JPH0119487B2 - - Google Patents

Description

[Detailed description of the invention]

An object of the present invention is to provide a cast-in-place reinforced concrete pile with a double pipe pile cap that can cope with ground displacement due to earthquakes and the like. In the seismic design of cast-in-place reinforced concrete piles currently being carried out, as shown in Figure 1,
Considering the situation in which a horizontal force P acts on the pile head, the pile cross section is designed to handle the bending moment generated by this. As a result, a large bending moment will occur near the pile cap of pile 1 (for example, approximately 10 m from the pile cap for a cast-in-place pile with a diameter of 1250 mm), so seismic design was performed only in the vicinity of the pile cap. There was virtually no seismic design for the middle and lower parts of Pile 1. In Fig. 1, a indicates the pile structure, and b indicates the bending moment distribution of the pile. However, when an earthquake actually occurs, it is beginning to be observed that large bending strains occur in locations other than the vicinity of the pile cap. For example, the second
As shown in the figure, the upper part is a soft alluvial layer with N≒O and the depth is approximately
A steel pipe pile with a diameter of 600 mm and a wall thickness of 9 mm was used as the straight pile 1a, and a steel pipe pile with a diameter of 600 mm and a wall thickness of 12 mm was installed as the oblique pile 1b in the ground with an N value distribution of N > 50 in an alluvial layer of 21 m or less. As shown in Figure a, a total of 64 piles have been driven, and the strain in each part of the piles when an earthquake occurs on this foundation has been observed. When the earthquake strength is 2.4 gal at the maximum foundation acceleration, the strain with respect to the depth of the straight pile 1a is shown in Figure 3b, and the strain with respect to the depth of the slanted pile 1b is shown in Figure 3c. That is, the maximum bending strain of the pile head of the straight pile 1a is 15.4μ, but the maximum bending strain of 8.9μ occurs even at the upper end of the support layer, which has not been taken into consideration in conventional seismic design. Based on these observation results, we can estimate the strain when the earthquake strength is 200 gal at the maximum foundation acceleration, as shown in Table 1.

