## 5 Conclusions  
In this paper, through the tensile bearing capacity test of single-limb single-plate and double-limb double-plate connection joints, the mechanical characteristics of the connection joints are obtained. The theoretical calculation expression of the stable bearing capacity of quadrilateral tower considering the influence of the axial stiffness of the joints is further derived by the energy method. Through numerical simulation, the influence of double-limb double-plate connection joint on the nonlinear stability bearing capacity of quadrilateral tower is further analyzed, and the following conclusions are obtained.  
* The scale test results of single-limb single-plate and double-limb double-plate connection joints show that the bearing capacity of double-limb double-plate connection joint can be increased by 26.1%. The load displacement curve of the joint shows that the elastic stiffness of double-limb double-plate connection joint is 3.12 times that of single-limb single-plate connection joint.
* The influence of the axial stiffness of the joint on the stable bearing capacity of the quadrilateral tower is studied by the energy method. The axial stiffness coefficient is introduced to express the influence of the change of the joint stiffness on the stable bearing capacity of the tower. The results show that with the increase of the axial stiffness coefficient, the stable bearing capacity of the tower first increases and then tends to be stable, and the axial stiffness coefficient has the greatest influence in the range of 0-0.5.
* Combined with the joint test and theoretical analysis, it is found that the DLDPJ can improve the stable bearing capacity of quadrilateral tower by 15.6%.
* The quadrilateral tower numerical model of the test model joint is established, and the whole process of nonlinear buckling of the tower is analyzed. Considering the influence of geometric nonlinearity of the tower and connection joints, it is found that the double-limb double-plate connection joint can improve the nonlinear stability bearing capacity of the transmission tower by 14.9%.  
Figure 18: Load displacement curve of tower bearing capacity.  
In this paper, the theoretical calculation expression of the stable bearing capacity of the quadrilateral tower considering the axial stiffness of the joint is proposed. The bearing capacity of the double-limb double-plate connection joint is studied from three aspects of the model test, theoretical analysis and numerical simulation, and the research results have been verified in the tower cell and have good engineering application value. However, this paper does not consider the influence of the number of real tower floors and tower inclination. Based on this, the next step will be to carry out the application research of double-limb double-plate connection joints in the full tower.  
Conceptualization, T.Z. and H.Y.; methodology, T.Z. and H.Y.; software, T.Z.; validation, T.Z., H.Y. and M.H.; formal analysis, T.Z. and H.Y.; investigation, L.Z. and P.H.; resources, L.Z., Z.L. and M.H.; data curation, Z.L. and M.H.; writing--original draft preparation, T.Z. and H.Y.; writing--review and editing, T.Z. and H.Y.; visualization, T.Z.; supervision, Z.L. and M.H.; project administration, Z.L. and M.H.; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript.  
The research work was funded by the National Natural Science Foundation of China (Project nos. 11572147 and 51568046).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.