Buckling Restrained Brace

Abstract

To provide a buckling restraining brace that is suitable for use by being incorporated in a frame of a wooden building or the like and is excellent in productivity.SOLUTION: A buckling restraining brace 100 includes a wooden restraining material 20 having a shaft shape, and a core material 10 inserted to an axial through hole 23 provided in the wooden restraining material 20, having a shaft shape, and formed of steel. The core material 10 includes a round steel 11 having a shaft shape, and a connector 15 provided in the end of the round steel 11.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a buckling-restrained brace. [Background technology]

[0002] Buckling-restrained braces, which have been designed to prevent buckling, have traditionally been used as braces to form building frames (column-beam frames, roof frames, etc.). Buckling-restrained braces come in a variety of stiffening configurations, including a steel core reinforced only with steel plates, a steel core reinforced with reinforced concrete (RC), and a steel core covered with steel and mortar.

[0003] Recently, efforts have been made to improve the fire resistance and earthquake resistance of wooden buildings (such as wooden houses, wooden warehouses, and wooden stadiums). Wooden houses inherently have advantages such as a high degree of freedom in floor plan and design, the soothing effect of natural wood, the humidity-regulating properties of wood, and generally lower construction costs compared to steel-framed or reinforced concrete structures, depending on the building's intended use (e.g., residential). However, the improved fire resistance and earthquake resistance are one factor driving increased interest in wooden buildings, including wooden houses. When incorporating the conventional buckling restrained braces described above into the frame of such a wooden house, wooden columns and beams are mixed with buckling restrained braces with metal or concrete stiffeners, resulting in an unbalanced appearance.

[0004] One possible solution is to cover the entire buckling restrained brace with a wooden or paper panel, making the metal or concrete stiffener invisible from the outside. However, this requires a great deal of work, which raises concerns about increased construction costs. Furthermore, conventional buckling restrained braces tend to be heavy because they use a lot of metal, concrete, mortar, etc., and installing heavy buckling restrained braces inside the lightweight wooden beams and columns that make up a wooden house is structurally unbalanced.

[0005] Patent Document 1 proposes a buckling restrained brace suitable for use within the framework of wooden buildings such as wooden houses. Specifically, this is a buckling restrained brace that has a core material and a pair of restraining members arranged along both sides of the core material, where the core material is made of steel and the pair of restraining members are made of wood, and the restraining members are made of laminated lumber, with the lamina stacked parallel to the core material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4901491 Summary of the Invention [Problem to be solved by the invention]

[0007] The buckling restraint brace described in Patent Document 1 requires processing laminated timber to create two wooden restraint members with L-shaped cross sections, then turning these upside down and connecting them with a core material sandwiched between them, making the buckling restraint brace not easy to manufacture.

[0008] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a buckling restraint brace that is suitable for use in being incorporated into the framework of a wooden building or the like and that is easy to manufacture. [Means for solving the problem]

[0009] In order to achieve the above object, one aspect of the buckling restrained brace according to the present invention is as follows: Axial wooden restraints; The wooden restraint material has a shaft-shaped steel core member inserted into an axial through-hole provided in the wooden restraint material.

[0010] According to this aspect, the buckling restrained brace is fabricated by inserting an axial steel core into an axial through-hole provided in an axial wooden restraint. This allows the buckling restrained brace to be fabricated using simple components, and the number of components is minimized, resulting in good manufacturability. Furthermore, because the core is inserted into the through-hole inside the wooden restraint and is surrounded by the wooden restraint, even when the buckling restrained brace of this aspect is applied to the frame of a wooden building, it does not have an out-of-place appearance with the frame components. Here, the wooden restraint may be formed from solid wood, laminated lumber with laminated laminas, LVL (Laminated Veneer Lumber), or the like. Furthermore, the wooden restraint may be formed from a single axial member, or, for example, from two axial members joined together.

[0011] In another aspect of the buckling restrained brace according to the present invention, The wooden restraint member is made of wood and is formed by a pair of restraint plates, The pair of restraint plates are characterized in that half-split holes that form the through holes are provided at corresponding positions on the contact surfaces thereof.

