Method for evaluating proof stress of concrete filled body filled in steel pipe pile

Abstract

To provide an evaluation method for determining the proof stress of a concrete filler in a hybrid steel pipe pile having a straight steel pipe pile body.SOLUTION: An evaluation method of the present invention is a method for obtaining a proof stress of a cylindrical concrete filled body in a steel pipe pile including a straight steel pipe pile main body, and includes a step of calculating a strength reduction coefficient which is a ratio of a surface area of a cylindrical concrete filled body to (a cross-sectional area of the concrete filled body * a concrete design basis strength of the concrete filled body), a step of verifying whether or not the strength reduction coefficient is less than 1, and a step of multiplying the concrete design basis strength by the strength reduction coefficient when the strength reduction coefficient is less than 1.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the strength of a concrete filled body in a steel pipe pile, and more particularly to a method for evaluating the strength of a concrete filled body in a knotless steel pipe pile. [Background technology]

[0002] Piles in which concrete is filled inside a steel pipe pile (sometimes referred to as "hybrid steel pipe piles" in this specification) are known. It is common for such hybrid steel pipe piles to have joints in the steel pipe pile body. This is to ensure the integrity of the steel pipe pile body and the concrete infill and to allow the steel pipes constituting the steel pipe pile body and the concrete infill body to each exert their inherent strength. These strengths can be calculated based on the material, diameter, length, and thickness of the steel pipe, as well as the surface area and material of the concrete infill body. Therefore, the jointed steel pipe pile body and the concrete infill body can be individually and appropriately designed to meet the requirements of the hybrid steel pipe pile. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-129671 Summary of the Invention [Problem to be solved by the invention]

[0004] Steel pipe pile bodies with knots have higher manufacturing costs than steel pipe pile bodies made of straight steel pipes without knots (hereinafter simply referred to as "straight steel pipe pile bodies" or "knotless steel pipe pile bodies"). Therefore, it has been considered to use knotless, i.e., straight steel pipes, as the steel pipe body of a hybrid steel pipe pile. However, in this case, it is difficult to ensure the integrity of the knotless steel pipe pile body and the concrete infill, and there is a risk that the concrete infill will shift relative to the steel pipe pile body when an external force is applied to the hybrid steel pipe pile. If the concrete infill shifts, its bearing strength cannot be calculated from the material and volume of the concrete. Therefore, the bearing strength of the hybrid steel pipe pile as a whole cannot be obtained, and hybrid steel pipe piles with straight steel pipe pile bodies have been hesitant to be used in the field. Even with a straight steel pipe pile body, if a bottom plate is firmly welded to the lower end and the concrete filler is supported by this bottom plate, it is possible to avoid displacement of the concrete filler relative to the steel pipe pile body. However, since the work of firmly welding a bottom plate to the lower end of the steel pipe pile body is time-consuming, it is desirable to avoid adopting such a bottomed steel pipe pile body. [Means for solving the problem]

[0005] The inventors have been conducting extensive research into a method for calculating the bearing capacity of the concrete infill in a hybrid steel pipe pile having a straight steel pipe pile body. f It was found that a calculation method using S and cross-sectional area S as parameters, in other words, the diameter D, radius D / 2, and length L of the concrete filler as parameters, is preferable. Note that the concrete design strength Fc of the concrete filler itself is determined from the material of the concrete filler.

[0006] The inventors conducted experiments on multiple samples (with different diameters D and lengths L) to determine the bearing strength of the concrete infill in a hybrid steel pipe pile with a straight steel pipe pile body, and attempted to develop a calculation formula that could express the obtained bearing strength. As a result, the surface area of ​​the cylindrical concrete filling material filled in the hybrid steel pipe pile consisting of straight steel pipes was f Ratio of S to (cross-sectional area S × concrete design strength Fc) c r p (referred to in this specification as "strength reduction coefficient") by a predetermined constant (referred to in this specification as "short-term allowable bond stress"), and c r p If the value is less than 1, the concrete design strength Fc (N / mm 2 ) and found that the yield strength of the concrete infill can be determined by multiplying it by Here, the strength reduction factor c r p The condition that is less than 1 is because this value c r p In the first place, the strength reduction coefficient c r p When is 1, it means that the concrete infill can exert its original concrete design strength Fc. This means that the concrete infill is attached to the steel pipe pile body, and the two are integrated, and can be treated in the same way as a nodular steel pipe pile body.

