JP2023001572A - Pile foundation and its design method - Google Patents

Abstract

The object of the present invention is to provide a pile foundation and a method of designing the same that can reduce material costs while maintaining sufficient cross-sectional strength.
A pile foundation (10) has a pile (2) in which a plurality of ready-made piles (21, 22, 23, 24) are connected in the axial direction. The pile foundation 10 includes a first prefabricated pile having a first cross-sectional yield strength against a first axial force acting on the pile head, and a peripheral surface of the first prefabricated pile disposed immediately below the first prefabricated pile. and a second prefabricated pile having a second cross-sectional strength against a second axial force obtained by subtracting the circumferential frictional resistance force from the ground acting on the first axial force.
[Selection diagram] Fig. 1

Description

The present invention relates to a pile foundation and its design method.

The piles used for pile foundations are embedded piles using ready-made piles, cast-in-place concrete piles, and the like. Embedded piles can be constructed by excavating the ground, forming soil cement in pile holes, and sinking prefabricated piles into the soil cement. Soil cement is formed by injecting and stirring cement milk into excavated soil in a pile hole. When the soil cement hardens, it is integrated with the prefabricated pile to form an embedded pile. On the other hand, cast-in-place concrete piles can be constructed by pouring concrete into pile holes drilled in the ground while sinking steel materials. As the concrete hardens, a cast-in-place concrete pile is formed.
The pile hole is usually formed to a depth where the tip of the pile reaches the bearing layer of the ground. If the bearing layer is several tens of meters below the ground surface, it is necessary to prepare piles long enough to reach the bearing layer. A prefabricated pile of several tens of meters can be prepared by connecting a plurality of prefabricated piles of a predetermined length (about 4 m to 15 m).

The axial resistance force that the pile receives from the ground includes the tip resistance force that acts from the ground (bearing layer) on the bottom end of the pile, and the peripheral frictional resistance force that acts from the ground on the pile peripheral surface. Therefore, when designing pile foundations, the pile diameter, type, and specifications of each pile should be determined so that the sum of the tip resistance and peripheral friction resistance of each pile exceeds the building load that the pile foundation actually supports. (In the case of ready-made piles, SC piles, PRC piles, PHC piles, joint piles, straight piles, etc.) are determined.
On the other hand, each pile has its own compressive strength and tensile strength as values indicating its breaking strength. In designing a pile foundation, the diameter and type of each pile are determined so that the compressive strength and tensile strength of the pile are larger than the axial force actually acting on each pile.

JP 2015-014107 A JP 2017-96037 A

Normally, when setting the pile diameter and type of pile, it is assumed that the axial force (pull-out force and push-in force) acting on the pile head acts on all cross sections over the entire length of the pile. Determine the diameter and type. For this reason, piles are used that have cross-sectional strength against the axial force acting on the pile head over the entire length of the pile.

However, since the peripheral frictional resistance from the ground acts on each pile, the axial force acting on the pile head decreases as it goes downward, and it does not act equally on the pile cross section located below. Absent. That is, the axial force acting on the pile cross section below is smaller than the cross section position by the amount of the circumferential frictional resistance applied from the ground to the pile in the upper section. Therefore, in the conventional design method, in which the axial force acting on all pile cross-sections over the entire length of the pile is assumed to be uniform, the pile cross-section located at the bottom has an over-specified state where the cross-sectional strength is unnecessarily large. higher cost.

SUMMARY OF THE INVENTION An object of the present invention is to provide a pile foundation and a method of designing the same, which can reduce material costs while maintaining sufficient cross-sectional strength.

A pile foundation according to one aspect includes a pile in which pile portions arranged in each of N axially aligned sections are connected in the axial direction. The N pile parts of this pile foundation are the top pile part having the first cross-sectional strength against the first axial force acting on the pile head of the pile, and all the (n-1)th pile parts from the top n-th pile portion having cross-sectional strength against the axial force obtained by subtracting the peripheral friction resistance force from the ground acting on the peripheral surface of the pile portion from the first axial force, where n is 2 ≤ n ≤ An integer that satisfies N.

According to the pile foundation of this aspect, it is possible to use the n-th pile portion having a smaller cross-sectional strength than the top pile portion while maintaining a sufficient cross-sectional strength, thereby reducing the material cost.

The top pile portion of the pile foundation has a bending resistance against the maximum acting bending moment acting on the top pile portion due to the horizontal load acting on the pile head, and the n-th pile portion , has the bending strength against the maximum acting bending moment acting on the n-th pile portion obtained by considering the ground resistance acting on the peripheral surface of all pile portions from the top to the (n-1)th pile portion.

