Research on safety performance of jacking tower under uneven settlement

This article takes a 220KV jack-up tower as the research object. By studying the safety performance of the jacking tower under the condition of uneven foundation settlement in the mined-out area, this paper uses the finite element software Midas GTS NX to establish the jacked-up transmission tower-foundation model and the jacked-up transmission tower line-foundation model. Based on the numerical simulation analysis method, the safety of jacking towers under different working conditions is studied.


1.Introduction
With the rapid development of our country's economy, energy has become an important engine for our country's development. Electric energy provides the normal operation of the power system and at the same time supports the transmission lines. With the development of my country's power construction, more and more transmission lines appear in areas with poor geology. Due to the complex geological conditions in various regions of our country, the uneven settlement of the foundation is more serious [1]. Transmission tower lines pose a major threat. Scholars at home and abroad have done a lot of research on the carrying capacity and damage characteristics of transmission towers [2][3][4][5][6][7][8]. However, as a new type of adjustable tower, compared with ordinary electric towers, the design of jacking towers still lacks systematic research on whether there will be structural defects. The safety performance of the structure under the uneven settlement of the foundation is worthy of further investigation. Based on a 220kv transmission tower transmission project as the research background, this paper uses the finite element software Midas GTS NX to establish the jacked-up transmission tower-foundation model and the jacked-up transmission tower line-foundation model, and analyze the performance of the jacking tower under different working conditions safety.

2.1.Model parameters
This model uses MIDAS GTS to establish a three-dimensional solid model of transmission tower-linefoundation. The model consists of four parts: transmission tower, wire, soil and cap. The transmission tower is a straight tower with a height of 57.1m and a root opening of 13.5m. The tower legs are made of Q345 angle steel, the diagonal material and cross-bars are made of Q235 angle steel, and the conductors are LGJ-500/45. The length and width of the platform are both 2m. The thickness is 0.5m, it is C30 concrete, the geometric dimensions of the soil are 20m in length and width, and the thickness is 10 m. It is simulated by three-dimensional solid elements, and the yield criterion is the classic Mohr-  Table 1. The jacking foot stand is 3m high, and the jacking model is shown in Figure 2. The three-tower two-line model of the finite element calculation model is shown in Figure 2. Among them, the No. 2 iron tower is the jacking tower.

2.2.Restrictions
When the transmission tower model of this model is established, the foundation grid group of the jacking tower is drawn according to the finite element model. According to the foundation conditions of the stress analysis, the left/right side of the model restricts the displacement in the X direction, the front/rear side restricts the displacement in the Y direction, and the bottom of the model restricts the displacement in the Z direction. The number of the foundation support of the tower is shown in Figure 3.  Figure 3.Tower leg support number

3.1.Destruction criterion
In the transmission tower structure, the structural material is mostly angle steel. The transmission tower model is established through Midas GTS NX. The rod material is Q235. Since each rod is connected to a different number of rods, the stress on each rod is And the direction is also different, so the stress in a single direction cannot be used to explain the stress of the structure. Instead, the equivalent stress is used in material mechanics to equate the stress experienced by the material in a complex state to the stress experienced in a unidirectional state. According to the fourth strength theory of material mechanics, the calculation expression of the corresponding strength judgment condition can be obtained as Equation 1. (1)

In the formula:
, , -Three principal stresses; -MisesRelative stress of yield criterion; [ ]-Material allowable stress; -Yield stress under uniaxial tension; -Safety factor, plastic material takes 1.5-2.2 For steel, the general design strength is 380 MPa. When a single rod is subjected to a stress of 380 MPa, it is considered that the rod will be damaged.

4.2.Working condition 2 and working condition 3 under the condition of double support settlement
The double supports A and B sink at the same time. When the settlement of the transmission tower increases, the top of the transmission tower is always in maximum deformation, and the lateral deformation is much smaller than the longitudinal deformation; with the increase of the settlement, No. 2 roof The maximum equivalent stress of the power transmission tower is linear with the settlement value as shown in Figure 6. When the settlement reaches 42mm, the maximum stress of the No.

4.3.Working condition 4 Settlement of A, B, D supports
After calculation and analysis, when the three tower foundations are sinking, the maximum stress of the key members of the No. 2 tower also increases with the increase of the settlement value, as shown in Figure 8. With the increase of the settlement value, the equivalent stress value on the tower leg member changes greatly, but the maximum equivalent stress of the main member of the tower body and the bottom cross member does not change very much. When the settlement value reaches 19mm, the tower leg of the tower The stress of the bar at A reaches 386MPa as shown in Figure 9, which exceeds the breaking strength of steel. At the same time, the No. 1 and No. 3 towers are less affected by the uneven settlement of the No. 2 tower, and the stress on the conductors on both sides of the No. 2 tower is also in a safe state.

5.Conclusion
Based on software analysis and comparison of results, it can be concluded.
(1) Under the settlement of single support, double support, and three support under working conditions, when the material of No. 2 jacking tower does not reach its strength, the settlement value of its rods has a linear relationship with the settlement value. Before reaching the yield strength, its uneven settlement has almost no effect on the adjacent No. 1 and No. 3 iron towers.
(2) When the three supports sink, as the settlement value increases, the deformation at the top of the transmission tower increases rapidly, and at the same time it will quickly exceed the safety range of its materials. Although the steel has not been damaged at this time, it is not suitable for continued work due to the excessive deformation of the transmission tower, and the stress of the cable may become too large during this period.
(3) In the case of uneven settlement, the location of the maximum stress change in the transmission tower is usually at the leg of the tower. At this time, the deformation at the tower leg will be greater than other parts. Therefore, protection should be carried out at the position of the legs of the transmission tower as much as possible to avoid accidents in time.