Connection structure and connection method for steel pipe

Abstract

To provide a connection structure and a connection method for steel pipes without requiring welding and bolts.SOLUTION: A connection structure 1 for steel pipes is configured to connect a first pipe end part 11 and a second pipe end part 22 by covering the first pipe end part 11 of a first steel pipe 10 with the second pipe end part 22 of a second steel pipe 20, wherein at least the second end part 22 of the second steel pipe 20, in which a slit 23 extended in a pipe axis direction and cut-out to an outside in the pipe axis direction, constraints the first pipe end part 11 of the first steel pipe 10 with compression load in a radial direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel pipe connection structure and connection method. [Background technology]

[0002] There is a pipe fitting in which a pipe fitting body made of molten resin is placed across the butted portion of two pipe members, and a notch is formed at the end of the pipe fitting body to release residual stress, thereby reducing the diameter and bringing the pipe fitting into contact with the two pipe members (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-46704 Summary of the Invention [Problem to be solved by the invention]

[0004] Steel pipe connection structures, such as those used to connect steel pipe piles at construction sites, require a certain level of strength against compressive, bending, and torsional loads. Welded joints are widely used as conventional steel pipe connection structures. However, they have drawbacks, such as their strength being affected by weather and the skill of the welder, the time required for on-site work, and their susceptibility to thermal stress. Another steel pipe connection structure is mechanical joints, in which steel pipes are fastened together with bolts. Compared to welded joints, mechanical joints offer superior on-site quality due to their strength being unaffected by weather and the skill of the welder, and their simplified construction and management. However, they have drawbacks, such as increased steel pipe processing costs, the formation of overhangs on the exterior and interior due to the bolts, and stress concentrations.

[0005] The present invention has been made in consideration of the above-mentioned problems, and has as its object to provide a steel pipe connection structure and connection method that do not require welding or bolts. [Means for solving the problem]

[0006] A first aspect of the present invention is a steel pipe connection structure in which a second pipe end of a second steel pipe is placed over a first pipe end of a first steel pipe to connect the first pipe end and the second pipe end, characterized in that at least the second pipe end of the second steel pipe has a slit formed therein that extends in the pipe axis direction and passes outward in the pipe axis direction, and the first pipe end of the first steel pipe is restrained by a radial compressive load.

[0007] According to the first aspect, the second end of the second steel pipe is placed over the first end of the first steel pipe, and at least the second end of the second steel pipe has a slit extending in the axial direction and passing outward in the axial direction, and the first end of the first steel pipe is constrained by a radial compressive load. This allows the first end of the first steel pipe to be constrained by the second end of the second steel pipe, and these pipes can be connected without the need for welding or bolts. Therefore, the first end of the first steel pipe can be evenly constrained by the second end of the second steel pipe, and these pipes can be connected without being affected by thermal stress and while preventing stress concentration.

[0008] As a second aspect of the present invention, in the first aspect, it is preferable that the first steel pipe is a lower pile, the second steel pipe is an upper pile, the first pipe end is an upper end of the first steel pipe, and the second pipe end is a lower end of the second steel pipe.

[0009] According to the second aspect, the upper end of the lower pile and the lower end of the upper pile can be connected without the need for welding or bolts. Therefore, the upper end of the lower pile can be evenly restrained by the lower end of the upper pile without being affected by thermal stress and while preventing stress concentration, and the two can be connected.

[0010] As a third aspect of the present invention, in the first aspect, it is preferable that the length of the slit in the axial direction of the second steel pipe is at least twice the outer diameter of the second steel pipe.

[0011] According to the third aspect, a sufficient reduction ratio can be obtained at the second pipe end of the second steel pipe, and the first pipe end of the first steel pipe can be securely restrained by the second pipe end of the second steel pipe without the need for welding or bolts.

[0012] As a fourth aspect of the present invention, in the first aspect, it is preferable that a diaphragm is provided on the inner surface of the second steel pipe, which abuts against the first pipe end of the first steel pipe in the pipe axis direction.

[0013] According to the fourth aspect, the axial force between the second steel pipe and the first steel pipe is received by the diaphragm, so that the second steel pipe can withstand a greater axial force acting between the second steel pipe and the first steel pipe.

[0014] As a fifth aspect of the present invention, in the first aspect, it is preferable that a high-friction body is provided on the outer surface of the first steel pipe in the pipe axial direction at least in an area overlapping with the second steel pipe.

[0015] According to the fifth aspect, the high friction body generates resistance to the circumferential torsional load between the first steel pipe and the second steel pipe, so that it is possible to exert large torsional resistance to the circumferential torsional load between the first steel pipe and the second steel pipe.

[0016] As a sixth aspect of the present invention, in the first aspect, it is preferable that a torsion receiving member is provided on the outer surface of the first steel pipe, the torsion receiving member being fitted into the slit and abutting against the base end of the slit.

[0017] According to the sixth aspect, the torsion-receiving member receives the circumferential torsional load between the first steel pipe and the second steel pipe, so that it is possible to exhibit large torsional resistance against the torsional load between the first steel pipe and the second steel pipe. In addition, because the torsion-receiving member receives the axial force between the first steel pipe and the second steel pipe, it is also possible to resist a larger axial force acting between the second steel pipe and the first steel pipe.

[0018] As a seventh aspect of the present invention, in the first aspect, it is preferable that a third pipe end of the second steel pipe opposite to the second pipe end is placed over a fourth pipe end of the third steel pipe, and the fourth pipe end of the third steel pipe is abutted against the first pipe end of the first steel pipe in the pipe axis direction, and the second steel pipe has a slit extending in the pipe axis direction over its entire length, and the first pipe end of the first steel pipe and the fourth pipe end of the third steel pipe are restrained with a radial compressive load.

[0019] According to the seventh aspect, the second end of the second steel pipe is placed over the first end of the first steel pipe, and the third end of the second steel pipe is placed over the fourth end of the third steel pipe, with the fourth end of the third steel pipe abutting against the first end of the first steel pipe in the axial direction, so that a slit extending in the axial direction over the entire length of the second steel pipe is formed in the second steel pipe, and the first end of the first steel pipe and the fourth end of the third steel pipe are constrained with a radial compressive load. This makes it possible to connect the first end of the first steel pipe by constraining the second end of the second steel pipe, and to connect the fourth end of the third steel pipe by constraining the third end of the second steel pipe, without the need for welding or bolts. Therefore, the first end of the first steel pipe can be connected to the second end of the second steel pipe by uniformly restraining them without the influence of thermal stress and preventing stress concentration, and the third end of the second steel pipe can be connected to the fourth end of the third steel pipe by uniformly restraining them without the influence of thermal stress and preventing stress concentration. Furthermore, because the fourth end of the third steel pipe abuts against the first end of the first steel pipe in the pipe axial direction, the connection strength between the third and first steel pipes against compressive loads is improved. In addition, the axial center of the first steel pipe can be aligned with the axial center of the second steel pipe.

[0020] As an eighth aspect of the present invention, in the seventh aspect, it is preferable that the first steel pipe is a lower pile, the third steel pipe is an upper pile, the first pipe end is the upper end of the first steel pipe, the second pipe end is the lower end of the second steel pipe, the third pipe end is the upper end of the second steel pipe, and the fourth pipe end is the lower end of the third steel pipe.

[0021] According to the eighth aspect, the upper end of the lower pile and the lower end of the upper pile can be restrained by the second steel pipe and connected together without the need for welding or bolts. Therefore, the upper end of the lower pile and the lower end of the upper pile can be connected together by being evenly restrained by the second steel pipe without being affected by thermal stress and while preventing stress concentration.

[0022] As a ninth aspect of the present invention, in the seventh aspect, it is preferable that a high-friction body is provided on the outer surface of the first steel pipe in an area overlapping with at least the second steel pipe in the pipe axis direction, and on the outer surface of the third steel pipe in an area overlapping with at least the second steel pipe in the pipe axis direction.

[0023] According to the ninth aspect, the high-friction body generates resistance to the circumferential torsional load between the first steel pipe and the second steel pipe, and the circumferential torsional load between the second steel pipe and the third steel pipe, so that it is possible to exhibit large torsional resistance to the circumferential torsional load between the first steel pipe and the second steel pipe, and it is also possible to exhibit large torsional resistance to the circumferential torsional load between the second steel pipe and the third steel pipe.

[0024] A tenth aspect of the present invention is a method for connecting steel pipes, comprising covering a first end of a first steel pipe with a second end of a second steel pipe to connect the first and second end of the steel pipe, wherein external compressive and internal tensile residual stresses are introduced in advance into at least the second end of the second steel pipe, and after covering the first end of the first steel pipe with the second end of the second steel pipe, a slit is formed in at least the second end of the second steel pipe, extending in the pipe axis direction and passing outward in the pipe axis direction, thereby reducing the diameter of at least the second end of the second steel pipe, and the first end of the first steel pipe is restrained by the second end of the second steel pipe with a radial compressive load.