[Table] In this case, the maximum strain at the top of the support layer of straight pile 1a is
1058μ, which causes the problem of exceeding the yield point of the pile. In this way, based on the observation results when an earthquake actually occurs, it has become clear that the currently used seismic design methods for piles are insufficient, so we conducted model vibration experiments to examine the distribution of strain that occurs in piles during earthquakes. A detailed investigation revealed the following: As shown in Figure 4, the experimental model consists of the ground, piles, and superstructure, and the ground model consists of a surface layer, a soft layer, an intermediate layer consisting of a sand layer with an N value of about 30,
The structure consists of a soft layer such as clay and a supporting layer, and the actual ground, foundation, and structure satisfy the law of similarity. Figure 5 shows the above experimental results, where a shows the bending strain distribution when using a cast-in-place pile model and applying vibration with an input acceleration of 10 gal and a ground resonance frequency of 3.3 Hz, and b shows the steel pipe pile model. The bending strain distribution is shown when vibration is applied at an input acceleration of 10 gal and a ground resonance frequency of 3.2 Hz. In addition, in the figure, the numerical value of Young's coefficient Em is
The Young's modulus of each ground layer used as a ground model is shown. As is clear from FIGS. 5a and 5b, the bending strain shows large values at the boundaries of the strata, such as the upper and lower ends of the intermediate layer and the upper end of the supporting layer. As mentioned above, based on the observation results and model experiment results during actual earthquakes, the only factor that causes the bending moment of a pile is the strain caused by the horizontal force P on the pile head as shown in Figure 6a. In addition, forced displacement of piles due to ground displacement during an earthquake as shown in b also has a large effect. Therefore, when designing the seismic resistance of piles, it is necessary to consider the combined bending moment of the bending moment due to ground displacement shown in c and the bending moment due to the horizontal force on the pile head shown in d, as shown in e. . By the way, for conventional cast-in-place reinforced concrete piles, for example, if the concrete allowable stress of the shaft is always 60Kg/cm ² , the maximum allowable bearing capacity of the ground at the tip is about 25Kg/cm ² , so the shaft In order to utilize the full allowable stress level of the concrete, the bottom of the pile tip is approximately doubled, and as a result, the normal vertical load supported by the pile increases approximately twice as much. The horizontal force that acts on the pile head during an earthquake is roughly expressed as (continuous vertical load x horizontal vibration), so as the constant vertical load increases, the horizontal force also increases, so in order for the pile head to withstand this horizontal force, In this case, it is necessary to increase the amount of reinforcing bars at the pile head, and therefore reinforcement cannot be arranged unless the diameter of the pile head is expanded as shown in FIG. Therefore, unless there is something to reduce the horizontal force from the superstructure, such as in a basement, piles with an expanded head and an expanded bottom should be used. In addition, in FIG. 7, 1 is a pile, 2 is concrete, 3 is an enlarged bottom part, 4 is an enlarged head part, and 5 is a reinforcing bar. An example of the pile head bending moment distribution generated by ground displacement during an earthquake for this expanded head pile is shown in Figure 8a. In the figure, A shows the bending moment distribution due to ground displacement of a non-expanded cast-in-place reinforced concrete pile with a shaft diameter of 1250 mm, and B shows the bending moment distribution due to ground displacement of an expanded cast-in-place reinforced concrete pile with an expanded head diameter of 1750 mm, an expanded head length of 8 m, and a shaft diameter of 1250 mm. The bending moment distribution due to displacement is shown. As shown in Figure b, the ground on which the piles were installed is about 5 m of fine sand with N=7, about 5 m to about 18 m of silt with N=3, and deeper than that is N=
It is made up of 50 gravels. In the example shown in Figure 8a, the pile cap bending moment is approximately 130 t·m for non-expanded piles, while it is approximately 430 t·m for expanded piles. That is, in the case of an expanded pile, the bending moment of the pile head caused by ground displacement is several times that of a non-expanded pile because the bending rigidity of the expanded head increases due to the expanded head. Therefore, when considering ground displacement during an earthquake, conventional cast-in-place reinforced concrete piles must be further expanded in order to withstand the pile head bending moment. A vicious cycle occurs in which the stiffness of the material further increases. An object of the present invention is to provide a cast-in-place reinforced concrete pile with a double pipe pile cap that solves the above-mentioned problems. The cast-in-place reinforced concrete pile with a double pipe pile head according to the present invention includes an internal steel pipe or steel pipe concrete (hereinafter referred to as internal steel pipe, etc.) whose pile head is implanted into the reinforced concrete pile of the lower pile as a vertical resistance member,
The horizontal resistance member consists of an external steel pipe whose pipe length is shorter than that of the internal steel pipe, etc., and by installing the external steel pipe so that it does not contact the internal steel pipe or the reinforced concrete pile at the bottom except at the head, bending moment due to ground displacement can be reduced. This is to keep the generation to a small level and support horizontal and vertical forces from the upper structure. The present invention will be explained below based on Examples. FIG. 9 is a longitudinal cross-sectional view of an embodiment of the present invention, in which 6 is an internal steel pipe, 7 is an external steel pipe, 8 is a steel plate for joining the inner and outer pipes, and 9
is a reinforced concrete pile. The head of the inner steel pipe 6 and the outer steel pipe 7 surrounding the inner steel pipe 6 are joined by an inner and outer pipe joining steel plate 8. In addition, the length of the external steel pipe 7 is shorter than that of the internal steel pipe 6, and the lower part of the internal steel pipe 6 is connected to the reinforced concrete pile 9 so that the external steel pipe 7 does not contact the internal steel pipe 6 and the reinforced concrete pile 9 of the lower pile except for the head part. be implanted in A gap 11 between the inner steel pipe 6 and the outer steel pipe 7 is filled with a void or a soft filler 10 such as soft earth or polyurethane to isolate the inner steel pipe 6 and the outer steel pipe 7. The internal steel pipe 6 and reinforced concrete pile 9 act as vertical resistance members, and the external steel pipe 7 acts as a horizontal resistance member. The bending moment distribution caused by ground movement when the internal steel pipe 6 is a steel pipe with a diameter of 600 mm and a wall thickness of 19 mm, the external steel pipe 7 is a steel pipe with a diameter of 1200 mm, a wall thickness of 12 mm, and a length of 6 m, and the reinforced concrete pile 9 has a pile diameter of 1250 mm is calculated as follows. 10
As shown in the figure. The ground conditions are the same as in Figure 8b. In the figure, C is the bending moment distribution of the external steel pipe 7, and D is the internal steel pipe 6 and the reinforced concrete pile 9.
shows the bending moment distribution. Further, point E indicates the boundary between the internal steel pipe 6 and the reinforced concrete pile 9. As is clear from the figure, the pile head bending moment of the external steel pipe 7 is approximately 40 t・m, and although it has almost the same function as the expanded head and expanded bottom cast-in-place reinforced concrete pile shown in Figure 8 a, Bending moments generated by ground displacement are becoming much smaller. In other words, in the case of the pile targeted in Figure 10,
The bending moment generated at the pile head due to ground displacement during an earthquake is approximately 12 t・m for the internal steel pipe and approximately 12 t・m for the external steel pipe.
40 t・m, and approximately 430 t・m of the reinforced concrete pile with expanded head and expanded bottom targeted in Figure 8 a.
Considering the value of m, it is clear that the economic design of the pile itself and the foundation beam can be achieved. In addition, in FIG. 9, the inner steel pipe 6 and the outer steel pipe 7 are not joined without using the inner and outer pipe joining steel plates 8,
A similar economical design can be achieved when the pipe is embedded in a footing, and the same is true when the internal steel pipe is made of steel pipe concrete. Further, FIG. 11 shows another embodiment of the present invention.
In the figure, the same symbols as in FIG. 9 indicate the same configurations. A positioning steel plate 12 is connected to the lower part of the internal steel pipe 6, and is connected to the reinforcing bars 5 of the reinforced concrete pile 9, thereby facilitating the planting of the pile head. As is clear from the above description, according to the present invention, the pile head is not made as rigid as an expanded reinforced concrete pile, and the bending moment of the pile head caused by ground displacement can be suppressed to a small level. It is possible to realize cast-in-place reinforced concrete piles that can handle increased horizontal forces and are economical in terms of vibration-resistant design, and the effects of implementation are significant.