A pile foundation according to another aspect includes a pile in which pile portions arranged in each of N axially aligned sections are connected in the axial direction. The N pile portions of this pile foundation include the (n−1)th pile portion having cross-sectional strength against the axial force acting on the upper end of the (n−1)th pile portion from the top, and (n−1) The axis that is placed directly under the th pile portion and acts on the upper end of the (n-1) th pile portion to apply the peripheral surface friction resistance force from the ground acting on the peripheral surface of the (n-1) th pile portion n th pile section having sectional strength for axial force minus axial force, where n is an integer satisfying 2≦n≦N.

According to the pile foundation of this aspect, it is possible to use the n-th pile portion with a smaller cross-sectional strength than the (n-1)th pile portion while maintaining sufficient cross-sectional strength, and the material cost can be reduced. .

The (n-1)th pile section of the above pile foundation is the maximum horizontal load acting on the (n-1)th pile section due to the horizontal load acting on the top of the (n-1)th pile section. The n-th pile portion has the bending strength against the acting bending moment, and the n-th pile portion is the maximum acting on the n-th pile portion obtained by considering the ground resistance acting on the peripheral surface of the (n-1)-th pile portion. It has a bending strength against the applied bending moment.

A method for designing a pile foundation according to another aspect is a method for designing a pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction. The cross-sectional strength of the th pile portion is subtracted from the axial force acting on the pile head of the pile by subtracting the circumferential frictional resistance force from the ground acting on the circumferential surface of all pile portions from the top to the (n-1)th pile portion. Set to section strength against axial force. n is an integer that satisfies 2≦n≦N.

According to the pile foundation design method of this aspect, it is possible to use the n-th pile portion with a smaller cross-sectional strength than the (n-1)-th pile portion while maintaining sufficient cross-sectional strength, and reduce material costs. It can be done cheaply.

According to the above pile foundation design method, the bending strength of the n-th pile from the top is obtained by considering the ground resistance acting on the peripheral surface of all the piles from the top to the (n-1)-th pile. It is set to the bending resistance against the maximum applied bending moment acting on the n-th pile section.

A method for designing a pile foundation according to another aspect is a method for designing a pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction. The cross-sectional strength of the th pile part is applied to the upper end of the (n-1)th pile part, and the peripheral frictional resistance force from the ground acting on the peripheral surface of the (n-1)th pile part from the top Set to the sectional strength against the axial force subtracted from the axial force. n is an integer that satisfies 2≦n≦N.

According to the pile foundation design method of this aspect, it is possible to use the n-th pile portion with a smaller cross-sectional strength than the (n-1)-th pile portion while maintaining sufficient cross-sectional strength, and reduce material costs. It can be done cheaply.

According to the above pile foundation design method, the bending resistance of the n-th pile from the top is calculated by considering the ground resistance acting on the peripheral surface of the (n-1)-th pile from the top. It is set to the bending strength against the maximum acting bending moment acting on the pile part of

ADVANTAGE OF THE INVENTION According to this invention, after maintaining sufficient cross-sectional strength, the pile foundation which can make material cost low, and its design method can be provided.

Drawing 1 is a schematic diagram for explaining axial force which acts on a pile of a pile foundation concerning an embodiment. FIG. 2 is a schematic diagram for explaining the axial force acting on each prefabricated pile when a pull-out force acts on the pile of FIG. FIG. 3 is a schematic diagram for explaining the axial force acting on each prefabricated pile when a pushing force acts on the pile of FIG. FIG. 4 is a diagram showing the relationship between the axial force N and the bending moment M acting on each cross-section in the axial direction of the pile of FIG. FIG. 5 is a diagram for explaining the relationship between the pull-out force PT acting on each prefabricated pile of FIG. 1 and the bending moment M. FIG. FIG. 6 is a diagram for explaining the relationship between the pushing force PC acting on each prefabricated pile of FIG. 1 and the bending moment M. FIG.

Hereinafter, embodiments will be described with reference to the drawings.
FIG. 1 shows the pull-out force and push-in force acting on the two piles 2L and 2R of the pile foundation 10 according to the embodiment (hereinafter, such axial force may be collectively referred to as axial force). It is the schematic for demonstrating. The pile foundation 10 of this embodiment has two piles 2L and 2R of substantially the same structure that support the weight of the building 100. As shown in FIG. The two piles 2L and 2R support the weight of the building 100 at approximately the same rate. Since the two piles 2L and 2R have substantially the same structure, they may be collectively referred to as the pile 2 in the following description. The number of piles 2 constituting the pile foundation 10 is not limited to two, and may be at least one, or may be three or more.

