JPH0214496B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a steel column base in which a steel column of a steel frame building is fixed to foundation concrete via a base plate. [Prior art] Conventionally, the base of a steel column in a building has been welded to the lower end of a steel column 2, as shown in Figure 7 (diagonal lines showing the cross section of the foundation concrete are omitted. The same applies to other figures). An easily constructed exposed type column base is used in which a base plate 1 made of a steel plate is fastened to an anchor bolt 3 fixed in foundation concrete 4. However, this column base had a drawback in that the column base underwent large rotational deformation in response to external force (bending moment generated in the column base), and as a result, the upper frame of the building had to bear a large amount of stress. Therefore, recently, as an improved column base, the anchor bolt 3 is fixed to the foundation concrete 4 using the hook part 3b at the lowermost end of the anchor bolt 3, as shown in FIG. 3b
The base plate 1 is set on the foundation concrete 4 and the anchor bolt 3 whose upper shaft portion 3a is an axially unrestricted portion 3c that is not attached to the concrete 4, and the base plate 1 is tightened with nuts 5. Steel frame column legs 6 in which tension is introduced into anchor bolts 3 have come to be used. [Problems to be Solved by the Invention] However, the degree of fixation (rotational rigidity) of this steel column base increases to some extent, and the stress borne by the upper structure of the building can be reduced to some extent, but as shown in Figure 9, the following problems occur. There are drawbacks. (1) When an external force (bending moment M) is applied to the column base, the base plate 1 deforms locally and in a complicated manner. (2) The value of the tension to be introduced into the anchor bolt 3 has not been clarified, and the effect of the introduced tension cannot be expected in each building, and of course cannot be reflected in the design. In some cases, it may also lead to a decline in the earthquake resistance of buildings. Here, the relationship between the rotational deformation θ when an external force such as an earthquake or bending moment M is applied to the conventional improved column base shown in Fig. 8 is the relationship between the local deformation of the column base hardware and the elongation of the anchor bolt. are intricately intertwined and become nonlinear as shown in FIG. Therefore, the strength performance and rotational rigidity performance of the column base are unstable, and it is virtually difficult to evaluate these performances at any design level, and this type of column base is generally considered unsuitable. There is a trend of adoption. However, the column base hardware used in the present invention (i.e., cast steel or forged steel with a protruding part in contact with the steel column and whose wall thickness gradually changes from the base to the end of the protruding part) is used in the present invention in addition to the conventional improved column base. According to experiments conducted by the present inventors, the relationship between M and θ of a column base is as shown in Figure 2a, when a certain amount of tension is applied to the anchor bolt. It becomes something simple. In other words, the relationship between M and θ of this column base is divided into areas from origin 0 to point A, from point A to point B, and beyond point B, where point A is the area on the side that is pulled by the bending moment. Point B is the point where the bottom surface of the column base hardware near the anchor bolt separates from the top surface of the foundation concrete, and point B is the point at which the anchor bolt yields. Further, the rotational rigidity (M/θ) of the column base is K (the inclination of OA) up to point A, and K' up to point B. moreover,
Bending moment (separation moment) value at point A
There is little general recognition that Ms varies depending on the magnitude of the tension introduced into the anchor bolt, and the relationship between the tension introduced into the anchor bolt and the separation moment Ms is quantitatively unclear. The dash-dotted line in FIG. 2a shows the relationship between M and θ when no anchor bolt introduction tension is applied to this column pedestal, and the rotational rigidity of the column pedestal at this time is K ₀ . It has been found through experiments by the inventors that this is approximately the same as the rotational rigidity between A and B of the column base shown in FIG. 2a. Therefore, the original effect of improving the rotational rigidity of the column base by applying tension to the anchor bolt is limited to the area OA in FIG. 2a. Here, it is generally accepted that the allowable bending moment when designing a column base should be up to the value My when the anchor bolt yields. However, if the column base is designed to have strength performance up to My, the rigidity of the column base can only be evaluated as the slope of the straight line connecting the origin 0 to point B, ie, K'. That is, the value K, which is the effect of the original tension introduced into the anchor bolt, cannot be expected. Conversely, if the rotational rigidity of the column base is designed as K, the allowable bending moment of the column base must be reduced from My to Ms. For this reason, with conventional column bases, the effect of introduced tension cannot be expected and cannot be reflected in the design. By the way, when designing a building, reducing the allowable bending moment of the column base from My to Ms and designing the rotational rigidity of the column base as K is a safer design. However, in reality, due to dry shrinkage of the foundation concrete, the tension introduced into the anchor bolt is released, and point A in Figure 2 a disappears and becomes like the dashed line, and the rotational stiffness may become K ₀ . , In this case, the effect of the tension introduced by the anchor bolt cannot be exhibited. Furthermore, if the amount of drying shrinkage of the foundation concrete exceeds the elastic strain of the initial tension of the anchor bolts, the anchor bolts may even loosen.
