Steel pipe piles, foundation structures, and construction methods for steel pipe piles

Abstract

The objective is to provide steel pipe piles that, when subjected to external forces such as earthquakes, can stably achieve a sufficient increase in strength with a relatively small amount of deformation after plastic deformation, and that can be expected to efficiently absorb energy on the steel side. [Solution] In the stress-strain relationship of the steel pipe pile, the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the outer surface of the steel pipe pile is in contact with dissimilar materials, and in the stress-strain relationship, the yield point σ y The formula satisfies equation (1), and the tensile strength σ t This satisfies equation (2). [Mathematics 1] TIFF2026168679000013.tif21170

Description

Technical Field

[0001] The present invention relates to a steel pipe pile, a foundation structure, and a construction method for a steel pipe pile.

Background Art

[0002] Steel pipe piles are used as foundation piles for civil engineering structures and building structures. With the increasing severity of earthquakes in recent years, steel pipe piles with excellent seismic performance during large earthquakes are desired. For steel pipe piles that support structures and resist forces acting during large earthquakes, for example, a structure as shown in Non-Patent Document 1 has been adopted.

Prior Art Documents

Non-Patent Document

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the conventional steel pipe pile as shown in the above-mentioned Non-Patent Document 1 has the following problems. When the steel pipe pile shown in the above Non-Patent Document 1 resists forces acting during a large earthquake, there is a problem that the proof stress possessed by the steel material cannot be fully exhibited within the deformation constraints such as displacement, inclination angle, and strain for maintaining the function of the structure supported by the steel pipe pile. That is, in Non-Patent Document 1, the second stiffness after plasticization in the stress-strain relationship of the steel pipe pile is 1 / 1000 to 1 / 100 of the first stiffness, and the increase in stress with respect to strain after plasticization is gradual. Therefore, in order to exert a predetermined proof stress against the force acting on the steel pipe pile during an earthquake, a relatively large deformation of the steel pipe pile is required, which may not satisfy the deformation constraints such as displacement, inclination angle, and strain for maintaining the function of the structure. In addition, there is a problem that the energy burden on the ground side with uncertain properties increases, and efficient energy absorption cannot be achieved on the steel material side with stable performance.

[0005] This invention has been made in view of the above-mentioned problems, and aims to provide a steel pipe pile, a foundation structure, and a method for constructing a steel pipe pile that, when subjected to external forces such as earthquakes, can stably achieve a sufficient increase in strength with a relatively small amount of deformation after the steel material has undergone plastic deformation, and that can be expected to efficiently absorb energy on the steel material side. [Means for solving the problem]

[0006] (1) Embodiment 1 of the steel pipe pile according to the present invention is characterized in that, in the stress-strain relationship of the steel pipe pile, the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the outer surface of the steel pipe pile is in contact with a dissimilar material.

[0007] In the steel pipe pile according to the present invention, the second stiffness in the stress-strain relationship of the steel pipe pile is greater than 1 / 100 of the first stiffness, so a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, the stress-strain relationship of steel pipe piles has a second stiffness greater than 1 / 100th of the first stiffness. This reduces the energy burden on the ground side, which has a high degree of uncertainty in its properties, and allows the stable steel material to efficiently absorb energy, thereby stably supporting the structure.

[0008] (2) Aspect 2 of the present invention is the steel pipe pile of Aspect 1, wherein the yield point σ of the stress-strain relationship is y The formula satisfies equation (1), and the tensile strength σ t It may also be characterized by the fact that it satisfies equation (2).

[0009]

number

[0010] In this invention, the stress-strain relationship of the steel pipe pile is such that the second stiffness is greater than 1 / 100 of the first stiffness, and the yield point σy The formula satisfies equation (1), and the tensile strength σ t By satisfying equation (2), it is possible to reliably expect a sufficient increase in strength with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe piles. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, the stress-strain relationship of steel pipe piles has a second stiffness greater than 1 / 100th of the first stiffness. This reduces the energy burden on the ground side, which has a high degree of uncertainty in its properties, and allows the stable steel material to efficiently absorb energy, thereby stably supporting the structure.

[0011] (3) Embodiment 3 of the present invention may be characterized in that, in the steel pipe pile of Embodiment 2, the above formulas (1) and (2) are satisfied in the state in which plastic deformation for steel pipe forming has been performed.

[0012] In this invention, steel pipe piles may be formed into a tubular shape by plastic deformation of steel plates. By having a predetermined stress-strain relationship after such steel pipe formation, a sufficient increase in strength can be reliably achieved with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, steel pipe piles that support structures often come into contact with dissimilar materials such as the ground on their outer surfaces. However, after plastic deformation for steel pipe formation, i.e., after the steel material has plasticized, it has a predetermined stress-strain relationship. Because the second stiffness is greater than 1 / 100 of the first stiffness, the energy burden on the ground side, which has a large uncertainty in its properties, is suppressed, and the energy is efficiently absorbed by the steel material side, which has stable performance, allowing the structure to be supported more reliably and stably.

[0013] (4) In aspect 4 of the present invention, it is preferable that the steel pipe pile of aspect 3 satisfies formulas (1) and (2) when at least a portion of it is buried in the ground.

[0014] In this invention, when a steel pipe pile is embedded in the ground at least partially, or when it is in service, it is subjected to construction loads and the weight of the structure it supports. However, because it subsequently has a predetermined stress-strain relationship, a sufficient increase in strength can be reliably achieved with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, while steel pipe piles are subjected to construction loads and the self-weight of the supported structure when at least a portion of them are buried in the ground or when they are put into service, they subsequently have a predetermined stress-strain relationship. Because the second stiffness is greater than 1 / 100th of the first stiffness, the energy burden on the ground side, which has a large uncertainty in its properties, is suppressed, and the energy is efficiently absorbed by the steel material side, which has stable performance, allowing the structure to be supported more reliably and stably.

