Foundation structure

Abstract

To provide a foundation structure capable of securing cross-sectional performance of a foundation even when a diameter of a reinforcement cage is reduced.SOLUTION: A transmission line tower 1 is supported in a gate-shape manner by a plurality of caisson type pile foundations 2. In concrete Con of the caisson type pile foundation 2, a reinforcement cage 3 is embedded. The reinforcement cage 3 is arranged eccentrically in the direction being separated from the center of gravity G of the transmission line tower 1 from a flat surface center c of the caisson type pile foundation 2. The reinforcement cage 3 is a telescopic type in which, for example, a hoop 32 and an axial direction steel 31 which is a flexible strand are combined so that an intersection angle becomes variable.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to the foundation structure of structures such as transmission towers.

Background Art

[0002] As a foundation structure of a transmission tower in a mountainous area or the like, when the supporting layer is deep or when it is desired to increase the pull-out resistance, generally a deep foundation is adopted (for example, Patent Document 1, etc.).

[0003] As shown in FIG. 7, the transmission tower 1 is supported in a portal shape by a plurality of columnar deep foundations 20, and a steel bar cage is embedded in the concrete of each deep foundation 20. A compressive force due to the dead load always acts on the deep foundation 20. The dead load is, for example, the weight of the transmission tower and electric wires.

[0004] The transmission tower 1 is strongly affected by the wind load W due to a typhoon or the like, and the design external force of the deep foundation 20 is dominated by the wind load W. When the wind load W is received from the normal state where it can be assumed that only compressive force is generated in the deep foundation 20, in the deep foundation 20 on the windward side, a pulling force T newly acts and at the same time an inward bending moment M is generated, and in the deep foundation 20 on the leeward side, a compressive force C newly acts and at the same time an outward bending moment M is generated.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Deep foundation is designed considering the sectional forces (axial force, bending moment, etc.) as described above. For example, the diameter of the deep foundation (hereinafter referred to as the deep foundation diameter) is determined from the aspects of ensuring weight and peripheral frictional force against the above-mentioned uplift force, and usually, the deep foundation diameter cannot be reduced further. The diameter of the steel reinforcement cage is reduced by the cover of the axial steel bars of the steel reinforcement cage from the aspect of ensuring durability. When the deep foundation diameter is 3,000 mm and the cover of the axial steel bars is 250 mm, the diameter of the steel reinforcement cage becomes 2,500 mm.

[0007] From the perspective of constructability, it is desirable to make the diameter of the steel reinforcement cage as small as possible if possible. By reducing the diameter of the steel reinforcement cage, the weight of the steel reinforcement cage is reduced and the construction becomes easier. In particular, the telescopic steel reinforcement cage described in Patent Document 1 is prefabricated at a factory or the like, loaded onto a large truck, and transported to the site. Therefore, there is also a need in terms of transportation to reduce the diameter of the steel reinforcement cage so that it can be accommodated within the width of a large truck (generally about 2,300 mm). On the other hand, when the diameter of the steel reinforcement cage is reduced, the sectional performance of the foundation generally decreases, and the reduction of the steel reinforcement cage needs to be carried out on the premise of ensuring the sectional performance.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a foundation structure or the like that can ensure the sectional performance of the foundation even when the diameter of the steel reinforcement cage is reduced.

Means for Solving the Problems

[0009] The present invention for solving the above-mentioned problems is a foundation structure of a structure, wherein the structure is supported in a portal shape by a plurality of columnar foundations, a steel reinforcement cage is embedded in the concrete of the foundation, and the steel reinforcement cage is eccentrically arranged in a direction away from the center of gravity of the structure from the plane center of the foundation.

[0010] In the present invention, in response to the characteristics of the cross-sectional performance of the foundation described later, that is, briefly speaking, in the foundation on the leeward side, there is a margin in the bearing capacity, in order to increase the bearing capacity when the foundation becomes the windward side, the reinforcing cage is eccentrically arranged outward (in the direction away from the center of gravity of the structure) as described above. Although the bearing capacity decreases when the foundation becomes the leeward side, as described above, originally there is a margin in the bearing capacity in the foundation on the leeward side, so the cross-sectional performance of the foundation is still determined by the bearing capacity when the foundation becomes the windward side. Thus, in the present invention, the cross-sectional performance of the foundation can be improved by the reasonable arrangement of the reinforcing cage, and as a result, it becomes possible to reduce the diameter of the reinforcing cage while ensuring the required cross-sectional performance.