[0012] According to this aspect, the wooden restraint member is formed from a pair of wooden restraint plates each having a half-split hole for forming the through hole, which improves the manufacturability of the through hole in the wooden restraint member. For example, it is not easy to bore a small-diameter through hole in the axial direction of a single shaft-shaped wooden restraint member with high precision, and the difficulty of processing becomes more pronounced as the length of the wooden restraint member increases.

[0013] Another aspect of the buckling restrained brace according to the present invention is: The pair of restraint plates are connected to each other at the contact surfaces via an adhesive.

[0014] According to this aspect, in a configuration in which the wooden restraint material is formed by a pair of restraint plates, the pair of restraint plates are connected via an adhesive, which further improves the manufacturability of the wooden restraint material.

[0015] In another aspect of the buckling restrained brace according to the present invention, The wooden restraining material is made of a driven or screwed material, and is characterized in that a split prevention means for preventing the wooden restraining material from splitting is embedded from the side of one of the restraining plates to the other of the restraining plates.

[0016] According to this aspect, in a configuration in which a wooden restraint member is formed by a pair of restraint plates, in addition to connecting the restraint plates with an adhesive, a crack prevention means made of a driven or screwed material is embedded (embedded by driving or screwing) from the side of one restraint plate to the other restraint plate, which effectively prevents the restraint plates from cracking and further functions as a fail-safe to prevent the restraint plates from separating at their contact surfaces (adhesion interfaces) in the event of a major earthquake, etc. For example, it is preferable to embed the crack prevention means in a staggered pattern along the length of the wooden restraint member.

[0017] Another aspect of the buckling restrained brace according to the present invention is: The core material is a round steel bar.

[0018] According to this aspect, the core material is round steel, which is as inexpensive as possible, and this, combined with good manufacturability, allows for reduction in manufacturing costs.

[0019] Another aspect of the buckling restrained brace according to the present invention is: The core material is characterized in that it is a steel pipe and a round bar that passes through the inside of the steel pipe.

[0020] According to this embodiment, the core material is a steel pipe and a round steel bar that penetrates the inside of the steel pipe, which prevents, for example, a buckled round steel bar from directly abutting against the wooden restraint material, causing the wooden restraint material to be pressed against the round steel bar and break.

[0021] In another aspect of the buckling restrained brace according to the present invention, A connector having a cross-shaped cross section made of a steel plate and connected to another member is joined to both ends of the core material, The end of the wooden restraint member is provided with a recess at a position corresponding to the connector so as not to interfere with the connector, A portion of the connector is housed in the recess.

[0022] According to this aspect, because connectors made of steel plates and having a cross-shaped cross section that are connected to other members are joined to both ends of the core material, when a buckling restrained brace is attached to a gusset plate and arranged on the structural surface of a building, the rigidity of the ends of the core material can be increased both inward and outward in the structural surface. In the gusset plate of the structural surface to which such connectors with a cross cross section are attached, fin stiffeners are attached to the gusset plate, and the multiple steel plates that form the connectors and are perpendicular to each other are joined to the gusset plate and the fin stiffener via splice plates using high-tension bolts or the like.

[0023] Furthermore, recesses are provided at the ends of the wooden restraints at positions corresponding to the connectors so that they do not interfere with the connectors, preventing damage to the ends of the wooden restraints due to pressure from the connectors. A gap is provided between the connectors and the recesses, and this gap is desirably set to a length that can accommodate the expansion and contraction of the core material when it expands and contracts in response to deformation of the structural face. The design of this gap is left to the discretion of the designer, and the amount of expansion and contraction of the core material is calculated based on the set story deformation angle, and the gap is set, for example, to be equal to or greater than the amount of expansion and contraction of the core material.

[0024] Furthermore, since the boundary region between the core material and the connector is a transition region where the planar and cross-sectional areas of the member change, a plastic region that is easily plasticized is formed in the core material side of this transition region, and this transition region can absorb the additional bending moment acting on the core material. The additional bending moment (or simply, additional bending) refers to the bending moment that can act on the wooden restraint member due to large deformation of the frame and buckling restrained brace, for example, during a major earthquake. In this way, in this embodiment, the additional bending moment acting on the core material can be effectively absorbed in the core material side region of the boundary region between the core material and the connector. [Effects of the Invention]