[0007] According to the study by the inventors, the strength reduction coefficient c r p is expressed as follows:

[0008]

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[0009]

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[0010] [Figure 1] Figure 1 shows the test specimen. [Figure 2] Figure 2 shows the experimental results of the two bending moments and axial force of specimen No. 1. [Figure 3] Figure 3 shows the calculation results (short-term strength) of the two bending moments and axial force for specimen No. 1. [Figure 4] Figure 4 shows the calculation results (ultimate strength) of the two bending moments and axial force for specimen No. 1. [Figure 5] Figure 5 shows the experimental results of the two bending moments and axial force of specimen No. 2. [Figure 6] Figure 6 shows the calculation results (short-term strength) of the two bending moments and axial force for specimen No. 2. [Figure 7] Figure 7 shows the calculation results (ultimate strength) of the two bending moments and axial force for specimen No. 2. [Figure 8] FIG. 1 is a schematic diagram showing the construction procedure for a steel pipe pile. DETAILED DESCRIPTION OF THE INVENTION

[0011] Figure 1 shows the specifications of the antibodies (test substances) used in the two tests. Each pile consists of a straight steel pipe pile body filled with concrete. A cavity of approximately 10 mm was provided between the bottom end of the steel pipe pile body and the bottom end of the concrete filler, allowing the concrete filler to move axially along the steel pipe pile body. A leg is attached to the bottom of the steel pipe pile body to allow the pile to stand on its own. A load is applied to the top of the free-standing pile at a position 10 mm off-center from its center. Specifically, the specifications of the test specimen are as follows:

[0012] [Table 1]

[0013] Table 2 shows the test results for the No. 1 specification specimens (2 specimens).

[0014] [Table 2] Figure 2 shows the relationship between bending moment and displacement. From the relationship between Table 2 and Figure 2, it can be seen that the resistance of the two test substances (antibodies) shows the same tendency.

[0015] Figure 3 shows the calculated relationship between bending moment and axial force (short-term strength), and Figure 4 shows the calculated relationship between bending moment and axial force (ultimate strength). Comparing FIG. 2 with FIG. 3 and FIG. 4, it can be seen that the experimental values ​​are greater than the calculated values.

[0016] Table 4 shows the test results for the No. 2 specification specimens (2 specimens).

[0017] [Table 3] Figure 5 shows the relationship between bending moment and displacement. From the relationship between Table 3 and Figure 5, it can be seen that the resistance of the two test substances (antibodies) shows the same tendency.

[0018] Figure 6 shows the calculated relationship between bending moment and axial force (short-term strength), and Figure 7 shows the calculated relationship between bending moment and axial force (ultimate strength). Comparing FIG. 5 with FIG. 6 and FIG. 7, it can be seen that the experimental values ​​are greater than the calculated values.

[0019] The calculated values ​​in Figures 3, 4, 6, and 7 were obtained as follows.

[0020]

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[0021] [Table 4] From Table 4, the minimum ratio of the experimental value to the calculated value was 1.77 in the short term and 1.07 in the ultimate state. As the outer diameter of the test specimen increases, the experimental value tends to approach the calculated value, and it is thought that as the outer diameter of the test specimen increases, the experimental value decreases. Therefore, in order to leave some leeway in the calculated value, we considered setting the bond between the straight steel pipe pile body and the concrete infill so that the ratio of the experimental value to the calculated value would be 1.2.

[0022] The bond stress X at which the ratio of the experimental value to the calculated value was 1.2 was X = 0.320. When this was substituted into the following formula (KS formula) proposed by the inventor, the strength reduction coefficient of the concrete filler was calculated, which was 0.91 for test piece No. 1 (Φ=89.1) and 0.90 for test piece No. 2.

[0023]

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[0024] The design standard strength Fc of the concrete infill in each test specimen was multiplied by this strength reduction coefficient, and the resulting value was added to the known design standard strength of the straight steel pipe pile body to obtain a second calculated value. A comparison of this value with the experimental value is shown in Table 5.

