JP7845665B2 - Stress transfer structure of cast-in-place steel pipe concrete piles - Google Patents

Description

This invention relates to a stress transmission structure for cast-in-place steel pipe concrete piles formed from steel pipes and concrete.

A cast-in-place steel pipe concrete pile is known, in which the upper part of the pile is made of steel pipe concrete and the lower part is made of cast-in-place concrete (see Patent Documents 1-3, etc.). In such a composite pile structure, a joint occurs at the boundary between the steel pipe concrete section and the reinforced concrete section.

Normally, piles are primarily subjected to vertical loads, and the presence of joints does not impede the vertical load. However, during an earthquake, if a large horizontal force acts on the pile head, bending moment and shear force will act on the pile, potentially making the joint a weak point.

Therefore, Patent Documents 1-3 provide a joint structure for reinforcing the joint between the steel pipe concrete section and the reinforced concrete section. Specifically, to increase the adhesion force (frictional resistance) with the concrete filling the interior of the steel pipe, protrusions are provided on the inner surface of the steel pipe.

Patent No. 5717118 Japanese Patent Publication No. 2006-138095 Special Publication No. 5-62171

However, creating a protrusion on the inner surface of a steel pipe can be difficult and time-consuming during manufacturing. On the other hand, Patent Document 3 describes a structure in which a rectangular opening is provided in the circumferential wall of the steel pipe, allowing concrete to flow around the outer circumference of the pipe.

A rectangular opening in side view allows for drilling from the outside of the steel pipe, which is more efficient than creating a protrusion on the inner surface of the pipe. However, since all four sides are cut, the number of steps required per opening is high, and creating multiple openings is quite time-consuming.

Furthermore, a challenge exists in that concrete bleeding tends to accumulate in the corners of rectangular openings, leading to the formation of voids in the hardened concrete. To prevent these voids from forming in the concrete, measures such as mixing special additives into the concrete are necessary.

Therefore, the present invention aims to provide a stress transmission structure for cast-in-place steel pipe concrete piles that is easy to manufacture, does not suffer from bleeding effects, and has excellent stress transmission performance.

To achieve the above objective, the stress transmission structure for a cast-in-place steel pipe concrete pile of the present invention is a stress transmission structure for a cast-in-place steel pipe concrete pile formed by a steel pipe and concrete, characterized in that multiple through holes, which are approximately circular in side view or polygonal in side view with their vertices facing upwards, are provided at intervals in the circumferential direction at the ends of a cylindrical steel pipe filled with concrete around its periphery and interior.

Here, the through-holes can be configured to be provided at equal intervals in the circumferential direction, from 4 to 32 locations, depending on the size of the steel pipe. Furthermore, the through-holes can be configured to be provided at intervals in the axial direction of the steel pipe, in any number of stages from 1 to 10.

Furthermore, in such cast-in-place steel pipe concrete piles, it is preferable that the end of the steel pipe be positioned near the boundary between the steel pipe concrete section and the reinforced concrete section, at the enlarged base of an enlarged-base pile, or at the enlarged diameter section of an intermediate enlarged-diameter pile.

Furthermore, the structure can be configured such that reinforcing pipes protruding from the outer circumference and into the interior of the steel pipe are inserted into the through-hole. Alternatively, the structure can be configured such that a reinforcing section for the opening is provided by welding steel material around the through-hole.

The stress transmission structure of the cast-in-place steel pipe concrete pile of the present invention, configured as described above, has multiple through-holes at intervals in the circumferential direction at the ends of a cylindrical steel pipe filled with concrete around its periphery and interior. These through-holes are approximately circular in side view or polygonal in side view, with their vertices facing upwards.

Through-holes that are roughly circular in side view or polygonal with their vertices on the upper side in side view can be easily drilled from the outside of the steel pipe. Furthermore, even when concrete is filled around these circular or polygonal through-holes, bleeding does not accumulate. Additionally, the connection between the concrete inside and outside the steel pipe through the through-hole increases the adhesion between the steel pipe and the concrete.

Furthermore, by providing 4 to 32 through-holes at equal intervals in the circumferential direction, depending on the size of the steel pipe, it becomes possible to obtain adhesive force without bias in the circumferential direction of the pile, and the reduction in the load-bearing capacity of the steel pipe 21 can be kept within an acceptable range.

Furthermore, by positioning the end of the steel pipe, which has a through-hole, near the boundary between the concrete-framed steel pipe section and the reinforced concrete section, it becomes possible to effectively transmit the tensile force of the reinforcing bars in the reinforced concrete section to the steel pipe in the concrete-framed steel pipe section, thereby improving stress transmission performance.

