Methodology for determining the design lengths of cross-grid elements of steel supports of overhead transmission lines

Introduction. The total capacity of power plants and the length of power grids in the Russian Federation are significantly increasing every decade due to the constant industrial development of cities and suburbs. This requires the involvement of huge material and labour resources in the sphere of power construction, so all possible ways to reduce the capital intensity of power grids of high and ultra-high voltage classes should be identified and implemented. For practical purposes, in addition to solving the stability problem itself, it is necessary to determine the combination of external loads (torque and longitudinal force) that predetermines the smallest possible value of the critical parameter.

Materials and methods. Due to the different lengths of the individual struts of the supports and the increasing forces in the girdles towards the base, the degree of pliability of the nodes to linear and angular displacements is not the same, so that only some struts lose stability. The paper considers a square-section tower with parallel rather than inclined girders, in which the lattice and girders have the same cross-sections, respectively, and an increasing torque and an unchanged longitudinal force applied with respect to the vertical axis of the support act on its free end. Due to the symmetry of the system and internal forces at the moment of loss of stability there will be a symmetric deformation of the struts losing stability. The problem was solved using the system of canonical equations of the displacement method in numerical and analytical formulation. The application of the described methodology for determining the design lengths of the grid struts is considered on the example of the lower section of the support 1P330-1.

Results. The considered fragment of the support in terms of structural solution is a spatial rod steel column, the nodes of which are not aligned in adjacent faces and consists of 12 panels. The structural elements of the section are bars made of single angles. The joints are bolted together. The canonical coefficients for the struts of each panel are determined and the stability equation is solved graphically, from which the design length coefficients are found.

Conclusions. The presented numerical and analytical method allows to determine the coefficients of design lengths of tower support shaft elements depending on the longitudinal force and the ratio of chord and strut stiffnesses. The obtained coefficients are approximately 10–15 % lower than the existing ones in the domestic standards. As a result, the reserve of bearing capacity of supports is revealed, which indicates the possibility of improving the methodology of solving the problem of stability of elements.

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