FIG. 1(a) shows a state (always) in which the weight of the building 100 equally acts as an axial force NL on the pile heads 2a of the two piles 2L and 2R. FIG. 1(b) shows that when an earthquake occurs, a horizontal force (horizontal force) directed from the left to the right of the drawing acts on the building 100, and the pile heads 2a of the two piles 2L and 2R are subjected to fluctuating axial forces in opposite directions. indicates the state in which -NT and NC act respectively. FIG. 1(c) shows a state in which the axial force NL in FIG. 1(a) and the fluctuating axial force −NT, NC in FIG. hour).

A pile foundation 10 of a building 100 has a plurality of (two in this embodiment) piles 2L and 2R built into a plurality of pile holes 1 provided in the ground G. The type, length, thickness, number, layout, etc. of the piles 2 are basically determined according to the weight and shape of the building 100, the state of the ground G, and the like.

Each pile 2 of the pile foundation 10 has a structure in which a plurality of (four in this embodiment) ready-made piles 21, 22, 23, and 24 are connected in the axial direction (vertical direction in the drawing). The ready-made piles 21, 22, 23, and 24 of each pile 2 function as pile portions arranged in each of a plurality of sections aligned in the axial direction. Since the joint structure of the uppermost prefabricated pile 21 to the building 100 and the connection structure of the prefabricated piles 21 to 24 are well-known techniques, detailed description thereof will be omitted here. The prefabricated piles 21 to 24 include, for example, concrete piles with shell steel pipes (SC piles), prestressed reinforced high-strength concrete piles (PRC piles), prestressed high-strength concrete piles (PHC piles), and the like.

The length of the pile 2 should be such that at least the tip of the pile 2 reaches the bearing layer of the ground G. Therefore, the pile hole 1 also needs to be formed to a depth that reaches the support layer. The length of the pile 2 can be changed by selecting the length and number of ready-made piles to be connected. The type and diameter of the prefabricated pile that constitutes the pile 2 may be appropriately selected based on the resistance required for the pile 2 (tip resistance and circumferential frictional resistance).

A pile circumference 4 is provided between the pile hole 1 and each pile 2 . The pile peripheral portion 4 includes a foot protection portion surrounding the tip of the pile 2 with soil cement and a pile peripheral portion surrounding the peripheral surface of the pile 2 with soil cement. That is, the pile peripheral portion 4 is formed by providing soil cement as a hardening material between the pile hole 1 and the pile 2 . When the soil cement provided on the pile circumference 4 hardens, the pile circumference 4 is integrated with the concrete pile 2 .

As shown in FIG. 1(a), only the vertically downward axial force NL determined by the weight of the building 100 acts on the piles 2L and 2R during normal times when an earthquake does not occur. In this case, the axial force NL acts on the pile heads 2a of the piles 2L and 2R.

When an earthquake occurs and horizontal force acts on the building 100 as indicated by arrows in FIG. Here, it is assumed that the horizontal force acts on the building 100 from left to right in the figure along the direction in which the two piles 2 are arranged.

When this horizontal force acts on the building 100 in the direction of the arrow Y, the pile 2L on the left side of the drawing receives -NT (fluctuating axial force) acting as a couple in the direction of pulling the pile head 2a upward. Also, at this time, a NC (fluctuating axial force) in the direction of pushing down the pile head 2a acts similarly as a couple on the pile 2R on the right side of the drawing.

In the first place, since the vertical downward normal axial force NL shown in FIG. 1A acts on each of the piles 2L and 2R, when the horizontal force shown in FIG. ), an axial force of NL-NT acts on the pile head 2a of the left pile 2L, and an axial force of NL+NC acts on the pile head 2a of the right pile 2R.

If the absolute value of the fluctuating axial force -NT acting on the pile head 2a of the left pile 2L when the horizontal force acts on the building 100 is greater than the normal axial force NL, it acts on the pile head 2a of the pile 2L when an earthquake occurs. The resulting axial force NL-NT is a pull-out force in the direction of pulling the pile head 2a upward. Such a pull-out force is likely to act on a pile foundation of a building with a large aspect ratio (4 or more).

At this time, the axial force NL+NC acting on the pile head 2a of the right pile 2R becomes a pushing force in the direction of pushing the pile head 2a downward. In this way, when a horizontal force acts on the building 100, either one of an axial pull-out force and a push-in force acts on the piles 2L and 2R of the pile foundation 10. As shown in FIG.

Concrete piles generally have a large compressive strength against an axial pushing force and a small tensile strength against a pull-out force. Therefore, when verifying the cross-sectional strength of the pile 2, it is often difficult to secure the cross-sectional strength on the pull-out side rather than on the pushing side.

As shown in FIG. 2, when the pull-out force PT1 (=NL-NT) acts on the pile head 2a of the pile 2L, the peripheral frictional resistance forces (Rf1, Rf2, Rf3, Rf4) acts as a resistance, and the pulling force is transmitted to the tip of the pile 2L while decreasing (PT1→PT2→PT3→PT4).