On the other hand, by positioning the end of the steel pipe with the through-hole in the enlarged base of the enlarged-base pile or the enlarged diameter of the intermediate enlarged-diameter pile, the pull-out resistance of the widened, inclined portion in contact with the ground can be effectively transmitted to the steel pipe in the concrete section of the steel pipe.

Furthermore, if reinforcing pipes protruding from the outer circumference and into the interior of the steel pipe are inserted into the through-hole, the reduction in the steel pipe's load-bearing capacity due to the drilling of the through-hole can be suppressed, and the bearing resistance can be increased. Here, the reduction in the steel pipe's load-bearing capacity can also be suppressed by providing an opening reinforcement section by welding steel material around the through-hole.

This is an explanatory diagram showing the overall configuration of the cast-in-place steel pipe concrete pile of this embodiment. This is an explanatory diagram showing the structure of the concrete pipe section of a cast-in-place steel pipe concrete pile. Figure 2A is an explanatory diagram illustrating the configuration of a steel pipe concrete section with a different number of through-holes. This is an explanatory diagram showing the configuration near the enlarged base of the enlarged base pile of Example 1. These diagrams illustrate the specimens used in a model experiment to confirm the effects of providing through holes. (a) is an explanatory diagram of a straight pile specimen, and (b) is an explanatory diagram of an enlarged-base pile specimen. This graph shows the relationship between displacement and load in relation to the experimental results. This graph shows the experimental results in relation to the size of the through-hole (total window area) and the maximum load. This graph shows the relationship between the size of the through-hole (total window area) and the shear stress, based on the experimental results. This is an explanatory diagram showing the configuration near the enlarged diameter portion of the intermediate enlarged diameter pile in Example 2. This is an explanatory diagram showing the overall configuration of another intermediate enlarged-diameter pile in Example 2. This is an explanatory diagram showing the configuration in which a reinforcing pipe is inserted into the through-hole of the steel pipe in Example 3. This is an explanatory diagram conceptually illustrating the function of the reinforcing pipe in Example 3. This is a conceptual diagram illustrating the function of reinforcing pipes in the case of enlarged-base piles. This is a conceptual diagram illustrating the function of a through-hole without reinforcing pipes. This is a conceptual diagram illustrating the function of a through-hole without reinforcing pipes in the case of an enlarged-base pile. This is an explanatory diagram showing the configuration in which an opening reinforcement portion is provided around the through-hole of the steel pipe in Example 4.

The embodiments of the present invention will be described below with reference to the drawings. Figure 1 is an explanatory diagram showing the overall configuration of the cast-in-place steel pipe concrete pile 1 of this embodiment. Figure 2A is an explanatory diagram showing the configuration of the steel pipe concrete section 2 of the cast-in-place steel pipe concrete pile 1.

First, to explain the overall structure, the cast-in-place steel pipe concrete pile 1, as shown in Figure 1, comprises a steel pipe concrete section 2 formed at the top and a reinforced concrete section 3 formed at the bottom. Furthermore, the cast-in-place steel pipe concrete pile 1 described in this embodiment is an enlarged-base pile, with a truncated conical enlarged base section 11 provided at the lower end of the reinforced concrete section 3.

The head of the cast-in-place steel pipe concrete pile 1 is connected to a reinforced concrete footing 6. The superimposed load acting on the foundation of a building or other structure is transmitted to the cast-in-place steel pipe concrete pile 1 via this footing 6.

The steel pipe concrete section 2 formed below the footing 6 is mainly composed of a cylindrical steel pipe 21 and concrete 4 filling its interior and exterior. Here, for example, a steel pipe material with a diameter of approximately 600 mm to 2500 mm can be used for the steel pipe 21.

Furthermore, multiple pile head anchoring bars 12 are attached to the outer surface of the upper end of the steel pipe 21 by welding or other means. These pile head anchoring bars 12, which protrude upward from the steel pipe 21, are embedded within the footing 6.

On the other hand, during an earthquake, the footing 6 moves significantly horizontally from the bottom of the pile, causing bending moments and shear forces to act on the joint between the steel pipe concrete section 2 and the reinforced concrete section 3.

Therefore, to counteract such forces, through-holes 22 are drilled near the lower end 21a of the steel pipe 21 to increase the adhesion strength with the concrete 4. These through-holes 22 are approximately circular in side view and multiple holes are provided at intervals around the circumferential direction of the steel pipe 21.

The diameter of the through-hole 22 should preferably be set to at least three times the maximum aggregate diameter of the concrete 4, taking into consideration factors such as the ease with which the concrete 4 can pass through and the magnitude of the adhesion force obtained at one location. As a guideline, a through-hole 22 with a diameter of approximately 75mm-250mm, for example, approximately 150mm in diameter, can be provided.

