RU2307212C2 - Pile foundation for seismic territories - Google Patents

Abstract

FIELD: construction, particularly pile foundations to be erected in seismic territories characterized by earthquake intensities of 9 or 10 points.

SUBSTANCE: pile foundation has intermediate pad arranged so that upper pad surface is flush with designed ground surface mark. During foundation erection two or three pile rows are arranged directly under grillage. One or two pile rows used for foundation base reinforcement outside foundation contour extend in both directions from foundation center. Sliding member having low friction coefficient is arranged at grillage base. The sliding member is covered with concrete mattress. Reinforcement bars of monolithic grillage used for independent column foundation erection may be formed as reinforcing cage. The reinforcing cage includes transversal reinforcement bars and free length of maim reinforcement bars.

EFFECT: decreased horizontal seismic impact to building superstructure.

2 cl, 3 dwg

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 highly seismic areas with earthquakes of 9 and 10 points, taking into account large - up to 1.6 m - residual soil displacements.

The construction of a pile foundation is known (Foundations, foundations and underground structures. M., 1985. Fig. 8.14), 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 (sub-column - in the case of a freestanding foundation or wall of the strip foundation), to which the load from the building is transferred. Such a foundation design has the following disadvantages. Horizontal piles of seismic grillage and the underground part of the foundation are transmitted by horizontal seismic action from the surrounding soil, 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) determine the decrease in the bearing capacity of such piles when calculating a special combination of efforts 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 construction of the foundation is a pile foundation with an intermediate cushion selected as a prototype (Foundations, foundations and underground structures. M., 1985. P.12.3 .; Ilichev V.A., Mongolov Yu.V., Shaevich V.M. Piled foundations in seismic regions. M., 1983). It consists of piles with heads, an intermediate cushion of bulk materials (crushed stone, gravel, sand of large and medium size) 40 ... 60 cm thick above the heads, compacted to the maximum possible density. A monolithic grillage rests on an intermediate cushion, above which in the strip foundation are the foundation walls, and in a separate foundation - a glass-type column, which forms a whole with the grill 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, are given the opportunity 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 load-bearing capacity of piles increased (in accordance with current SNiP 2.02.03-85 *. Pile foundations): 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 estimated soil resistance under the lower end of the pile R _eq not only does not decrease (see earlier, γ _eq1 = 0.7 ... 0.95), but increases - γeq ₁ = 1.2 (SNiP 2.02.03-85 *, item 11.14) . But such a foundation design has the following disadvantages. On the vertical surfaces of the grillage and the 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. For residual seismic displacements of the soil U ₀ , mm (Greiser V.M. preparation regarding grillage. In the case of a strip foundation under the wall, the use of a single-row arrangement of piles becomes impossible due to the eccentricity resulting from the residual displacements. Additionally, the sole will be applied grillage huge horizontal friction force reaching 40% of the vertical component of the seismic load N _a - coefficient of friction of the bulk material of the intermediate cushion concrete - 0.4 (Grounds, foundations and subterranean installations 1985, item 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 displacement of the soil, the friction forces will have the same direction with the forces acting on the vertical surfaces of the underground part of the foundation.

In addition, in highly seismic areas, residual soil displacements U ₀ (I), mm, sharply increase. If in 6-, 7-, 8-point areas they will be 1.2; 7.24 and 43.7 mm, respectively, then in 9-point and 10-point areas, they increase to 263 and 1585 mm. There are two dangers: if there is no cut-off of the underground part of the building relative to the above-ground part, which, as a rule, has a significant mass and, consequently, inertia, then the piles will simply “leave” from under the foundation (for example, by 1585 mm) and the grillage will be pressed to the base, which will lead to the destruction of the foundation, and then the building.

The technical result of the invention is the reduction of horizontal seismic effects on the building.