[0015] (5) Embodiment 5 of the present invention is preferably used in the steel pipe pile of Embodiment 3 or Embodiment 4 near the pile head that connects to a structure.

[0016] In this invention, although large stresses are generated near the pile head where it connects to a structure, making it prone to plastic deformation, the steel pipe pile used in that area has a predetermined stress-strain relationship. This allows for a sufficient increase in strength with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, the secondary stiffness being greater than 1 / 100th of the primary stiffness allows for reduced energy burden on the ground side, which has greater uncertainty in its properties, while the stable performance of the steel material efficiently absorbs energy, enabling stable support of the structure.

[0017] (6) Embodiment 6 of the foundation structure according to the present invention may be characterized in that it is constructed using at least a portion of one of the steel pipe piles from Embodiments 1 to 5.

[0018] In this invention, by constructing the foundation structure using steel pipe piles having a predetermined stress-strain relationship in at least a portion of the structure, it is possible to achieve a sufficient increase in strength with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe piles. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, by using at least some steel pipe piles with a predetermined stress-strain relationship to construct the foundation structure, the energy burden on the ground, which has a high degree of uncertainty in its properties, can be suppressed, while the energy can be efficiently absorbed by the steel material, which has stable performance, thereby stably supporting the structure.

[0019] (7) Embodiment 7 of the construction method for steel pipe piles according to the present invention is such that, in the stress-strain relationship of the steel pipe pile after plastic deformation for steel pipe forming, the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the yield point σ y The above equation (1) is satisfied, and the tensile strength σ t The present invention is characterized by comprising the steps of designing such that the above-mentioned formula (2) is satisfied, performing the plastic deformation, and bringing the outer surface of the steel pipe pile into contact with a dissimilar material.

[0020] In this invention, it is possible to construct steel pipe piles in which the second stiffness in the stress-strain relationship is greater than 1 / 100 of the first stiffness. Therefore, a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, after plastic deformation for steel pipe formation, i.e., after the steel material has plasticized, it has a predetermined stress-strain relationship. By constructing steel pipe piles where the second stiffness is greater than 1 / 100 of the first stiffness, the energy burden on the ground side, which has a large uncertainty in its properties, is suppressed, and energy is efficiently absorbed by the steel material side, which has stable performance, thereby enabling more reliable and stable support of the structure.

[0021] (8) Embodiment 8 of the construction method for steel pipe piles according to the present invention is such that, in the stress-strain relationship of the steel pipe pile after plastic deformation for steel pipe forming, the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the yield point σ y The above equation (1) is satisfied, and the tensile strength σ t The present invention is characterized by comprising the steps of: designing the steel pipe pile to satisfy the above-mentioned formula (2); performing the plastic deformation process; burying at least a portion of the steel pipe pile in the ground; and ensuring that the outer surface of the steel pipe pile comes into contact with a different material.

[0022] In this invention, it is possible to construct steel pipe piles in which the second stiffness in the stress-strain relationship of the steel pipe pile is greater than 1 / 100 of the first stiffness. Furthermore, although the steel pipe pile is subjected to construction loads and the self-weight of the supported structure when at least a portion of it is buried in the ground and when it is put into service, having a predetermined stress-strain relationship afterward allows for a more reliable increase in strength with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, while steel pipe piles are subjected to construction loads and the self-weight of the supported structure when at least a portion of them are buried in the ground or when they are put into service, they subsequently have a predetermined stress-strain relationship. By constructing steel pipe piles in which the second stiffness is greater than 1 / 100 of the first stiffness, the energy burden on the ground side, which has a large uncertainty in its properties, is suppressed, and the energy is efficiently absorbed by the steel material side, which has stable performance, allowing the structure to be supported with greater reliability and stability. [Effects of the Invention]

[0023] According to the steel pipe pile, foundation structure, and construction method of the steel pipe pile of the present invention, when subjected to external forces such as earthquakes, the steel material can undergo plastic deformation and then stably achieve a sufficient increase in strength with a relatively small amount of deformation, and efficient energy absorption can be expected from the steel material. [Brief explanation of the drawing]

[0024] [Figure 1]This figure shows the stress-strain relationship according to an embodiment of the present invention, where (a) is a figure showing an example of a roundhouse type, (b) is a figure showing an example of a bilinear type, (c) is a figure showing an example of a trilinear type, and (d) is a figure showing another example of a trilinear type. [Figure 2] This shows the stress-strain relationship between a conventional steel pipe pile (comparative example) and a steel pipe pile of the present invention (example). [Figure 3] This shows an example of an analytical model for steel pipe piles used in the examination of the embodiments. [Figure 4] This example shows the relationship between the strain of the steel pipe pile at the pile head and the horizontal load at the pile head. [Figure 5] This example shows the relationship between the strain of the steel pipe pile at the pile head and the cumulative energy sharing ratio. [Modes for carrying out the invention]

[0025] Hereinafter, a description of a steel pipe pile, foundation structure, and construction method for a steel pipe pile according to an embodiment of the present invention will be given based on the drawings.