Furthermore, if multiple through-holes 22 are provided at intervals in the circumferential direction, they can be provided at any number of intervals from one to five at the end 21a of the steel pipe 21, at intervals in the axial direction (pile axis direction). Figure 1 illustrates an example in which three stages of through-holes 22 are provided at the lower end 21a of the steel pipe 21.

Furthermore, as shown in Figure 2A, the through-holes 22 are provided at equal intervals in the circumferential direction. This figure illustrates an example where four through-holes 22 are provided at equal intervals in the circumferential direction. The number of through-holes 22 is not limited to this. For example, as shown in Figure 2B, eight through-holes 22 can be provided at equal intervals in the circumferential direction at the lower end 21a of the steel pipe 21A.

In other words, the through-holes 22 can be provided at equal intervals in the circumferential direction, in any number from 4 to 32, depending on the size of the steel pipe. By providing the through-holes 22 at equal intervals, the adhesion force obtained in the circumferential direction of the pile can be made uniform. For example, a steel pipe 21 with a diameter of 600 mm can have 4 through-holes 22 per row, while a steel pipe 21 with a diameter of 2500 mm can have 24 through-holes 22 per row. Typically, 4 to 8 through-holes 22 are provided at equal intervals in the circumferential direction.

On the other hand, the reinforced concrete section 3 formed at the lower part of the cast-in-place steel pipe concrete pile 1 comprises, as shown in Figure 1, a reinforcing cage 31 extending in the axial direction and concrete 4 filled around it.

This reinforcing cage 31 is manufactured in a cylindrical shape and is mainly composed of multiple main reinforcing bars 31b arranged at intervals in the circumferential direction as shown in Figure 2A, and annular stirrups 31c arranged at intervals in the pile axis direction as shown in Figure 1.

Furthermore, the upper end portion 31a of the reinforcing cage 31 is housed inside the steel pipe 21. The upper end portion 31a, housed in the lower part (end portion 21a) of the steel pipe 21, is positioned in approximately the same area as the three-tiered through-holes 22, as shown in Figure 1, thus effectively overlapping them.

For example, if the diameter of the main reinforcing bar 31b is d, the axial range (length) of the end 21a of the steel pipe 21 where the through-hole 22 is provided can be approximately 40d-45d. The diameter of the through-hole 22 can also be set considering the diameter (steel pipe size) of the steel pipe 21, ensuring that it is appropriately provided without compromising the load-bearing capacity of the steel pipe 21. Furthermore, the area around the through-hole 22 can be reinforced as needed, as described later.

Furthermore, by positioning the upper end 31a of the reinforcing cage 31 in approximately the same area as the area where the through-holes 22 are provided, and overlapping them, the tensile force of the main reinforcement 31b can be effectively transmitted to the steel pipe 21.

Furthermore, as shown in Figure 1, even when connecting the upper end 21b of the steel pipe 21 to the reinforced concrete section, a through-hole 22 can be provided to improve stress transfer performance with the reinforcement. Specifically, the lower part of the pile head reinforcement bar 121, whose upper part is embedded in the footing 6, is inserted into the cavity of the upper end 21b of the steel pipe 21, overlapping with the area where the through-hole 22 is provided. This allows the tensile force of the pile head reinforcement bar 121 to be effectively transferred to the steel pipe 21.

Furthermore, Figure 1 illustrates a case where three through-holes 22 are provided in the upper end 21b of the steel pipe 21. However, if there is concern about a reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through-holes 22, the reduction in load-bearing capacity of the end 21b can be suppressed by providing an opening reinforcement section 23, such as by welding an annular steel plate around the through-holes 22. This opening reinforcement section 23 can be provided by fixing an annular steel plate to either the outer or inner circumferential surface of the steel pipe 21, or by fixing annular steel plates to both the outer and inner circumferential surfaces of the steel pipe 21.

Next, the method for constructing the cast-in-place steel pipe concrete pile 1 of this embodiment will be described.
First, before construction, through holes 22, which are roughly circular in shape when viewed from the side, are drilled near the lower end 21a of the steel pipe 21, at the number and spacing specified in the design.

The through-hole 22, which is roughly circular in side view, can be easily drilled by simply marking the center of the circle on the outer surface of the steel pipe 21, then punching a punch at that center point and automatically rotating the gas cutting machine.