[Brief explanation of drawings]

Figure 1a is a schematic diagram of a cast-in-place pile based on the conventional seismic design method, b is its bending moment distribution diagram, and Figure 2
Figure 3 is a diagram of the ground conditions when pile strain was observed when an earthquake occurred, Figure 3a is a pile structure diagram when pile strain was observed when an earthquake occurred, and b is the bending strain distribution of straight piles at that time. Figures 1 and 2c are bending strain distribution diagrams of a diagonal pile, Figure 4 is a model diagram of a model vibration experiment, Figures 5a and b are bending strain distribution diagrams of a pile in a model vibration experiment, and Figure 6a is a diagram of a cast-in-place pile. Schematic diagram, b is a ground displacement diagram, c is a bending moment distribution diagram due to ground displacement, d is a bending moment distribution diagram due to horizontal force on the pile head, e is a bending moment distribution diagram that combines both, Figure 7 is an expanded cast-in-place diagram. A configuration diagram of a reinforced concrete pile, Fig. 8a is a bending moment distribution diagram due to ground displacement, b is a diagram of its ground structure, Fig. 9 is a structural diagram of an embodiment of the present invention, Fig. 10 is a bending moment distribution diagram due to ground displacement, FIG. 11 is a structural diagram of another embodiment of the present invention. 1: Pile, 2: Concrete, 3: Expanded bottom, 4:
Expanded head, 5: Rebar, 6: Internal steel pipe, 7: External steel pipe, 8: Jointed inner and outer pipe steel pipe, 9: Cast-in-place reinforced concrete pile.

Claims (1)

[Claims] 1 Consisting of an internal steel pipe of steel pipe or steel pipe concrete as a vertical resistance member to be implanted in the reinforced concrete pile of the lower pile, and an external steel pipe with a shorter pipe length than the internal steel pipe as a horizontal resistance member surrounding the internal steel pipe, and the external steel pipe is A cast-in-place reinforced concrete pile with a double pipe pile head, characterized in that the pile is installed so that parts other than the head do not come into contact with the internal steel pipe and the reinforced concrete pile of the lower pile.