[0026] The steel pipe piles according to this embodiment are intended to be driven into the ground and support structures as part of the foundation structure. The pile head of the steel pipe pile is joined to a part of the foundation structure (footing, etc.). In the stress-strain relationship of the steel pipe pile, the second stiffness E2, which represents the stiffness of the steel pipe pile after plastic deformation, is greater than 1 / 100 of the first stiffness E1, which represents the elastic stiffness, and the outer surface of the steel pipe pile is in contact with dissimilar materials.

[0027] Here, the steel pipe pile may be a plain steel pipe pile, or a pile that partially uses a steel pipe pile, such as a CFT pile (concrete-filled steel pipe pile), a steel pipe soil cement pile, an SC pile (precast concrete pile with outer steel pipe), an SPHC pile, an SPRC pile, an SSC pile, or a steel pipe-wrapped cast-in-place concrete pile.

[0028] The steel pipe pile may have a double-pipe structure. Further, the double-pipe structure may extend over the entire length of the pile or may be partial. The interior of the steel pipe pile may be filled with a filling material such as concrete or soil cement. Being filled with the filling material in this manner allows the strength to be increased efficiently with a relatively small deformation amount within the deformation constraint range.

[0029] In the stress-strain relationship, the steel pipe pile has a yield point σ y that satisfies Formula (1), and a tensile strength σ t that satisfies Formula (2).

[0030] [Formula]

[0031] Further, the steel pipe pile satisfies the above-described Formula (1) and Formula (2) in a state where plastic working for steel pipe forming has been performed.

[0032] Furthermore, the steel pipe pile satisfies the above-described Formula (1) and Formula (2) in a state where at least a part thereof is embedded in the ground. At this time, it is not necessary for all portions of the steel pipe pile to be embedded in the ground, and a protruding portion may be present.

[0033] Here, the structure supported by the steel pipe pile or the foundation structure using the steel pipe pile at least in part may be any of a civil engineering structure, a building structure, and other structures. Examples of civil engineering structures include road bridges, railway bridges, port facilities, airport facilities, and dams. Examples of building structures include government buildings, hospitals, schools, multi-unit residential buildings, high-rise buildings, data centers, factories, warehouses, and detached houses. Examples of other structures include gas holders, power transmission towers, and various types of facilities.

[0034] The steel pipe pile may or may not be coupled to the structure. When the pile and the structure are coupled, the pile head fixity degree may be a pile head fixed condition where rotation is completely constrained, a pile head free condition where rotation is not constrained, or a pile head semi-rigid condition where rotation is partially constrained. The steel pipe piles may be supported as either support piles or friction piles. Furthermore, the steel pipe piles may be either straight piles or inclined piles.

[0035] The dissimilar material in contact with the outer surface of the steel pipe pile may be ground, concrete, soil cement, or other materials. If the dissimilar material in contact with the outer surface of the steel pipe pile is ground, it may be sandy soil or cohesive soil, and it may or may not liquefy during an earthquake. By ensuring that the rigidity of the dissimilar material in contact with the outer surface of the steel pipe pile is less than or equal to the rigidity of the steel pipe pile, plastic deformation occurs efficiently in the steel pipe pile during an earthquake, allowing the stable steel material to absorb a large amount of energy and stably support the structure.

[0036] The plastic deformation for forming the steel pipe can be done by any method, whether cold or hot, and may be any of the following: spiral steel pipe, electric resistance welded steel pipe, UO steel pipe, drawn steel pipe, seamless steel pipe, or bent steel pipe. Spiral steel pipes, cold-worked in a spiral shape, are preferable because they offer excellent freedom in pile diameter during manufacturing. Electric resistance welded steel pipes are preferable because they offer excellent manufacturing efficiency. UO steel pipes are preferable because they offer high dimensional accuracy.

[0037] The method for burying steel pipe piles in the ground, at least partially, is not particularly limited and may include any of the following methods: impact method, vibration method, internal drilling method, steel pipe soil cement pile method, rotary pile method, press-in method, pre-boring method, earth drill method, reverse method, all-casing method, etc.

[0038] When steel pipe piles are embedded in the ground, at least partially, or when they are in service, they are subjected to construction loads and the weight of the structure they support. However, in the subsequent stress-strain relationship, the second stiffness E2 is greater than 1 / 100 of the first stiffness E1. Because the outer surface is in contact with a dissimilar material, when the steel pipe pile undergoes plastic deformation due to an earthquake or the like, a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the supporting structure. Since efficient energy absorption can be expected on the steel side, the structural support function of the steel pipe pile can be stably satisfied.

[0039] In the construction method for steel pipe piles described above, in the stress-strain relationship of the steel pipe pile after plastic deformation for steel pipe forming, the second stiffness, which represents the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which represents the elastic stiffness, and the yield point σ y The above equation (1) is satisfied, and the tensile strength σ t The process includes designing the structure to satisfy the above-mentioned formula (2), performing plastic deformation, burying at least a portion of the steel pipe pile in the ground, and ensuring that the outer surface of the steel pipe pile comes into contact with a dissimilar material. The step of burying at least a portion of the steel pipe pile in the ground can be omitted.

[0040] Next, we will explain the area around the pile head that connects to the structure. While large stresses and plastic deformation are likely to occur near the pile head where it connects to a structure, if the steel pipe pile used in that section has a predetermined stress-strain relationship, a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile.