[0025] According to a tenth aspect, external compressive and internal tensile residual stresses are introduced in advance into at least the second end of the second steel pipe, and the second end of the second steel pipe is then placed over the first end of the first steel pipe. A slit extending in the axial direction and outward in the axial direction is then formed in at least the second end of the second steel pipe, thereby reducing the diameter of at least the second end of the second steel pipe. The first end of the first steel pipe is then constrained by the second end of the second steel pipe with a radial compressive load, allowing the two pipes to be connected. This allows the first end of the first steel pipe to be constrained by the second end of the second steel pipe without the need for welding or bolts. Therefore, the first end of the first steel pipe can be constrained evenly by the second end of the second steel pipe, without the influence of thermal stress and while preventing stress concentration, allowing the two pipes to be connected.

[0026] As an eleventh aspect of the present invention, in the tenth aspect, it is preferable that before covering the first end of the first steel pipe with the second end of the second steel pipe, an inner part of the slit in the pipe axial direction is formed in at least the second end of the second steel pipe, and after covering the first end of the first steel pipe with the second end of the second steel pipe, an outer part of the slit in the pipe axial direction is formed in at least the second end of the second steel pipe.

[0027] According to the eleventh aspect, a part of the slit on the inside in the pipe axial direction can be formed in advance in a factory in at least the second pipe end of the second steel pipe, and when the first pipe end of the first steel pipe and the second pipe end of the second steel pipe are connected, the second pipe end of the second steel pipe can be placed over the first pipe end of the first steel pipe, and then the part of the slit on the outside in the pipe axial direction can be formed in at least the second pipe end of the second steel pipe. This reduces the work required at the construction site when connecting the first steel pipe and the second steel pipe.

[0028] As a twelfth aspect of the present invention, in the tenth aspect, it is preferable that external compressive and internal tensile residual stresses are introduced into the entire second steel pipe in advance, the second pipe end of the second steel pipe is placed over the first pipe end of the first steel pipe, a third pipe end of the second steel pipe opposite to the second pipe end is placed over a fourth pipe end of the third steel pipe, and the first pipe end of the first steel pipe and the fourth pipe end of the third steel pipe are butted in the pipe axial direction, and a slit extending in the pipe axial direction over the entire length of the second steel pipe is formed in the second steel pipe, thereby reducing the diameter of the second steel pipe, and the first pipe end of the first steel pipe is restrained by the second pipe end of the second steel pipe with a radial compressive load, and the fourth pipe end of the third steel pipe is restrained by the third pipe end of the second steel pipe with a radial compressive load.

[0029] According to the twelfth aspect, external compressive and internal tensile residual stresses are introduced into the entire second steel pipe in advance, the second end of the second steel pipe is placed over the first end of the first steel pipe, the third end of the second steel pipe is placed over the fourth end of the third steel pipe, and the first end of the first steel pipe and the fourth end of the third steel pipe are butted together in the axial direction. After that, a slit extending in the axial direction over the entire length of the second steel pipe is formed in the second steel pipe, thereby reducing the diameter of the second steel pipe, and the first end of the first steel pipe is restrained by the second end of the second steel pipe with a radial compressive load to connect them, and the fourth end of the third steel pipe is restrained by the third end of the second steel pipe with a radial compressive load to connect them. This allows the first pipe end of the first steel pipe to be restrained by the second pipe end of the second steel pipe, and the fourth pipe end of the third steel pipe to be restrained by the third pipe end of the second steel pipe, without the need for welding or bolts. Therefore, the first pipe end of the first steel pipe can be evenly restrained by the second pipe end of the second steel pipe, without the influence of thermal stress and preventing stress concentration, and the fourth pipe end of the third steel pipe can be evenly restrained by the third pipe end of the second steel pipe, without the influence of thermal stress and preventing stress concentration, and the fourth pipe end of the third steel pipe can be evenly restrained by the third pipe end of the second steel pipe, without the influence of thermal stress and preventing stress concentration. Furthermore, because the fourth pipe end of the third steel pipe abuts against the first pipe end of the first steel pipe in the pipe axial direction, the connection strength between the third steel pipe and the first steel pipe against compressive loads is improved.

[0030] As a thirteenth aspect of the present invention, in the tenth or twelfth aspect, it is preferable that after heating the second steel pipe, the second steel pipe is cooled in a state in which the cooling rate on the outer surface side of the second steel pipe is greater than that on the inner surface side, thereby introducing external compressive and internal tensile residual stresses into the second steel pipe in advance.

[0031] According to the thirteenth aspect, after heating the second steel pipe, it is cooled in a state in which the cooling rate on the outer surface side of the second steel pipe is greater than that on the inner surface side, so that even when residual stress is introduced into multiple steel pipes, they can be produced continuously and efficiently on a single line.

[0032] As a fourteenth aspect of the present invention, in the tenth or twelfth aspect, it is preferable that the heating temperature of the second steel pipe is 300°C or higher and the A3 transformation point or lower.

[0033] According to the 14th aspect, the heating temperature of the second steel pipe is set to 300°C or higher and the A3 transformation point or lower, thereby preventing a decrease in toughness due to the formation of martensite in the second steel pipe and making it possible to prevent brittle fracture due to scratches during construction. [Effects of the Invention]

[0034] According to the steel pipe connection structure and connection method of the present invention, steel pipes can be connected to each other without the need for welding or bolts. [Brief explanation of the drawings]

[0035] [Figure 1] 1A and 1B show a steel pipe connection structure according to a first embodiment of the present invention, in which FIG. 1A is a perspective view and FIG. 1B is a bottom view. [Figure 2] 1A and 1B are diagrams illustrating a steel pipe connecting method according to a first embodiment of the present invention, where (a) is a perspective view and (b) is a bottom view. [Figure 3] 1A and 1B are diagrams illustrating a steel pipe connecting method according to a first embodiment of the present invention, where (a) is a perspective view and (b) is a bottom view. [Figure 4] FIG. 4 is a perspective view showing a steel pipe connection structure of a second embodiment according to the present invention. [Figure 5] FIG. 10 is a perspective view showing a steel pipe connection structure of a third embodiment according to the present invention. [Figure 6] FIG. 10 is a perspective view showing a steel pipe connection structure of a fourth embodiment according to the present invention. [Figure 7] FIG. 10 is a perspective view illustrating a steel pipe connecting method according to a fifth embodiment of the present invention. [Figure 8] FIG. 10 is a perspective view illustrating a steel pipe connecting method according to a sixth embodiment of the present invention. [Figure 9] FIG. 13 is a perspective view illustrating a steel pipe connecting method according to a seventh embodiment of the present invention. [Figure 10]FIG. 10 is a perspective view showing a steel pipe connection structure of a seventh embodiment according to the present invention. [Figure 11] 10A and 10B show a steel pipe connection structure according to an eighth embodiment of the present invention, where (a) is a perspective view and (b) is a bottom view. [Figure 12] 10A and 10B are diagrams illustrating a steel pipe connecting method according to an eighth embodiment of the present invention, where (a) is a perspective view and (b) is a bottom view. [Figure 13] 10A and 10B are diagrams illustrating a steel pipe connecting method according to an eighth embodiment of the present invention, where (a) is a perspective view and (b) is a bottom view. [Figure 14] 1 is a side view schematically showing a heat treatment apparatus used in an embodiment of the present invention. [Figure 15] FIG. 2 is a diagram illustrating a method for calculating the diameter reduction amount of a steel pipe in an embodiment according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, a steel pipe connection structure and a connection method according to an embodiment of the present invention will be described with reference to the drawings.

[0037] [First embodiment] The steel pipe connection structure 1 and connection method of the first embodiment shown in Figure 1 are for connecting multiple steel pipes in series to form a single steel pipe pile that is driven into the ground in a substantially vertical direction. Here, the connection structure and connection method will be described as an example of connecting two steel pipe piles: a first steel pipe 10 serving as the lower pile and a second steel pipe 20 serving as the upper pile. The second steel pipe 20 shown in Figure 1(a) is used as the first pipe, and a second steel pipe of a similar structure with an inner diameter slightly larger than the outer diameter of the upper part of the first steel pipe is placed on top of it and connected. By appropriately repeating this connection upward, the steel pipe pile can be extended in the pipe axial direction.

[0038] As shown in FIG. 1(a), a first steel pipe 10 serving as a lower pile is cylindrical over its entire length in the axial direction, including a first pipe end 11 serving as an upper end. The second steel pipe 20, which is the upper pile, has a cylindrical main body 21 and a generally tapered second pipe end 22 that extends from one axial end of the main body 21 while reducing in diameter and aligning with the axial center of the main body 21. The main body 21 has an inner diameter slightly larger than the outer diameter of the first steel pipe 10. A slit 23 is formed in the second pipe end 22, extending in the axial direction of the pipe and extending outward in the axial direction. Here, the length of the slit 23 in the axial direction of the second steel pipe 20 is at least twice the outer diameter of the second steel pipe 20.