Meanwhile, at the construction site, pile holes 5 are excavated in the ground G using methods such as earth drilling or reverse drilling. Then, a steel pipe 21 is lifted above the pile holes 5 by a crane and installed in the upper part of the pile holes 5. At this stage, the inside of the steel pipe 21 is either hollow or filled with stabilizing fluid for excavation.

Next, the reinforcing cage 31 is lifted above the pile hole 5, inserted into the steel pipe 21, and then lowered. Although not shown in the diagram, a suspension bar longer than the steel pipe 21 is connected to the upper end 31a of the reinforcing cage 31. By suspending the reinforcing cage 31 via this suspension bar, the reinforcing cage 31 can be lowered to the position shown in Figure 1.

Then, by measuring the amount of protrusion of the reinforcing bars from the pile hole 5, the upper end 31a of the reinforcing cage 31 can be aligned with the area where the through-hole 22 of the end 21a of the steel pipe 21 is provided. After positioning the steel pipe 21 and the reinforcing cage 31 in the predetermined location in the pile hole 5 in this way, a tremie pipe is installed in the pile hole 5, and concrete 4 is poured using the tremie pipe. The concrete 4 is poured continuously from the bottom of the hole up to the upper end of the steel pipe 21.

Then, by pouring this concrete 4, a stress transmission structure for a cast-in-place steel pipe concrete pile 1 is constructed, comprising a reinforced concrete section 3 and a steel pipe concrete section 2, with the load-bearing capacity of the adhesion force between the concrete 4 and the steel pipe 21 ensured through the through-hole 22 drilled in the end 21a of the steel pipe 21.

Next, the operation of the stress transfer structure of the cast-in-place steel pipe concrete pile 1 of this embodiment will be explained.
In this embodiment, the stress transmission structure of the cast-in-place steel pipe concrete pile 1 is configured such that a plurality of through holes 22, which are approximately circular in side view, are provided at intervals in the circumferential direction at the end 21a of the cylindrical steel pipe 21, which is filled with concrete 4 around and inside.

The approximately circular through-hole 22, when viewed from the side, can be drilled from the outside of the steel pipe 21 using a gas cutting machine or similar tool, with minimal effort and ease. Furthermore, even when concrete 4 is filled around the circular through-hole 22, bleeding cannot accumulate around it, preventing the formation of voids in the concrete 4.

Furthermore, by connecting the concrete 4 inside and outside the steel pipe 21 through the through-hole 22, the adhesion force between the steel pipe 21 and the concrete 4 can be increased. In other words, by positioning the end 21a of the steel pipe 21 near the boundary between the reinforced concrete section 3 and the steel pipe concrete section 2, the tensile force of the main reinforcement 31b of the reinforcing cage 31 can be effectively transmitted to the steel pipe 21 of the steel pipe concrete section 2, thereby improving stress transmission performance.

Furthermore, if four to 32 through-holes 22 are provided at equal intervals in the circumferential direction of the steel pipe 21, depending on the size of the steel pipe, the adhesion force can be obtained without bias in the circumferential direction of the pile, and the reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through-holes 22 can be kept within an acceptable range.

The following describes a cast-in-place steel pipe concrete pile in an embodiment different from the cast-in-place steel pipe concrete pile 1 of the above-described embodiment, with reference to Figure 3. Note that parts identical or equivalent to those described in the above embodiment will be described using the same terms or reference numerals.

This first embodiment describes an enlarged-base pile 1A as a cast-in-place steel pipe concrete pile. The enlarged-base pile 1A of this first embodiment comprises a steel pipe concrete section 2A formed at the top and a truncated conical enlarged-base section 11A formed at the bottom.

The steel pipe concrete section 2A is mainly composed of a cylindrical steel pipe 21 and concrete 4 filled inside it. Here, for example, a steel pipe material with a diameter of approximately 600 mm to 2500 mm can be used for the steel pipe 21.

Furthermore, near the lower end 21a of the steel pipe 21, through-holes 22 are drilled to transmit the pull-out resistance of the enlarged base inclined portion of the enlarged base 11A to the steel pipe 21 via the concrete 4. These through-holes 22 are approximately circular in side view and multiple holes are provided at intervals around the circumferential direction of the steel pipe 21.

In this embodiment 1, the end 21a of the steel pipe 21 embedded inside the enlarged base 11A has multiple through holes 22 spaced circumferentially, forming one stage, and six stages of through holes 22 spaced axially along the steel pipe 21.

With this configuration of the stress transmission structure for the enlarged-base pile 1A, the end portion 21a of the steel pipe 21, which has a through-hole 22, is positioned at the enlarged base portion 11A of the enlarged-base pile 1A. This allows for effective transmission of the pull-out resistance of the inclined portion of the enlarged base portion 11A to the steel pipe 21.