[0011] The reinforcing cage is, for example, an expandable and contractible reinforcing cage in which a strip bar and a flexible strand are combined so that the crossing angle is variable. By reducing the diameter of the expandable and contractible reinforcing cage, it becomes easy to store the reinforcing cage prefabricated in a factory or the like in the cargo area of a large truck, which is preferable in terms of transporting the reinforcing cage.

[0012] In the present invention, the four foundations are respectively arranged at positions corresponding to the vertices of a rectangle in plan view, and the reinforcing cages of the respective foundations are eccentrically arranged in directions inclined with respect to both of the two orthogonal sides of the rectangle. Thereby, it becomes possible to obtain the effect of the present invention with respect to the wind loads applied from all directions.

[0013] The structure is, for example, a transmission tower. Thereby, the present invention can be applied to the foundation of a transmission tower. Since the foundation of a transmission tower often becomes large as a deep foundation, the present invention is particularly effective.

Effects of the Invention

[0014] According to the present invention, it is possible to provide a foundation structure or the like that can ensure the cross-sectional performance of the foundation even when the diameter of the reinforcing cage is reduced.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

[0017] FIG. 1 is a diagram showing an outline of the deep foundation 2 having the foundation structure according to the embodiment of the present invention and the power transmission tower 1 supported thereby. The power transmission tower 1 is a tower-shaped structure formed in a truss shape by steel frame members, and its legs are supported in a portal shape by a plurality of deep foundations 2 provided on the ground.

[0018] FIG. 2(a) is a diagram showing the inside of the deep foundation 2. The deep foundation 2 is a columnar foundation made of reinforced concrete, and a steel cage 3 is embedded in the concrete Con. Note that FIG. 2(b) shows a horizontal cross-section taken along the line a-a of FIG. 2(a).

[0019] The steel cage 3 is a cylindrical reinforcing member having axial steel bars 31 and hoop bars 32. The axial steel bars 31 are vertical steel bars, and a plurality of them are installed at intervals in the circumferential direction of the steel cage 3. The hoop bars 32 are steel bars arranged in an annular shape in plan view so as to surround the axial steel bars 31, and a plurality of stages are provided at intervals in the vertical direction.

[0020] In this embodiment, the steel reinforcement cage 3 is a telescopic steel reinforcement cage. The telescopic steel reinforcement cage 3 uses strands (wires thinner than PC steel) with flexibility instead of ordinary deformed bars as the axial steel bars 31, and the axial steel bars 31 and the stirrups 32 are joined by a joining member (not shown) at their intersection in the vertical plane so that the intersection angle between the axial steel bars 31 and the stirrups 32 can be varied. Therefore, by twisting the upper end of the steel reinforcement cage 3 as shown by the arrow A in Fig. 3(a), the steel reinforcement cage 3 contracts as shown in Figs. 3(b) and (c). On the other hand, as shown by the arrow B in Fig. 3(c), when the upper end of the steel reinforcement cage 3 is rotated in the direction opposite to the arrow A, the steel reinforcement cage 3 expands.

[0021] The telescopic steel reinforcement cage 3 is prefabricated in a factory or the like, loaded onto a large truck in a contracted state, and transported to the site. This steel reinforcement cage 3 is expanded at the construction site of the deep foundation 2 and placed in the excavation hole of the ground, and concrete Con is placed in the excavation hole, whereby the deep foundation 2 is constructed.

[0022] As shown in Fig. 2(a), in this embodiment, the steel reinforcement cage 3 is eccentrically arranged within the deep foundation 2. That is, the steel reinforcement cage 3 is eccentrically arranged in a direction away from the center of gravity G of the transmission tower 1, i.e., outward, from the plane center c of the deep foundation 2. Also, the diameter of the steel reinforcement cage 3 is smaller than the value obtained by subtracting the required cover for the axial steel bars 31 from the deep foundation diameter. When the deep foundation diameter is 3,000 mm and the required cover for the axial steel bars 31 is 250 mm, the diameter of the steel reinforcement cage 3 is set to a value smaller than 2,500 mm, for example, 2,000 mm. Therefore, even including the packaging of the cage and the lifting fixtures, etc., it can be fully loaded within the loading capacity of a large truck.