[0041] The pile head area, which is the connection point between the structure and the pile, is prone to plastic deformation due to large stresses during earthquakes. Therefore, by using the steel pipe pile of the present invention near the pile head, a stable and sufficient increase in strength can be expected with a relatively small amount of deformation, and efficient energy absorption can be expected on the steel side. Bending moments are likely to occur during earthquakes in the portion of the pile where the distance from the pile head in the depth direction is shallower than 3 / β. With the steel pipe pile of this embodiment, a stable and sufficient increase in strength can be expected with a relatively small amount of deformation after the steel material has undergone plastic deformation, and efficient energy absorption can be expected on the steel side, which is preferable. In particular, the portion of the pile where the distance from the pile head in the depth direction is shallower than 1 / β often experiences large bending moments during earthquakes, so using the steel pipe pile of this embodiment is preferable because a stable and sufficient increase in strength can be expected with a relatively small amount of deformation after the steel material has undergone plastic deformation, and efficient energy absorption can be expected on the steel side. Here, β is a characteristic value of the pile and is defined by formula (3).

[0042]

number

[0043] Here, in equation (3), D is the pile diameter (mm), k h The horizontal ground reaction coefficient (N / mm²) 3 ), E p The Young's modulus (N / mm²) of steel pipe piles is 2 ), I p This is the second moment of area (mm) of the steel pipe pile. 4 ) However, different unit systems are acceptable as long as the units are consistent across the constituent indices.

[0044] Horizontal ground reaction coefficient k h In calculating this, publicly known methods can be used. For example, the Architectural Institute of Japan's "Guidelines for the Design of Building Foundation Structures" (Architectural Institute of Japan: Guidelines for the Design of Building Foundation Structures, pp. 178-283, 2019), the Japan Road Association's "Specifications for Road Bridges and Commentary IV: Substructures" (Japan Road Association: Specifications for Road Bridges and Commentary IV: Substructures, pp. 226-316, 2017), the Railway Technical Research Institute's "Design Standards for Railway Structures and Others: Foundation Structures" (Railway Technical Research Institute: Design Standards for Railway Structures and Others: Foundation Structures, pp. 247-330, 2012), the Japan Port and Harbor Association's "Technical Standards for Port Facilities and Others (Volume 2)" (Japan Port and Harbor Association: Technical Standards for Port Facilities and Others (Volume 2), pp. 675-744, 2018), and Francis's formula (Francis, AJ: Analysis of Examples include "Pile Groups with Flexural Resistance," Journal of the Soil Mechanics and Foundations Division, ASCE, Vol.90, Issue 3, pp.10-32, 1964; the Kishida-Nakai method (Hideaki Kishida, Shoichi Nakai: Horizontal Resistance of Piles Considering Soil Failure, Transactions of the Architectural Institute of Japan, No. 281, pp.41-55, 1979.7); and methods presented by various organizations (Ministry of Land, Infrastructure, Transport and Tourism, Japan Society of Civil Engineers, Japanese Geotechnical Society, etc.).

[0045] Next, we will explain the constraints on deformation, such as displacement, tilt angle, and strain, necessary to maintain the function of the structure. To maintain the functionality of a structure, constraints on deformation such as displacement, inclination angle, and strain of steel pipe piles or foundation structures using steel pipe piles in part are set according to the required performance of the structure. The required performance varies depending on the structure, but is often determined from the perspective of protecting human life, continued use, and reparability. The constraints on deformation of steel pipe piles or foundation structures using steel pipe piles in part are indicated in various standards and guidelines corresponding to each structure.

[0046] For example, for building structures, deformation constraints are indicated in publications such as the Architectural Institute of Japan's "Guidelines for the Design of Building Foundation Structures" (Architectural Institute of Japan: Guidelines for the Design of Building Foundation Structures, 2019) and "Strength and Deformation Performance of Foundation Members" (Architectural Institute of Japan: Strength and Deformation Performance of Foundation Members, 2022); for road bridges, a civil engineering structure, the Japan Road Association's "Specifications for Road Bridges and Commentary IV: Substructures" (Japan Road Association: Specifications for Road Bridges and Commentary IV: Substructures, 2017); for railway bridges, the Railway Technical Research Institute's "Design Standards and Commentary for Railway Structures, etc.: Foundation Structures" (Railway Technical Research Institute: Design Standards and Commentary for Railway Structures, etc.: Foundation Structures, 2012); and for port facilities, the Japan Port and Harbor Association's "Technical Standards and Commentary for Port Facilities" (Japan Port and Harbor Association: Technical Standards and Commentary for Port Facilities, 2018). For other structures as well, deformation constraints are specified in various standards and guidelines for each type of structure. Furthermore, deformation constraints such as displacement, inclination angle, and strain for steel pipe piles or foundation structures that use steel pipe piles in at least part may be set individually and independently.

[0047] One constraint on the deformation of steel pipe piles is to limit the deformation to a level that does not compromise the axial force-holding performance required to support the structure's own weight. As a guideline for the amount of deformation that does not compromise the axial force-holding performance of the steel pipe pile, it is desirable to limit the deformation to below the critical strain (usually around a few percent strain) or critical curvature if the load-bearing capacity is reduced due to local buckling, and below the fracture strain (usually around 10% to 30% strain) if the load-bearing capacity is reduced due to fracture. Such deformation constraints may be imposed. For example, critical strain ε u This value is sometimes given by equation (4) as a conservative estimate of the occurrence of local buckling. In equation (4), t is the plate thickness (mm) and D is the outer diameter of the steel pipe pile (mm). Critical curvature φ u This can sometimes be given by equation (5), assuming plane stability. Furthermore, as a constraint on deformation of a foundation structure using steel pipe piles in at least part, constraints on deformation may be imposed by, for example, the rotation angle of the foundation structure (7 / 1000 or less, 3 / 100 or less, etc.), the amount of horizontal displacement, the amount of vertical displacement, the position and number of plastic hinges, etc., and these are determined according to the required performance of the structure.