In other words, as the concrete 4 poured into the pile hole 5 using a tremie pipe or the like rises from below, it fills the inside and outside of the steel pipe 21 through the through-hole 22, and the concrete 4 also fills the inclined portion of the enlarged base 11A that contacts the ground G, thus creating a structure in which the pull-out resistance is reliably transmitted to the steel pipe 21.

To effectively transfer the pull-out strength of the enlarged base 11A to the steel pipe 21, the end 21a of the steel pipe 21 is embedded down to the bottom of the enlarged base 11A, and through holes 22 are provided in approximately 3 to 5 stages, and in some cases, approximately 10 stages.

The following describes the experiments conducted to confirm the effects of the through-holes 22 in the cast-in-place steel pipe concrete pile 1 described in the above embodiment and in the enlarged-base pile 1A of this embodiment 1, with reference to Figures 4-7.

Figure 4 illustrates the steel pipe concrete section 2 (straight pile) of the cast-in-place steel pipe concrete pile 1 of the above embodiment, and the test specimens used in the experiment, which are modeled after the enlarged-base pile 1A of this embodiment 1. Figure 4(a) shows four straight pile test specimens. These test specimens are model-sized specimens, scaled down to approximately 1/10th the size of actual piles.

The specimen labeled "Straight-10" at the far left of Figure 4(a) is a steel pipe 21 with a diameter of 101.6 mm embedded in a cylindrical concrete 4 with a diameter of 125.0 mm. A through-hole 22 with a diameter of 15.0 mm is drilled 23.0 mm above the lower edge of the steel pipe 21 in the axial direction. Eight through-holes 22 are provided at equal intervals in the circumferential direction (eight directions), forming a single-stage through-hole configuration.

On the other hand, test specimen "Cho-11," located to the right of "Cho-10," has a configuration in which two more through-holes 22 are provided at 65.0 mm intervals in the axial direction from the lowest through-hole 22, for a total of three through-holes 22. The rest of the configuration is the same as "Cho-10."

Furthermore, the "Straight-12" specimen has a configuration in which four more through-holes 22 are provided at 32.0 mm intervals in the axial direction from the lowest through-hole 22, for a total of five through-holes 22. The rest of the configuration is the same as "Straight-10". In contrast, the "Straight-13" specimen has a configuration in which four through-holes 22 are provided at equal intervals in the circumferential direction (four locations in four directions), and the rest of the configuration is the same as "Straight-12".

Furthermore, the specimen labeled "Expand-4" in Figure 4(b) is a test specimen that simulates an expanded-base pile 1A, in which the lower part of a 101.6 mm diameter steel pipe 21 is embedded in concrete 4 with a truncated conical expanded base section 11A having a base diameter of 265.0 mm and a height of 145.0 mm.

The lower end of the enlarged base 11A is provided with a disc-shaped base with a height of 25.0 mm. A total of five through-holes 22 are provided, starting 23.0 mm axially above the lower edge of the steel pipe 21 embedded in the enlarged base 11A, and spaced 32.0 mm apart in the axial direction. Furthermore, within each stage, eight through-holes 22 are provided at equal intervals in the circumferential direction (eight directions).

Compression tests and pure bending tests were performed using these specimens. The reason for performing the compression test was that, given the shape of the specimens, applying compressive force would be disadvantageous for confirming the adhesion strength. In the pure bending test, if the required adhesion strength was present, failure would occur at reinforced concrete section 3, thus confirming the failure mode.

Figure 5 is a graph summarizing the experimental results of the pure bending test using each specimen, showing the relationship between displacement and load. These results show that yield strength was observed in all specimens. Furthermore, among the straight piles, the specimen with the most through-holes 22, "Straight-12," was found to have the greatest yield strength. Additionally, specimen "Enlarged-4" of the enlarged-base pile 1A was found to have an even greater yield strength than "Straight-12."

On the other hand, Figures 6 and 7 summarize the results of the compression test with the total window area ( ^mm² ), which is the total opening area of the through-holes 22, on the horizontal axis. Specifically, Figure 6 is a graph showing the experimental results in relation to the total window area _ΣWA and the maximum load P _max (kN). Figure 7 is a graph showing the experimental results in relation to the total window area _ΣWA and the shear stress τ _max ( ^mm² ).

As can be seen from the graph in Figure 6, in the case of straight piles, there is a linear proportional relationship between the increase in total window area and the increase in maximum load. Furthermore, it can be seen that with enlarged-base piles ("Enlarged-4"), a larger maximum load can be obtained than that of straight piles.