[0039] In the steel pipe connection structure 1 of the first embodiment, a first pipe end 11, which is the upper end of a first steel pipe 10, is covered with a second pipe end 22, which is the lower end of a second steel pipe 20. In this connection structure 1, the second pipe end 22 of the second steel pipe 20 abuts against the first steel pipe 10 over its entire circumferential range, as shown in FIG. 1(b) at its lower end, and restrains the first steel pipe 10 with a radial compressive load. Specifically, the second pipe end 22 of the second steel pipe 20 restrains the first pipe end 11, which is the upper end of the first steel pipe 10 shown in FIG. 1(a), with a radial compressive load. In this way, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected.

[0040] The steel pipe connection method of the first embodiment is a method for constructing the above-mentioned connection structure 1. Here, if the second steel pipe 20 is a normal steel pipe, the second pipe end 22 cannot be formed with the above-mentioned shape even if the slit 23 is machined. In the steel pipe connection method of the first embodiment, residual stress is generated by preheat-treating at least the second pipe end 22 of the second steel pipe 20, and then the slit 23 is formed by cutting, thereby releasing and redistributing the residual stress. As a result, the portion of the second steel pipe 20 where the slit 23 is formed is deformed in a direction that reduces its diameter, forming the second pipe end 22. Regarding the heat treatment, the second steel pipe 20 is heated and then cooled by water cooling on the outside and air cooling on the inside, introducing compressive residual stress on the outside and tensile residual stress on the inside due to the difference in cooling rate. Furthermore, the heating temperature of the second steel pipe 20 is set to 300°C or higher and the A3 transformation point or lower.

[0041] That is, in the steel pipe connecting method of the first embodiment, external compressive and internal tensile residual stresses are introduced in advance in at least the second pipe end 22 of the second steel pipe 20 by heat treatment at a factory. Then, at the construction site, the first steel pipe 10, which serves as a lower pile, is driven into the ground, and the second pipe end 22 of the second steel pipe 20, before the slit 23 is formed, is placed over the first pipe end 11, which is the upper end of the first steel pipe 10 protruding from the ground, as shown in FIG. 2(a). At this time, as shown in FIG. 2(b), a radial gap is formed between the second steel pipe 20 and the first steel pipe 10. Thereafter, as shown in FIGS. 3(a) and 3(b), a slit 23 is formed in at least the second pipe end 22 of the second steel pipe 20, extending in the pipe axial direction and piercing outward in the pipe axial direction. 1(a) and 1(b), at least the second pipe end 22 of the second steel pipe 20 is reduced in diameter, and the first pipe end 11 of the first steel pipe 10 is restrained by a radial compressive load at the second pipe end 22 of the second steel pipe 20. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected to each other, resulting in the connection structure 1 described above.

[0042] Here, the width of the slit 23 is set to a width that allows the second pipe end 22 to constrain the first pipe end 11 of the first steel pipe 10 with a predetermined radial compressive load, eliminating the difference between the inner diameter of the second steel pipe 20 before forming the slit 23 (i.e., the inner diameter of the main body portion 21 after forming the slit 23) and the outer diameter of the first steel pipe 10. In other words, the width of the slit 23 is set to a width that does not close the outer end of the slit 23 in the pipe axis direction, even when the second pipe end 22 of the second steel pipe 20 constrains the first pipe end 11 of the first steel pipe 10 with a predetermined radial compressive load. Note that when forming the slit 23, if the width is not set to be greater than the amount of diameter reduction, the cutter used to form the slit 23 will be pinched; therefore, a tool that can cut with a certain width, such as a milling cutter, is used.

[0043] According to the steel pipe connection structure 1 of the first embodiment, the second pipe end 22 of the second steel pipe 20 is placed over the first pipe end 11 of the first steel pipe 10, and at least the second pipe end 22 of the second steel pipe 20 has a slit 23 extending in the pipe axial direction and passing outward in the pipe axial direction, and the first pipe end 11 of the first steel pipe 10 is restrained by a radial compressive load. This makes it possible to connect the first pipe end 11 of the first steel pipe 10 by restraining it with the second pipe end 22 of the second steel pipe 20 without the need for welding or bolts. Therefore, the first pipe end 11 of the first steel pipe 10 can be connected by being evenly restrained with the second pipe end 22 of the second steel pipe 20 without being affected by thermal stress and while preventing stress concentration.

[0044] Furthermore, according to the steel pipe connection structure 1 of the first embodiment, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the second pipe end 22, which is the lower end of the second steel pipe 20, which is the upper pile, can be connected without the need for welding or bolts. Therefore, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, can be evenly restrained by the second pipe end 22, which is the lower end of the second steel pipe 20, which is the upper pile, without being affected by thermal stress and while preventing stress concentration, and these can be connected.

[0045] Furthermore, according to the steel pipe connection structure 1 of the first embodiment, the length of the slit 23 in the pipe axis direction of the second steel pipe 20 is more than twice the outer diameter of the second steel pipe 20, so that a sufficient reduction ratio can be obtained at the second pipe end 22 of the second steel pipe 20, and the first pipe end 11 of the first steel pipe 10 can be securely restrained by the second pipe end 22 of the second steel pipe 20 without the need for welding or bolts.

[0046] According to the method for connecting steel pipes of the first embodiment, external compressive and internal tensile residual stresses are first introduced into at least the second pipe end 22 of the second steel pipe 20, and the second pipe end 22 of the second steel pipe 20 is then placed over the first pipe end 11 of the first steel pipe 10. Then, a slit 23 extending in the pipe axial direction and passing outward in the pipe axial direction is formed in at least the second pipe end 22 of the second steel pipe 20. This reduces the diameter of at least the second pipe end 22 of the second steel pipe 20 to form the second pipe end 22, and the first pipe end 11 of the first steel pipe 10 can be restrained by the second pipe end 22 of the second steel pipe 20 with a radial compressive load, thereby connecting the first pipe end 11 of the first steel pipe 10. As a result, the first pipe end 11 of the first steel pipe 10 can be restrained by the second pipe end 22 of the second steel pipe 20, and the two pipes can be connected without the need for welding or bolts. Therefore, the first pipe end 11 of the first steel pipe 10 can be evenly restrained by the second pipe end 22 of the second steel pipe 20, and these can be connected without being affected by thermal stress and while preventing stress concentration.

[0047] Furthermore, according to the steel pipe connecting method of the first embodiment, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the second pipe end 22, which is the lower end of the second steel pipe 20, which is the upper pile, can be connected without the need for welding or bolts. Therefore, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, can be evenly restrained by the second pipe end 22, which is the lower end of the second steel pipe 20, without being affected by thermal stress and while preventing stress concentration, and these can be connected.

[0048] According to the steel pipe connecting method of the first embodiment, the second steel pipe 20 is heated and then cooled in a state in which the cooling rate on the outer surface of the second steel pipe 20 is greater than that on the inner surface. Therefore, even when residual stress is introduced into multiple steel pipes in a factory, the second steel pipes 20 can be manufactured continuously and efficiently on a single line.

[0049] According to the steel pipe connecting method of the first embodiment, the heating temperature of the second steel pipe 20 is set to 300°C or higher and the A3 transformation point or lower. By setting the temperature to 300°C or higher, the blue brittle region is avoided, and by setting the temperature to the A3 transformation point or lower, a decrease in toughness due to the formation of martensite in the second steel pipe 20 is prevented, making it possible to prevent brittle fracture due to scratches during construction.

[0050] [Second embodiment] Next, a steel pipe connection structure 1A and a connection method according to a second embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Fig. 4. Parts similar to those in the first embodiment will be designated by the same names and reference numerals.

[0051] In the steel pipe connection structure 1A of the second embodiment, an annular diaphragm 25 is fixed by welding to the inner surface of the end portion on the second pipe end portion 22 side of the main body portion 21 of the second steel pipe 20. Then, this diaphragm 25 abuts in the pipe axial direction against the first pipe end portion 11, which is the upper end portion of the first steel pipe 10 covered with the second pipe end portion 22 of the second steel pipe 20.

[0052] In the steel pipe connecting method of the second embodiment, external compressive and internal tensile residual stresses are introduced in advance in at least the second pipe end 22 of the second steel pipe 20 at a factory, and a diaphragm 25 is fixed to the inner surface of a predetermined position of the main body 21 of the second steel pipe 20, which will later become the end on the second pipe end 22 side. Then, at the construction site, a first steel pipe 10, which serves as a lower pile, is driven into the ground, and the second pipe end 22 of the second steel pipe 20 is placed over the first pipe end 11, which is the upper end of the first steel pipe 10 protruding from the ground, so that the diaphragm 25 abuts in the pipe axial direction. After that, a slit 23 is formed in the second pipe end 22 of the second steel pipe 20, extending in the pipe axial direction and extending outward in the pipe axial direction. This reduces the diameter of the second pipe end 22 of the second steel pipe 20 to form the second pipe end 22. Then, with the diaphragm 25 fixed to the second steel pipe 20 abutting against the first pipe end 11 of the first steel pipe 10, the first pipe end 11 is restrained by a radial compressive load at the second pipe end 22 of the second steel pipe 20. This connects the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20.

[0053] According to the steel pipe connection structure 1A of the second embodiment, the diaphragm 25 receives the compressive axial force between the second steel pipe 20 and the first steel pipe 10, so that the second steel pipe 20 can resist the large compressive axial force acting between it and the first steel pipe 10.

