JPS6047117A - Cast-in-place reinforced concrete pile - Google Patents

Abstract

PURPOSE:To control the bending moment of the pile head due to displacement of the ground to a minimum by providing a steel tube-clad reinforced concrete to the head of a steel tube having projections formed by rolling on its inside. CONSTITUTION:A pile 1 consists of concrete 2, reinforcing bars 5, and a steel tube 6 having projections in its inside. In the cast-in-place reinforced concrete pile with an expanded bottom and the head of steel tube-clad concrete having projections, the reinforcing bars are extended into the steel tube and fixed, and the head is made of steel tube-clad reinforced concrete with projections. The pile head can withstand increasing horizontal forces, the bending moment of the pile head due to displacement of the ground can be suppressed to a minimum, and the bonding strength between the steel tube and the concrete portion can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to cast-in-place reinforced concrete piles.

In the current seismic design of cast-in-place piles, as shown in FIG. 1, horizontal force P is applied to the pile head, and the design is based on the bending moment of the pile that is generated thereby.

This bending moment is large near the pile head, and at the bottom it is a small value that can be ignored in terms of design. In Fig. 1, (,) indicates the pile structure, and (b) indicates the moment distribution of the pile.

However, it is beginning to be observed that when an earthquake actually occurs, bending moments other than those indicated by the above design method occur. For example, as shown in Figure 2, the upper part is a soft alluvial layer with N of 0, and the upper part is a diluvial layer with a depth of about 21 m or less, where N is 0.
A straight pile 1a with a diameter of 60 mm is installed in the ground with a 50 ON value distribution.
0w+, wall thickness 9- steel pipe pile, diameter 600m as diagonal pile 1b
As shown in Figure 6 (a), a total of 64 steel pipe piles with a wall thickness of 12 mm and a wall thickness of 12 mm were driven, and when the strain in each part of the piles was observed when an earthquake occurred on this foundation, it was found that When the strength is the maximum foundation acceleration of 2.4-, the strain with respect to the depth of the straight pile 1a is as shown in FIG. 5(b), and the strain with respect to the depth of the slanted pile 1b is as shown in FIG. 5(c). In other words, in straight pile 1a, in addition to the bending strain at the pile head, the maximum &
A bending strain of 9μ was generated, and the latter strain was not taken into account in the seismic design method described above.

From this observation result, the strength of the earthquake is 200% of the maximum acceleration of the foundation.
If we estimate the distortion in the case of −, we get K as shown in Table 1.

Table 1 In this case, the maximum strain at the top of the support layer in garden section 1a is 1058μ
However, the problem arises that the yield point of the pile is exceeded.

In this way, the observation results from actual earthquakes have shown that the currently used seismic design methods for piles are inadequate, so we conducted model vibration experiments to examine the characteristics of strain that occurs in piles during earthquakes. A detailed investigation revealed the following: As an experimental model, as shown in Figure 4,
Considering the ground, piles, and superstructure, the ground model consists of the surface layer, the upper soft layer, the middle layer consisting of a sand layer with an N value of about 60, the lower soft layer, and the supporting layer, the actual ground, the foundation, and the structure. is a structure that satisfies 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 vibrations with an input acceleration of 10P+ and a ground resonance frequency of 6.5cm;
(b) shows the bending strain distribution when a steel pipe pile model is used and vibration is applied at an input speed of 10 pIs and a ground resonance frequency of 6.2 seconds. 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 Figures 5(a) and 5(b), large bending strains have occurred that cannot be shown using current seismic design methods. In other words, the factors that cause the bending moment of the pile are not only the strain caused by the water force P on the pile head as shown in Figure 6(a), but also the strain caused by the ground displacement during an earthquake as shown in Figure 6(b). Forced displacement also has a large effect. In the seismic design of this pile, as shown in (e), (c
It is necessary to consider the bending moment that is a combination of the bending moment due to ground displacement shown in ) and the bending moment due to the horizontal force on the pile head shown in (d).

By the way, conventional cast-in-place reinforced concrete piles, for example, have a constant r [capacitive compressive stress of 6 ckg/
If the N value of the support layer is 50, the ground bearing capacity at the tip is 25.
9/cdl(1/3 Therefore, the normal vertical load supported by the pile increases by about twice.