That is, when Rf1 is the circumferential frictional resistance force that the uppermost ready-made pile 21 constituting the pile 2L receives from the ground G, the pull-out force PT2 acting on the upper end of the second ready-made pile 22 from the top is PT2= PT1-Rf1. Similarly, if Rf2 is the peripheral friction resistance force that the second ready-made pile 22 from the top receives from the ground G, the pull-out force PT3 acting on the upper end of the third ready-made pile 23 from the top is PT3=PT2−Rf2. becomes. Furthermore, when Rf3 is the peripheral friction resistance force that the third ready-made pile 23 from the top receives from the ground G, the pull-out force PT4 acting on the upper end of the lowest ready-made pile 24 is PT4 = PT3 - Rf3. .

As shown in FIG. 3, when a pushing force PC1 (=NL+NC) acts on the pile head 2a of the pile 2R, the peripheral surface frictional resistance forces (Rf1, Rf2, Rf3, Rf4) acting on the peripheral surface of the pile 2R from the ground G. becomes a resistance, and the pressing force is transmitted to the tip of the pile 2R while decreasing (PC1→PC2→PC3→PC4).

That is, when Rf1 is the peripheral frictional resistance force that the uppermost ready-made pile 21 constituting the pile 2R receives from the ground G, the pushing force PC2 acting on the upper end of the second ready-made pile 22 from the top is PC2= PC1-Rf1. Similarly, if Rf2 is the circumferential friction resistance force that the second ready-made pile 22 from the top receives from the ground G, the pushing force PC3 acting on the upper end of the third ready-made pile 23 from the top is PC3=PC2−Rf2. becomes. Furthermore, if Rf3 is the peripheral friction resistance force that the third ready-made pile 23 from the top receives from the ground G, the pushing force PC4 acting on the upper end of the lowest ready-made pile 24 is PC4=PC3−Rf3. .

The term “peripheral frictional resistance” in this specification and claims refers to the frictional force between the pile periphery 4 and the pile hole 1 when the pile 2 is a knotted pile, and when the pile 2 is a straight pile. In some cases it refers to the frictional force between the pile 2 and the pile circumference 4 . For example, when the axial force acting on the pile head 2 a of the joint pile is gradually increased, the pile periphery 4 moves relative to the pile hole 1 before the pile 2 moves relative to the pile periphery 4 . When the axial force acting on the pile head 2 a of the straight pile is gradually increased, the pile 2 moves relative to the pile circumference 4 before the pile circumference 4 moves relative to the pile hole 1 . That is, the "peripheral frictional resistance" acts on different parts depending on whether the pile 2 is a knotted pile or a straight pile.

Further, the "peripheral frictional resistance force" in the present specification and claims has substantially the same value when the axial force acting on the pile head 2a is the pulling force and the pushing force. However, in practice, when the peripheral frictional resistance force when the pushing force is acting on the pile head 2a is 1, the peripheral friction when the pulling force of the same magnitude is acting on the pile head 2a The resistance is about 0.7-0.8. Therefore, it is necessary to interpret the meaning of "peripheral frictional resistance force" in consideration of the above differences.

When designing the pile 2, basically, the resistance in the pushing direction of the pile 2 (tip resistance and peripheral friction resistance) is the axial force NL or the pushing force PC1 ( = NL + NC), and the pull-out resistance of the pile (peripheral friction resistance) is greater than the pull-out force PT1 (= NL-NT) acting on the pile 2 from the building 100. Set the structure of 2L and 2R (pile length, pile diameter, type, etc.).

In addition, when designing the pile 2, the tensile strength against the pull-out force of each of the ready-made piles 21, 22, 23, and 24 constituting the pile 2 should exceed the axial force in the pull-out direction generated when the building 100 collapses. , ready-made piles 21, 22, 23, 24 are selected. Also, it is confirmed that the compressive strength against the pushing force of each of the prefabricated piles 21, 22, 23, and 24 exceeds the axial force in the pushing direction generated when the building 100 collapses.

In a conventional design, the compression of all prefabricated piles 21, 22, 23, 24 is assumed to be uniform on all prefabricated piles 21, 22, 23, 24, assuming that the pushing and pulling forces acting on the pile head 2a act uniformly. Yield strength and tensile strength were set. In this case, even the lowest ready-made pile 24 has a compressive strength exceeding the pushing force NL+NC acting on the pile head 2a, and a ready-made pile having a tensile strength exceeding the pulling force NL-NT acting on the pile head 2a. Since piles are used, it becomes an over-specification, and the material cost is unnecessarily high.