Furthermore, as can be seen from the graph in Figure 7, in straight piles, the shear stress tends to decrease as the total window area increases. However, for enlarged-base piles ("Enlarged-4"), a considerably larger shear stress is obtained compared to straight piles.

Thus, it was confirmed that, whether it is a cast-in-place steel pipe concrete pile 1 or an enlarged-base pile 1A, providing a through hole 22 in the end 21a of the steel pipe 21 embedded in the concrete 4 can be expected to increase the bond strength and pull-out strength.

Furthermore, since the other configurations and effects are substantially the same as those of the above embodiment or other examples, their explanation will be omitted.

The following description will refer to a cast-in-place steel pipe concrete pile in an embodiment different from the cast-in-place steel pipe concrete pile 1 of the above-described embodiment and the enlarged-base pile 1A of Example 1, with reference to Figures 8 and 9. Note that descriptions of parts that are the same or equivalent to those described in the above-described embodiment or Example 1 will be denoted by the same terms or reference numerals.

This second embodiment describes intermediate enlarged-diameter piles 1B and 1C, which are cast-in-place steel pipe concrete piles. As shown in Figures 8 and 9, the intermediate enlarged-diameter piles 1B and 1C of this second embodiment are equipped with an enlarged-diameter section 13 in the intermediate portion, not at the end in the pile axis direction.

The intermediate enlarged-diameter pile 1B shown in Figure 8 comprises a steel pipe concrete section 2B formed at the top of the pile, a reinforced concrete section 3B formed at the bottom, and an enlarged-diameter section 13 formed in the intermediate section that forms the boundary between them.

In other words, a widened diameter section 13 is provided between the straight pile-shaped steel pipe concrete section 2B and the reinforced concrete section 3B. The upper part of this widened diameter section 13, which connects to the steel pipe concrete section 2B, is formed in a truncated cone shape, while the lower part gradually tapers towards the diameter of the reinforced concrete section 3B.

The steel pipe concrete section 2B is mainly composed of a cylindrical steel pipe 21 and concrete 4 filled inside it. Here, for example, a steel pipe material with a diameter of approximately 600 mm to 2500 mm can be used for the steel pipe 21.

Furthermore, near the lower end 21a of the steel pipe 21, through-holes 22 are drilled to transmit the pull-out strength of the enlarged diameter inclined portion of the enlarged diameter section 13 to the steel pipe 21 via the concrete 4. These through-holes 22 are approximately circular in side view and multiple holes are provided at intervals around the circumferential direction of the steel pipe 21.

In the intermediate enlarged-diameter pile 1B of this embodiment 2, the end 21a of the steel pipe 21 embedded inside the enlarged-diameter section 13 has multiple through holes 22 spaced circumferentially, forming one stage, and four stages of through holes 22 spaced axially along the steel pipe 21.

On the other hand, the intermediate enlarged-diameter pile 1C shown in Figure 9 comprises a steel pipe concrete section 2C formed at the top of the pile, a reinforced concrete section 3C formed at the bottom, an enlarged-diameter section 13 formed in the intermediate section that forms the boundary between them, and a truncated conical enlarged-base section 11 formed at the lower end of the pile.

Furthermore, near the end 21a of the steel pipe 21 in the steel pipe concrete section 2C, through-holes 22 are drilled to transmit the pull-out strength of the enlarged diameter inclined section of the enlarged diameter section 13 to the steel pipe 21 via the concrete 4. These through-holes 22 are approximately circular in side view and multiple holes are provided at intervals around the circumferential direction of the steel pipe 21.

In the intermediate enlarged-diameter pile 1C of this embodiment 2, the end 21a of the steel pipe 21 embedded inside the enlarged-diameter section 13 has multiple through holes 22 spaced circumferentially, forming one stage, and four stages of through holes 22 spaced axially along the steel pipe 21.

With this configuration of the intermediate enlarged-diameter piles 1B and 1C, the end portion 21a of the steel pipe 21, which has a through hole 22, is positioned in the enlarged-diameter section 13 of the intermediate enlarged-diameter piles 1B and 1C. This allows for effective transmission of the pull-out resistance of the inclined portion of the enlarged-diameter section 13 that is in contact with the ground G to the steel pipe 21.

Furthermore, if the upper end 31a of the reinforcing cage 31 in the reinforced concrete sections 3B and 3C overlaps with the end 21a of the steel pipe 21, which has a through-hole 22, the tensile force of the main reinforcing bars 31b can be effectively transmitted to the steel pipe 21 in the concrete sections 2B and 2C.

Furthermore, since the other configurations and effects are substantially the same as those of the above embodiment or other examples, their explanation will be omitted.