The horizontal force on the pile head used in seismic design is roughly expressed as (continuous vertical load x horizontal seismic intensity).As the constant vertical load increases, the horizontal force also increases, so the pile head is affected by the bending moment caused by this horizontal force. In order to withstand the problem, the diameter of the pile head must be enlarged as much as possible, as shown in FIG. 7, and it is often impossible to design the pile head. Therefore, unless there is something to reduce the horizontal force from the superstructure, such as in a basement, the piles should be made with a corrugated crest when the bottom is widened. In addition, in FIG. 7, 1 is a pile, 2 is a concrete IJ-), 3 is an enlarged bottom part, 4 is an enlarged head part, and 5 is a reinforcing bar. An example of the calculated pile head bending moment distribution generated by ground displacement during an earthquake for this enlarged head is shown in Fig. 8(a). The calculation method is from the Japanese National Railways "Earthquake-resistant Design Guidelines (Draft) Explanation" July 1982. P
Apply mutatis mutandis the method on pages 54-75. In the figure, (a) shows the bending moment distribution due to ground displacement of a non-corrugated cast-in-place reinforced concrete pile with a pile diameter of 1250 m, and (b) shows an expanded head diameter of 1750 m, an expanded head length of 8 m, and a pile diameter of 1250 +.
This figure shows the bending moment distribution due to ground displacement of a cast-in-place reinforced concrete pile with a wave crest of m. In addition, the ground where the piles were installed had approximately 5 baits of fine sand with N=7, as shown in Figure (b).
The silt is about 5 meters from the bait to about 18 meters, and the deep part is N.
It consists of =50 gravels.

In the example shown in FIG. 8(a), the pile head bending moment is about 130 t-m for the non-corrugated pile, whereas it is about 430 t·m for the expanded head pile.

That is, in the case of an expanded pile head, the bending moment of the pile head caused by ground displacement is several times that of a pile without a wave crest, because the bending rigidity of the expanded head is greater than the wave crest. 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; This creates a vicious cycle in which the amount increases further.

An object of the present invention is to provide a cast-in-place reinforced concrete pile that solves the above-mentioned problems.

In order to achieve the above object, the cast-in-place reinforced concrete pile according to the present invention does not have a corrugated head, reduces or eliminates reinforcing bars in the head, and replaces the pile head with a steel pipe with protrusions on the inner surface. By using steel pipe-wrapped reinforced concrete or steel pipe-wrapped concrete, it is possible to cope with the increase in horizontal force on the pile head, suppress the bending moment of the pile head due to ground displacement, and further increase the bond stress between the steel pipe and the concrete part. The present invention is characterized in that it is configured so as to achieve the following. Note that the protrusions are formed by rolling.

The present invention will be explained below based on Examples.

Figure 9 is a longitudinal cross-sectional view of an embodiment of the present invention, (a) is an expanded cast-in-place reinforced concrete pile with a head made of steel pipe wrapped concrete with a protrusion, and (b) is a cast-in-place reinforced concrete pile with a head made of steel pipe wrapped with protrusion. 1 shows a cast-in-place reinforced concrete pile with an expanded bottom, 1 is a pile, 2 is concrete, 5 is an expanded bottom part, 5 is a reinforcing bar, and 6 is a steel pipe having a protrusion on the inner surface. Furthermore, (a)
In steel pipe-wrapped concrete, in addition to cases in which the reinforcing bars are stretched and fixed within the steel pipes, there are also cases in which the reinforcing bars are directly connected to the steel pipes by means of welding, etc.

In the following, in this embodiment, we will first show the advantages of wrapping a steel pipe around the head, and then discuss the necessity of providing projections on the inner surface of this steel pipe.

