RU2334843C2 - Aseismic pile base - Google Patents

Abstract

FIELD: building.

SUBSTANCE: invention refers to area of building and can be used at erection of the aseismic pile bases of buildings and constructions in seismic areas with intensity of earthquakes of 7, 8 and 9 points. The aseismic pile base consists of stilts set down in rows with head, an intermediate setting plate from grained materials and monolithic raft foundation. A sole of a monolithic reinforced-concrete raft foundation is placed on a mark of the planned ground surface; its top is combined with a mark of a sole of a building floor. There is a sliding layer between a raft foundation sole and an intermediate setting plate from grained materials. From the interior of a building a lateral and top horizontal surface of a raft foundation is separated from environment by a lightweight-aggregate concrete layer. The width of a raft foundation is accepted so that from exterior sides of extreme rows of stilts heads to exterior sides of a raft foundation in each side the distance is equal to the size of residual displacement from earthquake U₀ (I), mm, which is defined from expression lg U₀ = - 4.6 + 0.78 I, where I is an earthquake force, quantity of rows of stilts in a continuous footing being not less than two, and in separately standing base under a column quantity of stilts not less than four.

EFFECT: reduction of horizontal seismic impacts on an elevated part of a building.

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Description

The invention relates to the field of construction and can be used in the construction of earthquake-resistant pile foundations of buildings and structures in seismic areas with an earthquake intensity of 7; 8 and 9 points.

The construction of a pile foundation is known, consisting of a group of piles, a monolithic reinforced concrete grillage, rigidly connected to the heads of piles, located above the grillage of a part of the foundation (the column is in the case of a stand-alone foundation or the wall of a strip foundation), to which the load from the building is transferred. Such a foundation design has the following disadvantages. Seismic action from the surrounding soil is transmitted to piles, a monolithic grillage, and the underground part of the foundation, which oscillates during the passage of seismic waves and receives residual seismic displacements. Piles with low mass could potentially oscillate and move with the ground. But due to the rigid connection with the grillage and the entire mass of the building, which has tremendous inertia, shear forces arise in them. Current standards (SNiP 2.02.03-85 *. Pile foundations / Gosstroy of Russia. - M .: FSUE TsPP, 2004. - 46 p.) Determine the decrease in the bearing capacity of such piles when calculating a special combination of forces by about 25 ... 30 % The side surface of piles with a length h _d below the connection of piles with a grillage is turned off, the length of this section can reach 5.3 m (Foundations, foundations, and underground structures: Designer's Guide. M .: Stroyizdat, 1985. P.289). The calculated soil resistance under the lower end of the piles is reduced - R _eq = Rγ _eq1 . The coefficient of operating conditions γ _eq1 when piles are buried in semi-solid and refractory clay soils with an increase in the design seismicity of the building from 7 to 9 points decreases from 0.95 to 0.7.

The closest in technical essence to the claimed foundation design is a pile foundation with an intermediate cushion selected as a prototype (Foundations, foundations and underground structures: Designer Handbook. -M: Stroyizdat, 1985. P.12.3; Ilyichev VA, Mongolov Yu. V., Shaevich V.M. Pile foundations in seismic areas. M., 1983). It consists of piles with heads, an intermediate cushion of granular materials (crushed stone, gravel, sand of large and medium size) with a thickness of 40 ... 60 cm above the heads, compacted to the maximum possible density. A monolithic grillage rests on an intermediate pillow, above which in the strip foundation there are foundation walls, and in a separate foundation - a glass-type kneecap, which forms a whole with the grillage plate. When located between the sole of the grillage and the heads of the piles of the intermediate pillow, the piles, being not connected with the monolithic grillage and the entire mass of the building, will be able to oscillate and move together with the surrounding soil during an earthquake. They do not cause shearing forces, in turn, with horizontal residual displacements of the soil, additional horizontal force is not transmitted from the piles to the grillage. As a result, the bearing capacity of the piles increased (in accordance with the current SNiP 2.02.03-85 *. Pile foundations. M., 2004): the calculated soil resistance along the lateral surface f _eq acts along the entire length of the pile (that is, h _d = 0), and the calculated soil resistance under the lower end of the pile not only does not decrease (see earlier, γ _eq] = 0.7 ... 0.95), but increases - γ _eq1 = 1.2 (SNiP 2.02.03-85 *, Section 11.14). All parameters of the foundation, including the number of piles, the distance between the rows of piles and between piles in a row, are determined by the selection method: they are set by all parameters of the pile foundation, using the SNiP formula, determine the design load on the pile N, kN and check that it does not exceed the allowable load on the pile P = F _d / γ _k (see below), and so that the margin of bearing capacity is within 10%. The two equations mentioned are given in SNiP 2.02.03-85 *. Pile foundations. M., 2004 - formulas (2) and (3). The calculated load on the pile N, kN for the foundation with vertical piles is determined by the formula

where N _d is the calculated compressive force, kN;

M _x , M _y - calculated bending moments, kNm; relative to the main central axes x and the plan of piles in the plane of the sole of the grillage;

n is the number of piles in the foundation;

x _i , y _i - the distance from the main axes to the axis of each pile, m;

x, y are the distances from the main axes to the axis of each pile for which the calculated load is calculated, m.