[0025] As can be understood from the above description, the buckling restrained brace of the present invention is suitable for use in a frame such as a wooden building, and can provide a buckling restrained brace that is easy to manufacture. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a perspective view of an example of a core material that forms a buckling restrained brace according to an embodiment. [Figure 2] FIG. 10 is a perspective view of another example of a core material that forms a buckling restrained brace according to an embodiment. [Figure 3] FIG. 1 is an exploded perspective view of an example of a wooden restraint member that forms a buckling restrained brace according to embodiments. [Figure 4] FIG. 1 is a perspective view of an example of a buckling restrained brace according to embodiments. [Figure 5] 5 is a cross-sectional view of an example of an end portion of a buckling restrained brace, taken along the arrows VV in FIG. 4. FIG. [Figure 6] 6 is a cross-sectional view of an example of the central portion of a buckling restrained brace, taken along the line VI-VI in FIG. 4. FIG. [Figure 7] FIG. 10 is a perspective view of another example of a buckling restrained brace according to embodiments. [Figure 8] FIG. 10 is a perspective view of yet another example of a buckling restrained brace according to embodiments. [Figure 9]FIG. 1 is a diagram showing a state in which a buckling restraint brace according to an embodiment is incorporated into the frame of a wooden building or the like. [Figure 10] 1 is a diagram illustrating the deformation of the frame during a major earthquake and the additional bending moment at the buckling-restrained brace joint due to the deformation of the frame. [Figure 11] FIG. 10 is a diagram showing the overall buckling line of the buckling restrained brace. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, a buckling restrained brace according to an embodiment will be described with reference to the accompanying drawings. Note that in this specification and drawings, substantially identical components will be designated by the same reference numerals, and redundant description may be omitted.

[0028] [Buckling restrained brace according to the embodiment] First, an example of a buckling-restrained brace according to an embodiment will be described with reference to Figures 1 to 6. Figures 1 and 2 are both perspective views of an example of a core material forming a buckling-restrained brace according to an embodiment, and Figure 3 is an exploded perspective view of an example of a wooden restraining material forming a buckling-restrained brace according to an embodiment. Furthermore, Figure 4 is a perspective view of an example of a buckling-restrained brace according to an embodiment, Figure 5 is a cross-sectional view of an example of an end portion of a buckling-restrained brace taken along the arrows VV in Figure 4, and Figure 6 is a cross-sectional view of an example of a central portion of a buckling-restrained brace taken along the arrows VI-VI in Figure 4.

[0029] As shown in FIG. 1, the core material 10 has a shaft-shaped round steel bar 11 and a connector 15 formed by welding both ends of the round steel bar 11 together.

[0030] Two stoppers 13 protrude from the center of the round bar 11 in the longitudinal direction to position the round bar 11 inside the wooden restraining material described below.

[0031] The connector 15 has a cross-shaped cross section, with two steel plates 17 perpendicular to the steel plate 16 welded to both wide sides of the steel plate 16.

[0032] The core material 10 has a round steel bar 11 at the center of its longitudinal direction, and a connecting body 15 at the end of the longitudinal direction that has a larger cross-sectional area and dimensions than the round steel bar 11 and has greater rigidity, so that the round steel bar 11 at the center can be made into a region that is easily plasticized (plasticization region A), and further, the plasticization region A can be limited to the round steel bar 11 at the center.

[0033] Furthermore, as will be explained below, the steel plates 16, 17 that form the connector 15 each have bolt holes 16a, 17a for bolting via a splice plate to a gusset plate provided on the structural face or a fin stiffener (see FIG. 9) attached to the gusset plate. When the buckling restrained brace 100 is attached to the gusset plate so that the steel plate 16 of the connector 15 is arranged parallel to the structural face of the building, the connector 15 has a separate steel plate 17 that is perpendicular to the steel plate 16 that is parallel to the structural face, thereby increasing the rigidity of the end of the core material 10 in the direction outward from the structural face.

[0034] On the other hand, as shown in FIG. 2, another example of a core material 10A differs from the core material 10 in that a round bar 11 is inserted inside a steel pipe 18.

[0035] Two engagement grooves 19 are provided in the center of the interior of the steel pipe 18, and two shear stoppers 13 provided on the round bar 11 engage with the engagement grooves 19 to position the round bar 11 relative to the steel pipe 18. Although not shown, the round bar 11 and the steel pipe 18 may each have a hole communicating with each other at their respective central positions, and a locking pin may be inserted into the communicating hole from the outside of the steel pipe 18 to position the round bar 11 relative to the steel pipe 18. The engagement grooves 19 may also be loose grooves (loose holes).