[0023] The deep foundation 2 can be assumed to be in a state where only the compressive force due to the dead load acts at all times. However, when the transmission tower 1 receives the wind load from one direction D1, as described above, in the deep foundation 2 on the windward side (the left deep foundation 2), a tensile force newly acts, reducing the compressive force and simultaneously generating an inward bending moment M. In the deep foundation 2 on the leeward side, the compressive force increases due to the newly acting compressive force, and at the same time, an outward bending moment M is generated. "Inward" means the direction approaching the center of gravity G of the transmission tower 1, and "outward" means the direction away from the center of gravity G of the transmission tower 1.

[0024] Figures 4(a) and (b) show the yield line indicating the relationship between the axial force N and the bending moment M at the yield of the cross-section of the deep foundation 2 (more specifically, the yield of the axial steel 31) on a graph with the values of the axial force N and the bending moment M as two orthogonal axes. The axis indicating the value of the axial force N is called the N-axis, and the axis indicating the value of the bending moment M is called the M-axis.

[0025] The yield line indicated by the broken line in Figure 4(a) shows the relationship between the axial force N and the bending moment M when the center of the plane of the steel cage 3 is aligned with the center c of the plane of the deep foundation 2 (referred to as "central arrangement"). As the tensile force, that is, the tensile force, increases, the absolute value of the bending moment M decreases (yielding at a smaller bending moment M), and as the compressive force increases, the absolute value of the bending moment M increases (yielding at a larger bending moment M). Also, whether the bending moment M is inward or outward, there is no change in the absolute value of the bending moment M at the time of yield, and the yield line is symmetric with respect to the N-axis.

[0026] The point P on the graph indicates the sectional force of the deep foundation 2 at all times. As described above, a compressive force due to the dead load is constantly applied to the deep foundation 2. When the transmission tower 1 receives the wind load from one direction D1 (see Figure 2(a)), in the deep foundation 2 on the windward side, a tensile force acts, reducing the compressive force and generating an inward bending moment M. On the other hand, in the deep foundation 2 on the leeward side, the compressive force increases and an outward bending moment M is generated. The arrow extending from point P to the lower left indicates the former, and the arrow extending from point P to the upper right indicates the latter.

[0027] There is no difference in the changes in the axial force N and bending moment M of the deep foundation 2 (the increase in the axial force N and bending moment M per unit increase in wind load) ΔN and ΔM between the upwind side and the downwind side as the wind load increases. Therefore, from Fig. 4(a), when the wind load increases, the upwind deep foundation 2 reaches the yield line first. The sectional force of the deep foundation 2 at this time is shown by point P1 in Fig. 4(a). On the other hand, the sectional force generated in the downwind deep foundation 2 under the same wind load is shown by point P2 in Fig. 4(a), and there is a margin with respect to the yield line due to the outward bending moment M.

[0028] In this embodiment, by arranging the steel cage 3 eccentrically, as shown by the arrow in Fig. 4(b), the yield line (shown by the solid line) of the deep foundation 2 can be shifted in the direction where the inward bending moment M increases and the outward bending moment M decreases (the left side of the graph) compared to the case where the steel cage 3 is arranged at the center. As a result, the axial force N and bending moment M of the upwind deep foundation 2 will not reach the yield line unless the wind load becomes larger. The sectional force of the upwind deep foundation 2 at this time is shown by point P1' in Fig. 4(b), and the deep foundation 2 has a structure that can withstand a larger wind load. Note that the sectional force generated in the downwind deep foundation 2 under the same wind load is shown by point P2' in Fig. 4(b). Although the margin with respect to the yield line due to the outward bending moment M has become smaller, it still has not reached the yield line.