A single pile in the foundation should be calculated based on the condition

where N is the design load transferred to the pile, determined in accordance with formula (1);

F _d is the calculated bearing capacity of the soil of the base of a single pile, hereinafter referred to as the bearing capacity of the pile;

γ _to - the coefficient of reliability, taken equal to 1.2; 1.25; 1.4 depending on the method for determining the bearing capacity of piles.

When taking into account short-term loads (wind, crane, seismic) in combinations of forces, the load perceived by extreme piles can be increased by 20%:

Formula (3) - for the case when the moment acts in one plane. But such a foundation design has the following disadvantages. On the vertical surfaces of the grillage and underground parts of the foundation buried in the ground, horizontal seismic loads will act, which will be transmitted to the aboveground part of the building. With radial residual seismic displacements of the soil U _o mm, which are associated with the intensity of earthquakes I, points, and dependence (Greizer V.M. Seismic data on residual displacements during explosions and earthquakes // DAN. 1989. V.306. No. 4. P. 822-825):

pile displacements and intermediate preparation relative to the grillage will also occur. In the case of a strip foundation under the wall, the use of single-row arrangement of piles becomes impossible - between the center of the wall and the center of this row of piles appears - as a result of residual displacements according to (4) - eccentricity. Huge horizontal friction forces will be applied to the sole of the grillage, reaching 40% of the vertical component of the seismic load. N _a is the coefficient of friction of the bulk material of the intermediate cushion on concrete - 0.4 (Foundations, foundations and underground structures, 1985, p. 12.3). The design of the building is difficult, namely, the calculation of the stability (immovability) of the entire building. Usually, when an object is affected by horizontal forces, its displacement is prevented by friction forces along the base of the foundation, directed in the opposite direction to these horizontal forces. But during the process of soil displacement, the friction forces will have the same direction with the forces acting on the vertical surfaces of the underground part of the foundation.

The aim of this invention is to reduce horizontal seismic effects on the aerial part of the building.

The goal is achieved as follows. The sole of the monolithic reinforced concrete grillage is placed at the mark of the planned surface of the soil, its top is combined with the mark of the sole of the floor of the building, a sliding layer is placed between the sole of the grillage and the intermediate preparation, the side and upper horizontal surface of the grillage is separated from the environment by a layer of light concrete, and the width the grillage is taken so that from the outer faces of the heads of the extreme rows of piles to the outer faces of the grillage in each direction, the distance is equal to U _о (I), m m, which is determined from the dependence logU _o = -4.6 + 0.78I, where I is the earthquake score, and the number of rows of piles in the strip foundation is at least two, and in a separate foundation under the column the number of piles is at least four.

Features of the proposed foundation design are shown - see figure 1 - on the example of a pile earthquake-resistant strip foundation, depicted at the time of completion of its construction. Figure 2 shows the same foundation after the earthquake; the epicenter in this situation is located to the left of the foundation, the piles received an offset of U _о (I) to the right of the center of the wall.

An earthquake-resistant pile foundation (Fig. 1) consists of several rows of piles 1 (in this example, a foundation with a two-row arrangement of piles is shown), which are made with monolithic reinforced concrete heads 2. The foundation includes a monolithic reinforced concrete grillage 3, an intermediate cushion 4 of granular materials ( crushed stone, sand-gravel mixture, gravelly sand, coarse or medium coarse) compacted with thin layers to maximum density, and a sliding layer 5. To reduce horizontal seismic effects d, transferred to the building, the sole of the grillage is aligned with the mark of the planned surface DL, and the top of the grillage coincides with the mark of the sole of preparation for the floor of the first floor. As in the prototype, the main bearing element of the foundations - piles 1 with heads 2 - are disconnected from the grillage 3 by an intermediate cushion 4, which allows them to oscillate and receive residual displacements together with the surrounding soil. At the time of completion of the foundation between the geometric axis (center) of the foundation 6 and the geometric axis 7 of the grillage 3 and the walls of the aboveground part of the building 8, which (geometric axes 6 and 7) should ideally coincide, there may be a random eccentricity Δе associated with a deviation in production Center works 7 grillage from design position. Its value can be taken equal to 1 ... 2 cm. Since the working material of the grill 3 is reinforced concrete with the use of heavy concrete, the grill 3 will be a bridge of cold. In order to avoid condensation of moisture on the cold surfaces of the grillage 3 indoors, its lateral and upper surfaces are separated from the environment by a layer of light and warm concrete 9; the top of this layer coincides with the floor mark of building 10 - a mark of ± 0,000 in figures 1 and 2.

