JPH02266019A - Concrete pile with joint and its producing method and mold for its production - Google Patents

Abstract

PURPOSE:To increase the sectional area of the head and connecting part of a concrete pile and increase the bearing force of the front end and the bending strength against horizontal force, by providing a plurality of joints on the shell and using a concrete pile with the joints provided with the joints at least at one end of the pile. CONSTITUTION:A mold K, which are divided into two upper and lower semicircular parts to be opened and closed, is provided with joint-forming parts 32 having a larger diameter than a shell-forming part 31, with a fixed interval in the axial direction of the mold and further provided with a joint-forming part 32 at one end or both ends thereof. End metal tools 4 composed of an end 4a and a side plate 4b are fitted at the ends of the molding frame K. and a steel frame cage composed of a main frame 20 and a hoop joint 24 is set inside. Then ready made concrete is fed into the mold K to produce a concrete pile F with PC joints, which is composed of the shell 1 and a plurality of joints 2 with a larger dia. and also provided with the joints 3 at one or both pile ends and further the end metal tools 4 at the joints 3 of both ends.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a knotted concrete pile used as a foundation pile for supporting above-ground structures, a method for manufacturing the same, and a formwork for manufacturing the same.

[Prior Art] A conventional cylindrical knotted concrete pile (P') (hereinafter simply referred to as a knotted pile) has a cylindrical body (1゛) with a section on the outer periphery, as shown in FIGS. 19 and 20. Usually, a plurality of joints (2°) are provided at predetermined intervals in the axial direction, and an end fitting (4°) (4') is attached to the end of the pile. are used as friction piles that support structural loads using the pile's circumferential friction force.

[Problems to be Solved by the Invention] However, the conventional knotted pile (Fo) has a shape that does not have knotted parts at both ends, so when it is used as a foundation pile, it has the following disadvantages. There is.

■ When driving knotted piles, the area around the knotted pile (F") is filled with gravel, crushed stone, etc. and is used as a support (as shown in Figure 19), and the ground at the end of the pile is hard supporting ground. etc., the tip bearing capacity of the pile is also expected, but because the cross-sectional area of the tip of the pile is smaller than the joint, a large tip bearing capacity cannot be obtained.

■ In addition, when connecting multiple joint piles (Fo) to form a foundation pile as shown in Figure 20, they are connected by welding with the end fittings (4°) (4°) at the ends of each joint pile. However, it is smaller than the outer diameter of the connecting part or the joint diameter, and has weak bending strength.

■ In particular, when horizontal force acts on the two parts of a joint pile (Fo) driven into the ground,
2゛) acts as a support point to the ground.

Therefore, when connecting and using multiple knot piles (Fo),
It is desirable to connect the joints at the joints, which also have a large cross-sectional performance, so that a foundation pile that is safer and stronger against horizontal forces can be constructed. An end fitting (4゛) is attached to the end of a pile of the same diameter, and it is not possible to connect it at a joint.

Therefore, the first object of the present invention is to provide a knotted concrete pile that can obtain sufficient tip support capacity, has excellent bending strength at the connecting portion between knotted piles, and is safer and stronger against horizontal forces. This is what I am trying to do.

Furthermore, another object of the present invention is to provide a new manufacturing method for a concrete pile with the above-mentioned effects, particularly a manufacturing method and manufacturing mold suitable for manufacturing a prestressed knotted concrete pile in which the pile body is given bristles. It's about providing a framework.

[Means for Solving the Problems] The knotted concrete pile of the present invention which solves the above problems has a plurality of knots at predetermined intervals in the axial direction of the pile, and a knot at at least one end of the pile. The pile is characterized in that it has a section and an end fitting is attached to the node of the end of the pile.

In addition, in the manufacturing method of the present invention for manufacturing concrete piles, at least one of the cylindrical formwork is provided with a plurality of joint forming portions at predetermined intervals in the axial direction of the formwork. Insert or set the end fittings into the two joint molded parts, insert or lock the main bars that make up the reinforcing bar basket into the end plate locking holes of the end fittings, and then fill the formwork with ready-mixed concrete. This is to manufacture piles.

In the above manufacturing method, the joint molded portion into which the end fitting is fitted and set is usually the joint molded portion provided at at least one end of the mold. Furthermore, one or two end fittings may be fitted and set in one of the axially intermediate joint forming portions of the formwork with a slight spacing therebetween.

