About the design and construction of pier foundation?

On the design and structure of pier foundation, what is the specific content? Here, Zhong Da Consulting will answer your questions.

First, the scope of application of pier foundation:

Independent rigid foundations with a buried depth of more than 3m and a diameter of not less than 800mm, a ratio of buried depth to pier diameter of less than 6 or a ratio of buried depth to belled diameter of less than 4 can be designed as pier foundations. The effective length of pier should not exceed 5m.

Pier foundations are mostly used in multi-storey buildings. Because the foundation area is calculated according to the natural foundation design method, the single pier load test is omitted. Therefore, it is more popular in the case of tight construction period.

The pier foundation construction should adopt the method of digging (drilling) hole piles, with or without expanding the wall to form holes. Considering that when the buried depth is too large, it is unrealistic to design according to the pier foundation method, the limit of length-diameter ratio and the limit of effective length not exceeding 5m are stipulated to distinguish it from manual digging piles. When it exceeds the limit, it should be designed and tested according to the digging pile. From the bearing capacity analysis alone, the design method of pier foundation is safe.

Second, the design of pier foundation shall meet the following requirements:

1. The lateral friction of pier body is not considered in the calculation of the bearing capacity characteristic value of single pier or pier bottom area, and the bearing capacity characteristic value of modified bearing layer or determined according to shear strength index is adopted as the bearing capacity characteristic value of pier bottom resistance. The characteristic value of bearing capacity of rock bearing stratum is not modified by depth and width.

2. The determination of the bearing capacity characteristic value of the bearing layer shall conform to the provisions of Article 5.2.3 of the national standard Code for Design of Building Foundations (GB50007—2002).

The characteristic value of pier bottom bearing capacity of Grade A design buildings can be determined by plate load test of pier bottom in hole, deep plate load test and spiral plate load test. For the pier with small load, the vertical load test of single pier can also be directly carried out, and the characteristic value of bearing capacity of single pier can be directly determined according to the vertical load test method of single pile.

When the buried depth of the pier is more than 5m and the soil strength around the pier is relatively high, when the characteristic value of the bearing capacity of the bearing layer at the bottom of the pier is determined by formula calculation, indoor test, table lookup or other in-situ test methods (except load test), it can be multiplied by the adjustment coefficient 1. 1, and the rock foundation will not be adjusted.

3. The calculation of pier concrete strength shall comply with the provisions of Article 8.5.9 of the national standard Code for Design of Building Foundations (GB50007—2002).

4. The calculation of pier bottom pressure, soft substratum at pier bottom and settlement of single pier shall comply with the relevant provisions of chapter 5 of the national standard Code for Design of Building Foundation (GB50007—2002).

Three, the pier structure shall meet the following requirements:

1. The strength grade of pier concrete shall not be lower than C20.

2. When the pier adopts structural reinforcement, the longitudinal reinforcement should not be less than 8 φ 12 mm, the reinforcement ratio should not be less than 0. 15%, the length of longitudinal reinforcement should not be less than13 of pier height, and stirrup φ 8 @ 250 mm ..

3. For the pier foundation with one column and one pier, the connection between column and pier and the structure of pier cap (or cap) should be comprehensively determined according to the design grade, load size and coupling beam arrangement, and cap or pier can be directly connected with column. When the pier is directly connected with the column, the minimum distance between the column edge and the pier periphery should meet the requirements of Table 8.2.5-2 of the national standard Code for Design of Building Foundations (GB50007—2002), and the local bearing capacity should be checked. When the connection between column and pier cannot meet the requirements of fixed connection, coupling beams should be set in two directions, and the section and reinforcement of coupling beams should be determined by calculation.

The pier foundation under the wall is a multi-story brick-concrete structure building, and the horizontal force is not considered in the design. The connection between the foundation beam under the wall and the pier top only needs to consider the structural requirements, and the steel dowel connection can be adopted. A pier cap with the same cross section as the pier top can be set, and the bottom of the pier cap can be the same as the bottom elevation of the foundation beam, and can be poured with the foundation beam at one time. Setting a pier cap at the pier top can ensure the integral connection between the pier and the foundation beam, and its steel bar structure can refer to the beam-column connection at the top of the frame, and should meet the requirements of steel bar anchorage length.

4. The method of manual excavation and mechanical drilling should be adopted for the hole formation of pier foundation. The expansion diameter of pier bottom should not be greater than 2.5 times the diameter of pier body.

5. When the elevation of adjacent piers is consistent, the piers should be arranged according to the requirements of the upper structure and construction conditions, and the spacing between piers can be unlimited. When the bearing stratum fluctuates greatly, the relationship between the height difference of adjacent piers and the center distance of piers should be comprehensively considered to check the stability of the bearing stratum. If not, you can adjust the pier distance or pier bottom elevation.

6. The depth of the pier bottom entering the bearing stratum should not be less than 300 mm When the bearing stratum is moderately weathered, slightly weathered and unweathered rock, the pier bottom can be directly placed on the rock surface under the condition of ensuring the stability of the pier foundation. When the rock surface is uneven, it should be leveled or chiseled into steps.

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