When bending moments are repeatedly applied to the column base, the restoring force characteristics become a slip type as shown in Figure 3, which drastically reduces the earthquake resistance of the building. Therefore, the separation moment is thought to vary depending on the tension introduced into the anchor bolt.
If Ms is not set appropriately, the effect of the tension introduced by the anchor bolt cannot be expected, and the seismic performance of the building will deteriorate. For the reasons mentioned above, the conventional improved steel column bases not only cannot provide a high degree of fixation while taking full advantage of the strength performance of the anchor bolts, but also have great concerns in terms of ensuring the safety of the building. This is what is left behind. The present invention is intended to solve these problems, and aims to provide a steel column base with a high degree of fixation that can demonstrate the strength performance of anchor bolts, and to ensure the safety of buildings. It is something. [Means for Solving the Problems] The steel frame column base of the present invention has the following major features as shown in FIG. (a) The base plate 1 has a projecting part 1a that connects with the steel column 2 protrudes, and a slope is provided from the base 1b to the end 1c so that the base 1b is medium-height, and the column 2
Uses highly rigid cast or forged column base hardware that can be welded or bolted to the base. (b) The anchor bolt 7 is fixed to the foundation concrete 4 using an anchor frame or an anchor plate 8 in an unrestrained state with an unrestrained portion 7b that eliminates the adhesion between the shaft portion 7a of the anchor bolt 7 and the concrete. I did it. (c) To fix the base plate 1 to the foundation concrete 4, attach the base plate 1 to the anchor bolt 7.
After setting, a tension of 0.15 to 1.2 times the yield value was introduced into the anchor bolt 7 in order to create a separation moment in the column base. [Function] The steel column base in the present invention has a base plate 1
Since the rigidity of the base plate 1 is extremely high, its deformation can be extremely reduced, and the rotational deformation of the column base caused by the base plate 1 can be reduced to almost zero. In addition, by using the base plate 1 with high rigidity and at the same time introducing a predetermined tension of the anchor bolt 7, the mechanical mechanism of the column leg can be simplified, and the evaluation of the rotational rigidity of the column leg can be accurately grasped. This is what I did. According to experiments, the relationship between the external force (bending moment M) and rotational deformation θ of the steel column base according to the present invention is as follows:
This results in a simplified shape as shown by the solid line in FIG. 2a. In other words, the rotational rigidity (M/θ) of the column base is
The bottom surface 1e of the base plate 1 at the tension side anchor bolt position and the top surface 4 of the foundation concrete in Figure b
It changes in three sections: up to point A (FIG. 2a) where the tension-side anchor bolt 7 is separated, up to point B (FIG. 2a) where the tension-side anchor bolt 7 yields, and further thereafter. The rotational rigidity shown by the dashed line in Fig. 2a is the theoretical value of the rotational rigidity when the base plate 1 is a completely rigid body and no tension is introduced into the anchor bolt 7, and is geometrically given by equation (1). It will be done. K ₀ = E・A _B・l ² /L ...(1) K ₀ : Rotational rigidity of column base when no tension is introduced into anchor bolt (tm/rad) E: Young's modulus of anchor bolt (t/cm ² ) A _B : Total cross-sectional area of tension-side anchor bolt group (cm ² ) L: Distance between anchor bolts (cm) L: Effective embedment depth of anchor bolts (cm) According to the inventors' experiments, the present invention 0 of steel column base
The rotational stiffness K between ~A is based on the same theory as tensile bonding (2)
As shown in the formula, approximately (1+α) times as much force can be obtained as in the case where no tension is introduced into the anchor bolt. In this case, α is the spring constant ratio between the anchor bolt and the foundation concrete, and it was found that it approximates the value given by equation (3). K=(1+α)K ₀ ...(2) α=Ac/n・Ab...(3) Ac: Effective cross-sectional area of foundation concrete (cm ² ) Ab: Cross-sectional area of anchor bolt (cm ² ) n: Anchor The value of the Young's modulus ratio (1+α) between the bolt and the foundation concrete is approximately 5 to 6 for anchor bolts 7 made of commonly used SS41 material, and O
~A, by introducing tension to the anchor bolt 7, significantly higher rotational rigidity can be obtained. However, between A and B, base plate 1
When the bottom surface of the foundation concrete is separated from the top surface 4a of the foundation concrete, the anchor bolt changes to the state of simple tension, which is the same as when no tension is introduced, so the rotational rigidity is
