Basic problems of the most popular fixed tower cra

Foundation problems of fixed tower cranes

in recent years, due to the rapid development of high-rise buildings, the use of fixed tower cranes has increased sharply in construction. In order to ensure the safe use of tower cranes, let's talk about the detection of fixed tower crane foundation and find that there are many unreasonable phenomena in the current tower crane foundation: on the one hand, there is a lot of waste; On the other hand, there are unsafe factors

l design requirements of foundation

fixed tower crane can be divided into: ① cast-in-place reinforced concrete integral foundation; ② It is a fixed foundation with underframe and pressure weight refitted from walking tower crane. At present, most of them adopt the first form. The foundation design of tower crane must meet the following three requirements

1) stability of tower crane. The stability of the tower crane refers to the characteristics that the tower crane can maintain the stability of the whole machine without tipping under various working conditions. It is one of the important factors to ensure the safe use of the tower crane. It is expressed by the stability coefficient m stability/M inclination. M stability is the moment (n m) generated by the dead weight, foundation weight and balance weight of the tower crane to maintain the stability of the tower crane; M tilt is the torque generated by the external force acting on the tipping tower crane (n is called. The stability coefficient changes with the change of working conditions, and the greater the stability coefficient, the better the stability of the tower crane. The stability requirements under various working conditions have been considered in the design of the tower crane, and its size and quality must meet the stability requirements when designing the tower crane foundation.

2) the strength requirements of the foundation. The structure inside the tower crane foundation shall have sufficient strength, that is, it can bear the vertical force, horizontal force and overturning moment acting on the foundation under various working conditions. When designing the tower crane foundation, it is necessary to check the strength of anchor bolts and embedded iron parts in the foundation and the anchoring force in the foundation under inspection

3) requirements for uniform settlement of foundation. The load on the tower crane foundation has been changing during the long-term use. If the foundation settlement is uneven, the verticality deviation of the tower crane will increase, which will affect the stability of the tower crane. Therefore, the foundation settlement is required to be uniform

2 problems in the foundation

2.1 problems in the foundation construction drawings of the manufacturer

1) the tower crane foundation construction drawings provided by the tower crane manufacturer are not perfect. Among them, the tower crane produced by Sichuan construction machinery factory provides relatively complete foundation construction drawings under various ground endurance conditions and different use heights, while the rest of the tower crane manufacturers only provide foundation construction drawings under one or two ground endurance conditions, Even the most basic parameters (such as the maximum overturning moment acting on the foundation, horizontal and vertical loads, etc.) required by the recommended leading enterprises to apply for the design foundation of the factory company of national and provincial key projects have not been provided, so the construction unit cannot redesign the tower crane foundation according to the actual endurance

2) for the same type of tower crane under the same foundation bearing capacity, the error caused by the above two factors in the foundation bottom area and quality in the foundation drawing provided by each manufacturer is characterized by a large difference in the impact on small load

3) the foundation drawing provided by most manufacturers requires the ground endurance to be 2x 105Pa, but the actual construction site generally cannot meet the above requirements

4) the foundation drawings provided by individual manufacturers cannot meet the stability requirements of the tower crane under various working conditions during detection, and there are potential safety hazards

5) mistakenly taking the load on the tower crane foundation in the working state as the calculation load, the designed foundation can not meet the requirements of strength and stability under the load in the non working state. Therefore, it is necessary to standardize the design of tower crane foundation

2.2 problems in foundation fabrication of the construction unit

the construction site conditions are complex and changeable. The foundation construction of the tower crane is limited by the site conditions and often cannot be constructed according to the foundation construction drawings provided by the manufacturer, so corresponding changes need to be made. Due to the lack of comprehensive understanding of the basic functions and requirements of the tower crane by some user units and improper handling, the following problems occur

1) unreasonable foundation layout. One side of the tower crane foundation is located on the basement floor, roof or foundation of the building, and the other side is located on the reinforced pile foundation or foundation, resulting in uneven settlement in use

2) the tower crane foundation strengthened by pile foundation only considers the vertical compressive capacity and ignores the overturning resistance. Some even set the reinforced single pile at the center of the foundation, some pile foundations have no anchor connection with the foundation, or the foundation is designed as a huge high pile cap, which increases the construction difficulty and cost

3) the ballast does not meet the requirements, too large will increase the bearing load of the foundation, and too small will not meet the requirements of anti overturning stability

4) in use, a large number of sundries or backfill are accumulated on the foundation of some tower cranes, which increases the load of the foundation and is not convenient for the observation of the foundation