Figure 2 shows the same foundation at the time of the end of the active phase of the earthquake, when piles 1, intermediate preparation 4 will receive the same residual displacements (according to (4)), as well as the surrounding soil. The eccentricity between centers 7 (grillage 3 and walls 8 of the aboveground part of the building) and 6 piles 1 increased by the value of U _о (I) according to (4). For seven-, eight-, and nine-point earthquakes, the residual displacements are equal, respectively: U _o (7) = 7.24 mm, U _o (8) = 43.7 mm, U ₀ (9) = 263.0 mm. Given these displacements, the width of the grillage, which is recommended to be equal to the distance between the outer faces of the heads of the extreme rows of piles, is increased in each direction by the value U ₀ (I). And when determining the distance between the rows of piles 1 and between piles 1 in a row, not only the vertical component of the seismic load N _a , but also the additional seismic moment M _eq are taken into account.

In an 8-point seismic zone, in a building with a rigid structural design under a centrally loaded middle longitudinal wall at the level of the foot of the grillage (at the DL mark), the linear load to the main combination of forces q _I = 720 kN / m; bedding of soils: IGE-1 - fine sand, loose, h ₁ = 4.0 m; IGE-2 - plastic sandy loam, consistency I _L = 0.4, porosity coefficient e = 0.75; IGE-3 - semi-solid clay, I _L = 0.1, porosity coefficient e = 0.80. The soils of IGE-1 and IGE-2 belong (in accordance with Table 1 of SNiP II-7-81 *) to the III category, their total thickness - 6 m is more than half in a 10-meter base, therefore, the entire base belongs to III categories of seismic properties, and the seismicity of the object I = 8 + 1 = 9 points. Then (according to clause 3.37 of SNiP P-7-81 *) the vertical seismic load will be 30% of the corresponding vertical static load, that is, q _I , _eq = 720 · 1.3 = 936 kN / m. Having taken the mark of the bottom of the head at a depth of 0.9 m from the sole of the grillage and pile C7-30, 1.6 m deep in semi-solid clay, we obtain according to the formulas and tables SNiP 2.02.03-85 * bearing capacity F _d = 812.2 kN , and the permissible load on the pile P = F _d / γ _k = 812.2 / 1.4 = 580.2 kN. The corresponding values calculated for a special combination of efforts are the seismic: F _{d, eq} = 882.6 kN, P _eq = 630.4 kN, 1.2 P _eq = 756.5 kN. The calculated value of the pitch of the piles for the main combination of loads a = P / q _I = 580.2 / 720 = 0.806 m. Based on the main combination of loads, we take the two-row arrangement of piles (staggered): the distance between the rows of piles 2 y _N = 0, 45 m, the actual distance between the axes of the piles must be at least three diameters of the piles 3 × 0.3 = 0.9 m; L = (0.80 ² +0.45 ² ) ^0.5 = 0.92 m. In each row of piles after 1.6 m, the actual pitch of the piles is a = 0.80 m.

Calculation of a special combination of efforts. At the beginning of the earthquake, the residual displacements are close to zero (that is, only the vertical component of the seismic load N _a acts).

When a _N = 0,80 m average value of the load on the pile will be N _{d, eq} = 936 × 0,8 = 748,8 kN> P _eq = 630,4 kN, which is unacceptable. Pile pitch a _N (for N _eq > 0 and M _eq = 0) of a two-row pile foundation

and the distance between the two rows of piles

.

.

According to (5) and (6) a _N = 630.4 kN / 936 kN / m = 0.674 m, 2y _N = 0.596 m; we take a _N = 0.65 m and the distance between two rows of piles 2y _N = 0.65 m, for _N = 0.325 m the distance between the axes of the piles along the diagonal L = 0.65 × 2 ^0.5 = 0.92 m, which more than 0.9 m.

