JPS61242220A - Sc pile introduced temporary prestress - Google Patents

Abstract

PURPOSE:To raise the retained horizontal durability and axial pressure-durabilty of a pile by a method in which a prestress is introduced into the internal concrete of a steel tube and then removed as drying contraction and creeping phenomenon proceed in the concrete. CONSTITUTION:Many spiral projected lines 2 of slow slopes are formed on the inner surface of a steel tube 1 as an outer shell and on the upper and lower ends of a pile (a), an upper end plate 3 and a coupler end plate 4 are integrally connected to the tube 1. A tension is operated by using the formwork housing the tube 1 as a reactor to produce a desired tensile stress in the tube 1. Even when hair cracks develop, no extension of the cracks occurs by the restricting force of the projected lines 2, the tensile resistance of the concrete (b) is worked, and the retained horizontal durability of the pile (a) is increased. The pile (a) can withstand increased axial pressure and great bending moment to be encountered during the earthquake.

Description

[Detailed Description of the Invention] This invention relates to the improvement of conventional high-strength concrete piles with outer shell steel pipes (hereinafter referred to as SC piles), and its purpose is to improve the retention of SC piles by the special effects of the present invention. The objective is to provide a new SC pile with increased horizontal bearing capacity and axial pressure bearing capacity, and greater agricultural resistance.

When ordinary concrete is placed inside a steel pipe pile, which forms the outer shell, by centrifugal force, as the drying shrinkage of the concrete progresses, the steel pipe and concrete easily rub together, causing distortion only in the concrete. This pile does not have the load-bearing capacity of a composite pile. As a means to avoid this phenomenon, methods are used to mix an expansive material into the concrete and press the concrete to the inner surface of the steel pipe, or to provide a protrusion on the inner surface of the steel pipe and use this protrusion to restrict the free drying of the concrete. . The stress state between the steel pipe and concrete in this case will be analyzed below.

The strain of drying shrinkage of unrestrained concrete is 0.0.
Assume 003. In the case of a pile type in which the outer diameter of the SC pile is 50 cm and the thickness of the steel pipe is 4.5 mm, the cross-sectional area of the steel pipe is approximately 7 mm.
0 cm2, and the cross-sectional area of the steel pipe converted to concrete is 367.5 cm2 when calculated assuming an elastic modulus ratio of 5.25. On the other hand, the cross-sectional area of concrete is 986 cm2. In order for the steel pipe and concrete of an SC pile to behave as one and maintain the load-bearing capacity of a composite pile, the tensile force due to drying shrinkage that occurs in the concrete that is restrained by the steel pipe, and the tensile force on the steel pipe in response to the shrinkage of the concrete are required. The resulting compressive forces must be balanced. Based on this calculation, the compressive stress acting on the converted cross-sectional area of the steel pipe is 87.42 kgf/cm2,
The tensile stress acting on concrete is 32.58 kgf/
cm2, the experimental calculation is 87.42 x 367.5 = 3
2,126.85kg32.58×986=32,12
3.88 kg As mentioned above, the compressive force of the steel pipe is balanced with the tensile force of the concrete and becomes stable in this state. Also, the strain of the SC pile at this time is 0.00021855, and the compressive stress acting on the steel pipe is 87.42 x 5.25 = 4
58.955 kgf/cm2, that is, a compressive stress of approximately 460 kgf/cm2 is generated in the steel pipe due to the effect of drying shrinkage based on the unavoidable internal structure change of concrete. Moreover, no evidence to the contrary can be found that this compressive stress does not occur in steel pipes.

The above is an analysis of the stress state between steel pipes and concrete, focusing only on the drying shrinkage of concrete.
When the creep of concrete based on the vertical axial force of the superstructure is added to this drying shrinkage, the axial force shared by steel pipes, which is much less likely to creep than concrete, increases abnormally compared to the axial force shared by concrete. It is predicted that the actual compressive stress that will occur in the steel pipe will be much larger than the compressive stress that was previously assumed.

In reinforced concrete columns, the relationship between concrete creep and stress in the shaft reinforcement has already been clarified.
In SC piles, which basically have the same properties as reinforced concrete with a high reinforcement ratio, the abnormal increase in compressive stress that occurs in steel pipes is an undeniable phenomenon. The above-mentioned excessive pre-compressive stress generated in steel pipes is a major factor that clearly reduces the lead capacity and horizontal capacity of direct SC piles in the event of an earthquake. is the biggest challenge.

This invention aims to solve the above problems.

In this invention, as a method that can be implemented immediately, we first eliminate all effects of concrete drying shrinkage on steel pipes, and use means to reduce as much as possible the effects of concrete cleaves on steel pipes. Below, the general structure and manufacturing method of the new SC pile of the present invention will be explained. This SC pile introduces temporary prestress into concrete, so it is named a TPSC pile. The inner surface of the steel pipe 1, which forms the outer shell of this TPSC pile, is provided with multiple gently sloped spiral protrusions 2. At the upper and lower ends of the TPSC pile a, an upper end plate 3 and a joint end plate are provided. 4 is integrally welded to the steel pipe 1, and the symbols denote the required number of screw holes provided in the upper end plate 3 and the joint end plate 4. This TPS
The manufacturing method of C pile a will be easily understood by those skilled in the art without the need for detailed explanations or drawings of the device for pulling the steel pipe 1. Next, the manufacturing method will be briefly explained.