The following describes a cast-in-place steel pipe concrete pile 1 (1A-1C) of a different embodiment from the one described above in Embodiments and Examples 1 and 2, with reference to Figures 10-13B. Parts identical or equivalent to those described in the above Embodiments or Examples 1 and 2 will be described using the same terms or reference numerals.

In the cast-in-place steel pipe concrete pile 1 of the above embodiment, when there is concern about a reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through hole 22, an opening reinforcement portion 23 is provided by welding an annular steel plate around the through hole 22, as described above.

This third embodiment describes the configuration of the opening reinforcement for the through-hole 22, which can suppress the reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through-hole 22, and is also expected to increase the bearing resistance of the cast-in-place steel pipe concrete pile 1.

In other words, in the cast-in-place steel pipe concrete pile 1 of Example 3, reinforcing pipes 24 protruding from the outer circumference and into the interior of the steel pipe 21 are inserted into through holes 22 drilled in the steel pipe 21 and fixed to the steel pipe 21 by welding or other means.

In detail, as shown in Figure 11, the reinforcing pipe 24 is a pipe material such as a steel pipe having an outer diameter ( _d2 ) about the same as the diameter of the through hole 22 drilled in the steel pipe 21, and has a length ( _t1 + _t2 + _t3 ) that protrudes outward from the outer side of the steel pipe 21 (see _t2 ) and also protrudes inward from the inside of the steel pipe 21 (see _t3 ).

As described in the above embodiment, the diameter of the through hole 22 is preferably set to three times or more the maximum aggregate diameter of the concrete 4, taking into consideration the ease with which the concrete 4 can pass through and the relationship with the magnitude of the adhesion force obtained at one location. Therefore, when inserting a reinforcing pipe 24 into the through hole 22, it is preferable that the inner diameter (d ₁ ) of the reinforcing pipe 24 is set to three times or more the maximum aggregate diameter of the concrete 4.

For example, a reinforcing pipe 24 with an inner diameter (d ₁ ) of about 150 mm and a length of about 50 mm is inserted into a through hole 22 with a diameter of about 167.5 mm, causing a protrusion of about t ₂ = 20 mm on the outer circumference side of the steel pipe 21, and also causing a protrusion of about t ₃ = 20 mm on the inner side of the steel pipe 21. The lengths of the protrusions of the reinforcing pipe 24 (t ₂ , t ₃ ) can be set considering the relationship between the gap between the pile hole 5 and the outer surface of the steel pipe 21, and the size of the reinforcing cage 31 that passes through the steel pipe 21.

On the other hand, Figure 12 is a conceptual diagram illustrating the function of the reinforcing pipe 24 in the case of the enlarged-base pile 1A described in Embodiment 1. As shown in Figures 13A and 13B, even if only through holes 22 are provided in a steel pipe 21 with a wall thickness _t1 = 10 mm, the stress transmission structure of the cast-in-place steel pipe concrete piles 1, 1A can be constructed by the compressive stress _σB and shear stress τ of the concrete 4 filled around and inside the steel pipe 21 through the through holes 22.

Furthermore, as in this embodiment 3, by passing a reinforcing pipe 24 through the through hole 22, the area receiving compressive stress σ _B from the concrete 4 on the central side of the through hole 22 can be widened according to the length of the reinforcing pipe 24, as shown by the upward arrow in Figure 11.

In addition, the protruding portions ( _t2 , _t3 ) of the reinforcing pipe 24 can also receive the compressive stress _σB of the concrete 4, thereby significantly increasing the bearing resistance of the cast-in-place steel pipe concrete pile 1.

In particular, as shown in Figure 12, in the enlarged base portion 11A of the enlarged base pile 1A, concrete 4 is sufficiently filled around the reinforcing pipe 24 (t ₂ : see reference numeral in Figure 11) that protrudes from the outer circumference of the steel pipe 21, so the compressive stress σ _B of the protruding portion can be expected more reliably.

Furthermore, since the other configurations and effects are substantially the same as those of the above embodiment or other examples, their explanation will be omitted.

The following description will refer to Figure 14 and describe a different embodiment of the cast-in-place steel pipe concrete pile 1 (1A-1C) from the previously described embodiment and Example 1-3. Parts identical or equivalent to those described in the previous embodiment or Example 1-3 will be described using the same terms or reference numerals.

In the previously described embodiment of the cast-in-place steel pipe concrete pile 1, when there is concern about a reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through hole 22, an opening reinforcement portion 23 is provided by welding an annular steel plate around the through hole 22. This embodiment 4 will be explained in more detail with reference to Figure 14.