[0048]

number

[0049] Figure 1 shows stress-strain relationships, where (a) is an example of a roundhouse type, (b) is an example of a bilinear type, (c) is an example of a trilinear type, and (d) is another example of a trilinear type. Because steel pipe piles often undergo plastic deformation during the manufacturing process, they generally have a roundhouse-type stress-strain relationship, as shown in Figure 1(a), which does not have a clearly defined yield plate. Yield point σ yThis represents the strength at yield. As shown in Figure 1(a), in the case of a roundhouse type stress-strain relationship, the yield point is often defined by the 0.2% offset proof strength, etc., but it can also be defined by the elastic limit strength, the general yield method, or the slope factor method. When using the slope factor method, the yield point is defined as the point where the gradient of the stress-strain relationship becomes 1 / 3 of the initial gradient.

[0050] Tensile strength σ t This refers to the maximum strength that can be achieved before fracture under tensile force. The yield ratio is the ratio of the yield point to the tensile strength σ y / σ t Therefore, a smaller yield ratio is preferable because it allows for a greater increase in strength after plastic deformation.

[0051] The first stiffness E1 refers to the elastic stiffness (Young's modulus). The first stiffness E1 is determined using the common value for carbon steel, which is 205,000 N / mm². 2 or 200,000 N / mm 2 It may be a representative value shown in various standards and regulations, or it may be secant stiffness (the slope of the line from the origin to a certain point in the stress-strain relationship), or tangential stiffness (the slope of the tangent at a certain point in the stress-strain relationship), or it may be the average stiffness within a certain range of the elastic range, and can be selected as appropriate.

[0052] The second stiffness E2 refers to the stiffness after plastic deformation. The second stiffness E2 can be determined as follows: it may be the slope of a straight line connecting the yield point and the tensile strength in the stress-strain relationship; it may be the slope from the yield point onward when the stress-strain relationship is modeled as a bilinear type as shown in Figure 1(b) or a trilinear type as shown in Figures 1(c) and (d), such that the area enclosed by the stress-strain relationship up to a certain strain is equivalent to the area enclosed by the relationship; it may be the tangential stiffness outside the elastic range; it may be the secant stiffness outside the elastic range; or it may be the average stiffness over a certain range outside the elastic range; the choice can be made as appropriate. The stress-strain relationship targeted in this embodiment is not limited to the roundhouse type, but may be any shape, such as a shape having a yield shelf, upper yield point, and lower yield point, or a multi-bend line shape such as a bilinear or trilinear type.

[0053] Work hardening refers to the phenomenon in which the strength and hardness of a material increase during the process of plastic deformation. In this embodiment, a larger work hardening gradient in the stress-strain relationship is preferable. Methods to increase the work hardening gradient include, for example, increasing the amount of additive elements used when manufacturing steel materials, but even in such cases, it is desirable to ensure weldability during steel pipe forming and weldability during longitudinal jointing at construction sites.

[0054] Next, we will describe a steel material in which the second stiffness E2 is greater than 1 / 100th of the first stiffness E1, as described above. One example of a steel material that can exhibit a second stiffness E2 greater than 1 / 100th of the first stiffness E1 is DP (Dual Phase) steel. DP steel has a microstructure consisting of soft ferrite and hard martensite, with both materials well-balanced and dispersed within the steel sheet. The presence of ferrite allows for high elongation at a low yield ratio, while the presence of martensite allows for greater stress, resulting in significant work hardening and thus a high second stiffness E2. Other combinations of ferrite and martensite are also possible as composite structures for DP steel. Among the steel structures created during temperature changes, ferrite is the softest, followed by pearlite and bainite, and martensite is the hardest. Because composite structures can be created by combining these relative differences in properties, combinations such as ferrite and bainite, or ferrite and pearlite are also possible. When manufacturing small-diameter or thick-walled steel pipe piles, it is possible to differentiate the stress-strain relationship of the steel pipe pile by modifying the microstructure. For example, by increasing the amount of ferrite compared to ordinary DP steel to prioritize formability, or by increasing the amount of bainite and martensite compared to ordinary DP steel to further enhance the second stiffness E2 due to work hardening. Furthermore, by adding manganese or silicon to ferrite, or by adding niobium to strengthen it by precipitation, the second stiffness E2 due to work hardening can be increased, and the stress-strain relationship of the steel pipe pile can also be differentiated by such added elements.

[0055] Other steel materials that exhibit a high secondary stiffness E2 due to work hardening include TRIP (Transformation Induced Plasticity) steel. TRIP steel retains approximately 30% to a few percent of austenite that transforms into martensite upon deformation within a bainite or ferrite-bainite matrix. Although TRIP steel does not exhibit as much work hardening in the initial stages of deformation as DP steel, it exhibits work hardening up to the high-strain region and maintains a high secondary stiffness E2. This is due to a microstructural change in which the amount of hard martensite is initially small and increases with deformation, and this is achieved by the work hardening transformation of retained austenite.

[0056] While DP steel and TRIP steel were given as examples, any other steel material is acceptable as long as the second stiffness E2 in the stress-strain relationship of the steel pipe pile is greater than 1 / 100 of the first stiffness E1. This includes STK400 / SKK400, STK490 / SKK490, stainless steel, etc. In the stress-strain relationship of the steel material, the second stiffness E2 is greater than 1 / 100 of the first stiffness E1, and the steel pipe pile, which is in contact with a dissimilar material on its outer surface, can be expected to achieve a sufficient increase in strength with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the supporting structure when it undergoes plastic deformation due to an earthquake, and efficient energy absorption can be expected on the steel side, thus stably satisfying the supporting function of the structure as a steel pipe pile.

