to this.

As is apparent from FIGS. 1 and 2, the reinforcing bars 5 provided on the outer circumference of the anchor bolt 4 are composed of a plurality of vertical reinforcing bars 51 and a plurality of horizontal reinforcing bars 52. The upper end portion of the vertical reinforcing bar 51 has a hook shape, but this is for strengthening the fixation to the base concrete and may have a shape other than the hook shape.

The second feature of the present invention is that the distance between the reinforcing bar 5 and the anchor bolt 4 is 12 which is the diameter D of the anchor bolt 4.
That is within the point. This distance is the distance between the axis of the anchor bolt 4 and the axis of the reinforcing bar 5. When the distance between the reinforcing bar 5 and the anchor bolt 4 is larger than 12 which is the diameter D of the anchor bolt, the reinforcing effect of the reinforcing bar 5 becomes insufficient. As an ordinary anchor bolt, 19-72 mm
Since the diameter is about the same, it is preferable that the distance between the reinforcing bar 5 and the anchor bolt 4 is within 225 mm.

A third feature of the present invention is that the total cross-sectional area (Ar) of the reinforcing bar satisfies the following relationship with the total cross-sectional area (Ab) of the anchor bolt 4. Ar ≧ (bF / γF) · Ab (where bF is the yield stress of the material of the anchor bolt 4 and γF is the yield stress of the material of the reinforcing bar 5)

The above equation can be transformed into Ar × γF ≧ Ab × bF. This formula means that the total tensile yield strength of the reinforcing bar 5 must be larger than the total tensile yield strength of the anchor bolt 4. Thereby, the steel pillar 1 can be supported without causing the destruction of the basic concrete 3.

FIG. 3 is a sectional view taken along line BB of FIG. As is clear from FIG. 3, in the steel column base having the reinforcing bars 5 satisfying the requirements of the present invention, the horizontal cross-sectional area of the rising portion 31 of the basic concrete 3 is slightly larger than the area of the bottom plate of the column base metal fitting 2. Only. Therefore, the base concrete 33 is less likely to protrude from the building in the steel column base portion.

In the steel column base having the above structure, due to the bending moment M generated in the column base due to an earthquake or a windstorm,
A pulling force T is generated on each anchor bolt 4. At that time, in the concrete of the rising portion 33 of the basic concrete 3, from the end of the fixing plate 41 of the anchor bolt 4
The cone breaking surface 6 generated 45 ° above is the vertical rebar
The pulling force T of the anchor bolt 4 is transmitted to the vertical reinforcing bar 51 from the intersecting point 51. As described above, the tensile yield strength (Ar · γF) of the reinforcing bar 5 in the vertical direction is larger than the tensile yield strength (Tmax = Ab · bF) of the anchor bolt 4, so that the anchoring force of the anchor bolt 4 is set to the vertical of the foundation rising portion. It can be secured by the reinforcing bar 51 in the direction.
As shown in FIG. 3, in the rebar column base of this embodiment, the anchor bolts and the rebars are arranged in the same direction in both the X and Y directions, so that the bending moment M acts in either direction due to an earthquake or the like. But just like the anchor bolt 4
Will be secured.

FIG. 4 is a sectional view showing another embodiment of the present invention. In the figure, the same members as those in FIG. 1 are given the same numbers as those in FIG. 5 is a sectional view taken along line CC of FIG. 4, and FIG. 6 is a sectional view taken along line DD of FIG.

In this embodiment, H is used as the steel column 1.
Since the shape steel is used, the bottom plate of the column base metal member 2 has a rectangular shape. In this case, the outer peripheral reinforcing bar 5 has a requirement (b) in both the X direction and the Y direction (the distance between the reinforcing bar 5 and the anchor bolt 4 is within 12 times the diameter D of the anchor bolt 4).
Must be met. FIG. 6 shows that the vertical reinforcing bars 51 enter in a circle having a radius r (within 12D) centered on each anchor bolt 4. Moreover, it is natural that the requirement (c) (the total tensile yield strength of the reinforcing bar 5 is larger than the total tensile yield strength of the anchor bolt 4) must be satisfied.