[0036] The steel pipe 18 is a member that prevents the wooden restraining material from being damaged when the round steel bar 11 is plastically deformed and the deformed round steel bar 11 directly presses against the wooden restraining material.

[0037] As shown in Figure 3, the wooden restraint material 20 is formed by a pair of restraint plates 21, and at corresponding positions (central positions in the width direction) on the abutment surfaces 22 of each restraint plate, half-split holes 24 are provided to form through holes 23 (see Figure 4) in which the round steel bars 11 are accommodated.

[0038] Furthermore, half recesses 26 extending in the short direction of the constraining plate 21 are provided on the abutment surfaces 22 at both ends of the constraining plate 21. When a pair of constraining plates 21 abut against each other at the abutment surfaces 22, a recess 25 is formed by the two corresponding half recesses 26 (see FIG. 4), and the steel plate 16 forming the connecting body 15 is accommodated in the recess 25.

[0039] A pair of restraint plates 21 are connected to each other at the abutment surface 22 in a position where the round steel bar 11 is accommodated in the through hole 23 and the steel plate 16 is accommodated in the recess 25, thereby forming a wooden restraint material 20 with the core material 10 sandwiched inside.

[0040] The restraint plate 21 is made of solid wood or laminated wood formed by stacking multiple lamina parallel to the contact surface 22 and bonding them together. The lamination direction of the lamina may be perpendicular to the contact surface 22, and is not particularly limited. As will be described in detail below, the cross-sectional area, cross-sectional rigidity, Young's modulus, etc. of the wooden restraint member 20 are set so as to prevent global buckling of the buckling-restrained brace. This Young's modulus is determined by the wood material. Examples of wood materials include cypress, red pine, larch, fir, and Yezo spruce.

[0041] Although not shown in the figures, the wooden restraint material may be formed from a single axial piece of wooden material. However, since it is not easy to bore a small diameter through hole in the axial direction with high precision in such a single axial piece of wooden restraint material, it is preferable to form it from a pair of restraint plates 21 as shown in the figures, from the standpoint of ease of manufacturing the through hole and processing precision.

[0042] As shown in Figure 4, wooden restraint members 20 are formed by bonding the contact surfaces 22 of a pair of restraint plates 21 with adhesive 30, and a buckling restrained brace 100 is formed that includes a core member 10 and wooden restraint members 20. The adhesive 30 can be a urethane-based adhesive, an epoxy-based adhesive, or the like.

[0043] 5, at the end of the wooden restraint member 20, a predetermined gap G is provided between the through hole 23 and recess 25 and the round bar 11 and steel plate 16 housed therein. Here, the wooden restraint member 20 may have no gap G, or may have gaps only on the left and right sides of the steel plate 16.

[0044] In this way, by having a gap G between the through hole 23 and recess 25 and the round steel 11 and steel plate 16, when the buckling restraint brace 100 incorporated into the frame is deformed during an earthquake, the round steel 11 and steel plate 16 come into contact with the through hole 23 and recess 25, preventing damage to the restraint plate 21 due to pressure being applied to their wall surfaces.

[0045] On the other hand, as shown in FIG. 6, in the center of the wooden restraint member 20, the round bar 11 is accommodated in the through-hole 23 without any gaps.

[0046] Although not shown in the drawings, a gap may be present between the round steel bar 11 and the through-hole 23, and this gap may be blocked by the unbonded material.

[0047] The buckling restraint brace 100 is manufactured by inserting round steel bars 11 into axial through holes 23 provided on the abutment surfaces 22 of a pair of restraint plates 21, and by bonding the abutment surfaces 22 together with adhesive 30. As a result, the simple configuration of components and simple connection method make it easy to manufacture.

[0048] Furthermore, a pair of restraint plates 21 are connected to form a wooden restraint member 20 with a closed structure, and the core member 10 made of round steel bars 11 is surrounded by the wooden restraint member 20. Therefore, even when the buckling restraint brace 100 is applied to the frame of a wooden building, there is no risk of it appearing out of place with the frame components.

[0049] 7 and 8 show modified examples of the buckling restraint brace.