It is the same value as K ₀ in equation (1) and is less than the value between 0 and A. As mentioned above, in the construction industry, the allowable bending moment of the column base is generally determined by the yield value of the entire column base, so the allowable bending moment of the column base of the present invention is My( anchor bolt 7
is the value of the bending moment at which the yield occurs. Therefore, if the allowable bending moment of the column base is My and the separation moment Ms is less than My, the rotational rigidity of the column base must be designed only as K′, and it is impossible to ensure a high rotational rigidity K. It becomes difficult. On the other hand, in order to guarantee high rotational rigidity K, it is necessary to design the column base by lowering the allowable bending moment from My to Ms, which causes a disadvantage in terms of strength. Therefore, in order to ensure a high rotational rigidity K and a high allowable bending moment My for the column base, Ms and My may be set to the same value. By the way, a column base is constructed by fixing an anchor bolt in concrete with its shaft part in an unrestrained state and a column base metal fitting connected to a column by welding with a nut, and a bending moment is applied to this column base to form a column base. According to experiments conducted by the present inventors in pursuit of the locus of rotational deformation of metal objects, it was found that the separation moment Ms is related to the value of the tension introduced into the anchor bolt and is approximately given by equation (4). Ms＝T ₀・l ...(4) T ₀ : Value of tension introduced into anchor bolt (t/cm ² ) l : Distance between anchor bolts (cm) Also, the allowable bending moment My of the column base is calculated by formula (5). given by. My=Ty・l=σy・A _B・l...(5) Ty: Anchor bolt yield tensile force (t/cm ² ) σy: Anchor bolt yield stress (t/cm ² ) Therefore, Ms=My. In order to do this, T ₀ =
It is sufficient to set Ty = σy・A _B , and this is the method to maximize the performance of the anchor bolt in terms of allowable bending moment and rotational rigidity. The relationship between the bending moment of the external force on the column base and rotational deformation at this time is shown by the solid line in FIG. In addition,
The broken line in the figure is the case where Ms<My. Therefore, basically, the effect of applying the introduced tension to the anchor bolt occurs at 1 or less (not including 0) times the yield stress σy of the anchor bolt. However, it has been known that the tension introduced into the anchor bolt is released by drying shrinkage and creep of the concrete. The results of the anchor bolt tension unloading test conducted by the present inventors are shown in FIG. The test shown in Figure 10 investigated changes over time in the tension of anchor bolts (ratio of tension after a predetermined period of time to initial tension introduced = T/T ₀ ). A specimen corresponding to the steel column base of an actual steel frame building, as shown in Figure 11, is made of bolts (No. 5 in Table 1 below).
This test was carried out using a test specimen. Through this experiment, it was discovered that there is a certain regularity in the stress release of anchor bolts. That is,
When tension is introduced into anchor bolts of foundation concrete that have been placed for more than four weeks, as is the case in normal use, the stress release stabilizes in about four days and reaches about 20%.
This corresponds well with the amount of retightening of anchor bolts after the above-mentioned period of time has elapsed after pouring foundation concrete, which the present inventors have usually experienced. Therefore, from the above experimental results, if an introductory tension that generates a stress level of 1.2σy, which is 20% more than σy, is applied to the anchor bolt, that value will eventually change over time.
It is 1.0σy, which is ideal. Therefore, in the present invention, 1.2σy was set as the upper limit value of the stress degree due to the introduced tension. However, according to the "Structural Calculation Guidelines and Explanations" issued by the Japan Building Center on February 1, 1981, from the standpoint of maintaining a high level of safety, the allowable stress of anchor bolts is based on anchor bolts made of JIS standard materials. When used, 1.1σy is set as the upper limit allowable value. Therefore, unless the upper limit permissible value in the above guideline is revised, in actual construction, in order to efficiently and safely demonstrate the performance of the anchor bolt, it is necessary to apply an introductory tension to the anchor bolt that generates a stress level of up to 1.1σy. is desirable. In addition, the lower limit of the stress due to the anchor bolt introduction tension according to the present invention is 0.15σy for the following reason.