On the other hand, in this embodiment, the plurality of ready-made piles 21, 22, 23, 24 are selected so that the compressive strength and tensile strength (cross-sectional strength) gradually decrease toward the tip of the piles 2L, 2R. I tried to connect and use them in this order. For this reason, according to this embodiment, it is possible to reduce the material cost of the pile foundation 10 while maintaining a sufficient cross-sectional strength over the entire length of the pile 2 .

That is, in this embodiment, as the uppermost prefabricated pile 21, one having a compressive yield strength exceeding the pushing force NL+NC and having a tensile yield strength exceeding the pulling force NL-NT is used. In addition, as the second prefabricated pile 22, it has a compressive strength exceeding the pushing force NL + NC - Rf1 minus the peripheral frictional resistance Rf1 acting on the top prefabricated pile 21, and the peripheral frictional resistance Rf1 is subtracted. A material having a tensile yield strength exceeding the pulling force NL-NT+Rf1 was used.

In addition, as the third prefabricated pile 23, it has a compressive strength exceeding the pushing force NL + NC - Rf1 - Rf2 obtained by further subtracting the peripheral frictional resistance Rf2 acting on the second prefabricated pile 22, and the peripheral frictional resistance Rf2 was further subtracted, and the one having a tensile yield strength exceeding the pull-out force NL-NT+Rf1+Rf2 was used. Furthermore, as the lowest prefabricated pile 24, it has a compressive strength exceeding the pushing force NL + NC - Rf1 - Rf2 - Rf3 obtained by further subtracting the peripheral frictional resistance Rf3 acting on the third prefabricated pile 23, and has a peripheral friction A material having a tensile yield strength exceeding the pull-out force NL-NT+Rf1+Rf2+Rf3 obtained by further subtracting the resistance Rf3 is used.

By the way, when a horizontal force acts on the building 100 when an earthquake occurs, a bending moment M acts on the piles 2L and 2R of the pile foundation 10 in addition to the above-described fluctuating axial forces -NT and NC. A bending moment M is generated in each section of the pile 2 which is continuous in the axial direction. The bending moment M is generally largest at the pile head 2a fixed to the building 100, and gradually decreases toward the tip of the pile 2 due to ground resistance acting from the ground G on the periphery of the pile.

The "soil resistance" referred to here means the horizontal resistance from the ground that acts in a direction perpendicular to the axial direction of the pile. works.

When designing the bending resistance of the prefabricated piles 21, 22, 23, and 24 of each pile 2, the above-described ground resistance acting on the periphery of the pile is taken into consideration. In this case, the ground resistance considered is the ground resistance acting on the pile circumference above the pile cross-section where the maximum acting bending moment acts on each of the prefabricated piles 21, 22, 23, 24. For example, when the maximum bending moment acting on the third prefabricated pile 23 from the top acts on the cross section of the pile between the upper end and the lower end of the prefabricated pile 23, the ground resistance to be considered is the top prefabricated pile Ground resistance acting on the circumference of the pile 21, ground resistance acting on the circumference of the second ready-made pile 22, and the pile from the upper end of the third ready-made pile 23 to the cross section of the pile where the maximum bending moment acts. It becomes the ground resistance acting on the periphery.

FIG. 4 is a relationship diagram (NM diagram) showing the relationship between the axial force N and the bending moment M, which schematically shows the bending resistance performance of the pile 2. As shown in FIG. In FIG. 4, the vertical axis indicates the bending moment M, and the horizontal axis indicates the axial force N. As shown in FIG. The plus side (right side in the figure) of the horizontal axis is the compressive axial force, and the minus side (left side in the figure) is the tensile axial force. The bending resistance performance of each section of the pile 2 is evaluated by this NM diagram. Such an NM diagram exists for each type of ready-made pile. When the prefabricated pile is a concrete pile as in this embodiment, it is weak against tensile force and strong against compressive force, so as shown in FIG. 4, the peak of the envelope curve in the NM diagram shifts to the compression side.

The axial force at the point where the bending moment M does not act means the breaking strength of the pile 2, the value of the axial force on the plus side indicates the breaking compressive strength, and the value of the axial force on the minus side indicates the breaking tensile strength. indicates If even a little bending moment acts on the pile 2 while such an axial force is acting on the pile 2, the pile 2 will be destroyed.

All points on the envelope curve other than the two points (two ends of the envelope curve in the NM diagram) where the bending moment M is not acting are points where the bending moment M is acting in addition to the axial force. be. In other words, in designing the pile 2, not only the above-mentioned axial tensile strength and compressive strength should be considered, but also the bending strength against the bending moment M acting on each prefabricated pile. Each prefabricated pile can be selected so that the bending strength of the prefabricated pile under the axial force (pushing force or pulling force) acting on the prefabricated pile exceeds the maximum bending moment M acting on each prefabricated pile. desirable.