[0029] Even if the direction in which the transmission tower 1 receives the wind load becomes the direction D2 (see Fig. 2(a)) opposite to the previous one, only the upwind deep foundation 2 and the downwind deep foundation 2 are interchanged, and the effect obtained by the eccentric arrangement of the steel cage 3 remains unchanged.

[0030] When viewed in a plane, the power transmission tower 1 usually has four legs. As shown in Fig. 5, the deep foundation 2 has a total of four provided at positions corresponding to each leg and hitting the vertices of a rectangle in plan view. The steel reinforcement cage 3 is arranged eccentrically outward from the plane center c of each deep foundation 2. However, the eccentric direction b of the steel reinforcement cage 3 is inclined with respect to both of the two orthogonal sides of the rectangle so as to be able to cope with the wind loads applied from all directions. If the rectangle is a square, the angles α and β formed by the eccentric direction b of the steel reinforcement cage 3 and the two orthogonal sides of the rectangle are 45°.

[0031] Fig. 6(a) is a graph showing the results of actually analyzing the effect of the eccentric arrangement of the steel reinforcement cage 3. As the analysis conditions, the deep foundation diameter was 3,000 mm, the design standard strength of the concrete Con was 30 MPa, and the number of axial steel bars 31 (PC steel wire with a cross-sectional area of 277.1 mm 2 , tensile strength of 1,860 N / mm 2 in the wire) of the steel reinforcement cage 3 was 24.

[0032] The analysis targets were six types: [Normal reinforcement], [Reduced diameter reinforcement], [Large eccentric arrangement (+250)], [Large eccentric arrangement (-250)], [Small eccentric arrangement (+177)], and [Small eccentric arrangement (-177)]. The outlines of these are shown in Fig. 6(b).

[0033] [Normal reinforcement] and [Reduced diameter reinforcement] are those in which the diameter of the steel reinforcement cage 3 (based on the position of the axial steel bar 31) is 2,500 mm and 2,000 mm respectively, and these are centrally arranged within the deep foundation 2.

[0034] [Large eccentric arrangement (+250)] and [Large eccentric arrangement (-250)] are those in which the diameter of the steel reinforcement cage 3 is 2,000 mm, and this is eccentrically arranged by shifting 250 mm upward and downward on the windward side and leeward side in the direction D where the wind load is applied within the deep foundation 2, corresponding to the arrangement of the steel reinforcement cage 3 in Fig. 2.

[0035] [Small Eccentric Arrangement (+177)][Small Eccentric Arrangement (-177)] have the same diameter of the steel bar cage 3, which is 2,000 mm, and are eccentrically arranged by shifting 250 mm each to the windward side and the leeward side in the direction inclined 45° from the direction D where the wind load acts in the deep foundation 2. The eccentric distance along the above direction D is 250×1 / 2 1 / 2 = 177 mm. This corresponds to the arrangement of the steel bar cage 3 in Fig. 5.

[0036] From the graph in Fig. 6(a), first, comparing [Normal Reinforcement] and [Reduced Diameter Reinforcement], it can be seen that when the diameter of the steel bar cage 3 is reduced, the bending moment M at yield is slightly reduced.

[0037] Regarding the effect of eccentric arrangement, by setting [Small Eccentric Arrangement (+177)] and [Large Eccentric Arrangement (+250)], the (inward) bending moment M at yield increases compared to [Reduced Diameter Reinforcement], and the increase amount is larger for [Large Eccentric Arrangement (+250)]. Also, in these cases, by eccentrically arranging the steel bar cage 3 on the tensile edge side (the lower side in Fig. 6(b)), the increase in the axial steel material 31 arranged in the tensile region of the cross-section also contributes to the improvement of the cross-section performance. On the other hand, in the cases of [Small Eccentric Arrangement (-177)] and [Large Eccentric Arrangement (-250)], the (outward) bending moment M at yield decreases compared to [Reduced Diameter Reinforcement], and the decrease amount is larger for [Large Eccentric Arrangement (-250)]. The diameter of the steel bar cage 3 and the amount of steel material in the axial direction are determined considering the above relationships so as to obtain the cross-section performance required for the deep foundation 2.