Fig. 9(b) Shaft diameter 1250 according to the present invention as shown in K
The bending moment distribution due to ground displacement of an expanded cast-in-place reinforced concrete pile in which a steel pipe 6 with a protrusion of diameter 1250 mm, wall thickness 23 mm, and length 8 mm is wrapped around the head of the pile 1 of 8 mm is as follows.
This is the passage shown in Figure (c). As is clear from this, the bending moment of the pile head is approximately 4 in the case of a wave crest pile ((b) in the figure).
50 t-m, whereas in the case of the present invention where the head is made of steel pipe wrapped reinforced concrete, it is about 260 t-m.
I was able to reduce the weight by about 40 inches.

FIG. 10 shows the cast-in-place reinforced concrete pile according to the present invention and the conventional cast-in-place reinforced concrete pile with a wave crest.
When installed on the ground shown in Figure (b), unit long-term axial force (1) for the diameter of the general part of the pile, unit length rrL
) This is an example of the construction cost of Atashi's pile cap parts calculated based on the magazine "Construction Prices" published in March 1981. The concrete of the pile with the diameter covered in Figure 10 has a compressive strength of F
c = 240 kg/ca.

The reinforcing bars are 5D60 and the steel pipes are 5TK41, and the design takes into consideration the sum of the horizontal force acting on the pile cap and the bending moment generated by ground displacement during an earthquake. The maximum reinforcing bar ratio was set at 6%, and the rusting ratio was set at 1. Further, the price of the flat steel plate is used as the price of the steel plate with projections, but the cost here has the character of an approximate value.

In Figure 10, (a) shows the case where the head part is made of steel pipe wrapped concrete, (e) and (f) show the case where the head part is made of steel pipe wrapped reinforced concrete, and (e) shows the bending that occurs in the lower part of the reinforced concrete part. When the moment is set to 50tm and reinforcing bars arranged against this are raised on the steel pipe winding part, (
(F) is the same case when reinforcing bars for 150Lm are erected. (G) is the case of conventional cast-in-place reinforced concrete with a wave crest.The diameter of the general pile on the horizontal axis is the same as that below the wave crest. As is clear from the diagram, the conventional pile with a wavy crest (G) means the diameter of the general part, whereas the material cost increases significantly as the diameter of the general part of the pile increases.
For the piles according to the present invention), (e), and (f), the material cost is almost constant regardless of the diameter of the general pile part, and the diameter of the general pile part is 15
In the case of the 00 earthquake, it was approximately 2/6.1800mm of the conventional wave crest pile.
If it exceeds, it becomes 172 or less.

The advantages of wrapping a steel pipe around the head have been shown above. Next, we will discuss the necessity of adding protrusions to the inner surface of this steel pipe.

In the secondary design, which is the ultimate design, we will examine the adhesive strength required between the steel pipe and concrete.

The collapse mechanism near the head of a concrete IJ-toe section wrapped around a steel pipe differs depending on whether it is due only to the horizontal force of the pile cap, only to ground displacement, or when both act simultaneously. Regarding the adhesion strength required between concrete, the value for the collapse mechanism when only horizontal force acts on the pile cap is the largest value. Therefore, consider a situation where only horizontal force acts on the pile cap.

The bending moment and shear force distribution in the ultimate state due to the horizontal load on the pile head are calculated using the BROMS method. For example, on a sandy ground (surface layer) with N=60, a diameter of 1500 and a wall thickness of 14
m, concrete compressive strength Fc 24 ok/t:
Figure 11 shows the distribution of bending moment and shear force of steel pipe wrapped concrete of RL. In the figure, (a) shows the shear force distribution with respect to the depth of the pile, and (b)
) shows the bending moment distribution with respect to the pile depth. According to this Promus method, the surface layer is N=10 or N=60
On the sandy ground, the diameter is 1000+e++, 1500+m, 2
Table 2 shows the pile head bending moment (Mp), the pile head shear force Qu, and the depth t at which the bending moment becomes zero when the pile head is 000 mm.

By the way, the necessary adhesion stress degree τ between the steel pipe 6 and the concrete 2 can be requested based on the condition shown in FIG. 12. In Figure 12, (a)
is a front view of the steel pipe-wrapped concrete IJ-to part, (b) is a plan view of the steel pipe-wrapped concrete part, (c) is the pile head bending moment distribution, and (d) is the stress in the concrete when compressive force is applied. Show the distribution. Note that 7 is the cross-sectional area of the conc IJ) on which compressive stress is applied, and 8 is the neutral axis.