[0054] [Third embodiment] Next, a steel pipe connecting structure and connecting method according to a third embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Fig. 5. Parts similar to those in the first embodiment will be designated by the same names and reference numerals.

[0055] In the steel pipe connection structure 1B of the third embodiment, a high-friction body 30, such as a rubber material, adhesive, or aluminum layer, which increases the friction coefficient, is provided on the outer surface of the first steel pipe 10, which serves as the lower pile, in an area that overlaps with at least the second steel pipe 20 in the pipe axial direction. If the high-friction body 30 is a rubber material, it is, for example, glued to the outer surface of the first steel pipe 10; if the high-friction body 30 is an adhesive, it is, for example, painted on the outer surface of the first steel pipe 10; and if the high-friction body 30 is an aluminum layer, it is, for example, thermally sprayed on the outer surface of the first steel pipe 10.

[0056] In the steel pipe connecting method of the third embodiment, a high-friction body 30 is provided in advance, for example, in a factory, on the outer surface of the first pipe end 11 side of the first steel pipe 10. Then, at the construction site, the first steel pipe 10, which serves as a lower pile, is driven into the ground, and the second pipe end 22, which is the lower end of the second steel pipe 20, is placed over the high-friction body 30 on the outer surface of the first pipe end 11 side, which is the upper end of the first steel pipe 10 protruding from the ground, and a slit 23 is formed in at least the second pipe end 22 of the second steel pipe 20, extending in the pipe axial direction and passing outward in the pipe axial direction. Alternatively, at a construction site, a first steel pipe 10 serving as a lower pile is driven into the ground, a high-friction body 30 is provided on the outer surface of the first pipe end 11, which is the upper end of the first steel pipe 10 protruding from the ground, and a second pipe end 22, which is the lower end of the second steel pipe 20, is covered with the high-friction body 30, and a slit 23 extending in the pipe axis direction and outward in the pipe axis direction is formed in at least the second pipe end 22 of the second steel pipe 20. By either of these methods, at least the second pipe end 22 of the second steel pipe 20 is reduced in diameter to form the second pipe end 22, and the first pipe end 11 of the first steel pipe 10 is restrained by a radial compressive load at the second pipe end 22 of the second steel pipe 20 via the high-friction body 30. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected via the high-friction body 30.

[0057] According to the third embodiment of the steel pipe connection structure 1B, the high friction body 30 generates resistance to the circumferential torsional load between the first steel pipe 10 and the second steel pipe 20, so that it is possible to exert large torsional resistance to the circumferential torsional load between the first steel pipe 10 and the second steel pipe 20.

[0058] In the steel pipe connection structure 1B of the third embodiment, the second steel pipe 20 can also be provided with the diaphragm 25 of the second embodiment.

[0059] [Fourth embodiment] Next, a steel pipe connecting structure and connecting method according to a fourth embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Fig. 6. Parts similar to those in the first embodiment will be designated by the same names and reference numerals.

[0060] In the fourth embodiment of the steel pipe connection structure 1C, a torsion receiving member 35 is fixed to the outer surface of the first steel pipe 10, which is the lower pile, and fits into the slit 23 of the second steel pipe 20, which is the upper pile, and abuts the upper end, which is the base end, of the slit 23.

[0061] In the steel pipe connecting method of the fourth embodiment, at a construction site, a first steel pipe 10 serving as a lower pile is driven into the ground, and a second steel pipe 20 serving as a lower end is placed over a first pipe end 11 serving as an upper end of the first steel pipe 10 protruding from the ground. A slit 23 extending in the pipe axial direction and passing outward in the pipe axial direction is then formed in at least the second pipe end 22. This restrains the first pipe end 11 of the first steel pipe 10 with a radial compressive load by the second pipe end 22 of the second steel pipe 20. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected. A torsion-receiving member 35 is then fixed by welding to the outer surface of the first steel pipe 10 serving as the lower pile, so as to fit into the slit 23 of the second steel pipe 20 serving as the upper pile and abut against the upper end, which is the base end of the slit 23.

[0062] According to the steel pipe connection structure 1C of the fourth embodiment, the torsion receiving member 35 can exert large torsional resistance against a circumferential torsional load between the first steel pipe 10 and the second steel pipe 20. Moreover, because the torsion receiving member 35 abuts against the upper end portion, which is the base end portion of the slit 23, and receives the axial force in the compressive direction between the second steel pipe 20 and the first steel pipe 10, the second steel pipe 20 can also resist a larger axial force acting between it and the first steel pipe 10.

[0063] In the steel pipe connection structure 1C of the fourth embodiment, it is also possible to provide the diaphragm 25 of the second embodiment on the second steel pipe 20, and it is also possible to provide the high-friction body 30 of the third embodiment between the first steel pipe 10 and the second steel pipe 20.

[0064] [Fifth embodiment] Next, a steel pipe connecting method according to a fifth embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Fig. 7. Parts similar to those in the first embodiment will be designated by the same names and reference numerals.

[0065] In the steel pipe connecting method of the fifth embodiment, as shown in Fig. 7, for example, at a factory, an inner portion 23a of a slit 23 in the pipe axis direction is formed in at least a second pipe end portion 22 of a second steel pipe 20, which serves as an upper pile, that will later be placed over the first pipe end portion 11 of a first steel pipe 10, which serves as a lower pile. That is, at the construction site, the second pipe end portion 22, which is the lower end portion of the second steel pipe 20, which serves as an upper pile, is to be placed over the first pipe end portion 11, which is the upper end portion of the first steel pipe 10, which serves as a lower pile. However, before this, an inner portion 23a of a slit 23 in the pipe axis direction is formed in at least the second pipe end portion 22 of the second steel pipe 20, which serves as an upper pile.

[0066] Then, at the construction site, the first steel pipe 10, which serves as the lower pile, is driven into the ground, and the second pipe end 22, which serves as the lower end of the second steel pipe 20, which serves as the upper pile, is placed over the first pipe end 11, which serves as the upper end of the first steel pipe 10 protruding from the ground, and the remaining axially outer portion of the downward slit 23 is formed in at least the second pipe end 22 of the second steel pipe 20. As a result, the downward slit 23 is formed as shown in FIG. 1 , and the first pipe end 11 of the first steel pipe 10 is restrained by the second pipe end 22 of the second steel pipe 20 with a radial compressive load. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected.

[0067] According to the steel pipe connecting method of the fifth embodiment, an inner portion 23a of the slit 23 in the pipe axis direction is formed in at least the second pipe end 22 of the second steel pipe 20 in advance at a factory. Then, when connecting the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 at a construction site, the second pipe end 22 of the second steel pipe 20 is placed over the first pipe end 11 of the first steel pipe 10, and then the outer portion of the slit 23 in the pipe axis direction is formed in at least the second pipe end 22 of the second steel pipe 20. This makes it possible to form a slit 23 that extends downward. This reduces the amount of work required at the construction site when connecting the first steel pipe 10 and the second steel pipe 20.

[0068] In the steel pipe connecting method of the fifth embodiment, it is also possible to provide the diaphragm 25 of the second embodiment on the second steel pipe 20, it is also possible to provide the high friction body 30 of the third embodiment between the first steel pipe 10 and the second steel pipe 20, and it is also possible to provide the torsion receiving member 35 of the fourth embodiment on the first steel pipe 10.

[0069] [Sixth embodiment] Next, a steel pipe connecting method according to a sixth embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Fig. 8. The same parts as those in the first embodiment will be designated by the same names and reference numerals.

[0070] In the steel pipe connecting method of the sixth embodiment, as shown in Fig. 8, at least the second pipe end 22 of the second steel pipe 20 serving as the upper pile, which will later be placed over the first pipe end 11 of the first steel pipe 10 serving as the lower pile, is formed in a broken line shape in advance, for example, at a factory. That is, at the construction site, the second pipe end 22, which is the lower end of the second steel pipe 20 serving as the upper pile, is to be placed over the first pipe end 11, which is the upper end of the first steel pipe 10 serving as the lower pile, but before this, at least the second pipe end 22 of the second steel pipe 20 serving as the upper pile is formed in a broken line shape in a broken line shape in the circumferential direction.

[0071] Then, at the construction site, the first steel pipe 10, which serves as the lower pile, is driven into the ground, and the second pipe end 22, which serves as the lower end of the second steel pipe 20, which serves as the upper pile, is placed over the first pipe end 11, which serves as the upper end of the first steel pipe 10 protruding from the ground. The remaining portion of the inner edge of the dashed-line slit 23 is then formed so as to connect a portion 23b of the inner edge of the dashed-line slit 23 to at least the second pipe end 22 of the second steel pipe 20. This forms a downward-extending slit 23, as shown in FIG. 1 , and the first pipe end 11 of the first steel pipe 10 is restrained by a radial compressive load at the second pipe end 22 of the second steel pipe 20. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected.