The present invention will be described in more detail with reference to the following specific examples.

Example 1 Using the anchor bolts and reinforcing bars (reinforcing bars) shown in FIGS. 7 and 8 and Table 1, each test piece was prepared. The size of the test piece and the distance between the anchor bolt and the reinforcing bar are as shown. The vertical reinforcing bars 51 in the rising portion 33 are the test bodies B-1 to B-1.
For B-3, a diameter of 19 mm (D1
9), and for B-4 there are 6 diameters of 19 mm (D1
9) consists of. The horizontal rebar 52 has a diameter of 19 mm.
SD35 material. Furthermore, the anchor bolt has a diameter of 42 mm.
(Φ42) SS50 material. The rebar in the basic concrete 3 is SD35 material with a diameter of 35 mm (D35) in the vertical part.
The horizontal part is SD35 material with a diameter of 25 mm (D25).

A strain gauge was attached to each bar and the tensile stress applied to the bar was measured. The F of the concrete used
c was 210 kg / cm ² . The mechanical properties of each member are shown in Tables 2-4.

[0022] Table 1 specification specimen symbol number e of the specimen _{(mm) e 0 (mm)} e 1 (mm) T B-1 3 75 75 100 0.52 B-2 1 150 75 100 0.52 B-3 1 225 75 100 0.52 B-4 1 150 75 75 1.55 ( _{Note) T = (n · a r} · f r) / (a a · f a) where n:添筋the number a _r: cross-sectional area of添筋one a _a: cross-sectional area of the anchor bolt one f _r:添筋tensile yield strength f _a: tensile yield strength of the anchor bolt

Table 2 Mechanical Properties of Steel Material Yield Stress Degree Tensile Strength Elongation Type Material (Kgf / mm ² ) (Kgf / mm ² ) (%) D13 SD30 38.8 55.3 18 D19 SD30 38.2 58.2 16 D25 SD30 37.3 61.1 15 φ42 SS50 33.0 53.7 34

Table 3 Results of tensile test of anchor bolt Thread yield Yield shaft Yield screw fracture Anchor bolt (tf) (tf) (tf) φ42 36.5 45.7 60.3

Table 4 Mechanical properties of concrete Concrete age during curing test Compressive strength Type of elastic modulus method (days) (Kgf / cm ² ) (Kgf / cm ² ) Normal concrete sealing 14 225 2.28 × 10 ⁵

(1) Relationship between load and slip-out deformation For a test body consisting of the anchor bolt, reinforcing bar and concrete of the above strength, a tensile force T is applied to the anchor bolt while the upper surface of the foundation beam is simply supported, The slip-out and strains of the reinforcement and the anchor bolt were measured. FIG. 9 shows the relationship between the pull-out load (T) of the test body and the slip-out deformation (δ) of the anchor bolt. Table 5 shows the main loads in FIG.

Table 5 Adhesive fracture load of cone-shaped reinforcement bars Maximum load test load (tf) (tf) (tf) Failure type B-1-1 10 32.2 32.2 Adhesive failure of reinforcement bar B-1-2 15 31.6 31.6 Adhesive failure of rebar B-1-3 19 32.5 32.5 Adhesive failure of rebar B-2 12 31.4 31.4 Adhesive failure of rebar B-3 19 27.9 31.7 Adhesive failure of rebar B-4 27-60.9 Anchor bolt thread Break

The three bodies B-1 to B-3 having different distances (e) from the anchor bolts to the reinforcement bars have a total tensile yield strength of the reinforcement bars which is only about 52% of that of the anchor bolts. In addition, the add-on bar has adhesive failure after tensile yielding. These three
Comparing the T-δ curves of the body, the adhesion failure load of the added muscle is e
The tendency is that the larger the value becomes, the smaller the value becomes.
Is not so different from the case of e = 75, 150 mm. Therefore, e is 225
It is considered that reinforcement by reinforcement is effective even at about mm. on the other hand,
The total tensile strength of the reinforcement bars is about the tensile strength of the anchor bolt.
B-4, which has been reinforced to be 1.5 times, has the same properties as the tensile load-elongation relationship of anchor bolts.
It can be seen that the overall properties are governed by the properties of the anchor bolt. Based on the comparison of the three B-1 bodies, it can be judged that the number of strain gauges attached to the reinforcing bar has little effect on the adhesive fracture resistance of the reinforcing bar.