[0057] To increase the secondary rigidity due to work hardening, additive elements are sometimes added to steel materials. However, in order to use them as steel pipe piles, it is necessary to ensure weldability during steel pipe forming and weldability during longitudinal jointing at the construction site, and carbon equivalent C eq The smaller the value, the better, preferably 0.6% or less, and even more preferably 0.5% or less. Furthermore, the weld cracking susceptibility composition PCM is also desirable to be as low as possible, preferably 0.3% or less. C eq and P CM These are defined by equations (6) and (7), respectively. However, C eq and P CM Various evaluation formulas have been proposed, and any of them may be used. Here, C is carbon, Mn is manganese, Si is silicon, Ni is nickel, Cr is chromium, Mo is molybdenum, V is vanadium, Cu is copper, and B is boron. eq and P CM In formulas (6) and (7), all units are in percentages.

[0058]

number

[0059] As explained above, with regard to steel pipe piles, the second stiffness in the stress-strain relationship of the steel pipe pile is greater than 1 / 100 of the first stiffness. Therefore, a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of deformation such as displacement, inclination angle, and strain necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, in this embodiment, the steel pipe piles supporting the structure are in contact with dissimilar materials such as the ground on their outer surfaces, and the second stiffness of the stress-strain relationship of the steel pipe piles is greater than 1 / 100 of the first stiffness. This suppresses the energy burden on the ground side, which has a large uncertainty in its properties, and allows the steel material, which has stable performance, to efficiently absorb energy and stably support the structure.

[0060] Furthermore, in the steel pipe pile according to this embodiment, the yield point σ in the stress-strain relationship is y The above equation (1) is satisfied, and the tensile strength σ t This satisfies equation (2). In this configuration, the stress-strain relationship of the steel pipe pile is such that the second stiffness is greater than 1 / 100 of the first stiffness, and the yield point σ y The formula satisfies equation (1), and the tensile strength σ t By satisfying equation (2), it is possible to reliably expect a sufficient increase in strength with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe piles. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, the stress-strain relationship of steel pipe piles has a second stiffness greater than 1 / 100th of the first stiffness. This reduces the energy burden on the ground side, which has a high degree of uncertainty in its properties, and allows the stable steel material to efficiently absorb energy, thereby stably supporting the structure.

[0061] Furthermore, in the steel pipe pile according to this embodiment, the above-mentioned equations (1) and (2) are satisfied in the state after plastic deformation for steel pipe forming has been performed. In this configuration, although steel pipe piles may be formed into a tubular shape by plastic deformation of steel plates, having a predetermined stress-strain relationship after such steel pipe formation allows for a more reliable increase in strength with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe pile. Furthermore, steel pipe piles that support structures often come into contact with dissimilar materials such as the ground on their outer surfaces. However, after plastic deformation for steel pipe formation, i.e., after the steel material has plasticized, it has a predetermined stress-strain relationship, and the second stiffness is greater than 1 / 100 of the first stiffness. This reduces the energy burden on the ground side, which has a large uncertainty in its properties, and allows the stable steel material to efficiently absorb energy, enabling more reliable and stable support of the structure.

[0062] Furthermore, in the steel pipe pile according to this embodiment, equations (1) and (2) described above are satisfied when at least a portion of it is buried in the ground. With this configuration, the steel pipe piles are subjected to construction loads and the weight of the supporting structure when at least a portion of them are embedded in the ground or when they are put into service. However, because they subsequently have a predetermined stress-strain relationship, a sufficient increase in strength can be reliably expected with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe piles. Furthermore, while steel pipe piles are subjected to construction loads and the weight of the supported structure when at least a portion of them are embedded in the ground or when they are put into service, they subsequently have a predetermined stress-strain relationship, and the second stiffness is greater than 1 / 100 of the first stiffness. This reduces the energy burden on the ground side, which has a large uncertainty in its properties, and allows the stable steel material to efficiently absorb energy, enabling the structure to be supported more reliably and stably.

[0063] Furthermore, the steel pipe pile according to this embodiment is used near the pile head where it connects to a structure. With this configuration, large stresses are generated near the pile heads where they connect to the structure, making them prone to plastic deformation. However, because the steel pipe piles used in that section have a predetermined stress-strain relationship, a sufficient increase in strength can be expected with a relatively small amount of deformation within the constraints of displacement, inclination angle, strain, etc., necessary to maintain the function of the structure supported by the steel pipe piles. Furthermore, while steel pipe piles supporting structures often come into contact with dissimilar materials such as the ground on their outer surfaces, the secondary stiffness being greater than 1 / 100th of the primary stiffness allows for reduced energy burden on the ground side, which has greater uncertainty in its properties, while the stable performance of the steel material efficiently absorbs energy, enabling stable support of the structure.

[0064] Next, an example of a steel pipe pile implemented to support the effects of the above-described embodiment will be explained below.

[0065] (Examples) In this embodiment, the effectiveness of the steel pipe pile of this embodiment was confirmed by comparing it with a conventional steel pipe pile. In this embodiment, we examined a case where a steel pipe pile with standard conditions—a pile diameter D of 1200 mm, a plate thickness t of 24 mm, and a diameter-to-thickness ratio D / t of 50—is embedded in a site with an N-value of 5 and used to support a structure. In this case, the dissimilar material in contact with the outer surface of the steel pipe pile is the ground. Although this embodiment considers a single pile, it may also consist of multiple piles.