[0072] According to the steel pipe connecting method of the sixth embodiment, at least the second pipe end 22 of the second steel pipe 20 is pre-formed in a factory with a dashed line-shaped portion 23b of the inner edge of the slit 23 on both sides of the second pipe end 22 in the circumferential direction. Then, when connecting the first pipe end 11 of the first steel pipe 10 and the second pipe end 20 of the second steel pipe 20 at the construction site, the second pipe end 22 of the second steel pipe 20 is placed over the first pipe end 11 of the first steel pipe 10, and the remaining portion of the slit 23 is then formed on at least the second pipe end 22 of the second steel pipe 20 so as to connect the dashed line-shaped portion 23b of the inner edge of the slit 23. This allows the formation of a downwardly extending slit 23. This reduces the amount of work required at the construction site when connecting the first steel pipe 10 and the second steel pipe 20.

[0073] In the steel pipe connecting method of the sixth embodiment, it is also possible to provide the diaphragm 25 of the second embodiment on the second steel pipe 20, it is also possible to provide the high friction body 30 of the third embodiment between the first steel pipe 10 and the second steel pipe 20, and it is also possible to provide the torsion receiving member 35 of the fourth embodiment on the first steel pipe 10.

[0074] [Seventh embodiment] Next, a steel pipe connecting structure and connecting method according to a seventh embodiment will be described, focusing on differences from the sixth embodiment, mainly with reference to Figures 9 and 10. Parts similar to those in the sixth embodiment will be designated by the same names and reference numerals.

[0075] 9, in the steel pipe connecting method of the seventh embodiment, as in the sixth embodiment, at least the second pipe end 22 of the second steel pipe 20 serving as the upper pile is formed in a dashed line shape on both sides of the circumferential direction of the inner edge of the slit 23 in the second pipe end 22, which will later be placed over the first pipe end 11 of the first steel pipe 10 serving as the lower pile, for example, in advance at a factory. That is, at the construction site, the second pipe end 22, which is the lower end of the second steel pipe 20 serving as the upper pile, is to be placed over the first pipe end 11, which is the upper end of the first steel pipe 10 serving as the lower pile, but before this, at least the second pipe end 22 of the second steel pipe 20 serving as the upper pile is formed in a dashed line shape on both sides of the circumferential direction of the inner edge of the slit 23.

[0076] Furthermore, for example, in a factory, a torsion bearing member 40 is fixed by welding or the like to at least a predetermined position of the first pipe end 11 of the first steel pipe 10, which serves as the lower pile, on which the second pipe end 22 of the second steel pipe 20, which serves as the upper pile, will later be placed. That is, at the construction site, the second pipe end 22, which is the lower end of the second steel pipe 20, which serves as the upper pile, will be placed over the first pipe end 11, which is the upper end of the first steel pipe 10, but before this, the torsion bearing member 40 is fixed to a predetermined position of the first pipe end 11, which is the upper end of the first steel pipe 10, which serves as the lower pile. Here, the torsion bearing member 40 has a tapered wedge shape such that its width in the circumferential direction of the first steel pipe 10 narrows toward the outside, i.e., the upper side, of the first steel pipe 10 in the pipe axial direction.

[0077] Then, at the construction site, the first steel pipe 10, which serves as the lower pile, is driven into the ground, and the second pipe end 22, which serves as the lower end of the second steel pipe 20, is partially placed over the first pipe end 11, which serves as the upper end of the first steel pipe 10 protruding from the ground, so that the position between the portions 23b on both sides of the circumferential direction of the second pipe end 22 at the inner edge of the slit 23, which has been formed in a dashed line shape beforehand, is aligned with the torsion receiving member 40 in the circumferential direction, and the second steel pipe 20 is pressed into the first steel pipe 10 by a construction machine to further increase the amount of covering. Then, the torsion receiving member 40 cuts through the remaining portion of at least the second pipe end 22 of the second steel pipe 20, which serves as the upper pile, so as to connect the portions 23b of the inner edge of the slit 23, which has been formed in a dashed line. As a result, a downward slit 23 is formed as shown in Figure 10, and the first pipe end 11 of the first steel pipe 10 is restrained by a radial compressive load at the second pipe end 22 of the second steel pipe 20. As a result, the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected. When the slit 23 is formed, the second pipe end 22 of the second steel pipe 20 has a portion that was inside the inner edge of the slit 23 turned up, and this portion is removed to form the slit 23.

[0078] According to the steel pipe connecting method of the seventh embodiment, at a factory, a portion 23b of the inner edge of the slit 23 on both sides in the circumferential direction of the second pipe end 22 is formed in a dashed line shape at at least the second pipe end 22 of the second steel pipe 20, and a torsion receiving member 40 is fixed to the first pipe end 11 of the first steel pipe 10. Then, when the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20 are connected at a construction site, the second pipe end 22 of the second steel pipe 20 is placed over the first pipe end 11 of the first steel pipe 10. At this time, the torsion receiving member 40 forms the remaining portion of at least the second pipe end 22 of the second steel pipe 20 so as to connect the portion 23b of the inner edge of the slit 23 formed in a dashed line, thereby forming a slit 23 that passes downward. This reduces the amount of work required at the construction site when connecting the first steel pipe 10 and the second steel pipe 20. In this case, the torsion bearing member 40 forming the slit 23 generates resistance to a circumferential torsional load between the first steel pipe 10 and the second steel pipe 20, and also generates resistance to a large axial force in the compressive direction acting between the first steel pipe 10 and the second steel pipe 20.

[0079] In the steel pipe connecting method of the seventh embodiment, it is also possible to provide the diaphragm 25 of the second embodiment on the second steel pipe 20. Also in the steel pipe connecting method of the fifth embodiment, it is also possible to form a part 23a of the slit 23 on the inner side in the pipe axial direction in at least the second pipe end 22 of the second steel pipe 20 in advance at a factory, and to fix a torsion receiving member 40 to a predetermined position on the first pipe end 11 of the first steel pipe 10 by welding or the like, as in the seventh embodiment. When connecting the first pipe end 11 of the first steel pipe 10 and the second pipe end 22 of the second steel pipe 20, the second pipe end 22 of the second steel pipe 20 is fitted over the first pipe end 11 of the first steel pipe 10 by using a construction machine to press the second steel pipe 20 into the first steel pipe 10, and the torsion receiving member 40 breaks the outer portion of the slit 23 in the pipe axial direction in the second pipe end 22 of the second steel pipe 20 to form the slit.

[0080] In the first to seventh embodiments, the heat treatment for introducing the above-mentioned residual stress into the second steel pipe 20 may be performed on the entire second steel pipe 20, or may be performed only on the second pipe end portion 22, which is the portion to be reduced in diameter. Furthermore, after the heat treatment for introducing the above-mentioned residual stress into the entire steel pipe, it is also possible to cut this steel pipe circumferentially and divide it into multiple parts in the pipe axial direction, and weld these parts to the end portion of another steel pipe, thereby forming the second pipe end portion 22 with this other steel pipe forming the main body portion 21.

[0081] [Eighth embodiment] Next, a steel pipe connecting structure and connecting method according to an eighth embodiment will be described, focusing on differences from the first embodiment, mainly with reference to Figures 11 to 13. Parts similar to those in the first embodiment will be designated by the same names and reference numerals.

[0082] The eighth embodiment of the steel pipe connection structure 1E shown in FIG. 11 and the connection method shown in FIGS. 11 to 13 also involve connecting multiple steel pipes in series to form a single steel pipe pile that is driven into the ground in a substantially vertical direction. Here, the steel pipe pile is described using an example of a connection structure 1E and connection method, which includes a single first steel pipe 10 (lower pile) similar to that of the first embodiment, a second steel pipe 50 whose lower end, a second pipe end 51, is connected to a first pipe end 11 (upper end) of the first steel pipe 10, and a third steel pipe 60 (upper pile) whose upper end, a third pipe end 52, on the opposite side of the second pipe end 51, is connected to a fourth pipe end 61 (lower end). Here, the fourth pipe end 61 of the third steel pipe 60 abuts against the first pipe end 11 of the first steel pipe 10 in the pipe axial direction. It is possible to lengthen the steel pipe pile in the pipe axis direction by appropriately repeating the same connecting structure 1E of the second steel pipe 50 and the third steel pipe 60 upward.

[0083] The third steel pipe 60, which is the upper pile, is cylindrical. The third steel pipe 60 has the same inner diameter as the first steel pipe 10 and the same outer diameter as the first steel pipe 10. The second steel pipe 50 has an inner diameter equal to the outer diameter of the first steel pipe 10 and the outer diameter of the third steel pipe 60, and an outer diameter larger than the outer diameters of the first steel pipe 10 and the third steel pipe 60. The second steel pipe 20 has a slit 53 formed therein that extends over the entire length in the axial direction of the pipe.