In the section t until the bending moment Mp becomes zero,
Assuming that the bending moment distribution is a linear distribution as shown in FIG. 12(c) and the shear force distribution is constant, it is assumed that the axial force during an earthquake is (constant axial force x (1±1)). Under these conditions, the adhesion stress degree τb-req required for the steel pipe and the concrete part to be integrated is calculated as follows.

Here, Fc is the compressive strength of concrete, Ac is the cross-sectional area of concrete where compressive stress is acting, φ is the length of the contact area between the concrete part where compressive stress is acting and the steel pipe, and t is the bending moment. is the depth at which becomes zero.

Table 2 shows the required adhesion stress degrees τb and req obtained by applying equation (1) when the above pile diameters are 1000 m, 1500 m, and 2000 m.

As a result, the required bond stress between the steel pipe and the concrete part is 2
It will be 0 to 60 Yu/-.

The following six conditions must be satisfied regarding adhesive stress, including the above-mentioned required adhesive strength.

■ Required adhesion strength is 50 to 9/d or more.

■ In the event of a severe earthquake where secondary design is the target, 1!1
As shown in Figure (&), a state in which the adhesive stress strength is reached repeatedly occurs, but in contrast to this state, the strength does not deteriorate as shown in (b), and the strength increases as shown in (c). It is necessary to maintain it.

■ It is thought that skin separation between the concrete and the steel pipe may occur due to the shrinkage of concrete over time, so prevent the deterioration of bond strength due to this.

The adhesion strength between a steel pipe using a normal flat steel plate and concrete is 2 to 10 kg/cfl. Also, after reaching the strength
It is presumed that the properties shown in FIG. 16(b) will be exhibited under reverse loading. Furthermore, unless special measures such as expanding concrete are taken, it is difficult to prevent the adhesive strength from deteriorating due to shrinkage of concrete over time.

On the other hand, it will be shown below that the above six conditions can be solved using a steel pipe with projections. The degree of adhesion stress between the protruded steel pipe and concrete was obtained as an experimental value of 50 kg/d in the steel pipe-concrete adhesion strength test shown in FIG.

In other words, the adhesion strength required under condition ① is 20 kg/
I am fully satisfied with the CD. In Figure 14, 2 is concrete IJ-), 9 is a steel pipe, and 10 is concrete 2 and steel pipe 9.
In this case, the pulling force increases monotonically.

Next, we will talk about maintaining strength in the combed state, which is required under condition ①. In a test block 12 in which a steel plate 11 with protrusions is provided in concrete 2 as shown in FIG.
FIG. 16 shows the degree of adhesion stress when repeatedly applying a pulling force to the steel plate 11 with projections.

In FIG. 16, δp indicates the amount of coverage at the top of the test block in FIG. 15, and δf indicates the amount of coverage at the bottom. In FIG. 15, the reinforcing bars 5, both vertical bars and hoop bars, were 5R24 and had a diameter of 6 baits.

As shown in Fig. 16, the adhesive strength between the protruded steel plate and concrete does not deteriorate even under reverse loads. As shown in FIG. 16, this test was not a double-sided test, but a single-side swing test, but it is easily expected that the same strength could be maintained for both double-side swings.

Next, we will discuss the prevention of adhesive strength deterioration due to shrinkage of concrete over time, which is required in the last condition (2). The amount of shrinkage over time varies depending on various conditions such as the amount of cement, but it is sufficient to consider 4×10 as the maximum volumetric strain. As an example, considering a pile with a diameter of 2000 m, as shown in Figure 17 (a), in the case where it contracts uniformly toward the center, the gap 16 between the steel pipe 9 and the concrete 2 is 0.4 tax, (
As shown in b), in the case of eccentric contraction, the gap is 1
6 is 0.8 gourd. Here, if the height h of the protrusion is set to be equal to or greater than these values as shown in (C), deterioration of the adhesive strength due to shrinkage over time can be prevented.

Next, an example of a steel pipe with projections according to the present invention is shown in FIG. 18. In the figure, 14 is a protrusion.

