1.

INTRODUCTION

The branch and plate belled cast-in-place pile is a new type of variable cross-section pile foundation in recent years. On the basis of ordinary bored cast-in-place pile, the bearing plate is formed at different parts along the pile body with special equipment, and then the enlarged head is formed at the pile end through excavation and other processes. It has good bearing performance, especially the uplift bearing performance has been greatly improved. Compared with ordinary equal diameter cast-in-place pile foundation, the bearing area of pile shaft side surface and pile end is increased by the branch and plate belled pile. At the same time, the bearing plate of pile shaft can make full use of the foundation bearing capacity of better soil layer, so that the pile depth design can be greatly shortened[1].

At present, the research on the bearing capacity of the branch and plate belled pile of transmission line is less, mainly the research on the bearing capacity of the branch and plate belled pile.Song[2] used ABAQUS finite element software to establish the finite element models of single pile and pile group of branch and belled pile, respectively, and studied the influence of factors such as the angle of branch and disc forming, the diameter of branch and disc, the angle of disc forming of enlarged head, the diameter of enlarged head, the disc spacing between branch and enlarged head on the bearing performance of branch and disc belled pile, and analyzed the adjustment coefficient of uplift bearing capacity of branch and disc belled pile.

With regard to the study of the influence of pile soil structure interaction on the dynamic response of the structure, lihongnan et al. Established the overall finite element model of the transmission tower and analyzed the seismic response by using viscoelastic artificial boundary to simulate the soil boundary; Fan[3,4] studied the influence of pile soil structure interaction on wind-induced response of high-rise buildings by using the lumped mass method. Ilaria venanzi [5]studied the influence of soil foundation structure interaction on wind-induced response of high-rise buildings by using finite element method. Jendoubia and legeronf[6] studied the effect of soil structure interaction on rigid transmission towers under wind and impact loads; Liu[7] used ABAQUS software to establish an overall model under the consideration of PSSI effect to study the wind-induced vibration response analysis under different soil conditions. Ke [8]used an integrated finite element model of “blade nacelle tower foundation” considering the centrifugal force of blades, and the soil structure interaction (SSI) effect is considered by setting mass springs and dampers on the interface between foundation and soil.The results show that the natural vibration period of the transmission tower increases with the increase of soil flexibility after considering PSSI effect, the displacement of the main nodes of the tower body is the largest, and the acceleration and stress of the main control points are reduced to varying degrees.

To sum up, the current research on the bearing capacity of the belled pile is only about its bearing capacity, and the research on the dynamic characteristics of the belled pile is relatively lacking. Especially when the analysis of wind-induced vibration response of transmission lines is based on the consideration of consolidation foundation support, it can not better reflect the actual situation, and the overall modeling analysis is required, and the impact analysis of PSSI effect of transmission towers with branch and belled piles has not been studied. In view of this, this paper will use ANSYS software to establish the overall finite element model of the power transmission pole, and conduct time history analysis by simulating the wind speed. Firstly, the dynamic characteristics of the power transmission pole structure when the foundation is consolidation and PSSI is considered are compared, and then the different effects of the common equal diameter piles and the new branch belled piles on the dynamic characteristics of the structure are compared. It is hoped that the research on the dynamic characteristics of the new pile type and other foundation conditions in this paper can provide some meaningful reference conclusions for the wind resistance design and pile foundation design of the actual transmission line project.

2.

MODELING OF DISTRIBUTION POLE-FOUNDATION-STAKE AROUND SOIL

Three groups of comparative models are established, the first group is the simplified consolidated foundation model, the second group is the traditional equal diameter pile foundation model, and the third group is the new branch and belled pile foundation model. The structural parameters of each model are shown in Table 1. Figure 1 shows the model of traditional equal diameter pile foundation and belled pile foundation.

Figure 1.

Finite element model of traditional

Table 1.

Structural modeling parameters(m)

2.2

Distribution pole model

The distribution pole is the most common pole type of 10kV line represented by ϕ 190×12×O×BΥ. That is, the pole diameter is 190mm and the length is 12m. O represents the cracking load of 8kN, and BY represents the partially prestressed pole. The taper of the pole is 1/75, and the roughness of class A. The circular concrete pole has the characteristics of simple structure, less steel consumption and reasonable stress, so it is widely used in 110kV and below transmission lines. The conductor suspended above is LGJ 185/10, with a sectional area of 193.4mm2, an outer diameter of 18mm, a maximum horizontal span of 60m, and a unit line mass of 584kg/km. The elevation structure of the pole is shown in Figure 2.

Figure 2.

Elevation of distribution pole pile foundation and new branch belled pile

The finite element model of distribution pole is established in ANSYS. The pole is defined as beam189 element. The material properties are: elastic modulus e=45.4Gpa, poisson’s ratio υ= 0.2, density ρ= 2500kg/m3, and the damping ratio of the structure is 0.05. The wind load transmitted by the conductor is equivalent to that transmitted by the conductor, and concentrated nodal force is applied at the point where the conductor bears force on the pole.