[0050] The buckling restraint brace 100A shown in Figure 7 differs from the buckling restraint brace 100 in that the splitting prevention means 40, consisting of a driven or screwed material, is embedded from the side of one restraint plate 21 to the other restraint plate 21.

[0051] Materials to be driven into include nails, and materials to be screwed into include wood screws and bolts.

[0052] In the illustrated example, split prevention means 40 are provided in a staggered arrangement at the left and right positions on the upper and lower surfaces of both restraint plates 21 .

[0053] In this way, in addition to connecting the restraint plates 21 together with adhesive 30, the anti-splitting means 40 is embedded from the side of one restraint plate 21 to the other restraint plate 21, which effectively prevents the restraint plates 21 from splitting, and further functions as a fail-safe that prevents the restraint plates 21 from separating at the contact surfaces 22 in the event of a major earthquake, etc.

[0054] On the other hand, the buckling restrained brace 100B shown in FIG. 8 differs from the buckling restrained brace 100 in that it includes the core material 10A shown in FIG.

[0055] The through hole 23 (the half-split hole 24 forming it) is machined to a size that can accommodate the steel pipe 18, and the steel pipe 18 is accommodated in the through hole 23, and the round bar 11 is accommodated inside the steel pipe 18.

[0056] In this way, since the wall of the through hole 23 and the round steel bars 11 are not in contact with each other, it is possible to prevent, for example, a buckled round steel bar 11 from directly contacting the wall of the through hole 23, causing the wooden restraint material 20 to be pressed against the round steel bars 11 and damaged.

[0057] [Framework incorporating buckling restraint braces] Next, an example of a building frame incorporating a buckling restrained brace according to an embodiment will be described with reference to Figures 9 and 10. Here, Figure 9 is a diagram showing a state in which a buckling restrained brace according to an embodiment is incorporated into the frame of a wooden building or the like. Also, Figure 10 is a diagram explaining the deformation of the frame during a major earthquake and the additional bending moment at the buckling restrained brace joint resulting from the deformation of the frame. Note that the buckling restrained brace shown in the figure may be incorporated into the frame of a steel (S) building, a reinforced concrete (RC) building, or a steel reinforced concrete (SRC) building, in addition to the frame of a wooden building.

[0058] The frame S shown in Figure 9 is formed from wooden columns C and beams B that constitute a wooden building or the like. Gusset plates GP made of flat steel are attached to the two diagonally positioned corners. Fin stiffeners FS are welded to the surface of the gusset plates GP so that they are perpendicular to the surface. The fin stiffeners FS are joined to the gusset plates GP so that their center L3 intersects with the intersection O between the column center L1 of the column C and the beam center L2 of the beam B. The buckling restrained brace 100 is also arranged linearly passing through both diagonally positioned intersections O.

[0059] The steel plate 16 that forms the connection body 15 between the gusset plate GP and the core material 10 is joined by a high-tension bolt via a splice plate SP, and the steel plate 17 that forms the connection body 15 between the fin stiffener FS is joined by a high-tension bolt via a splice plate SP.

[0060] As shown in Figure 10, when a large earthquake occurs, deformation of the structural plane can cause an additional bending moment, as shown in equation (1) below, to act on the buckling-restrained brace joint, assuming that the joint is rigid.

[0061]

number

[0062] According to the buckling restrained brace 100, a gap G is provided between the round steel bars 11 of the core member 10 and the through holes 23 of the wooden restraining members 20. This allows the gap G to absorb the deformation of the core member 10 when the structural surface to which the buckling restrained brace 100 is attached undergoes significant deformation, preventing additional bending moments from acting on the wooden restraining members 20. Furthermore, a gap G is also provided between the steel plates 16 of the connectors 15 and the recesses 25 of the wooden restraining members 20. This allows the gap G to absorb the expansion and contraction of the core member 10 when it expands and contracts in response to deformation of the structural surface, preventing the expanding and contracting core member 10 from contacting the wall surfaces of the recesses 25 and further compressing them, damaging the wooden restraining members 20. However, the buckling restrained brace may also be configured without the gap G as described above.

[0063] [Study of overall buckling] Next, we will explain the design method for preventing global buckling of buckling-restrained braces.