(2) Load-strain relationship As shown in FIG. 8 (b), the load at the position where the above-mentioned hypothetical cone fracture surface intersects the reinforcing bar for two reinforcing bars at different positions on B-4. -The strain curve was determined. This is shown in FIG. According to this, the two reinforcements show almost the same load-strain relationship. From this fact, it is estimated that all six supplementary muscles exerted their effects substantially evenly. In addition, the maximum strain of the added reinforcement at the yield of the anchor bolt shaft is about the yield strain.
It was 3/4. From this result, when the amount of added reinforcement for ensuring the yield of the anchor bolt shaft portion is calculated backward, the total tensile yield strength should exceed the yield strength of the anchor bolt shaft portion.

From the above experiment, the following was found. (a) When the position of the reinforcing bar is 225 mm from the anchor bolt, the reinforcing bar is effective as a reinforcing bar. (b) If the total tensile yield strength of the reinforcement exceeds the yield strength of the anchor bolt, anchoring strength of the anchor bolt is secured. (c) If the total tensile yield strength of the reinforcement is about 1.5 times the tensile yield strength of the anchor bolt, its properties are governed by the properties of the anchor bolt.

Example 2 The test body is a column base made of a square steel pipe column having a side of 120 cm and is reduced to 1/2, and the number of anchor bolts is 8 (HB-8) and 16 ( HB-16) type column base metal fittings. The shape of each column base metal fitting is shown in Figure 11 to Figure 13.
Fig. 14 shows the details of the test specimen. When HB-8 is used, the reinforcing bar 51 has a diameter of 25 mm (D25) is 40, and
When B-16 is used, the reinforcement 51 has a diameter of 19 mm (D19)
Was 68. Steel column 1 is 600 mm x 600 mm x 32 mm
It was a prism made of SM50 material. Here, the tensile strength of the added reinforcement of both test specimens is designed to be equal to or more than the tensile strength of the anchor bolt {(102% (HB-8), 116% (HB-16)}. Has unbonded the surface and introduced an initial tension of about σ = 0.5 tf / cm ^{2. The} results of the material tests of the used steel and concrete are shown in Tables 6 to 8. For loading, the test specimen foundation beam is used. While being fixed to the test bed, a gradually increasing horizontal force was applied to the stigma, and a bending moment (M = P × h) of positive and negative repetitions and a shearing force were simultaneously applied to the pedestal part. It shows in Table 9.

Table 6 Mechanical Properties of Steels Yield Stress Degree Tensile Strength Elongation Type Material (Kgf / mm ² ) (Kgf / mm ² ) (%) D13 SD30 37.9 54.7 21 D19 SD30 38.2 58.2 16 D25 SD30 37.3 61.1 15 φ42 SS50 33.0 53.7 34 φ56 SS50 33.2 56.1 29

Table 7 Tensile test results of anchor bolt alone Shaft cross-sectional area Thread yield Shaft yield Shaft fracture Anchor bolt (mm ² ) (tf) (tf) (tf) φ42 1385 36.5 45.7 60.3 φ56 2410 67.6 79.5 112.0

Table 8 Mechanical Properties of Concrete Concrete Age Curing Test Age Compressive Strength Elastic Modulus Classification Method (days) (Kgf / cm ² ) (Kgf / cm ² ) Normal Concrete Standard 28 208 2.30 × 10 ⁵ Normal Concrete Sealed 28 201 2.19 x 10 ⁵