Figure 14 shows a configuration in which an opening reinforcement section 23 is provided on the outer surface of the steel pipe 21 by welding an annular steel plate around the through hole 22. In short, the opening reinforcement section 23 is composed of an annular steel plate having an inner diameter equal to the diameter of the through hole 22 and a width suitable for reinforcement.

Furthermore, although not shown in the illustrations, the opening reinforcement is not limited to annular steel plates. For example, if the through holes 22 are densely drilled in the steel pipe 21, the opening reinforcement can be created by drilling multiple holes of the same diameter as the through holes 22 in a single rectangular steel plate, aligning the holes with the positions of the through holes 22, and welding the steel plate to the steel pipe 21. In this case, a single strip-shaped steel plate may be fixed to the through holes 22 arranged in the axial direction of the steel pipe 21, or a single ring-shaped or arch-shaped steel material may be fixed to the through holes 22 arranged in the circumferential direction of the steel pipe 21.

In this way, by welding steel material around the through hole 22 to provide an opening reinforcement section, the reduction in the load-bearing capacity of the steel pipe 21 due to the drilling of the through hole 22 can be suppressed.
Furthermore, the other configurations and effects are substantially the same as those of the above embodiment or other examples, so their explanation will be omitted.

While embodiments and examples of the present invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and examples. Any design modifications that do not depart from the spirit of the present invention are included within the scope of this invention.

For example, the above embodiments and examples described a case where four or eight through-holes 22 are provided at equal intervals in the circumferential direction of the steel pipe 21. However, the invention is not limited to this, and the number of through-holes 22 provided at intervals in the circumferential direction can be set to any number between four and thirty-two, depending on the size of the steel pipe.

Furthermore, while the above embodiments and examples described use cast-in-place steel pipe concrete piles 1 (1A, 1B, 1C) with three, four, and six through-holes 22 spaced axially at the end 21a of the steel pipe 21 as examples, the invention is not limited to these. The number of through-holes 22 spaced axially at the pile can be set to any number between one and ten.

Furthermore, while the above embodiments and examples described a case where through-holes 22 with a substantially circular shape in side view are provided at the ends 21a and 21b of the steel pipe 21, the invention is not limited to this. Polygonal through-holes with vertices formed on the upper side in side view can also be provided at the ends 21a and 21b of the steel pipe 21. For example, even with through-holes where the vertices of a polygon such as a triangle, rhombus, pentagon, or hexagon are positioned on the upper side, bleeding cannot accumulate around the vertices of the through-hole, thus preventing the formation of voids in the concrete.

1: Cast-in-place steel pipe concrete pile 1A: Enlarged base pile (cast-in-place steel pipe concrete pile)
11A: Enlarged base section 1B, 1C: Intermediate enlarged diameter pile (cast-in-place steel pipe concrete pile)
13: Enlarged diameter section 2, 2A-2C: Steel pipe Concrete section 21, 21A: Steel pipe 21a: End section 21b: End section 22: Through hole 23: Opening Reinforcement section 24: Reinforcement pipe 3, 3B, 3C: Reinforced concrete section 4: Concrete

Claims (4)

A stress transmission structure for a cast-in-place steel pipe concrete pile formed by steel pipes and concrete,
The end of the cylindrical steel pipe, which is filled with concrete around and inside, is provided with multiple through holes that are approximately circular in side view or polygonal in side view with their vertices facing upwards, spaced apart in the circumferential direction .
A stress transmission structure for a cast-in-place steel pipe concrete pile, characterized in that reinforcing pipes protruding from the outer circumference and into the interior of the steel pipe are inserted into the aforementioned through-hole . The stress transmission structure for a cast-in-place steel pipe concrete pile according to claim 1, characterized in that the through-holes are provided at equal intervals in the circumferential direction from 4 to 32 locations, depending on the size of the steel pipe. The end of the steel pipe is positioned near the boundary between the concrete steel pipe section and the reinforced concrete section.
The stress transmission structure for a cast-in-place steel pipe concrete pile according to claim 1 or 2, characterized in that the through holes are provided in any number of stages, from one to five, at intervals in the axial direction of the steel pipe, in order to improve the stress transmission performance between the steel pipe and the reinforcing bars of the reinforced concrete section. The end of the steel pipe is positioned at the enlarged base portion of the enlarged base pile or the enlarged diameter portion of the intermediate enlarged diameter pile,
The stress transmission structure for a cast-in-place steel pipe concrete pile according to claim 1 or 2, characterized in that the through holes are provided in any number of stages from one to ten at intervals in the axial direction of the steel pipe in order to transmit the pull-out strength of the enlarged base or the enlarged diameter to the steel pipe.