[0066] Figure 2 shows the stress (N / mm²) of steel pipe piles in a conventional comparative example and in an example. 2 This shows the stress-strain (%) relationship. As an example, we assume a bilinear stress-strain relationship (see Figure 1(b)) which is commonly used in design. The conventional steel pipe pile is SKK490, and the yield point σy = 325 N / mm 2 , tensile strength σ t = 490 N / mm 2 Above, first rigidity E1=205,000N / mm 2 , second stiffness E2=E1 / 100=2,050N / mm 2 The stress-strain relationship is as follows. The steel pipe pile in the example is 590DP, and the yield point σy = 325 N / mm 2 , tensile strength σ t = 590 N / mm 2 Above, first rigidity E1=205,000N / mm 2 , second stiffness E2=E1 / 10=20,500N / mm 2 Let the following be the stress-strain relationship.

[0067] When designing for earthquake resistance against major earthquakes, the deformation of steel pipe piles is sometimes constrained to a level where the axial force-holding performance that supports the self-weight of the structure is not lost. In this case, the deformation constraint is set such that the strain generated in the steel pipe pile is defined as the limit strain ε. u The following points should be noted. Here, the critical strain ε u ε is a value that conservatively evaluates the occurrence of local buckling in steel pipe piles, u It may be given as =0.44t / D (where t is the plate thickness and D is the pile diameter). Based on the current pile specifications, the limit strain ε u This results in 0.88%, meaning that the design aims to keep the strain below 0.88%.

[0068] Figure 3 shows the analysis model 1 of the steel pipe pile 10 used in the study of this embodiment. The analysis used the analysis model 1 shown in Figure 3, and was performed using a static pushover analysis in which the horizontal force at the pile head 10a of the steel pipe pile 10 (arrow F shown in Figure 3) was gradually increased. The steel pipe pile 10 was modeled as a beam element 11 (fiber model), and the ground as a spring element 12. For the steel pipe piles of the comparative example and the example, the stress-strain relationships shown in Figure 2 were applied to the fiber model, respectively. The steel pipe pile 10 was divided into sections with nodes at 200 mm intervals in the depth direction, and a ground spring 12 was connected to each node. The pile length L of the steel pipe pile 10 was set to a sufficiently long 100 m to simulate the conditions of a semi-infinite long pile. The pile head 10a was set as a fixed pile head condition, and the lower end 10b was set as a boundary condition using a horizontal pin roller 13.

[0069] Further details were determined by referring to standards and papers such as "Guidelines for the Design of Building Foundation Structures" (Architectural Institute of Japan, 2019) and "Strength and Deformation Performance of Foundation Members" (Architectural Institute of Japan, 2022). The relationship between horizontal ground reaction force and horizontal displacement of a ground spring, and the degree of plastic horizontal ground reaction force, are evaluated using the method proposed by Kishida and Nakai (Hideaki Kishida, Shoichi Nakai: Horizontal resistance of piles considering ground failure, Transactions of the Architectural Institute of Japan, No. 281, pp. 41-55, 1979.7). This is based on the horizontal ground reaction coefficient k, which is the initial stiffness of the horizontal ground reaction force p. h This is given by equation (8) proposed by Francis (Francis, AJ: Analysis of Pile Groups with Flexural Resistance, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol.90, Issue 3, pp.10-32, 1964), and the plastic horizontal ground reaction force p y This is a bilinear evaluation method where the point where the deformation modulus of the ground is reached is defined as the inflection point. s This is evaluated using formula (9). Plastic horizontal ground reaction force p y As shown in equation (10), the values ​​of equations (11) and (12) are compared at each depth and the smaller value is adopted. The internal friction angle φ of the ground is evaluated using Osaki's formula shown in equation (13) (Osaki, Yoshihiko: "Tokyo Ground Map", Gihodo Publishing, pp. 18-19, 1959). The earth pressure at rest coefficient K0 is evaluated using Yerkee's formula shown in equation (14).

[0070]

number

[0071]

number

[0072]

number

[0073]

number

[0074] Here, k h : Horizontal ground reaction coefficient (kN / m 3 ), E s : Modulus of deformation of the ground (kN / m 2 ), D: Outer diameter of steel pipe pile (m), N: N-value of the ground, E p : Young's modulus of steel pipe piles (kN / m 2 ), I p : Second moment of area of ​​steel pipe pile (m 4 ν: Poisson's ratio of the ground (assumed to be 0.33 for sandy soil), p: horizontal ground reaction force (kN / m 2 ), p y :Plastic horizontal ground reaction force (kN / m 2 ), γ': Effective unit weight of the ground (kN / m 3 ), z: depth (m), K0: coefficient of earth pressure at rest (=1-sinφ), K A : Active earth pressure coefficient (=(1-sinφ) / (1+sinφ)), φ: internal friction angle (°), α, β: wedge angle (for sandy soil, α=φ / 2, β=π / 4+φ / 2).

[0075] As shown in Figure 2, the analysis results show that, up to a deformation constraint strain of 0.88%, neither the conventional steel pipe pile in the comparative example nor the steel pipe pile of the present invention in the example can utilize their full tensile strength. However, in the case of the steel pipe pile in the comparative example, since the second stiffness E2 is E1 / 100, the tensile strength is 339.8 N / mm up to a strain of 0.88%. 2 The stress exerted is limited to this. In the case of the steel pipe pile in the example, the second stiffness E2 is E1 / 10, so up to a strain of 0.88%, it is 472.9 N / mm 2 It can exert stress. Thus, the steel pipe piles of this embodiment can achieve a stable and sufficient increase in strength with a relatively small amount of deformation after plastic deformation.