Screw the required number of threaded steel rods into the screw holes 5 of each of the upper end plate 3 and the joint end plate 4, pass the threaded steel rods of the joint end plate 4 through the through holes in the end face formwork on the fixed side, and tighten them with nuts. Tightening,
The joint end plate 4 is fixed to the stationary side end face formwork. In addition, the threaded steel rod of the upper end plate 3 is passed through the through hole of the tension plate of the tensioning device and tightened with a nut, the upper end plate 3 is fixed to the tension plate, and the steel pipe 1
A tensioning device is operated using the main body frame containing the steel pipe as a reaction force to generate the required tensile stress in the steel pipe 1, and this tensioning device is fixed to the end face formwork on the tensioning side. Next, concrete is supplied into the steel pipe 1 from the fixed side using a concrete pump, and then the concrete is compacted by centrifugal force. The autoclave curing of the concrete and the introduction of prestress based on the tensile stress of the steel pipe 1 are performed in the same manner as for conventional prestressed high-strength concrete piles.

The thus obtained steel pipe 1 has an outer diameter of 50 cm and a plate thickness of 84 cm.
Initial tensile stress degree σ of steel pipe 1 of TPSC pile a set to 5 mm
Let s be 1400 kgt/cm2. The degree of distortion εs1 at this time is εs1=σs/Es=1400/2.1×106=0.
00066666E3: Elastic modulus of steel pipe (kgf/cm
2) The elastic modulus of high-strength concrete 6 is 4 after completion and hardening.
00,000 kgf/cm2, but the elastic modulus Ec1 at the time of stress introduction is assumed to be 375,000 kgf/cm2 since the concrete is not considered to be in a completely hardened state.

According to a trial calculation, the tensile stress of steel pipe 1 is 1000 kgf/
cm2, it balances the compressive stress of concrete b.

The distortion degree εs2 of the steel pipe 1 at this time is εs2=1000
/2.1×106=0.00047619 The distortion degree εc of concrete b at this time is εc=εs1−εs2 =0.00066666−0.00047619=0.
00019047 Therefore, the compressive stress σc of the concrete corresponding to the above distortion degree εc is σc=εc×Ec1 =0.00019047×3.75×105=71.4
The balance between the total tensile force T of the 2625 kgf/cm2 steel pipe 1 and the total compressive force C of the concrete b is estimated.

T=1000×70=70,000kgC=71.42
625 x 986 = 70.426 kg As mentioned above, both forces are almost balanced and stable. In other words, when prestressing is introduced, the tensile stress of steel pipe 1 increases by 400% from the initial tensile stress level.
kgf/cm2 decreases to 1000 kgf/cm2.

Next, the decrease in the tensile stress of the steel pipe 1 due to drying shrinkage of the concrete b is calculated. If the degree of distortion of drying shrinkage of concrete b is assumed to be 0.0003, the corresponding steel pipe 1
The reduced tensile stress σ31 is σ31=0.0003×2.1
×106=630kgf/cm2 Therefore, the tensile stress σ32 remaining in the steel pipe 1 is σ32=1000-630=37
0 kgf/cm2 The stress σ32 of the steel pipe 1 above corresponds to the vertical axial force of the superstructure acting on the TPSC pile a installed underground and the creep of concrete due to residual prestress, and the residual tension of the steel pipe 1 From the point in time when stress σ32 is eliminated in accordance with the creep of concrete b, it is assumed that the creep occurring in concrete 6 will be extremely small.

The degree of distortion of concrete at the end of creep can be obtained by keeping each condition constant, so the above σ3
By conducting experiments in which 2 is varied in various ways, it will be possible to ascertain an almost accurate value, and thereby a correct empirical formula will be obtained. From the above, the essential difference between the TPSC pile of the present invention and the conventional SC pile is that the steel pipe 1 of the TPSC pile a is not affected by drying shrinkage of concrete at all, and is only slightly affected by creep of concrete. As a result, the increase in the compressive stress of the steel pipe due to the total shrinkage of concrete will be very small or almost zero. The total increase in compressive stress due to concrete creep is added to the total increase in compressive stress due to concrete creep, and the axial load burden from the simple calculation of the superstructure acts on this, so in some cases, the constant compressive stress of the steel pipe is likely to be around 800 kgf/cm2. The difference mentioned above is that after manufacturing both piles,
It does not appear in a bending test performed within a few days. If both piles are left under pressure equivalent to the axial force of the superstructure for at least one year, and then a bending test is performed while the concrete is shrinking, a clear difference between the two piles will appear. Dew. It is expected that the gap between the two will further widen in two or three years. In the above explanation, drying shrinkage and creep of concrete were explained separately, but in SC piles that are used soon after manufacture, the shrinkage of both occurs almost simultaneously, so both can be considered as Although these cannot be distinguished, the rate of progression of these shrinkages becomes extremely slow within about 3 years, and as a result, the total shrinkage of concrete will be as explained separately. In other words, the TPSC pile of the present invention applies tensile force in advance to a steel pipe that will receive a large compressive force in the future due to drying shrinkage and creep of concrete, and offsets the tensile force of the steel pipe with the shrinkage that occurs in the concrete. This prevents harmful and unnecessary compressive stress from occurring in steel pipes.