[0038] As described above, in the present embodiment, in response to the characteristics of the cross-sectional performance of the deep foundation 2 described above, that is, in the deep foundation 2 on the leeward side, there is a margin in bearing capacity, the reinforcing cage 3 is eccentrically arranged outward to increase the bearing capacity when the deep foundation 2 becomes the windward side. Although the bearing capacity decreases when the deep foundation 2 becomes the leeward side, as described above, since there is a margin in bearing capacity in the deep foundation 2 on the leeward side originally, the cross-sectional performance of the deep foundation 2 is still determined by the bearing capacity when the deep foundation 2 becomes the windward side. Thus, in the present embodiment, the cross-sectional performance of the deep foundation 2 can be improved by the reasonable arrangement of the reinforcing cage 3. As a result, after ensuring the required cross-sectional performance, it becomes possible to reduce the diameter of the reinforcing cage 3 as described above to reduce the weight, and it is also possible to reduce the amount of the axial steel material 31.

[0039] Also, by reducing the diameter of the reinforcing cage 3, a large space is created between the inner surface of the excavation hole and the reinforcing cage 3, and a sufficient space for the worker to move up and down can be secured in this space. Thereby, the lifting equipment such as a ladder can be made high-spec and reliable (for example, the one used when excavating the excavation hole), and it becomes easier for the worker to move up and down.

[0040] Also, in the present embodiment, by reducing the diameter of the telescopic reinforcing cage 3, it becomes easy to store the telescopic reinforcing cage 3 prefabricated at a factory or the like in the bed of a large truck, which is preferable in terms of transporting the reinforcing cage 3.

[0041] Also, in the example of FIG. 5, the four deep foundations 2 are arranged at the positions corresponding to the vertices of a rectangle in plan view, and the reinforcing cages 3 of the respective deep foundations 2 are eccentrically arranged in the directions inclined with respect to both of the two orthogonal sides of the rectangle. Therefore, the above-described effects can be obtained with respect to the wind loads applied from all directions.

[0042] However, the present invention is not limited to the above-described embodiment. For example, the application target of the present invention is not limited to the deep foundation 2, and any columnar foundation may be used. For example, it is also possible to apply it to a pile foundation. Also, although the deep foundation 2 of the present embodiment has a circular cross-section, it is also possible to apply it to columnar foundations having other cross-sectional shapes.

[0043] In addition, in this embodiment, the expandable and contractible reinforcing cage 3 is used, but the reinforcing cage 3 may be a normal one that is not expandable and contractible.

[0044] Moreover, the present invention is particularly effective for the foundation of the transmission tower 1 which is often enlarged as the deep foundation 2. However, the structure supported by the foundation is not limited to the transmission tower 1, and any structure supported in a portal shape by a plurality of foundations may be used. For example, it may be a water supply tower, a building, or the like.

[0045] Also, the load applied to the structure is not limited to the wind load, and the same effect can be obtained when other horizontal loads such as seismic loads are applied. The foundation is also not limited to those buried in the ground.

[0046] As described above, the preferred embodiments of the present invention have been described with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that those skilled in the art can conceive of various modification examples or correction examples within the scope of the technical idea disclosed in the present application, and it is naturally understood that those also belong to the technical scope of the present invention.

Explanation of Reference Numerals

[0047] 1: Transmission tower 2, 20: Deep foundation 3: Reinforcing cage 31: Axial steel material 32: Stirrup

Claims

1. A basic structure of a structure, wherein the structure is supported in a portal shape by a plurality of columnar bases, a reinforcing cage is embedded in the concrete of the base, and the reinforcing cage is eccentrically arranged in a direction away from the center of gravity of the structure from the plane center of the base. The basic structure is characterized by this.

2. The basic structure according to claim 1, characterized in that the reinforcing cage is an extensible and contractible reinforcing cage in which a strip bar and a flexible strand are combined so that the intersection angle is variable.

3. Four of the said bases are arranged at respective positions corresponding to the vertices of a rectangle in plan view, and the reinforcing cages of each base are eccentrically arranged in a direction inclined with respect to both of the two orthogonal sides of the rectangle. The basic structure according to claim 1 is characterized by this.

4. The basic structure according to claim 3, characterized in that the structure is a transmission tower.

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