[0076] Figure 4 shows the relationship between the strain of the steel pipe pile at the pile head and the horizontal load at the pile head. In Figure 4, the horizontal axis represents the strain (%) of the steel pipe pile at the pile head, and the vertical axis represents the horizontal load (kN) at the pile head. The solid line represents the steel pipe pile of the example, and the dashed line represents the steel pipe pile of the comparative example. As shown in Figure 4, in the case of the comparative example steel pipe pile, the second stiffness E2 is E1 / 100, so the horizontal resistance force exerted is limited to 2713kN up to a strain of 0.88%. In the case of the steel pipe pile of the example, the second stiffness E2 is E1 / 10, so the horizontal resistance force can be exerted as much as 3524kN up to a strain of 0.88%. Thus, in the steel pipe pile of the example, a stable and sufficient increase in horizontal load can be expected with a relatively small amount of deformation after plastic deformation, and it can efficiently resist the horizontal forces of an earthquake.

[0077] Figure 5 shows the relationship between the strain of the steel pipe pile at the pile head and the cumulative energy sharing ratio. In Figure 5, the horizontal axis represents the strain (%) of the steel pipe pile at the pile head, and the vertical axis represents the cumulative energy sharing ratio (%). The solid line shows the energy absorption on the ground side and the steel pipe pile side in the case of the steel pipe pile of the example, and the dashed line shows the energy absorption on the ground side and the steel pipe pile side in the case of the steel pipe pile of the comparative example. As shown in Figure 5, in the case of the steel pipe pile of the comparative example, the second stiffness E2 is E1 / 100, so up to a strain of 0.88%, the energy absorption on the steel pipe pile side is 39.7% and the energy absorption on the ground side is 60.3%. In the case of the steel pipe pile of the example, the second stiffness E2 is E1 / 10, so up to a strain of 0.88%, the energy absorption on the steel pipe pile side is 43.2% and the energy absorption on the ground side is 56.8%, meaning that a larger proportion of the energy can be absorbed on the steel pipe pile side. Thus, in the steel pipe piles of this embodiment, the energy burden on the ground side, which has a large uncertainty in its properties, is suppressed, and the energy is efficiently absorbed by the steel material, which has stable performance, thereby stably supporting the structure.

[0078] The embodiments of the steel pipe pile, foundation structure, and construction method for the steel pipe pile according to the present invention have been described above. However, the present invention is not limited to the embodiments described above, and it is possible to replace the components in the embodiments described above with well-known components as appropriate, without departing from the spirit of the present invention.

[0079] For example, in this embodiment, in the stress-strain relationship, the yield point σ y The above equation (1) is satisfied, and the tensile strength σ t While the company employs steel pipe piles that satisfy equation (2), it is not limited to those that satisfy equations (1) and (2).

[0080] Furthermore, the steel pipe pile is not limited to being a steel pipe pile that has undergone plastic deformation for steel pipe forming, where the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and satisfies the above-mentioned equations (1) and (2).

[0081] Furthermore, the steel pipe pile is not limited to being embedded in the ground to the extent that at least a portion of it is embedded, and that the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and that it satisfies the above-mentioned equations (1) and (2).

[0082] Furthermore, it is not limited to steel pipe piles used near the pile head where they connect to a structure.

[0083] Regarding units, including those shown above, different unit systems are acceptable as long as they are consistent across the constituent indices. For example, regarding the unit "N / mm 2 This includes not only unit systems like "MPa" but also unifications like "mm" and "m".

Claims

1. A steel pipe pile characterized in that, in the stress-strain relationship of the steel pipe pile, the second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the outer surface of the steel pipe pile is in contact with a dissimilar material.

2. In the aforementioned stress-strain relationship, Yield point σ ｙ This satisfies equation (1), Tensile strength σ ｔ The steel pipe pile according to claim 1, wherein the formula (2) is satisfied. [Math 1]

3. The steel pipe pile according to claim 2, wherein the formulas (1) and (2) are satisfied in a state in which plastic deformation for steel pipe forming has been performed.

4. The steel pipe pile according to claim 3, wherein at least a portion of it is buried in the ground and satisfies formulas (1) and (2).

5. A steel pipe pile according to claim 3, used near the pile head where it connects to a structure.

6. A foundation structure characterized by being constructed using at least a portion of the steel pipe piles described in any one of claims 1 to 5.

7. In the stress-strain relationship of steel pipe piles after plastic deformation for steel pipe forming, The second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the yield point σ ｙ The formula satisfies equation (1), and the tensile strength σ ｔ The process of designing such that equation (2) is satisfied, The process of performing the aforementioned plastic deformation, The process involves the outer surface of the steel pipe pile coming into contact with a different material, A method for constructing steel pipe piles, characterized by having [a certain feature]. [Math 2]

8. In the stress-strain relationship of steel pipe piles after plastic deformation for steel pipe forming, The second stiffness, which refers to the stiffness after plastic deformation, is greater than 1 / 100 of the first stiffness, which refers to the elastic stiffness, and the yield point σ ｙ The formula satisfies equation (1), and the tensile strength σ ｔ The process of designing such that equation (2) is satisfied, The process of performing the aforementioned plastic deformation, A step of burying at least a portion of the steel pipe pile in the ground, The process involves the outer surface of the steel pipe pile coming into contact with a different material, A method for constructing steel pipe piles, characterized by having [a certain feature]. [Math 3]

Images