[0064] When designing a buckling-restrained brace, the following formula (2) must be satisfied to prevent global buckling of the buckling-restrained brace.

[0065]

number

[0066] Here, the bending moment acting on the center of the restraint plate can be expressed by the following equation (3).

[0067]

number

[0068] The condition for preventing the overall buckling of the wooden restraint member is to satisfy the following equation (4).

[0069]

number

[0070] Equation (4) is shown in Figure 9 as the global bending stress curve for the buckling-restrained brace. In Figure 9, the upper side of the global bending stress curve is the safe zone, and the lower side is the dangerous zone. The design axial force of the wooden restraint member, the Euler load, the length of the general part of the core member, and the yield bending strength of the wooden restraint member are set so that they fall within the safe zone. Note that the global bending stress curve for the buckling-restrained brace shown in Figure 9 applies to both global buckling in the weak axis direction of the core member and global buckling in the strong axis direction.

[0071] In addition to examining the relationship between the yield bending strength of the wooden restraint material and the bending moment acting on it, it is also advisable to examine the fact that the short-term allowable bending strength of the wooden restraint material will be greater than the bending moment acting on it when the core material yields (formula omitted).

[0072] <Study on the compressive failure of wooden restraints> Next, we will explain how to consider the failure of wooden restraints due to the core material sinking into them. To prevent the wooden restraints from failing, we use the following formula (5): Verify satisfaction.

[0073]

number

[0074] Here, in addition to examining the relationship between the compressive strength of the restraining plate and the stiffening force acting on it, it is also advisable to examine the fact that the short-term allowable compressive strength of the restraining plate will be greater than the stiffening force acting when the core material yields (formula omitted).

[0075] It should be noted that the present invention is not limited to the configurations shown here, and other embodiments may be possible in which other components are combined with the configurations described in the above embodiments. In this regard, the present invention can be modified within the scope of the present invention, and can be appropriately determined depending on the application form. [Explanation of symbols]

[0076] 10, 10A: Core material 11: Round steel 13:Slip prevention 15: Connector 16,17: Steel plate 16a, 17a: Bolt holes 18: Steel pipe 19: Engagement groove 20: Wooden restraint 21: Restraint plate 22: Contact surface 23:Through hole 24:Half hole 25: Recess 26: Half-split recess 30: Adhesive 40: Split prevention measures 100, 100A, 100B: Buckling restraint brace A: Plasticization region S: Frame (composition) C: Pillar B: Beam GP: Gusset plate FS: Fin stiffener SP: Splice plate

Claims

1. Axial wooden restraints; a shaft-shaped steel core member inserted into an axial through-hole provided in the wooden restraint member; A connector having a cross-shaped cross section made of a steel plate and connected to another member is joined to both ends of the core material, The end of the wooden restraint member is provided with a recess at a position corresponding to the connector so as not to interfere with the connector, A portion of the connector is housed in the recess, The core material is a steel pipe and a round bar that penetrates the inside of the steel pipe, This buckling restraint brace is characterized in that two loose grooves, which are engagement grooves, are provided at corresponding positions on the steel pipe, holes are provided in the round steel bar at positions corresponding to the engagement grooves, and the round steel bar is positioned relative to the steel pipe by inserting a pin into the communicating hole formed by the two engagement grooves and the holes.

2. Axial wooden restraints; a shaft-shaped steel core member inserted into an axial through-hole provided in the wooden restraint member; The wooden restraint member is made of wood and is formed by a pair of restraint plates, half-split holes that form the through holes are provided at corresponding positions on the contact surfaces of the pair of restraint plates, the pair of restraint plates are connected at the contact surfaces via an adhesive; a plurality of split prevention means for preventing the wooden restraining material from splitting, which are made of a material to be driven or screwed into, are disposed in a staggered pattern in the longitudinal direction of the restraining plates at left and right positions on the surfaces of both the restraining plates, and are embedded from one of the restraining plates to the other; The core material is a steel pipe and a round bar that penetrates the inside of the steel pipe, This buckling restraint brace is characterized in that two loose grooves, which are engagement grooves, are provided at corresponding positions on the steel pipe, holes are provided in the round steel bar at positions corresponding to the engagement grooves, and the round steel bar is positioned relative to the steel pipe by inserting a pin into the communicating hole formed by the two engagement grooves and the holes.

Images