Furthermore, the formwork for manufacturing the above-mentioned knotted concrete piles is provided with a plurality of knotted parts at predetermined intervals in the axial direction of the cylindrical form, and at least one end of the formwork. It is characterized by having a joint molded part.

In this case, the diameter of the joint molded portion at any position into which the end fitting is fitted can be made larger than the diameter of the other joint molded portions.

[Function] According to the knotted concrete pile of the present invention having the above configuration, at least one end of the pile has a knot, so
The cross-sectional area of the tip is larger than that of conventional knotted piles, and even when used as a friction pile, greater tip support can be expected. Also, when these knotted concrete piles are connected and used, by welding and connecting the end fittings that are attached to the joints of the pile ends, the joint cross-sectional area becomes larger, and the horizontal force is increased accordingly. The bending strength is increased.

According to the above-mentioned manufacturing method of the present invention, the end fitting is fitted and set into at least one joint molded part of the formwork, so when closing the cylindrical formwork or applying tension to the main reinforcement of the reinforcing bar cage. When the end fitting is attached, the end fitting will not be tilted or excessively misaligned, and will be reliably held in a predetermined position.

For example, when applying tension to the main reinforcement of a reinforcing bar cage, even if the end fitting moves in the direction of the pile axis as the main reinforcement extends, the end fitting is fitted into the joint forming part, so The fitting comes into contact with the side surface of the joint molded part, and further movement is restricted and held in a predetermined position. Furthermore, even if the main reinforcement touches the insertion hole of the end fitting during tensioning, there is no risk of the end fitting being displaced.

[Example] Next, embodiments of the present invention will be described with reference to the drawings.

Fig. 1 shows a knotted concrete pile (P) according to the present invention, in which a plurality of knots (2) having a larger diameter than the body (1) are provided at predetermined intervals in the pile axis direction. At the same time, a joint (3) is provided at one or both ends of the pile as shown in the figure, and an end plate (4a) and a side plate (4b) are provided at this end joint (3).
) is attached to the end fitting (4).

Figure 2 shows the knotted concrete pile (1'
) and an example of a formwork (K) as a manufacturing method and manufacturing apparatus are shown. Based on this, the so-called PC knotted concrete pile (F) (hereinafter referred to as
In the description of the embodiment, a case of manufacturing a joint pile (simply referred to as a joint pile) will be described below.

First, the manufacturing formwork (K) used for this consists of a closable cylindrical formwork (K) that is divided into upper and lower semicircular halves, and corresponds to the joint pile (P) to be manufactured. , a plurality of joint molded parts (32) having a larger diameter than the body molded part (31) are provided at predetermined intervals in the axial direction of the mold, and joint molded parts (33) are also provided at both ends. ) is provided. Both ends of this formwork (K) are closed with fixed plates (10) (1,0), and centrifugal tires (36) for transmitting rotation are installed on the outer periphery of the formwork (K) at required intervals in the axial direction. It is being

In this example, the end fitting (4) and the reinforcing bar cage (B) that are attached to the end of the pile are set in the cylindrical form (K) with the following structure, and the tension is Given.

That is, the main bars (20) of the reinforcing bar cage (B) are cut to a certain length, each end is helded, and is knitted into a basket shape with hoop bars (24) and the like. (2
1) indicates the settling part.

The end fitting (4) is formed by an end plate (4a) and a side plate (4b) and has a cap shape.As shown in Fig. 4, the end plate (4a) has two large and small holes. The same number of connected gourd-shaped fastening holes (4c) are provided at the same positions as the main reinforcing bars (20). The small hole part of this fastening hole (4c) is a tapered hole (4d), and a tapered seat is provided so that the heading part (21) of the main reinforcing bar (20) is engaged with the large hole part. The part is the above-mentioned settling part (
21) is a screw hole (4e) for a bolt with a diameter that can be inserted therethrough, and is adapted to engage a tension bolt (12) to be described later.

That is, the heading part (21) of the main reinforcement (20) is inserted through the large hole of the end plate (4a), that is, the bolt screw hole (4e), and the end fitting (4) is slightly rotated. When the metal fitting (4) or the reinforcing bar cage (B) is pulled, it is locked in the hetting part (21) or the seat of the tapered hole (4d).