[0084] In the steel pipe connection structure 1E of the eighth embodiment, a second pipe end 51, which is the lower end of a second steel pipe 50, is placed over a first pipe end 11, which is the upper end of a first steel pipe 10, which serves as a lower pile. In this connection structure 1E, the second pipe end 51 of the second steel pipe 50 abuts against the first steel pipe 10 over its entire circumferential range, as shown in FIG. 11(b), and restrains the first steel pipe 10 with a radial compressive load. Specifically, the second pipe end 51 of the second steel pipe 50 restrains the first pipe end 11, which is the upper end of the first steel pipe 10 shown in FIG. 11(a), with a radial compressive load. This connects the first pipe end 11, which is the upper end of the first steel pipe 10, which serves as a lower pile, and the second pipe end 51, which is the lower end of the second steel pipe 50.

[0085] In the steel pipe connection structure 1E of the eighth embodiment, the third pipe end 52, which is the upper end of the second steel pipe 50 opposite the second pipe end 51, is placed over the fourth pipe end 61, which is the lower end of the third steel pipe 60, which serves as the upper pile. In this connection structure 1E, the third pipe end 52 of the second steel pipe 50 restrains the fourth pipe end 61 of the third steel pipe 60 over its entire circumferential range with a radial compressive load. This connects the third pipe end 52 of the second steel pipe 50 and the fourth pipe end 61 of the third steel pipe 60. Moreover, the fourth pipe end 61, which is the lower end of the third steel pipe 60, abuts against the first pipe end 11, which is the upper end of the first steel pipe 10, in the pipe axial direction.

[0086] Therefore, the second steel pipe 50 has a slit 53 extending in the pipe axial direction along its entire length, and restrains the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, with a radial compressive load.

[0087] The steel pipe connection method of the eighth embodiment is a method for constructing the above-mentioned connection structure 1E. Here, if the second steel pipe 20 is a conventional steel pipe, even if a slit 53 is formed, it will not be possible to restrain the first pipe end 11, which is the upper end of the first steel pipe 10 (the lower pile), or the fourth pipe end 61, which is the lower end of the third steel pipe 60 (the upper pile). In the connection method of the eighth embodiment, the second steel pipe 20 is preheat-treated to generate residual stress, and then the slit 53 is formed by cutting, thereby releasing and redistributing the residual stress. This causes the portion of the second steel pipe 50 where the slit 53 is formed to deform in a direction that reduces its diameter. Regarding the heat treatment, the second steel pipe 50 is heated and then cooled by water cooling on the outside and air cooling on the inside. Due to the difference in cooling rate, external compressive and internal tensile residual stresses are introduced throughout the second steel pipe 50. Furthermore, the heating temperature of the second steel pipe 50 is set to 300°C or higher and below the A3 transformation point.

[0088] That is, in the eighth embodiment of the steel pipe connecting method, external compressive and internal tensile residual stresses are introduced into the entire second steel pipe 50 at a factory. Then, at the construction site, a first steel pipe 10 serving as a lower pile is driven into the ground, and the second steel pipe 50 is placed over the first steel pipe 10 protruding from the ground, as shown in FIG. 12(a). At this time, as shown in FIG. 12(b), a radial gap is formed between the second steel pipe 50 and the first steel pipe 10. Next, as shown in FIG. 12(a), a third steel pipe 60 serving as an upper pile is placed on the first steel pipe 10 serving as the lower pile so that their axes are aligned, and the fourth pipe end 61, which is the lower end of the third steel pipe 60, is butted against the first pipe end 11, which is the upper end of the first steel pipe 10, in the pipe axial direction. Then, with the first pipe end 11 of the first steel pipe 10 and the fourth pipe end 61 of the third steel pipe 60 abutting against each other in the axial direction, the second pipe end 51, which is the lower end of the second steel pipe 50, is placed over the first pipe end 11, which is the upper end of the first steel pipe 10, which serves as the lower pile, and the third pipe end 52, which is the upper end of the second steel pipe 50 opposite the second pipe end 51, is placed over the fourth pipe end 61, which is the lower end of the third steel pipe 60, which serves as the upper pile. Thereafter, a slit 53 is formed over the entire length of the second steel pipe 50, as shown in Figures 13(a) and 13(b). 11(b), the second pipe end 51, which is the lower end of the second steel pipe 20, is reduced in diameter and abuts against the first steel pipe 10, which is the lower pile, over the entire circumferential length, thereby restraining the first pipe end 11, which is the upper end of the first steel pipe 10 shown in FIG. 11(a), with a radial compressive load, and the third pipe end 52, which is the upper end of the second steel pipe 50, is reduced in diameter and restrains the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, with a radial compressive load. As a result, the first pipe end 11 of the first steel pipe 10 is connected to the second pipe end 51 of the second steel pipe 50, and the third pipe end 52 of the second steel pipe 50 is connected to the fourth pipe end 61 of the third steel pipe 60, thereby realizing the above-mentioned connection structure 1E.

[0089] The width of the slit 53 is set so that the difference between the inner diameter of the second steel pipe 50 before forming the slit 53 and the outer diameters of the first steel pipe 10 and the third steel pipe 60 can be eliminated, and the first pipe end 11 of the first steel pipe 10 and the fourth pipe end 61 of the third steel pipe 60 can be constrained with a predetermined radial compressive load. In other words, the width of the slit 53 is set so that the slit 53 does not close over its entire length even when the first pipe end 11 of the first steel pipe 10 and the fourth pipe end 61 of the third steel pipe 60 are constrained with a predetermined radial compressive load by the second steel pipe 50. Note that when forming the slit 53, if the width is not greater than the amount of diameter reduction, the cutter used to form the slit 53 will be pinched, so a tool that can cut with a certain width, such as a milling cutter, is used.

[0090] According to the eighth embodiment of the steel pipe connection structure 1E, the second pipe end 51 of the second steel pipe 50 is placed over the first pipe end 11 of the first steel pipe 10, and the third pipe end 52 of the second steel pipe 50 is placed over the fourth pipe end 61 of the third steel pipe 60, and the fourth pipe end 61 of the third steel pipe 60 is abutted against the first pipe end 11 of the first steel pipe 10 in the pipe axis direction, so that a slit 53 extending in the pipe axis direction over the entire length of the second steel pipe 50 is formed and the first pipe end 11 of the first steel pipe 10 and the fourth pipe end 61 of the third steel pipe 60 are restrained with a radial compressive load. This allows the first pipe end 11 of the first steel pipe 10 to be restrained by the second pipe end 51 of the second steel pipe 50, thereby connecting them, and the third pipe end 52 of the second steel pipe 50 to be restrained by the fourth pipe end 61 of the third steel pipe 60, thereby connecting them, without the need for welding or bolts. Therefore, the first pipe end 11 of the first steel pipe 10 can be evenly restrained by the second pipe end 51 of the second steel pipe 50, without being affected by thermal stress and while preventing stress concentration, and the third pipe end 52 of the second steel pipe 50 can be evenly restrained by the fourth pipe end 61 of the third steel pipe 60, without being affected by thermal stress and while preventing stress concentration, thereby connecting them. Furthermore, because the fourth pipe end 61 of the third steel pipe 60 is abutted against the first pipe end 11 of the first steel pipe 10 in the pipe axial direction, it is possible to resist a greater axial force acting between the third steel pipe 60 and the first steel pipe 10. In addition, the axial centers of the first steel pipe 10, the second steel pipe 50, and the third steel pipe 60 can be aligned.

[0091] According to the steel pipe connection structure 1E of the eighth embodiment, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, can be restrained by the second steel pipe 50 and connected together without the need for welding or bolts. Therefore, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, can be connected together by being evenly restrained by the second steel pipe 50 without being affected by thermal stress and while preventing stress concentration.

[0092] According to the eighth embodiment of the steel pipe connecting method, external compressive and internal tensile residual stresses are introduced into the entire second steel pipe 50 in advance, the second pipe end 51 of the second steel pipe 50 is placed over the first pipe end 11 of the first steel pipe 10, the third pipe end 52 of the second steel pipe 50 is placed over the fourth pipe end 61 of the third steel pipe 60, and the first pipe end 11 of the first steel pipe 10 and the fourth pipe end 61 of the third steel pipe 60 are butted against each other in the pipe axial direction, and a slit 53 extending in the pipe axial direction over the entire length of the second steel pipe 50 is formed. As a result, the diameter of the second steel pipe 50 is reduced, and the first pipe end 11 of the first steel pipe 10 is restrained by the second pipe end 51 of the second steel pipe 50 with a radial compressive load to connect them, and the fourth pipe end 61 of the third steel pipe 60 is restrained by the third pipe end 52 of the second steel pipe 50 with a radial compressive load to connect them. As a result, without the need for welding or bolts, the first pipe end 11 of the first steel pipe 10 can be restrained by the second pipe end 51 of the second steel pipe 50 to connect them, and the third pipe end 52 of the second steel pipe 50 can be restrained by the fourth pipe end 61 of the third steel pipe 60 to connect them. Therefore, the first pipe end 11 of the first steel pipe 10 can be evenly restrained by the second pipe end 51 of the second steel pipe 50, without being affected by thermal stress and while preventing stress concentration, and the two can be connected, and the fourth pipe end 61 of the third steel pipe 60 can be evenly restrained by the third pipe end 52 of the second steel pipe 50, without being affected by thermal stress and while preventing stress concentration. In addition, because the fourth pipe end 61 of the third steel pipe 60 is abutted against the first pipe end 11 of the first steel pipe 10 in the pipe axial direction, it can also resist a greater axial force acting between the third steel pipe 60 and the first steel pipe 10.