As a design considering the bending strength of each prefabricated pile, the design when the pull-out force NL-NT and the bending moment M act on the pile head 2a, and the design when the pushing force NL+NC and the bending moment M act on the pile head 2a. should be considered.

In the drawing design, as shown in FIG. , 22, 23, 24, PT1, PT2, PT3, PT4 (Fig. 5(a)) are plotted on the NM diagram (indicated by "O" in Fig. 5(c) ). In FIG. 5(c), for comparison, the bending moments M1, M2, M3, and M4 are taken as coordinates in the conventional design assuming that the same pull-out force PT1 acts on all the prefabricated piles 21, 22, 23, and 24. Points are indicated by "x".

The NM diagram shows the envelope curves (1), (2), (3) and (4) of four types of prefabricated piles with different yield strengths. The prefabricated pile with the envelope curve (1) having the highest yield strength is, for example, a concrete pile with an outer shell steel pipe (SC pile), and the prefabricated pile with the envelope curve (2) is, for example, a prestressed reinforced high-strength concrete pile (PRC pile ) is about VI to III, and the ready-made pile of the envelope curve (3) is, for example, prestressed reinforced high-strength concrete pile (PRC pile) II to I, or prestressed high-strength concrete pile (PHC pile ), and the ready-made pile of the envelope curve (4) is, for example, A prestressed high-strength concrete pile (PHC pile).

The axial force PT1 acting on the top ready-made pile 21 closest to the ground surface is the pull-out force NL-NT acting on the pile head 2a, and the maximum value of the bending moment M acting on the top ready-made pile 21 is A bending moment M1 acting on the pile head 2a. The point where this coordinate is plotted is between the envelope curves (1) and (2), as indicated by the "o" in the top panel of FIG. 5(c). Since "O" exists outside the envelope curve (2), the ready-made pile of the envelope curve (1) must be used as the top ready-made pile 21.

The maximum value of the axial force acting on the second ready-made pile 22 from the top is the pull-out force PT2 acting on the upper end of the ready-made pile 22, and the maximum value of the bending moment M acting on the second ready-made pile 22 is the ready-made pile 22. is the bending moment M2 acting on the upper end of . The point where this coordinate is plotted is between the envelope curves (2) and (3), as indicated by "O" in the second diagram from the top in FIG. 5(c). Therefore, as the second prefabricated pile 22, the prefabricated piles of envelope curves (1) and (2) can be used, but it is preferable to use the prefabricated pile of envelope curve (2), which is cheaper in cost.

On the other hand, in the conventional design, since the pull-out force acting on the second prefabricated pile 22 is PT1, the coordinates of the crossing point "x" with the bending moment M2 are outside the envelope curve (2). That is, in the conventional design, the prefabricated pile of envelope curve (2) cannot be used as the second prefabricated pile 22 from the top.

The maximum value of the axial force acting on the third ready-made pile 23 from the top is the pull-out force PT3 acting on the upper end of the ready-made pile 23, and the maximum value of the bending moment M acting on the third ready-made pile 23 is the ready-made pile 23. is a bending moment M3 acting on the portion between the upper end and the lower end of . The point where this coordinate is plotted is between the envelope curves (3) and (4), as indicated by "o" in the third diagram from the top in FIG. 5(c). Therefore, the ready-made piles of the envelope curves (1), (2), and (3) can be used as the third ready-made pile 23, but it is preferable to use the ready-made pile of the envelope curve (3), which is cheaper in cost. is.

On the other hand, in the conventional design, the pull-out force acting on the third ready-made pile 23 is PT1, so the coordinates of the crossing point "x" with the bending moment M3 are outside the envelope curve (3). That is, in the conventional design, the prefabricated pile of envelope curve (3) cannot be used as the third prefabricated pile 23 from the top.

The maximum value of the axial force acting on the lowest ready-made pile 24 is the pull-out force PT4 acting on the upper end of the ready-made pile 24, and the maximum value of the bending moment M acting on the lowest ready-made pile 24 is the ready-made pile 24. is the bending moment M4 acting on the upper end of . The point where this coordinate is plotted is inside the envelope curve (4), as indicated by "o" in the bottom diagram of FIG. 5(c). Therefore, as the lowest ready-made pile 24, all of the ready-made piles of the envelope curves (1) to (4) can be used, but it is preferable to use the ready-made pile of the envelope curve (4), which is cheaper in cost. .

On the other hand, in the conventional design, since the pull-out force acting on the ready-made pile 24 is PT1, the coordinates of the crossing point "x" with the bending moment M4 are outside the envelope curve (4). That is, in conventional designs, the prefabricated pile of envelope curve (4) cannot be used as the bottom prefabricated pile 24 .