In addition, as shown in FIG. 6, a gourd is engaged with a tension bolt (12) on a fixed plate 10 provided at one end of the formwork (K) (the left end in FIG. 2). shaped engagement hole (1,
0a) are provided in the same number and at the same position as the main reinforcing bars (20). As a result, the tension bolt (12) is screwed into the bolt screw hole (4e) leaving a certain length from the bolt head, and the head of the tension bolt (12) is attached to the fixing plate (10). ), insert it through the large hole of the gourd-shaped engagement hole (I Oa), and rotate and pull the reinforcing bar (B) instead.
The heads of the tension bolts (1, 2) engage with the gourd-shaped engagement holes (IOa), that is, the fixing plate (10).

Further, the other end of the main bar (20) of the reinforcing bar (B) is engaged with the following structure.

(11) is a tension plate with approximately the same diameter as the inner diameter of the formwork (K);
As shown in the figure, a gourd-shaped engagement hole similar to that of the fixing plate 10 (
11, a) are provided in the same number and at the same position as the main reinforcing bars (20), and a fastening screw hole (Ilb>) for fastening the tension rod (13) is provided in a part of the center.

End fitting (4), main reinforcement (20) and tension bolt (12)
The engagement is carried out in the same manner as described above. Also, the end plate (
4) and the tension bolt (12) screwed to the tension plate (11).
) is also engaged in the same way as the engagement between the fixing plate <10)1 ] and the tension bolt (2).

Furthermore, the tightening screw holes (1, 1, l) of the tension plate (11))
One end of the tension rod (13) is screwed into the rod hole (16) of the other fixing plate (10) of the formwork (K). It protrudes to the outside of the formwork (K) through.

(14) is a locking nut that is screwed onto the tension rod (3), and after tensioning the reinforcing bar (B) with an appropriate tensioning device via the rod, tighten the locking nut (14) and tighten the reinforcing bar. (B), the tension rod (13) is locked to the formwork (K) in a tensioned state.

Setting is performed as described above, but in particular, when producing a knotted pile (F) having both ends of the pile as knotted parts (2) as shown in FIG. 1, it is carried out as follows.

As explained above, first, both ends of the reinforcement capacity (B),
Alternatively, an end fitting (4) whose outer diameter is approximately equal to the inner diameter of the joint molded portion (33) at the end of the formwork is attached to one end of the main reinforcement (2).
0) so that it locks into the flange portion (21) or the tapered hole (4d) of the end plate (4a). Also, the end plate (4a
) is the tension bolt (12) in the bolt screw hole (4e).
Screw in and set, leaving a certain length from the head.

The reinforcing bar capacity (B) in the above state is connected to the lower formwork (
34) Install in the lower formwork (34) so as to be located at the end knot molding part (33).

In this case, the tension bolt (1) attached to the end fitting (4)
2), each head of which has a fixing plate (10) and a tension plate (
It is installed so as to be located in the large hole part of the gourd-shaped engagement hole (10a) (Ila) of 11).

Next, fix the reinforcing bar capacity (13) on the left end of the fixing plate (
10), and the head of the tension bolt (12),
Insert into the large hole part of the gourd-shaped engagement hole (1,0a),
The head of the tension bolt (12) is made to protrude outside from the fixing plate (10).

In this state, move the tension plate (11) on the right side of the reinforcing bar (B) to the left, and pass the head of the tension bolt (12) through the gourd-shaped engagement hole (Ila) to the right side of the tension plate (II) in the drawing. make it stand out.

Next, rotate the reinforcing bar (B) slightly so that the flange part (2I) of the main bar (20) is located in the small hole part of the gourd-shaped engagement hole (], Oa, ) (1, 1a). When the reinforcement rod (B) and the tension rod (13) are moved to the right, the left and right flanges (21) of the main reinforcement (20)
It will be locked in the small hole portion of the gourd-shaped engagement hole (1, 0a) (], ia) of the fixing plate (10) and tension plate (11).

As described above, when the end fittings (4), reinforcing bars (B) and lower formwork (34) are set, the lower formwork (34)
) is filled with a predetermined amount of ready-mixed concrete 1, and as shown in Figure 13, after closing it by covering it with the second formwork (35),
5) Fasten the lower formwork (34) at the flange. At this time, the end fitting (4) is connected to the joint molded part (33) of the formwork (K).
Since the end fitting (4) is fitted and set, the end fitting (4) does not tilt or shift, and is held in a predetermined position.