[0093] Furthermore, according to the steel pipe connecting method of the eighth embodiment, it is possible to connect the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, to the second pipe end 51, which is the lower end of the second steel pipe 50, without the need for welding or bolts, and it is also possible to connect the third pipe end 52, which is the upper end of the second steel pipe 50, to the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile. Thus, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, can be evenly restrained by the second pipe end 51, which is the lower end of the second steel pipe 50, without being affected by thermal stress and while preventing stress concentration, and the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, can be evenly restrained by the third pipe end 52, which is the upper end of the second steel pipe 50, without being affected by thermal stress and while preventing stress concentration, and these can be connected. In addition, since the fourth pipe end 61 of the third steel pipe 60, which is the upper pile, is abutted against the first pipe end 11 of the first steel pipe 10, which is the lower pile, in the pipe axial direction, it can also resist a greater axial force acting between the third steel pipe 60 and the first steel pipe 10.

[0094] According to the eighth embodiment of the steel pipe connecting method, the second steel pipe 50 is heated and then cooled in a state in which the cooling rate on the outer surface of the second steel pipe 50 is greater than that on the inner surface thereof, so that even when residual stress is introduced into multiple second steel pipes 50 in a factory, continuous and efficient production can be carried out on a single line.

[0095] According to the eighth embodiment of the steel pipe connecting method, the heating temperature of the second steel pipe 50 is set to 300°C or higher and the A3 transformation point or lower. Therefore, by setting the temperature to 300°C or higher, the blue brittle region is avoided, and by setting the temperature to the A3 transformation point or lower, a decrease in toughness due to the formation of martensite in the second steel pipe 50 is prevented, and brittle fracture due to scratches during construction can be prevented.

[0096] In the eighth embodiment, high-friction bodies 30 similar to those in the third embodiment may be provided on the outer surface of the first steel pipe 10 in an area overlapping with at least the second steel pipe 50 in the pipe axis direction, and on the outer surface of the third steel pipe 60 in an area overlapping with at least the second steel pipe 50 in the pipe axis direction. This allows the high-friction bodies 30 to generate resistance to a circumferential torsional load between the first steel pipe 10 and the second steel pipe 50, and the high-friction bodies 30 to generate resistance to a circumferential torsional load between the third steel pipe 60 and the second steel pipe 50. Therefore, it is possible to exert a large torsional resistance to a circumferential torsional load between the first steel pipe 10 and the third steel pipe 60.

[0097] Furthermore, in the eighth embodiment, a torsion receiving member that fits into the slit 53 of the second steel pipe 50 may be fixed to the outer surface of the first steel pipe 10 by welding or the like, and a torsion receiving member that fits into the slit 53 of the second steel pipe 50 may be fixed to the outer surface of the third steel pipe 60 by welding or the like. In this way, the torsion receiving member of the first steel pipe 10 generates resistance to a circumferential torsional load between the first steel pipe 10 and the second steel pipe 50, and the torsion receiving member of the third steel pipe 60 generates resistance to a circumferential torsional load between the third steel pipe 60 and the second steel pipe 50. Therefore, it is possible to exert large torsional resistance to a circumferential torsional load between the first steel pipe 10 and the third steel pipe 60. In this case, it is also possible to provide high-friction bodies 30 between the first steel pipe 10 and the second steel pipe 50 and between the third steel pipe 60 and the second steel pipe 50 .

[0098] Furthermore, in the eighth embodiment, the intermediate portion of the slit 53 in the pipe axis direction may be formed in advance, for example, at a factory, in the pipe axis direction of the second steel pipe 20. That is, at the construction site, the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, is covered with the second pipe end 51, which is the lower end of the second steel pipe 50, and the third pipe end 52, which is the upper end of the second steel pipe 50, is covered with the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, and the fourth pipe end 61 of the third steel pipe 60 is brought into contact with the first pipe end 11 of the first steel pipe 10 in the pipe axis direction. However, before this, a part of the intermediate portion of the slit 53 in the pipe axis direction is formed in the second steel pipe 50.

[0099] Then, at the construction site, the first steel pipe 10, which serves as the lower pile, is driven into the ground, and the first pipe end 11, which serves as the upper end of the first steel pipe 10 protruding from the ground, is covered with the second pipe end 51, which serves as the lower end of the second steel pipe 50, and the third pipe end 52, which serves as the upper end of the second steel pipe 50, is covered with the fourth pipe end 61, which serves as the lower end of the third steel pipe 60, which serves as the upper pile, and the fourth pipe end 61 of the third steel pipe 60 is abutted against the first pipe end 11 of the first steel pipe 10. In this state, the outer portion of a slit 53 extending downward in the pipe axis direction is formed in the lower end of the second steel pipe 50, and the outer portion of a slit 53 extending upward in the pipe axis direction is formed in the upper end of the second steel pipe 50. As a result, the first pipe end 11 of the first steel pipe 10 is restrained by the second pipe end 51 of the second steel pipe 50 with a radial compressive load, and the third pipe end 52 of the second steel pipe 50 restrains the fourth pipe end 61 of the third steel pipe 60 with a radial compressive load. This makes it possible to reduce the amount of work required at the construction site when connecting the first steel pipe 10, the second steel pipe 50, and the third steel pipe 60.

[0100] Furthermore, in the eighth embodiment, dashed lines may be formed in advance, for example, at a factory, on both circumferential sides of the inner edge of the slit 53 of the second steel pipe 50. That is, at the construction site, the second pipe end 51, which is the lower end of the second steel pipe 50, is placed over the first pipe end 11, which is the upper end of the first steel pipe 10, which is the lower pile, and the third pipe end 52, which is the upper end of the second steel pipe 50, is placed over the fourth pipe end 61, which is the lower end of the third steel pipe 60, which is the upper pile, so that the fourth pipe end 61 of the third steel pipe 60 abuts against the first pipe end 11 of the first steel pipe 10. However, before this, dashed lines are formed in the inner edge of the slit 53 of the second steel pipe 50 on both circumferential sides of the second steel pipe 50.

[0101] Then, at the construction site, the first steel pipe 10, which serves as the lower pile, is driven into the ground, and the first pipe end 11, which serves as the upper end of the first steel pipe 10 protruding from the ground, is covered with the second pipe end 51, which serves as the lower end of the second steel pipe 50, and the third pipe end 52, which serves as the upper end of the second steel pipe 50, is covered with the fourth pipe end 61, which serves as the lower end of the third steel pipe 60, which serves as the upper pile, and the fourth pipe end 61 of the third steel pipe 60 is abutted against the first pipe end 11 of the first steel pipe 10. Then, the remaining part of the second steel pipe 50 is formed so as to connect parts of both circumferential sides of the inner edge of the dashed-line slit 53, thereby forming the slit 53 over the entire length. As a result, the first pipe end 11 of the first steel pipe 10 is restrained by the second pipe end 51 of the second steel pipe 50 with a radial compressive load, and the third pipe end 52 of the second steel pipe 50 restrains the fourth pipe end 61 of the third steel pipe 60 with a radial compressive load. This makes it possible to reduce the amount of work required at the construction site when connecting the first steel pipe 10, the second steel pipe 50, and the third steel pipe 60.

[0102] Furthermore, if residual stress is introduced into a steel pipe by internal compression and external tension, it is possible to manufacture a steel pipe that can be expanded by inserting a slit. Utilizing this method, internal compression and external tension residual stress is first introduced into the first pipe end 11, which is the upper end of the first steel pipe 10, which serves as the lower pile. Then, the second pipe end 22, which is the lower end of the second steel pipe 20, which serves as the upper pile and does not have a slit 23, is placed over this first pipe end 11. A slit is then formed in the first pipe end 11, and the first pipe end 11 is expanded in diameter, allowing the first pipe end 11 to push open the second pipe end 22, which is the lower end of the second steel pipe 20 and does not have a slit 23, from the inside, creating a connected structure. It is also possible to insert an inner pipe with internal compressive and external tensile residual stresses into the butt joint between the first steel pipe 10 (the lower pile) and the third steel pipe 60 (the upper pile), and form a slit in this inner pipe along its entire length in the pipe axis direction to expand the inner pipe from the inside, thereby forming a connection structure between the first steel pipe 10 and the third steel pipe 60 using the inner pipe. In these cases, the steel pipes are air-cooled on the outside and water-cooled on the inside during the heating and cooling process. [Example]

[0103] A specific example using an actual steel pipe will be explained. The original dimensions of the steel pipe are a diameter of 193.7 mm, a thickness of 9.5 mm, and a length of 600 mm. The chemical composition of the steel pipe is shown in Table 1.