On the other hand, in the design for pushing, as shown in FIG. Points with the coordinates of the maximum pushing forces PC1, PC2, PC3, and PC4 acting on the piles 21, 22, 23, and 24 (Fig. 6(a)) are plotted on the NM diagram ("○" in Fig. 6(c) ). In FIG. 6(c), for comparison, the bending moments M1, M2, M3, and M4 are taken as coordinates in the conventional design assuming that the same pushing force PC1 acts on all the prefabricated piles 21, 22, 23, and 24. Points are indicated by "x".

The NM diagram shows the envelope curves (1), (2), (3) and (4) of four types of prefabricated piles with different yield strengths. The prefabricated pile with the envelope curve (1) having the highest yield strength is, for example, a concrete pile with an outer shell steel pipe (SC pile), and the prefabricated pile with the envelope curve (2) is, for example, a prestressed reinforced high-strength concrete pile (PRC pile ) is about VI to III, and the ready-made pile of the envelope curve (3) is, for example, prestressed reinforced high-strength concrete pile (PRC pile) II to I, or prestressed high-strength concrete pile (PHC pile ), and the ready-made pile of the envelope curve (4) is, for example, A prestressed high-strength concrete pile (PHC pile).

The axial force PC1 acting on the top ready-made pile 21 closest to the ground surface is the pushing force NL+NC acting on the pile head 2a, and the maximum value of the bending moment M acting on the top ready-made pile 21 is the pile head. is the bending moment M1 acting on 2a. The point where this coordinate is plotted is on the envelope curve (3), as indicated by the "o" in the top panel of FIG. 6(c). Therefore, the ready-made piles of envelope curves (1) and (2) can be used as the top ready-made pile 21, and the ready-made pile of envelope curve (1) selected by the drawing-related design can be used. It could be confirmed.

The maximum value of the axial force (pushing force) acting on the second ready-made pile 22 from the top is the pushing force PC2 acting on the upper end of the ready-made pile 22, and the maximum value of the bending moment M acting on the second ready-made pile 22. is the bending moment M2 acting on the upper end of the prefabricated pile 22; The point where this coordinate is plotted is inside the envelope curve (4), as indicated by "o" in the second figure from the top in FIG. 6(c). Therefore, as the second ready-made pile 22, all of the ready-made piles of the envelope curves (1) to (4) can be used, and the ready-made pile of the envelope curve (2) selected by the drawing-related design can be used. It could be confirmed.

The maximum value of the axial force acting on the third ready-made pile 23 from the top is the pushing force PC3 acting on the upper end of the ready-made pile 23, and the maximum value of the bending moment M acting on the third ready-made pile 23 is the ready-made pile 23. is a bending moment M3 acting on the portion between the upper end and the lower end of . The point where this coordinate is plotted is inside the envelope curve (4), as indicated by "o" in the third figure from the top in FIG. 6(c). Therefore, as the third ready-made pile 22, all of the ready-made piles of the envelope curves (1) to (4) can be used, and the ready-made pile of the envelope curve (3) selected by the drawing-related design can be used. It could be confirmed.

The maximum value of the axial force acting on the lowest ready-made pile 24 is the pushing force PCT4 acting on the upper end of the ready-made pile 24, and the maximum value of the bending moment M acting on the lowest ready-made pile 24 is the ready-made pile 24. is the bending moment M4 acting on the upper end of . The point where this coordinate is plotted is inside the envelope curve (4), as indicated by "o" in the bottom diagram of FIG. 6(c). Therefore, as the lowest ready-made pile 24, all of the ready-made piles of the envelope curves (1) to (4) can be used, and the ready-made pile of the envelope curve (4) selected by the drawing-related design can be used. was confirmed.

As described above, according to this embodiment, in the pile 2 in which a plurality of ready-made piles 21 to 24 are connected, appropriate tensile strength, compressive strength, and A plurality of ready-made piles 21 to 24 were set so as to have bending resistance. For this reason, it is possible to design without using a ready-made pile having cross-sectional strength exceeding the axial forces PC1 and PT1 acting on the pile head 2a even near the tip of the pile, as in the conventional case. We were able to keep costs down.

Concrete piles tend to have high compressive strength against pushing force in the axial direction and weak tensile strength against pulling force. Therefore, when designing piles, it is necessary to verify the pull-out side more severely than the push-in side. In other words, it can be said that the above-described design method of the present embodiment has a greater advantage on the extraction side than on the compression side.