Table 9 Loading procedure Cycle load (tf) Deformation (rad) 1 cMy / 2-2, 3 cMy-4, 5-1 / 200 6, 7-1 / 100 8-1 / 50 9-1 / 20 (Note): cMy Allowable bending strength of column base

Column base moment M and rotation angle R of both test bodies
As a result of measuring and, the following can be said regarding the relationship between the two. (a) The MR relationship of both test bodies is dominated by the effect of the outermost edge anchor bolt, and the MR relationship shows almost elastic behavior at the yield of the screw part, and reaches the maximum yield strength due to the yield of the shaft part. (b) After the yield of the screw part, the MR relationship has a slip-type restoring force characteristic, but in the HB-8 test piece, the loading was completed R = 1 /
No decrease in proof stress was observed up to 20 rad, confirming the effectiveness of reinforcement of reinforcement. On the other hand, in HB-16, the reinforcing bar adhered and fractured during repeated loading in the large deformation region after R = 1/50 rad, and the load bearing capacity decreased. Note that the attachment failure of the reinforcing bar was caused by the fact that the concrete at the rising edge was peeled off due to the vertical crack just below the compression end of the column base metal.

Further, FIG. 15 shows the relationship between the tensile force acting on the anchor bolt and the pull-out deformation of the anchoring portion, in comparison with the result of the reinforcement test. The following can be said from this figure. (c) In order to expect a sufficient restraining effect by the reinforcement even when the anchor bolt anchorage part is pulled out, in order to expect a sufficient restraint effect of the reinforcement, the reinforcement with a yield strength of 105% or more, preferably 110% or more of the anchor bolt shaft yield strength. It is necessary to arrange.

Evaluation of Column Base Bending Strength / Elastic Rigidity Only the anchor bolts on the outermost edge row on the tension side are considered for the bending strength and elastic rigidity of the column base using a large column base metal fitting in which anchor bolts are arranged in multiple stages. The conventional evaluation formula (1)
It was calculated by (2) and compared with the experimental results. The results are shown in Table 10. From the table, the proof strength of the column base using a large column base metal
It is considered that the rigidity can also be evaluated on the safe side by the conventional evaluation formula. (a) Bending strength evaluation formula cMu = n ・ Tu ・ dt + {(n ・ Tu ・ D) / 2} ・ {1-2 ・ (n ・ Tu / Nu) ² } ・ ・ ・ (1) where Nu: Column Compression strength of leg concrete n: Number of anchor bolts on the outermost edge on the tension side Tu: Yield strength of anchor bolts dt: Distance from column core to the position of action of Tu D: Width of column pedestal (b) Elastic rigidity evaluation formula cK = {(Ee ・ n ・ Ab) / L} ・ (dt + dc) ・ dt (2) However, dc: From column core, compression reaction force acting position Ee: Equivalent Young's modulus of anchor bolt (Ee = 0.85 ・ E) ) L: Shaft length of unbonded anchor bolt Ab: Shaft cross-sectional area per anchor bolt

[0039] Table 10 column base flexural strength and elasticity stiffness Bending Strength (tf) elastic stiffness (10 ³ tf / rad) Found Calculated Found Calculated Specimen ^{eMu 1) cMu eMu / cMu eK} 2) cK eK / cK HB-8 223.0 212.8 1.05 47.7 35.4 1.35 HB-16 246.0 205.1 1.20 57.1 44.3 1.29 ( Note): 1) outermost yield load 2 of the anchor bolt shank) points on the M-R curve during cMy and the origin Slope of connecting straight line

From the above results, it is possible to evaluate the bending strength and rigidity of the column base using a large column base metal on the safe side by the conventional evaluation formulas (1) and (2), and in the actual column base. Also, it was confirmed that reinforcement of the reinforcement is effective.

[0041]

As is apparent from the above description, in the steel column base of the present invention, even if the area of the basic concrete is made small, the anchoring bolt anchoring strength can be secured by the action of the reinforcing bar. The effect is obtained. (1) The design of the building is good. (2) Construction becomes simple. (3) Construction is possible when the boundary between adjacent land is close. (4) The mass of concrete becomes small and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing a steel column base according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a sectional view taken along line BB of FIG. 1;

FIG. 4 is a longitudinal sectional view showing a steel column base according to another embodiment of the present invention.