In this way, the tension rod (13) is pulled from the outside of the formwork (K) in the axial direction, that is, in the right direction in the figure.
) to be nervous. In this case, the flange part (21) is attached to the left end of the main reinforcement (20) of the reinforcing bar (B) inside the formwork.
), when pulled, the main bar (20) stretches and tension is applied to the reinforcing bar (B), and in this state, the locking nut (14) screwed into the tension rod (I3) , and lock it to the right side fixing plate (10) at the end of the formwork (K).

When tensioned as described above, the end fitting (4) on the tension plate <11) side is locked to the hetting part (21) of the main reinforcement (20), so as the main reinforcement (20) extends, it moves to the right. Move to. Therefore, as shown in FIG. 2, the joint molded portion (33) that connects the end fitting (4) is formed to a length that takes into account the amount of movement thereof.

Also, at this time, even if the end fitting <4) moves in the pile axis direction as the main reinforcement (20) extends, the joint formed part (
The end fitting (4) fitted into the joint part (33)
33), further movement is restricted and held in a predetermined position.

The formwork (K) in the above state is placed on the rotating roller (41) of the centrifugal molding device shown in Fig. 13, and the formwork (K) is centrifugally formed by being rotated at high speed in the same manner as before. , followed by steam curing.

The mold is removed after being hardened by steam curing by loosening the tension on the tension rod (13).
), the locking nuts 1-(1, 4) were loosened, the force of the reinforcing bar cage (B) trying to shrink applied prestress to the pile body, and it engaged with the end fitting (4) of the pile. Tension bolt (
12) and demold. As a result, joints (3) (3) are provided at the ends, and end fittings (4) (4) are attached to these joints (3) (3) as shown in Figure 1 or Figure 16. A pile (17) can be manufactured.

Figure 3 shows an example of manufacturing a joint pile (P) in which only one end of the pile is a joint (3). A corresponding end fitting (40) is attached to the section using the same means as described above, and the formwork (
K), and is engaged and connected to the fixing plate (10) with tension bolts (12), set, and tensioned in the same manner as above, and centrifugal forming is performed. As a result, the joint pile (1?) shown in Fig. 14 has a joint (3) only at one end and has an end fitting (4) attached to the joint (3).
can be manufactured.

FIG. 5 also shows that in the above embodiment, the end fitting (4)
Knot molded part (33) at the end of the formwork (K) into which it is fitted
The figure shows an example in which the diameter of the joint is larger than the diameter of the intermediate joint forming part (32), and in this case also, the main reinforcement (20) of the reinforcing bar basket (13) and the end fitting ( 4) and the fixing plate (10) are engaged in the same manner as described above. As a result, as shown in Fig. 17, it is possible to manufacture a knotted pile (l?) in which the diameter of the knot (3) at the end of the pile is larger than that of the middle knot (2), and according to this knotted pile (F), the pile The tip supporting force is further improved.

FIG. 8 shows an embodiment in which the ends of the main bars (20) of the reinforcing bar basket (B) are threaded.

In the case of this embodiment, as shown enlarged in FIGS. 10 and 11, the main reinforcement (20) of the reinforcing bar cage (B) is cut to a certain length, each end is threaded, and a hoop is formed. It is organized into a cage shape with threads (24), etc., and a seated nut (23) is screwed into the threaded part (22) so that the threaded part of the end protrudes. The end fitting (4) is formed of an end plate (4a) and a side plate (4b) as in the above embodiment, and the end plate (4a)
The same number of insertion holes (4r) are provided at positions corresponding to the main reinforcing bars (20).

The protruding threaded portion (22) is connected to the insertion hole (4).
The end fitting (4) is fitted so that it passes through
is screwed into place.

Regarding the fixed plate (10) and tension plate (11,),
The fixing plate (1
0) and the tension plate (11) are engaged with the internally threaded tension bolt (14) in the same manner as in the case of the hedging process.