Next, we prepared a TP with protrusions 2 on the inner surface of the steel pipe 1 shown in Fig. 1.
A feature of SC pile a is that when the pile is subjected to bending force during an earthquake, the steel pipe 1 and the concrete b literally behave as one unit due to the action of the ridges 2, causing hair-like cracks in the concrete b on the tensile side. Even if this occurs, the cracks will not expand due to the restraining force of the protrusions 2, and the tensile resistance of most of the concrete b with a healthy structure will work effectively, and the horizontal bearing capacity will particularly increase. can give. Considering this valuable feature together with the rare feature that harmful pre-compression force does not act on the steel pipe 1 of the outer shell, it is found that the plate thickness is 4.5 mm and 6 mm.
The seismic capacity of the TPSC pile a of the present invention using the steel pipe 1 is as follows:
It is expected that these piles will correspond to conventional SC piles using higher grade steel pipes with plate thicknesses of 6 mm and 9 mm, respectively. Note that the ridges 2 provided on the steel pipe 1 are not necessarily limited to spiral ridges, but may also be hoop-shaped ridges that are perpendicular to the axial direction of the pile, or lattice-shaped ridges as seen on Kitahashima steel plates.

The solid line part shown in Figure 1 shows the TPSC pile a after concrete b has finished shrinking, and the part shown by the dashed-dotted line (enlarged) shows the state when the initial tension is applied to the steel pipe 1. shows the elongation of steel pipe 1. In Fig. 2, a steel band 7 is welded to the outer surface of the end of the steel pipe 1 on the fixed side, and this steel band 7 is brought into contact with the end of the main body frame 8, so that one end of the steel pipe 1 is easily attached to the main body frame. , and pull the upper end plate 3 at the other end.
This is a simplified pre-tensioning process for the steel pipe 1. In addition, when using a steel pipe 1' with a smooth inner surface as shown in Fig. 2, the thickness of the steel pipe 1' becomes thinner at the time of initial tensioning, the inner diameter of the steel pipe 1' expands slightly, and the axial direction of the steel pipe increases as the concrete shrinks. As the elongation of the steel pipe decreases, the inner diameter of the steel pipe returns to its original size, exerting some force on the integration of the steel pipe 1' and the concrete b, but it is not clear whether it will become a decisive force for integration. So,
In this case, it is desirable to mix an expanding agent into the concrete. However, when using a steel pipe 1 with protruding ridges 2 protruding from the inner surface as shown in Fig. 1, the concrete b and steel pipe 1 are integrated together due to the structure of a large number of ridges 2, as well as the above-mentioned restoring action of the inner diameter of the steel pipe. , so it is not necessarily necessary to mix in an intumescent material.

As mentioned above, the TPSC pile of the present invention is a very simple method in which concrete is cast and formed inside the steel pipe with initial tension applied to the steel pipe, and it is possible to create a TPSC pile using a very simple means of pouring and forming concrete into the steel pipe while applying an initial tension to the steel pipe. It does not generate compressive stress and has the feature of exceptionally increasing the horizontal bearing capacity and axial pressure bearing capacity of foundation piles that are installed underground and receive the axial force of the superstructure, and can withstand a dramatic increase in axial force during earthquakes. In addition, it can be piled to withstand huge bending moments during earthquakes, making it highly useful as earthquake-resistant foundation piles.

[Brief explanation of drawings]

Figure 1 is a vertical cross-sectional view of the TPSC pile of the present invention, including a partial external view, and Figure 2 is a partial external view showing a steel band connected to the end of the steel pipe fixed to the main body frame on the fixed side. It is a local sectional view. In the drawings, reference numerals a... TPSC piles, 1 and 1'... steel pipes on the outer shell, 2... protrusions provided on the steel pipes, 3... upper end plate, 4... joint end plate, 5... screw holes provided in the end plate, 6...High strength concrete,
7... Steel band connected to the end of the steel pipe, 8... Main body frame.

Claims (1)

[Claims] (1) A prestress based on the tensile stress of the steel pipe, which is generated by applying a tension below the yield point to the steel pipe that forms the outer shell of the pile, is introduced into the concrete inside the steel pipe, and this prestress is applied to the drying of the concrete. An SC pile incorporating a false prestress characterized by being able to be eliminated as shrinkage and creep progress.