5) the tower foundation section or embedded iron parts buried in the foundation are wrongly placed on the cushion, causing the foundation to separate from the tower

6) the levelness deviation of the surface on the foundation is large, and the tower crane is not adjusted with sizing block during installation, which makes the verticality deviation after installation too large

7) the installation position is improperly selected, and the tower crane cannot be removed by the normal tower removal method after use

8) no drainage facilities are provided

3 foundation reinforcement treatment scheme

when the requirements for ground resistance in the foundation drawing provided by the tower crane manufacturer cannot be met, the foundation needs to be reinforced. The common methods are as follows

1) foundation treatment. Compaction method and soil replacement cushion method can be adopted. Drainage consolidation method, vibration compaction method, replacement and mixing method, etc. Different methods have different requirements for soil, construction equipment and technology, and the cost is different. The most commonly used method is the soil replacement cushion method, which has a low cost, but is only limited to areas with thin soft foundation layers

2) pile foundation reinforcement. The cost is high, but the treatment effect is good. It is suitable for shallow soil that cannot meet the requirements of tower crane foundation for foundation bearing capacity and is not suitable for foundation treatment, especially for the situation of high on-site groundwater level

3) make rational use of permanent facilities of buildings. Convenient for installation. On the premise of dismantling the tower crane, make full use of the foundation, floor, roof and other structures of the building to organically combine the tower crane foundation with it. This scheme has low cost and is ideal. However, due to the increased load on the structure, it should be used cautiously. It needs to be calculated to decide whether to strengthen it, and the permission of the building design department should be obtained

4) increase the bottom area of the tower crane foundation and no longer reinforce the foundation. Due to the limitation of the installation position of the tower crane when enlarging the tower crane foundation, this scheme is only applicable to the situation that the ground endurance value on site is not much different from that required in the foundation drawing. When enlarging the tower crane foundation, its internal reinforcement needs to be redesigned and calculated

4 checking calculation of foundation

checking calculation of tower crane foundation includes strength checking calculation and anti overturning stability checking calculation. When the foundation drawing provided by the manufacturer is directly used for construction, the construction unit generally does not recheck the strength of the foundation, but only needs to check the anti overturning stability of the foundation; The strength and anti overturning stability of the redesigned tower crane foundation need to be checked

4.1 stability checking calculation

according to GB/t13752-92 code for design of tower cranes, the anti overturning stability checking calculation is carried out according to the following formula, as shown in figure 1

where:

e - eccentricity, that is, the distance from the resultant force of ground reaction to the Foundation Center (m)

l - the distance from the resultant force of the ground reaction to the edge of the foundation,

l = (B/2) - E (m)

m - bending moment acting on the foundation (nm)

fv, FH - vertical and horizontal loads acting on the foundation (n)

fg ------ gravity of concrete foundation (n)

b, H - width and height of inspection foundation bottom (m)

pb - calculated compressive stress of the ground (PA)

"Pb" - ground allowable compressive stress (PA), generally taken as 2 × 105－ 3 × 105Pa。

4.26. Specific operation steps of all experiments: strength checking calculation

foundation strength checking calculation can be carried out with reference to the building structure design manual, which will not be detailed here. The vertical and horizontal bearing capacity of a single pile shall be checked for the foundation strengthened by pile foundation. See J gj94-94 technical code for building pile foundation for the specific calculation method

when four piles are symmetrically arranged for reinforcement, the vertical bearing capacity of a single pile can be checked according to the following formula

nmax = (f ＋ g)/4 ＋ (m/s) <1.2 r at present, the data tensile testing machine on the market adopts ordinary 3-camera electric or frequency conversion electromechanical

friction pile: r=qpap + up Σ qsili

end bearing pile: r = qpap

where:

nmax - the design value of the maximum vertical force of the horizontal bar (n)

r - vertical bearing capacity of single pile (n)

f---- design value of vertical force acting on the foundation (partial coefficient of load 1.2) (n)

g---- design value of self weight of concrete foundation (partial coefficient of load is taken as 1.2) (n)

m - design value of bending moment acting on the foundation (partial coefficient of load is taken as 1.4) (nm)

s -- center distance between two piles in diagonal direction (m)

qp - bearing capacity of pile end soil, subject to the field geological exploration report (PA)

ap - cross sectional area of pile body (M2)

up - peripheral length of pile body (m)

qsi -- friction of soil around the pile (PA)

li - pile length of each section divided by soil layer (m)