Introduction to earthquake resistance of pile foundation?

Speaking of earthquake resistance of pile foundation, how to stipulate the basic situation of earthquake resistance of pile foundation at this stage, and what are the requirements for earthquake resistance structure of pile foundation? The following is the seismic related content of pile foundation combed by Zhong Da Consulting Bianxiao Company, and the basic situation is as follows:

In order to help relevant personnel understand the earthquake resistance of pile foundation, Zhong Da consulted and compiled relevant information, and the basic situation is as follows:

Pile foundation is a deep foundation (see figure) composed of piles and caps connecting the top of piles, or a single pile foundation connected by columns and piles, referred to as pile foundation. If the pile body is completely buried in the soil and the bottom of the pile cap is in contact with the soil, it is called low pile cap pile foundation; If the upper part of the pile body is exposed to the ground and the bottom of the cap is exposed to the ground, it is called high cap pile foundation. Building pile foundation is usually a low pile cap pile foundation. Pile foundation is widely used in high-rise buildings.

In order to help construction enterprise personnel understand the requirements of drawing pile foundation completion drawings, Zhong Da consulted and compiled relevant seismic data of pile foundation, and the basic situation is as follows:

(1) reinforcement of cast-in-place pile body

When there are liquefied soil or soft-hard interbeds in the pile length range, there are significant differences in seismic wave velocities in these soil layers with different properties, which leads to drastic changes in soil displacement near the soil layer interface, and the internal force of foundation pile is greater with soil deformation. According to the investigation of earthquake damage, it is found that this kind of bending-shear damage is not more serious than the damage of pile head, so it can be considered that the internal force of foundation pile caused by foundation soil displacement is in an order of magnitude with the pile head. In engineering practice, when the reinforcement of pile head is sufficient, extending longitudinal reinforcement and stirrup to the vicinity of liquefied soil or soft and hard interlayer should ensure its safety.

Therefore, it is stipulated that the reinforcement range of piles in liquefied soil is from the top of the pile to the bottom of the liquefied layer, and the depth into the stable soil layer is not less than α/4.0, and the diameter and spacing of longitudinal reinforcement and stirrup are the same as that of the pile top. When there are soft and hard layers in the pile length, the diameter and spacing of longitudinal reinforcement and stirrup should be the same as the pile top within 2 days below the soft layer.

(2) Prestressed concrete hollow pile

Prestressed concrete hollow piles are often used in areas where the overlying soil layer is weak, and the soil layers in these areas are mostly sand, silt, cohesive soil and muddy soil. In some areas, pipe piles pass through soft soil layer and are placed on bedrock, which makes the foundation piles bear large differential deformation of soil layer and produce large bending and shearing internal forces under earthquake action. In addition, the precast hollow pile has less reinforcement, fine reinforcement, small concrete section and low flexural and shear bearing capacity; The damage investigation of Hanshin earthquake also shows that precast hollow piles are seriously damaged, so it is not suitable to use precast hollow piles in areas above 8 degrees. It is not suitable for sites with liquefied soil layer (except for hollow pile squeezing to eliminate soil liquefaction) and extremely soft soil layer.

(3) Connection between pile caps and reinforced concrete columns

For multi-pile caps, the longitudinal main reinforcement of the column should be anchored into the cap, and should not be less than 35 times the diameter of the longitudinal main reinforcement (non-seismic); Under the action of earthquake, according to the investigation of earthquake damage, the longitudinal reinforcement of the column is not pulled out of the pile cap in the earthquake damage of building engineering, but the reinforcement is pulled out at the root of the viaduct column, which shows that it has good integrity for the building structure with more statically indeterminate times. However, for those frame structures located in high-intensity areas without basements, the longitudinal bars at the root of columns still need to be anchored reliably. Therefore, the anchorage length of longitudinal reinforcement should be multiplied by the coefficient 1. 15 for columns with seismic grade I and II. For columns with seismic grade III, the anchorage length of longitudinal main reinforcement shall be multiplied by the coefficient 1.05.

When the height of the pile cap does not meet the anchorage requirements, the vertical anchorage length should not be less than 20 times the diameter of the longitudinal main reinforcement according to the seismic requirements; The lower part of longitudinal reinforcement should be bent 90 degrees to the column axis.

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