The specified technical result is achieved as follows. The top of the intermediate cushion is placed at the level of the planned ground surface near the DL building and leveled with a thin - 3 ÷ 5 cm - cement mortar layer, a sliding element of two layers of material with a low coefficient of friction is placed on it and it is covered with a concrete preparation layer, placed directly under the grill in the direction under consideration n _R rows of piles - two or three, which is determined by comparing the load on the foundation and the bearing capacity of the piles, the width of the grill B _{R is} taken equal

where A _b is the pitch of piles in the direction in question,

except n _R rows of piles, disposable directly under grillage, arrange n _Coll rows of piles of perimeter reinforcement base in each direction from the base center with the step of piles A _b: 9-point zone of at least one row, and a 10-point area - at least two rows, the distance between the extreme rows of piles located under the grillage and the extreme rows of piles of marginal reinforcement take at least the value of residual seismic displacements of the soil U ₀ (I _max ), mm with a possible earthquake of maximum intensity I _max for a given object:

and the distance between the piles in the row and between the rows of piles is taken taking into account the possible additional seismic moment:

where N _a is the vertical component of the seismic load, e _{eq, max} is the possible maximum distance between the center of the grillage and the center of the group of piles located under the grillage, defined by the equation

where A _og - the width of the head;

the width of the intermediate cushion, a thin layer of cement mortar along its top, a sliding element of two layers of material with a low coefficient of friction is taken so that they extend beyond the axis of the extreme row of contour reinforcement piles to each side from the center of the foundation by ΔB, defined by the equation:

In an earthquake-resistant pile foundation for highly seismic areas, the reinforcement of a monolithic grillage of a separate foundation for a column is made in the form of an armature frame, including transverse reinforcement and the release of working reinforcement for connection with the working reinforcement of the column.

The features of the proposed foundation design are shown on the example of a pile strip foundation. In Fig.1 and Fig.2 shows a cross section and a fragment of the plan of the pile strip foundation at the time of completion of its construction. Figure 3 shows the same foundation after the earthquake; the piles received an offset of U ₀ (I) to the right of the center of support of the aerial part.

The pile foundation for highly seismic areas (see Fig. 1) consists of several rows of piles 1 and 2 with heads 3. Directly under the grillage 4, on which the outer wall 5 of the aerial part of the building rests, there are at least two rows of piles 1. In this embodiment three rows of piles 1 are shown; the number of rows of piles 1 - usually two or three - depends on the ratio of the bearing capacity of the piles and the load transferred to the foundation. Piles of marginal reinforcement 2 were also used; the number of rows of piles 2 in each direction from the center of the foundation — at least one row at objects with an earthquake intensity of 9 points and at least two rows — in a 10-point zone. Between the grillage 4 and the headers of the piles 3 there is an intermediate cushion 6, the top of which is raised and combined with the mark of the planned soil surface DL at the building to reduce the horizontal seismic effects transmitted to the aerial part of the building. Friction forces on the sole of the grillage can reach 40% of the vertical component of the load during seismic displacements of the soil. To reduce them, the top of the intermediate cushion 6 is leveled with a thin - 3 ÷ 5 cm - layer of cement mortar 7, a sliding element 8 of two layers of material with a low coefficient of friction (for example, of two layers of fluoroplastic) is placed on it and closed with a layer of concrete preparation 9 thickness, for example, 100 mm. Since the reinforced concrete grill 4 will be a cold bridge in the cold weather near the outer wall of building 5, the side and upper horizontal surface of the grill will be separated from the environment by the insulation layer 10, for example, a layer of light concrete, the top of which is combined with the floor mark of the building - mark ± 0,000. In the design and construction of the pile foundation for highly seismic areas, the position of the axis 11 of the wall 5 of the aboveground part of the building and the grill 4 are combined with the axis 12, the center n _{R of} piles 1, located directly under the grill 4. The distance from the extreme row of piles 1, located directly under the grill, to extreme number of piles 2 of perimeter base reinforcement in each direction from the center of foundation should not be less than the residual seismic ground displacements and the piles 1 and 2 during an earthquake while the maximum intensity I _max, in Possible at the site. For example, with a distance between rows of piles of 0.8 m (see figure 1 and figure 2), the specified distance is 0.8 * 2 = 1.6 m, which is more than the maximum seismic displacement of soil and piles in a 10-point zone - 1,585 m.