2.3

Foundation-stake around soil model

Drucker Prager (DP) model is used to consider the material nonlinearity of the soil around the pile, and solid186 solid element is used as the element. According to the field measurement data provided by the Design Institute, it is estimated that the elastic modulus of the soil around the pile is 40 MPa, the Poisson coefficient is 0.35, the density is 1820 kg/m3, the cohesion is 24000pa, the internal friction angle is 12.1 °, the expansion angle is 0 °, and the pile-soil friction coefficient is 0.5. The length and width of the soil area of the pile foundation is about 30 times the pile diameter, and the height is about 1.7 times the pile length. The total size of the soil area is 5m × 5m × 5m. Constraints of six degrees of freedom are imposed on the bottom and surrounding surface of soil around the pile. The overall modeling graph is shown in Figure 3.

Figure 3.

Finite element model for integral modeling of distribution pole

3.

WIND LOAD SIMULATION OF DISTRIBUTION POLE LINE SYSTEM

The pole in the engineering background is located in the coastal area of Fujian Province, China. The extreme wind speed corresponding to different return periods can be calculated by calculating the extreme wind speed in this area. The 30 return period is 30.8m/s, the 50 return period is 33.4m/s, and the 100 return period is 37.3m/s. The wind speed of the distribution pole is simulated. Dvenport spectrum is used for the wind speed spectrum along the wind direction. The wind speed with a 50 year return period is taken as the reference wind speed, that is, 33.4m/s. The number of sampling points is set to 250, the roughness coefficient is set to 0.00129, the wind profile index is set to 0.05, and the time interval is 0.2S. The wind speed time history of fluctuating wind at each point is obtained by harmonic synthesis method.

The fluctuating wind speed time history of each simulation point is superimposed with the average wind speed of each point to obtain the total wind speed time history of each simulation point. The pole is divided into 10 wind zones for wind load calculation, and the wind load of the conductor is simulated by the calculated concentrated nodal force. The total wind speed time history of each simulation point is converted into the wind load time history according to the following formula:

Where, μ_sis the shape coefficient of the structure, the wire diameter is greater than or equal to 17mm, the shape coefficient of the conductor is taken as 1.1, and the shape coefficient of the distribution pole is calculated as 0.6 according to the load specification; V_t(t) is the total wind speed at each simulation point; A_t is the windproof area of each loading point. For conductors, the windproof area is the product of the length and diameter of each simulated representative. For distribution poles, the windproof area is the projected area of each component in the area represented by each loading point.

4.

TIME HISTORY ANALYSIS OF WIND INDUCED VIBRATION RESPONSE

The simulated wind load time history is applied to the distribution pole under three different foundation conditions, and the wind-induced vibration response is analyzed. The response results are extracted by the ANSYS time history post processor. Figure 4 shows time history of node displacement at the top of distribution pole with consolidation foundation. It can be seen from figure 4 that the peak displacement of the pole is 0.113m without considering the effect of PSSI. Figure 5 shows time history of node displacement at the top of distribution pole with equal diameter pile foundation. It can be seen that the peak value of rod top displacement is 0.209m. It can be concluded that after considering the PSSI effect, the rod top displacement increases significantly, with an increase of 84.9%. Figure6 shows time history of node displacement at the top of distribution pole with branch plate belled pile foundation. It can be seen that the peak displacement of the pole top is 0.197m, which is lower than that under the working condition of the equal diameter pile, but compared with the consolidation support working condition, the displacement still increases by 73.9%.

Figure 4.

Time history of node displacement at the top of distribution pole with consolidation foundation

Figure 5.

Time history of node displacement at the top of distribution pole with equal diameter pile foundation

Figure 6.

Time history of node displacement at the top of distribution pole with branch plate belled pile foundation

Within the height range of 1-10m of the pole, the displacement, acceleration and bending moment of the corresponding node and element are extracted at an interval of 1m. Table 4 shows the peak displacement of 10 nodes under three working conditions, table 5 shows the peak acceleration of 10 nodes under four working conditions, and table 6 shows the peak bending moment of 10 units under three working conditions.

Table 2.

Peak displacement of main control node (m)

Table 3.

Peak acceleration of main control node/(m/s2)

Table 4.

Peak bending moment of main control unit (N • m)

Comparing the consolidated foundation structure with the other two integral structures considering PSSI, it can be seen from table 2 that after considering PSSI, the peak displacement of each node of the rod body is greater than that of the consolidation foundation condition. It can be seen from table 4 that the bending moment of the consolidation foundation distribution pole without considering the PSSI effect is generally small, with a maximum difference of 28%. Therefore, it can be concluded that the PSSI effect should be considered in the superstructure design of the transmission line to ensure the reliability of the structural design.

By comparing the working conditions of equal diameter pile and branch belled pile considering the effect of PSSI, it can be seen from table 2 that the peak displacement of each point of the rod body under the condition of branch belled pile is less than that under the condition of equal diameter pile, and the maximum difference is 16%, the higher the difference is, the smaller the difference is, and the difference between the end and top is 6%. It can be seen that the branch belled pile can reduce the peak displacement of structural wind-induced vibration response, and has the greatest impact on the peak displacement of the bottom of the structure; From the comparison of bending moments in Table 4, it can be seen that the bending moments at various parts of the rod body can be moderately reduced by 14% at most.

5.

CONCLUSION

In this paper, the distribution line pole in poor soil area is taken as the research object, and the differences in dynamic characteristics and wind-induced vibration response among the simplified consolidation constraint pole model, the directly buried single pile pole model and the new branch and belled pile pole model are compared and analyzed