Below, fresh concrete is filled into the formwork (K), and the main reinforcement (
The process of applying tension to 20) is the same as described in 1. However, in the case of screw machining, the end fitting (4) and the end of the main reinforcement (20) are not locked, so the process of applying tension to If the main bar (
20) only extends. Therefore, the length of the internal threads (-=I tension bolts (1, 7) on the side of the tension plate (11) must be set to a length that takes into account the above amount of elongation, as shown in the figure. , main] 8 Even if the reinforcement (20) or internally threaded tension bolt (17) comes into contact with the end fitting (4), the end fitting (4)
4) is fitted into the inside of the joint molded portion (33), so that it is held in a predetermined position without tilting or shifting.

Therefore, by performing centrifugal forming in the same manner as described above, a knotted pile (as shown in FIG. P
) can be obtained. In this case, a seated nut (2
3) is prevented by fixing it on the concrete.

Furthermore, as shown in FIG. 8, two end fittings (4
) (4) with a slight gap, and concrete 1.
・After curing and hardening, cut the antibody between the end fittings (4) (4), with one formwork (K),
It is possible to easily and economically manufacture two or more knotted piles (I') in which the end of the pile is a knotted part (3) and an end fitting (4) is attached. 9 Although not shown, one end fitting (4
) are fitted and set in the same manner as above, it is possible to manufacture a joint pile (1?) shorter than the formwork (K) without using the entire length of the formwork.

Figure 9 shows a method of manufacturing a joint (3) in which the end fitting (9) has a shape corresponding to the side surface shape of the joint molded part (33). 1?) is obtained. In this case as well, the engagement means can be implemented in the same manner as above, but in particular, the engagement means can be implemented in a good condition without any displacement of the end fitting (4) fitted and set into the joint molded part (33). can.

In each of the above embodiments, the knot molding portion (3
3), rather than making the inner diameter of the end of the formwork the same as the diameter of the node as shown in Fig. 2 or 11, it is better to make it the same diameter as the body (1゛) as shown in Fig. 8 or 9. The stability of the end fitting (4) will be better if it is left in place.

Also, in each example of ``nee'', the tension pol 1 - (1, 2)
Alternatively, attach tension bolts (17) with internal threads to the formwork (K
), but as shown in Fig. 12, the hetting part (2
1) is attached to the joint molding part (33).
If set to , the end fitting (4) comes into contact with the joint (33) and does not move in the axial direction, so it can be used as a reaction force of tension.

The knotted pile (P) manufactured as described above can be used as a friction pile by filling the circumference of the pile with gravel, crushed stone, etc., as shown in Fig. 14, or as a friction pile as shown in Fig. 15. ) are used as foundation piles by connecting them.In this case, because there are knots at the ends, a large bearing capacity can be expected at the pile tip as a 1V pile. By welding the parts together, the cross-sectional area of the connected parts is large, the bending strength is improved, and a safe and strong foundation pile can be obtained.

[Effects of the Invention] As described above, according to the knotted concrete pile of the present invention, due to the presence of knots at least at one end, even when used as a friction pile,
The pile tip bearing capacity increases. Furthermore, by welding and connecting these knotted concrete piles with the end fittings of the joints at the ends of the piles, the joint cross-sectional area becomes larger, and the bending strength against horizontal force increases accordingly. Therefore, it is possible to construct foundation piles that are safer and stronger than conventional ones.

According to the above-mentioned manufacturing method of the present invention, the end fittings are fitted and set into the joint molded portions of the formwork, so when closing the formwork or applying tension to the reinforcing bar cage, the end fittings It is reliably held in a predetermined position without being tilted or misaligned.

Therefore, for example, by manufacturing by using a formwork having a knot-forming part at least one end and fitting and setting an end fitting into the knot-forming part of this end, at least one pile end can be formed. To easily and reliably manufacture a knotted concrete pile having the above-mentioned effect in which an end fitting is fitted to a knot without slipping, and its judicial precision is also improved.

It is also possible to manufacture the pile by fitting and setting two end fittings back to back with a slight gap between them in the joint molded part located at the intermediate position. In this case, after manufacturing the pile, the two end fittings By cutting the antibody, two or more stakes with jointed ends can be manufactured simply and economically using one mold. It is also possible to manufacture the set by fitting one end metal fitting into the molded part of the joint located inward from the end of the formwork. Can you manufacture piles?