[0104] [Table 1]

[0105] As shown in Figure 14, this steel pipe 100 was heated to 830°C by high-frequency induction heating using a high-frequency induction heating device 101, then held at 850°C for 7 minutes in an atmospheric furnace 102, and then water-cooled from the outside using a cooling device 103 with a cap placed on the end of the pipe to prevent water from flowing into the inner surface. A 100mm wide slit was formed in the cooled steel pipe 100 along its entire axial length by gas cutting, and the outer diameter was measured before and after the slit formation. The results are shown in Table 2.

[0106] [Table 2]

[0107] The outer diameter was determined by measuring and averaging the diagonal diameters at the top and bottom ends of the steel pipe 100. It was confirmed that the outer diameter of the steel pipe was reduced by about 5 to 7 mm before and after the slit formation.

[0108] At this time, it is known that the residual stress introduced into the steel pipe 100 can be calculated as shown in the following formula (1).

[0109] σ t =(E / (1-ν 2 ))t×((1 / D0)-(1 / D))…(1) formula

[0110] where σ t : Residual stress (N / mm 2 ), E: Young's modulus of steel pipe (N / mm 2 ), ν: Poisson's ratio of the steel pipe, t: steel pipe thickness (mm), D0: steel pipe outer diameter before deformation (mm), D: steel pipe outer diameter after deformation (mm).

[0111] As described above, a residual stress distribution is formed in the steel pipe 100 by heating the steel pipe 100 and then cooling it at different cooling rates on the inner and outer surfaces, and then a slit is formed in the axial direction of the pipe to reduce the diameter of the steel pipe 100. By placing an upper pile with a larger diameter over the lower pile and then forming a slit in the upper pile, it is possible to form a joint that connects two steel pipes without using welding or bolts.

[0112] As shown in Figure 1, when forming a slit 23 in the second pipe end 22 (the lower end of the second steel pipe 20), the upper pile, it is considered that the axial length of the slit 23 must be sufficient to prevent the tip of the upper pile from achieving sufficient diameter reduction due to the restraint of the portion without the slit 23. Although this depends on the dimensions of the second steel pipe 20 (the upper pile), the introduced residual stress, and the difference in outer diameter between the second steel pipe 20 (the upper pile) and the first steel pipe 10 (the lower pile), for steel pipe sizes and residual stresses similar to those in the example described above, providing an axial length of the slit 23 of approximately twice the outer diameter is considered to achieve a reduction of approximately 1 cm at the tip of the upper pile and to be able to restrain the first steel pipe 10 (the lower pile). A calculation example is shown in Table 3.

[0113] Here, the calculation of the diameter reduction amount was performed by considering the steel pipe 110 shown in Figure 15(a) as a cantilever beam on which a distributed load due to residual stress acts, with the portion without slit 111 as shown in Figure 15(b) as the fixed end 112, and assuming that a distributed load due to residual stress acts on the portion 113 where slit 111 is formed, as shown by the arrow in Figure 15(b). Then, as shown in Figure 15(c), for the left half 114 of the steel pipe 110, the second moment of area of ​​the beam was calculated as a rectangle, the beam width was calculated as the plate thickness, and the beam depth was calculated as the circumferential length of the plate center, and the same calculation was performed for the right half 115 of the steel pipe 110.

[0114] x=((wl 4 ) / (8EI))×2…(2) formula w=σ t ×t…(3) formula I=(t×((π(D0-t)-s) / 2) 3 ) / 12…(4) formula where x is the amount of diameter reduction (mm), w is the distributed load due to residual stress (N / mm), l is the slit length (mm), s is the slit width (mm), I is the second moment of area for the diameter reduction (mm 4 ), is.

[0115] [Table 3]

[0116] From Table 3, it can be seen that if the length of slit 111 in the axial direction of steel pipe 110 is made at least twice the outer diameter of steel pipe 110 before reduction, a sufficient reduction ratio can be achieved in steel pipe 110, and steel pipe 110 can reliably restrain other steel pipes inside without the need for welding or bolts. [Explanation of symbols]

[0117] 1,1A~1E connection structure 10 First Steel Pipe 11 first tube end 20 Second Steel Pipe 22 second tube end 23 Slit 25 diaphragm 30 High friction body 35,40 Torsion bearing member 50 Second Steel Pipe 51 Second pipe end 52 Third pipe end 60 Third Steel Pipe 61 Fourth pipe end

Claims

1. A steel pipe connection structure in which a first pipe end of a first steel pipe is covered with a second pipe end of a second steel pipe to connect the first pipe end and the second pipe end, A steel pipe connection structure characterized in that at least the second pipe end of the second steel pipe has a slit extending in the pipe axis direction and passing outward in the pipe axis direction, and the first pipe end of the first steel pipe is restrained with a radial compressive load.

2. The steel pipe connection structure according to claim 1, characterized in that the first steel pipe is a lower pile, the second steel pipe is an upper pile, the first pipe end is an upper end of the first steel pipe, and the second pipe end is a lower end of the second steel pipe.

3. 2. A steel pipe connection structure according to claim 1, characterized in that the length of the slit in the axial direction of the second steel pipe is at least twice the outer diameter of the second steel pipe.

4. The steel pipe connection structure according to claim 1, characterized in that a diaphragm is provided on the inner surface of the second steel pipe, which abuts against the first pipe end of the first steel pipe in the pipe axial direction.

5. 2. A steel pipe connection structure according to claim 1, characterized in that a high-friction body is provided on the outer surface of the first steel pipe in the axial direction of the pipe at least in an area overlapping with the second steel pipe.

6. 2. A steel pipe connection structure according to claim 1, characterized in that a torsion receiving member is provided on the outer surface of the first steel pipe, which fits into the slit and abuts against the base end of the slit.

7. a third pipe end portion of the second steel pipe opposite to the second pipe end portion is fitted onto a fourth pipe end portion of the third steel pipe, and the fourth pipe end portion of the third steel pipe is abutted against the first pipe end portion of the first steel pipe in the pipe axial direction; The steel pipe connection structure according to claim 1, characterized in that the second steel pipe has a slit extending in the axial direction over its entire length, and the first pipe end of the first steel pipe and the fourth pipe end of the third steel pipe are restrained by a radial compressive load.

8. The steel pipe connection structure described in claim 7, characterized in that the first steel pipe is a lower pile, the third steel pipe is an upper pile, the first pipe end is the upper end of the first steel pipe, the second pipe end is the lower end of the second steel pipe, the third pipe end is the upper end of the second steel pipe, and the fourth pipe end is the lower end of the third steel pipe.

9. A steel pipe connection structure as described in claim 7, characterized in that high-friction bodies are provided on the outer surface of the first steel pipe in an area that overlaps with at least the second steel pipe in the pipe axis direction, and on the outer surface of the third steel pipe in an area that overlaps with at least the second steel pipe in the pipe axis direction.

10. A method for connecting steel pipes, comprising: covering a first pipe end of a first steel pipe with a second pipe end of a second steel pipe to connect the first pipe end and the second pipe end; Preliminarily introducing external compressive and internal tensile residual stresses into at least the second pipe end portion of the second steel pipe; After or when the second pipe end portion of the second steel pipe is covered onto the first pipe end portion of the first steel pipe, A method for connecting steel pipes, characterized in that a slit extending in the pipe axis direction and passing outward in the pipe axis direction is formed in at least the second pipe end of the second steel pipe, thereby reducing the diameter of at least the second pipe end of the second steel pipe, and restraining the first pipe end of the first steel pipe with a radial compressive load at the second pipe end of the second steel pipe.

11. Before the second pipe end portion of the second steel pipe is placed over the first pipe end portion of the first steel pipe, a part of the slit on the inner side in the pipe axial direction is formed in at least the second pipe end portion of the second steel pipe, 11. The method for connecting steel pipes according to claim 10, characterized in that after the second pipe end portion of the second steel pipe is covered onto the first pipe end portion of the first steel pipe, an outer portion of the slit in the pipe axial direction is formed in at least the second pipe end portion of the second steel pipe.

12. Preliminarily introducing external compressive and internal tensile residual stresses into the entire second steel pipe; The second pipe end of the second steel pipe is placed over the first pipe end of the first steel pipe, and a third pipe end of the second steel pipe opposite to the second pipe end is placed over a fourth pipe end of a third steel pipe, and the first pipe end of the first steel pipe and the fourth pipe end of the third steel pipe are butted against each other in the pipe axial direction, 11. A method for connecting steel pipes according to claim 10, characterized in that a slit extending in the axial direction over the entire length of the second steel pipe is formed in the second steel pipe to reduce the diameter of the second steel pipe, and the first pipe end of the first steel pipe is restrained by the second pipe end of the second steel pipe with a radial compressive load, and the fourth pipe end of the third steel pipe is restrained by the third pipe end of the second steel pipe with a radial compressive load.

13. A method for connecting steel pipes as described in claim 10 or claim 12, characterized in that after heating the second steel pipe, it is cooled in a state where the cooling rate on the outer surface of the second steel pipe is greater than that on the inner surface, thereby introducing external compressive and internal tensile residual stresses into the second steel pipe in advance.

14. 13. The method for connecting steel pipes according to claim 10 or 12, wherein the heating temperature of the second steel pipe is set to 300°C or higher and the A3 transformation point or lower.

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