It should be noted that the present invention is not limited to the above-described embodiments, and various modifications can be made in the implementation stage without departing from the scope of the invention. In addition, various inventions are included in the above-described embodiments, and various inventions can be extracted by combinations selected from a plurality of disclosed constituent elements. For example, even if some constituent elements are deleted from all the constituent elements shown in the embodiments, if the problem can be solved and effects can be obtained, the configuration with the constituent elements deleted can be extracted as an invention.

In the above-described embodiment, it is assumed that the target design external force is a large earthquake. In that case, it is necessary to use values corresponding to the external force setting for the pile resistance, compressive strength, tensile strength, and bending strength.

Further, in the above-described embodiment, the case where the present invention is applied to the pile foundation 10 using the embedded pile in which the pile 2 connecting the plurality of ready-made piles 21, 22, 23, 24 is buried in the ground has been described. However, the present invention is not limited to this, and can also be applied to pile foundations using cast-in-place concrete piles or driven piles.

In the case of cast-in-place concrete piles and driven-in piles, the structure is different from that of prefabricated piles, which can be clearly divided into sections by connecting multiple piles. For cast-in-place concrete piles and driven piles, it is necessary to appropriately set several sections by providing transition points in strata and changes in the amount of steel material.

1 Pile hole 2, 2L, 2R Pile 2a Pile head 4 Pile periphery 10 Pile foundation 21, 22, 23, 24 Prefabricated pile 100 Building G Ground M M1 to M4...Bending moment NL...Regular axial force -NT...Variable axial force in tensile direction NC...Variable axial force in pushing direction PT1 to PT4...Pulling force PC1 to PC4...Pushing force Rf1 to Rf4 … Circumferential frictional resistance.

Claims (8)

A pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction,
The N pile portions are
a top pile portion having a first cross-sectional strength against a first axial force acting on the pile head of the pile;
The n-th pile portion having a cross-sectional strength against the axial force obtained by subtracting the circumferential frictional resistance force from the ground acting on the circumferential surface of all pile portions from the top to the (n-1)th pile portion from the first axial force. and including
The n is an integer that satisfies 2 ≤ n ≤ N,
pile foundation. The top pile portion has a bending strength against the maximum acting bending moment acting on the top pile portion due to the horizontal load acting on the pile head,
The n-th pile portion is the maximum acting bending acting on the n-th pile portion obtained by considering the ground resistance acting on the peripheral surface of all pile portions from the top to the (n-1)th pile portion. Having bending strength against moment,
The pile foundation according to claim 1. A pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction,
The N pile portions are
The (n−1)th pile portion having cross-sectional strength against the axial force acting on the upper end of the (n−1)th pile portion from the top;
The (n-1)-th pile portion is placed directly under the (n-1)-th pile portion, and the peripheral surface frictional resistance force from the ground acting on the peripheral surface of the (n-1)-th pile portion is the (n-1)-th pile. an nth pile section having a cross-sectional strength against the axial force subtracted from the axial force acting on the top of the section;
The n is an integer that satisfies 2 ≤ n ≤ N,
pile foundation. The (n-1)th pile portion is the maximum acting bending moment acting on the (n-1)th pile portion due to the horizontal load acting on the upper end of the (n-1)th pile portion. It has a bending resistance against
The n-th pile portion has a bending resistance against the maximum acting bending moment acting on the n-th pile portion, which is obtained by considering the ground resistance acting on the peripheral surface of the (n-1)-th pile portion. having
The pile foundation according to claim 3. A method for designing a pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction,
The cross-sectional strength of the n-th pile portion from the top, and the circumferential frictional resistance force from the ground acting on the circumferential surface of all the pile portions from the top to the (n-1)th pile. set to the sectional strength for the axial force subtracted from the force,
The n is an integer that satisfies 2 ≤ n ≤ N,
How to design pile foundations. The bending strength of the n-th pile portion is applied to the n-th pile portion obtained by considering the ground resistance acting on the peripheral surface of all the pile portions from the top to the (n-1)th pile portion. set to the bending strength for the maximum acting bending moment,
The method for designing a pile foundation according to claim 5. A method for designing a pile foundation including piles in which pile portions arranged in each of N sections arranged in the axial direction are connected in the axial direction,
The cross-sectional strength of the n-th pile portion from the top, and the peripheral frictional resistance force from the ground acting on the peripheral surface of the (n-1)-th pile portion from the top is the upper end of the (n-1)-th pile portion. set to the sectional strength against the axial force subtracted from the axial force acting on
The n is an integer that satisfies 2 ≤ n ≤ N,
How to design pile foundations. The bending strength of the n-th pile portion from the top is the maximum acting on the n-th pile portion obtained by considering the ground resistance acting on the peripheral surface of the (n-1)-th pile portion from the top. is set to the bending strength against the applied bending moment of
The method for designing a pile foundation according to claim 7.

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