In addition, by using a formwork in which the diameter of the joint molded part into which the end fitting is fitted and set is larger than the other joint molded parts, the joint is made larger in diameter than the joint of the pile end or the middle joint. It is possible to manufacture concrete piles, and by using these piles, the tip bearing capacity of the foundation piles and the connection strength between the piles are further improved.

[Brief explanation of drawings]

Figure 1 is a partial vertical cross-sectional side view of a concrete pile with joints in which end fittings are attached to the joints at both ends of the pile, and Figure 2 is a plan view showing an example of the manufacturing method of a concrete pile with joints. Figure 3 is a partially cutaway plan view showing a method for manufacturing a knotted concrete pile with only one end as a knot, and Figure 4 (a> (b) shows the end of the joint produced by the above method. A front view and a cross-sectional view showing the fittings; Fig. 5 is a partial plan view showing a method for manufacturing a concrete pile with joints I and J in which the diameter of the end joint is larger than that of the middle joint; Figure 7 is a front view of the fixed plate, Figure 7 is a front view of the tension plate, Figure 8 is a plan view showing an example of the method for manufacturing knotted concrete piles in which the ends of the main reinforcement are threaded, and Figure 9 is a front view of the tension plate. 10(a, Hb) is a partial plan view showing another embodiment, and FIG. FIG. 12 is a partial plan view showing another embodiment; FIG. 13 is a front view showing the entire centrifugal molding device;
Figure 14 and Figures 1-5 are cross-sectional views showing foundation pile structures using knotted piles with knots at one end, and Figures 16 to 18.
The figures are front views showing knot piles with different shapes of end knots, and FIGS. 19 and 20 are cross-sectional views showing foundation pile structures using conventional knot piles, respectively. [Explanation of symbols] (1)...Body part (2)...Knot part (3)...Knot part (4)...End fitting (4a)... ... End plate (4b) ... Side plate (4C) ... Gourd-shaped fastening hole (4d) ... Taper hole (4e) ... Screw hole (4F) ...Insertion hole <10)...Fixing plate (1,0a)...Gourd-shaped engagement hole (11)...Tension plate (Ila)...Gourd-shaped engagement hole (Ilb)
... Cotton spinning screw hole (I2) ... Tension bolt (13) ... Tension rod (I4) ... Locking nut (15) ... Centrifugal Tire (16)...Rod hole (17)...Inner thread (=J tension bolt (20)...
...Main bar (21,)...Heading part (22)...Threaded part (23)...Seat ζj nut (24)...Hoop bar (31) )...Body molded part (32)...Middle joint molded part (33)...End joint molded part (34)...Lower formwork (35)...''2 formwork (r()... ・Reinforced bar basket (l?)...Knotted concrete (K) ・Formwork 1・Pile (knotted pile) Figure 6 4C Figure 19 Figure 20 4' Procedural Official Book (Method) Contents of Amendment July 31, 1999 Specification Box Page 24, Line 15 [Figure 10 (a) (b)

Claims (1)

[Claims] 1. A plurality of joints are provided at predetermined intervals in the axial direction of the pile, a joint is provided at at least one end of the pile, and an end fitting is attached to the joint of the pile end. A knotted concrete pile characterized by being crowned. 2. Fitting and setting an end fitting into at least one joint molding part of an openable/closable cylindrical mold frame having a plurality of joint molding parts provided at predetermined intervals in the axial direction of the mold; A method for manufacturing a knotted concrete pile, which involves inserting or locking the main reinforcement constituting a reinforcing bar cage into the end plate locking hole of a metal fitting, and then filling the inside of the formwork with ready-mixed concrete to manufacture the pile. 3. The method for manufacturing a knotted concrete pile according to claim 2, wherein the knot molded portion into which the end fitting is fitted and set is a knot molded portion provided at at least one end of the formwork. 4. The joint forming part into which the end fittings are fitted and set is one of the joint forming parts in the axially intermediate part of the formwork, and one or two end fittings are fitted and set into the joint forming part. A method for manufacturing a knotted concrete pile according to claims 2 and 3. 5. A plurality of joint molding parts are provided at predetermined intervals in the axial direction of an openable and closable cylindrical form, and the joint molding part is provided at at least one end of the mold. Formwork for manufacturing knotted concrete piles. 6. The formwork for manufacturing a knotted concrete pile according to claim 5, wherein the diameter of the knot-formed portion at an arbitrary position is larger than that of other joint-formed portions.