Figure 2 shows a fragment of the plan of the pile foundation with an intermediate cushion 6 at the time of completion of the construction. As shown above, the foundation can be built in a 10-point zone. Piles 1 and 2 are staggered, they have all the parameters (section and length of piles, piles head marks, pitch of piles) are taken the same. In particular, the same pitch of piles in rows 1 and 2 is taken: for residual seismic displacements of the soil together with piles, the rows of contour reinforcement piles 2 become piles 1 located directly under the grill 4. During residual seismic displacement of the soil together with piles 1 and 2, favorable and unfavorable combinations of the arrangement of piles under the grill 4. arise. When the piles move (to the right or left) by A _b / 2, there will be four rows of piles under the grill. This is a favorable arrangement of piles: in this example, the bearing capacity of the foundation should be provided by three rows of piles, which at the time of their construction are located symmetrically with respect to the geometric axis of the wall 5.

Figure 3 shows the pile foundation after the maximum residual seismic displacement of the soil and piles to the right. Two rows of piles that were before the displacement by the piles of the contour reinforcement 2, are located under the grillage and perceive the load transmitted by the grillage 4, that is, they became piles 1. This is the position of the piles when the outer face of the head of the third left pile 1 coincides with the outer face of the grillage 4, it is characteristic that the eccentricity - the distance between the geometric axis 11 of the wall and the grillage and the geometric axis of 12 piles 1 located under the grillage will be maximum - e = e _max and is determined by equation (4).

When building the foundation (see Fig. 1), the width of the intermediate cushion 6, a thin layer of cement mortar 7 along its top, the sliding element 8 of two layers of material with a low coefficient of friction is taken so that they extend beyond the axis of the extreme row of contour reinforcement piles in each side from the center of the foundation by ΔВ, determined by equation (5). This size is determined from the condition that even at a critical value of the residual seismic displacements, when the piles take the position shown in Fig. 3, there was a sliding element 8 under the entire grill 4 and horizontal seismic effects were transmitted to the grill 4 and the aerial part of the building 5 .

Claims (2)

1. A pile foundation for highly seismic areas, consisting of rows of piles with heads, an intermediate pillow made of granular materials, a monolithic grillage, characterized in that the top of the intermediate pillow is placed at the level of the planned ground surface near the DL building and is leveled with a thin - 3 ÷ 5 cm - a layer of cement mortar, a sliding element of two layers of material with a low coefficient of friction is placed on it and cover it with a layer of concrete preparation, directly under the grill ayut in this direction n _R rows of piles - two or three, which is determined from the comparison of the load on the foundation and the bearing capacity of piles grillage width B _R is taken equal to

wherein A _b - step piles in this direction, except n _R rows of piles, disposable directly under grillage, arrange n _Coll series of foundation piles of perimeter reinforcement in each direction from the base center with the step of piles A _b: 9-point area at least one series , and in the 10-point zone - at least two rows, while the distance between the extreme rows of piles located under the grillage and the extreme rows of piles of marginal reinforcement take at least the value of residual seismic displacements of the soil U _o (I _max ), mm in case of a possible earthquake and maximum intensity I _max for this object

and the distance between piles in a row and between rows of piles is taken into account the possible additional seismic moment

where Na is the vertical component of the seismic load, e _eq , _max is the possible maximum distance between the center of the grillage and the center of the group of piles located under the grillage, defined by the equation

where A _og - the width of the head; the width of the intermediate cushion, a thin layer of cement mortar along its top, a sliding element of two layers of material with a low coefficient of friction are taken so that they extend beyond the axis of the extreme row of piles of marginal reinforcement in each direction from the center of the foundation by ΔB, defined by the equation

2. An earthquake-resistant pile foundation for high-seismic areas according to claim 1, characterized in that the reinforcement of a monolithic grillage of a freestanding foundation for a column is made in the form of an armature frame, including transverse reinforcement and the release of working reinforcement for connection with the working reinforcement of the column.

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