[0025] [Table 5] The results in Table 5 show that there is no significant difference between the second calculated values ​​and the experimental values, and all calculated values ​​are greater than the experimental values. The results in Table 5 are values ​​when the bond stress X is set to 0.320, and if this value is made smaller, the second calculated value will also be smaller. In other words, the difference between the experimental value and the second calculated value will be larger. This difference can be considered as the safety factor of the second calculated value. Here, the inventor proposes X=0.225 based on a higher safety factor and "Architectural Institute of Japan: Guidelines for Design and Construction of Concrete-Filled Steel Pipe Structures (2008), p. 201." If there is flexibility in the safety factor, the bond stress X in the following range can be adopted. 0.200≦X≦0.320(N / mm 2 )

[0026] As explained above, the KS calculation formula newly proposed by the inventors makes it possible to easily calculate the strength reduction factor of the concrete filled into the body of a straight steel pipe pile with a sufficient safety factor, using the diameter D, radius D / 2, and length L of the concrete filled body as parameters. Since the design strength of the concrete filler is known, the actual strength (bearing strength) of the concrete filler filled into the body of a straight steel pipe pile can be obtained by multiplying this by the strength reduction coefficient calculated using the KS formula. The strength (bearing strength) of the steel pipe pile body is also known, so the bearing strength of a straight steel pipe pile filled with concrete can be calculated by adding the two together. At steel pipe pile installation sites, the straight steel pipe pile body to be filled with concrete is often installed last.

[0027] Figure 8 shows the construction procedure. In Figure 8A, a first straight steel pipe pile is embedded in the ground G, leaving the pile head. A cylindrical backing 11 is attached to the pile head by welding or other methods, and a disk-shaped lid (made of steel plate) is placed on top of it. To prevent the lid from shifting, simple spot welding may or may not be used. This lid can be easily installed as long as it has the function of preventing the concrete from falling before hardening.

[0028] Next, as shown in Figure 8B, the lower end of the second straight steel pipe pile 20 is connected to the upper end of the first straight steel pipe pile 10 via a backing 11 according to a standard method. Thereafter, the second straight steel pipe pile 20 is embedded in the ground, leaving its head uncovered (Figure 8C). At this time, it is preferable to cover the head of the pile with a removable cover. The ground G can also be covered with a surface layer S of crushed stone or concrete. In FIG. 8D, concrete is filled into the second straight steel pipe pile 20 to produce the completed structure shown in FIG. 8E.

[0029] The present invention is not limited to the above-described embodiments and drawings, and any design modifications are possible within the scope of understanding of those skilled in the art. [Explanation of symbols]

[0030] 10...First straight steel pipe pile 11...Backing 20...Second straight steel pipe pile

Claims

1. A method for obtaining the bearing capacity of a cylindrical concrete infill in a steel pipe pile having a straight steel pipe pile body, comprising: A step of calculating a strength reduction coefficient, which is the ratio of the surface area of ​​the cylindrical concrete filler to (the cross-sectional area of ​​the concrete filler x the concrete design standard strength of the concrete filler); verifying whether the strength reduction factor is less than 1; When the strength reduction coefficient is less than 1, multiplying the concrete design strength Fc by the strength reduction coefficient; A method for evaluating a concrete filler, comprising:

2. The strength reduction factor is obtained as follows: [Equation 1] Here, c r u : Concrete strength reduction factor F c : Design strength of concrete (N / mm 2 ) c D: Outer diameter of concrete (mm) L: concrete filling length (mm) x: Short-term allowable bond stress (N / mm 2 ) The method according to claim 1, wherein x is greater than or equal to 0.225 and less than or equal to 0.

320.

3. Executing the method of claim 1 or 2 to evaluate the first bearing capacity of the concrete filler; Evaluating a second bearing strength of the steel pipe pile body; Evaluating the overall strength of the steel pipe pile based on the first strength and the second strength; A method for evaluating the bearing capacity of a steel pipe pile equipped with a straight steel pipe.

4. The evaluation of claim 3 is carried out for the upper pile of the buried steel pipe pile, During construction, the upper pile and the lower pile are joined using joints, Construction direction: A cover is placed on top of the joint to prevent the filling concrete from falling.

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