5 is a cross-sectional view taken along line CC of FIG.

6 is a cross-sectional view taken along line DD of FIG.

FIG. 7 is a cross-sectional view showing a test body used in Example 1,
(a) shows the cross section along the longitudinal direction, (b) shows the EE cross section of (a).

8 is a sectional view taken along line FF showing the positional relationship between the anchor bolt and the reinforcing bar at the rising portion of the test body of FIG.
(a) shows the case of one rebar on each side of the anchor bolt, and (b) shows the case of three rebars on each side of the anchor bolt.

FIG. 9 is a tensile force T and an extraction deformation amount δ in Example 1.
It is a graph which shows the relationship with.

FIG. 10 is a graph showing a relationship between a tensile force T of a reinforcing bar and a strain ε in Example 1.

FIG. 11 is a column base metal used in Example 2 (HB-8).
It is a top view which shows the shape dimension of.

FIG. 12 is a column base metal used in Example 2 (HB-16).
It is a top view which shows the shape dimension of.

13 is a vertical cross-sectional view of the column pedestal shown in FIGS. 11 and 12. FIG.

FIG. 14 is a vertical cross-sectional view showing a test body in Example 2.

FIG. 15 is a graph showing the relationship between the pull-out force T and the pull-out deformation amount δ of the fixing plate in Example 2.

FIG. 16 is a schematic plan view showing a region of a cone breaking surface in a steel column base using eight anchor bolts.

FIG. 17 is a schematic plan view showing a cone breaking surface in a steel column base using eight anchor bolts.

FIG. 18 is a schematic cross-sectional view showing a region of a cone fracture surface in a steel column base using four anchor bolts.

FIG. 19 is a schematic cross-sectional view showing a cone breaking surface in a steel column base using four anchor bolts.

[Fig. 20] Fig. 20 is a schematic plan view showing the appearance of basic concrete when the steel column base of the present invention is used.

FIG. 21 is a schematic plan view showing the appearance of basic concrete when a conventional steel column base is used.

[Explanation of symbols]

 1 Steel column 2 Column leg hardware 3 Basic concrete 4 Anchor bolt 5 Reinforcing bar (reinforcing bar) 6 Cone fracture surface

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kuniaki Sato 1-2-7, Moto-Akasaka, Minato-ku, Tokyo Kashima Construction Co., Ltd. (72) Inventor Akio Tomita 1-2-7, Moto-Akasaka, Minato-ku, Tokyo No. Kashima Construction Co., Ltd. (72) Inventor Shozo Maeda 1-2-7 Moto-Akasaka, Minato-ku, Tokyo Kashima Construction Co., Ltd. (72) Shunichi Yamada 1-2-7 Moto-Akasaka, Minato-ku, Tokyo No. Kashima Construction Co., Ltd. (72) Inventor Toshio Saeki No. 19-1 Tobita, Chofu-shi, Tokyo Kashima Construction Co., Ltd. Technical Research Institute (72) Inventor Yoshihiro Nakamura 1-2-7 Moto-Akasaka, Minato-ku, Tokyo No. Kashima Construction Co., Ltd.

Claims (1)

[Claims] 1. A column base metal member joined to a lower end of a steel frame column,
In a steel column base fixed on the foundation concrete with anchor bolts and nuts embedded in the foundation concrete, (a) Reinforcing bars are embedded in the outer periphery of the anchor bolt in the foundation concrete, and (b) The distance from the anchor bolt is within 12 times the diameter of each anchor bolt, (c) The total cross-sectional area (Ar) of the reinforcing bars is the total cross-sectional area (Ab) of the anchor bolts arranged at the outermost edge. On the other hand, Ar ≧ (bF / γF) · Ab (where bF is the yield stress of the anchor bolt material and γF is the yield stress of the reinforcing bar material). Steel column base.