And so. In other words, it is clear from numerous experimental results that when the stress level due to the tension introduced into the anchor bolt is 0, the column base assumes a slip type as shown in Figure 3, and the bending moment generated in the column base is positive or negative. In the reverse direction, even if no large bending moment is generated in the column base, the column base undergoes extensive rotational deformation. And it is a well-known fact that this leads to an adverse effect on the superstructure. In order to make the column base have good performance (spindle-shaped restoring force characteristics) as shown in FIG. 4, it is necessary to prevent the anchor bolt from completely releasing the introduced tension. In other words, when the stress level due to the tension introduced into the anchor bolt is at a low level of 0.1 to 0.2σy, the stress release from the anchor bolt is approximately 0.05σy due to drying shrinkage of the concrete, and approximately 0.02σy due to creep of the concrete. The present inventors have experimentally discovered that a total stress release of about 0.07σy occurs. Therefore, if the safety factor is set to approximately 2 and the stress level due to the introduced tension is set to a minimum of 0.15σy, the stress level due to the introduced tension of the anchor bolt will not be completely released over time (0.08σy will remain), and the performance of the column base will be affected. Also, the good performance shown in FIG. 4 can be ensured. Therefore, in the present invention, the lower limit of the stress level due to the tension introduced into the anchor bolt is set to 0.15σy. This can prevent a decline in the seismic performance of the building due to loosening of the anchor bolts. [Example] The rotational rigidity of a steel column base having the dimensions shown in FIGS. 11a and 11b and Table 1 was measured, and the results are also shown in Table 1. In Table 1, K ₀ is the rotational stiffness of the conventional steel column base when no tension is introduced into the anchor bolt, and K is the rotational rigidity of the steel column base according to the present invention when tension is introduced into the anchor bolt. . As can be seen from Table 1, the rotational rigidity K of the steel column base according to the present invention is extremely high, being 5 to 6 times the rotational rigidity K ₀ of the conventional one.

〔Effect of the invention〕

As described above, the steel column base of the present invention is
The stress level due to the tension introduced into the anchor bolt is 0.15.
Since it was set to a value of ~1.2σy, it was possible to demonstrate the strength performance of the anchor bolt, and the rotational rigidity of the column base could also be evaluated highly. In addition, good performance (spindle-shaped restoring force characteristics) can be ensured against repeated positive and negative stresses (bending moments generated in column bases) during earthquakes, etc., and a decline in the earthquake resistance of the building can be prevented.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a steel column base showing an embodiment of the present invention, FIG. 2a is a diagram showing the relationship between the bending moment of external force and rotational deformation of the steel column base according to the present invention, and FIG. Similarly, a schematic diagram of rotational deformation, FIG. 3 is a restoring force characteristic diagram of a conventional steel column base in which tension is introduced into the anchor bolt, FIG. 4 is a restoring force characteristic diagram of a steel column base according to the present invention, and FIG. An enlarged view of the base plate portion of a steel column base according to the present invention, FIG. 6 is a sectional view showing another embodiment of the present invention, FIG. 7 is a sectional view showing an example of a conventional steel column base, and FIG. 8 is an anchor. A cross-sectional view of a conventional steel column base with tension introduced into the bolts.
Figures 9a and b are front sectional views and side sectional views showing the state when an external force (bending moment) is applied to a conventional steel column base in which tension is introduced into the anchor bolt, and Figure 10 is a front sectional view and a side sectional view showing the state in which tension is introduced into the anchor bolt. Figures 11a and 11b are diagrams showing changes over time. Figures 11a and 11b are a plan view and side sectional view of a specimen in an example of the present invention. Figures 12 and 13 are diagrams showing the bending moment and rotational deformation of the steel column base due to external forces, respectively. FIG. 1: Base plate, 1a: Projection, 1b: Base, 1c: End, 2: Steel column, 4: Foundation concrete, 6: Steel column base, 7: Anchor bolt, 7
a: shaft portion, 7b: unrestricted portion, 8: anchor frame (anchor plate).

Claims (1)

[Claims] 1. The steel column is joined by welding or bolts to a cast or forged column base hardware that has a protruding part that connects with the steel column and whose wall thickness gradually changes from the base to the end of the protruding part. , the column base hardware is tied to an anchor bolt buried and fixed in the foundation concrete in an unrestrained state in the axial direction, and in order to create a separation moment in the column base, the anchor bolt is 0.15 to 1.2 times the yield stress. A steel frame column base characterized by being fastened by applying an introduced tension such that a stress degree of .