In the above embodiment, the present invention is applied to a cast-in-place reinforced concrete pile with an expanded bottom, but it goes without saying that the present invention can also be applied to a cast-in-place reinforced concrete pile without an expanded bottom. Further, although an example of the protrusion is shown in FIG. 18, the present invention is not limited to this, and other shapes and dimensions of nine dimensions can also be used.

As is clear from the above description, according to the present invention, by eliminating or reducing the reinforcing bars on the pile head and using a steel pipe wrapped with protrusions, the corrugated reinforced concrete pile does not have to be stiff, and the pile head is not stiffened. It is possible to suppress the bending moment of the pile head due to displacement, and even when the bottom is expanded, it is possible to handle the increased horizontal force due to the bottom expansion. Furthermore, by adding protrusions to the steel pipe, the adhesion strength between the steel pipe and concrete increases.
It is also tough and can be expected to provide good adhesion to the protrusions of the steel pipe even if the concrete and steel pipe become separated due to shrinkage of the concrete over time. Therefore, the effect is significant when applied to cast-in-place piles designed for earthquake resistance.

[Brief explanation of the drawing]

Figure 1 (a) is a schematic diagram of a cast-in-place pile using the conventional design method, (b) is its bending moment distribution diagram, Figure 2 is a diagram of the ground conditions when pile strain was observed during an earthquake, and Figure 6 (a
) is a pile structure diagram when strain of the pile was observed during an earthquake, (
b) is the bending strain distribution diagram of the garden area at that time, (c) is the bending strain distribution diagram of the slanted pile, Figure 4 is the model diagram of the model vibration experiment, and Figure 5 is the diagram of the bending strain distribution of the garden.
Figures (a) and (b) are bending strain distribution diagrams of piles in model vibration experiments, Figure 6 (a) is a schematic diagram of cast-in-place piles, and (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, (,) is a bending moment distribution diagram that combines both, and Figure 7 is a wave crest diagram. Configuration diagram of cast-in-place reinforced concrete piles,
Figure 8(a) is a bending moment distribution diagram due to ground displacement.
(b) is a diagram of the ground conditions, Figures 9 (a) and (b) are longitudinal cross-sectional views of the embodiment of the present invention, Figure 10 is a distribution diagram of material costs with respect to the diameter of the pile-crotch, and Figure 11 ( ,) is a shear force distribution diagram using the Promus method, (b) is a bending moment distribution diagram, Figure 12 (a) is a front view of steel pipe wrapped concrete, (b) is a plan view, and (c) is a bending diagram. moment distribution map,
(d) is a stress distribution diagram of steel pipe and concrete, Figure 16 is a characteristic diagram of adhesive stress, (a) is a characteristic diagram of change in adhesive stress with time, (b) and (c) are the relative relationship between steel pipe and concrete. In the characteristic diagram of displacement and adhesion stress, (b) is inferior in strength to the bending after reaching strength. (c) is a characteristic diagram when no deterioration occurs,
Figure 14 is a block diagram of the steel pipe-concrete adhesive strength test block, Figure 15 (a) is a plan view of the steel plate with protrusion and concrete adhesive stress test block, (b) is a side view, and Figure 16 is the 15 Diagram showing the relationship between adhesive stress and relative displacement when repeated pulling force is applied to the block shown in the figure, Figure 17 (a)
, (b) and (c) are state diagrams of concrete shrinkage over time in the case of a diameter of 2000 mm, and FIG. 18 is a configuration diagram of a steel pipe with projections. 1; Pile, 2; Concrete, 6; Expanded bottom part, 5; Rebar, 6
1 Steel pipe with protrusion, 14; protrusion. Agent Patent Attorney Sanro Kimura (o) (b) Fig. 8 (al lbn @ 1 demo mate (t, m) → -N I direct [+1 snoring 15 Fig. 1/+1 (be1. 1 Figure 17 (a) (b) (C)

Claims (1)

[Claims] A cast-in-place reinforced concrete pile characterized by using a steel pipe having protrusions formed by rolling on the inner surface, and having a head made of a steel pipe, rolled reinforced concrete, or steel pipe wrapped concrete.