Currently, it is believed that the direction of action of seismic forces can be arbitrary; we accept the worst case scenario - residual displacements will be normal to the longitudinal axis of the wall and foundation. With a two-row arrangement of piles in the strip pile foundation from the condition - the maximum load on the pile N _st.max (in accordance with clause 3.10; 3.11 SNiP 2.02.03-85 *, M., 2004) should not exceed 1.2P _eq - should :

where a _NM is the pitch of the piles, taking into account the actions of N and M; a _NM ≤a _N.

Taking into account (5), we have relations showing how the pitch of the piles and the distance between the rows of piles depend on the seismic eccentricity, which is proportional to the bending moment -M _eq = N _a e _eq :

When designing in 7- and 8-point seismic zones, we encounter cases of "small eccentricities": from (8) it follows that for e _eq ≤ 0.2 for _{N, i the} pitch of the piles and _NM can be taken to be the same as a _N. Accepted 0.2u _{N, I} = 0.2 × 0.325 = 0.065 m. Take Δе = 0.01 m, then in the 7-point zone e _eq = 0.01 + 0.0073 = 0.0173 m <0.2u _{N, i} = 0.065 m. And in the 8-point zone e _eq = 0.01 + 0.0434 = 0.0534 m <0.2y _{N, I} = 0.065 m. Therefore, in 7- and 8-point seismic zones due to the small residual seismic displacement parameters a _N = 0.65 m and 2y _N = 0.65 m, taken into account the action of only the vertical component of the seismic load (or at the initial moment of the earthquake, when U ₀ (I) = 0 ) remain unchanged. The 9-point seismic zone e _eq = 0,273 m> 0,2u _{N, I} = 0, 065 m, the distance between piles in a row between two rows of piles to take into account (8) and (9). Increase the distance between the rows of piles 2y _NM = 0.80 m, for _NM = 0.40 m; then, according to (8), the pitch of the piles is a _NM = 0.460 m. We accept a _NM = 0.45 m, 2 at _NM = 0.80 m (note that when calculating the main combination of loads, the following dimensions were taken: a _N = 0.80 m , 2y _NM = 0.45 m, that is, when taking into account M _{eq, the} need for piles increased 1.78 times).

We verify that inequality (3) holds. The calculated wall section with a length l = 0.45 m × 6 = 2.7 m (6 piles in a staggered pattern, between the rows of piles - 0.8 m), load N _{d, eq} = 936 × 2.7 = 2527.2 kN bending moment from the wall plane: М _eq = 2527.2 kN × 0.273 m = 689.9 kN.m according to (3).

N _{max / min} = 2527.2 / 6 ± 689.9 × 0.4 / 6 × 0.4 ² = 412.2 ± 287.46 kN.

The average load on the pile N _{d, eq} = 412.2 kN <P _eq = 630.4 kN; N _max = 699.46 kN <1.2P _eq = 756.5 kN. The pile is not loaded at (57.04 / 756.5) × 100% = 7.5% <10%.

A freestanding foundation for the column is more expedient to carry out not of the glass type, but on the releases of reinforcement from the slab part of the grillage. The sizes b and l of a monolithic grillage increase in each of the four sides by U ₀ (I) in comparison with the distance between the outer faces of the heads of the extreme rows of piles (the last sizes are according to the recommendations: Foundations, foundations, and underground structures: Designer's Guide. -M. : Stroyizdat, 1985, p. 291 - excluding residual displacements of soil and piles during earthquakes). The number of piles in the foundation for the column is at least four, since when calculating the pile foundation, the residual displacement of the soil together with piles by the value U ₀ (I) is possible in any direction.

Claims (2)

1. An earthquake-resistant pile foundation consisting of rows of piles with heads, an intermediate cushion of granular materials, a monolithic grillage, characterized in that the sole of the monolithic reinforced concrete grillage is placed at the mark of the planned surface of the soil, its top is combined with the mark of the sole of the floor of the building, between the sole of the grillage and an intermediate cushion of granular materials have a sliding layer, on the inside of the building a side and upper horizontal surface of the grillage they are separated from the environment by a layer of light concrete, while the width of the grillage is taken so that from the outer faces of the heads of the extreme rows of piles to the outer faces of the grillage in each direction, the distance is equal to the value of the residual displacement from the earthquake U ₀ (I), mm, which is determined from the dependence log U ₀ = -4.6 + 0.78 I, where I is the earthquake score, and the number of rows of piles in the strip foundation is at least two, and in a separate foundation under the column the number of piles is at least four. 2. The earthquake-resistant pile foundation according to claim 1, characterized in that the reinforcement of a monolithic grillage of a freestanding foundation under the column is made in the form of an armature frame, including transverse reinforcement and releases of working reinforcement for connection with the working reinforcement of the column.

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