Guidelines for Electrical Transmission Line Structural Loading

This chapter addresses structural loadings to be applied to transmission lines in the interest of reliable and cost-effective designs that comply with regulations, standards, and prescribed design methods. It discusses the principal systems of a transmission line—the wire system and structural support system—including some unique aspects of transmission line design, such as tolerance of failure and linear exposure. This chapter also explores load and resistance factor design; weather-related loads, such as those resulting from extreme wind, high-intensity wind, and extreme ice with concurrent wind; and mean recurrence intervals for weather-related loads. A unique aspect of structural design of electrical transmission line facilities is the acceptability of some level of risk failure; the acceptable level of risk of failure often depends on the importance of the transmission line considered

This chapter discusses weather-related loads on transmission line structures and wires. These loads are associated with wind, or a combination of ice and wind, referred to as ice with concurrent wind. Specific aspects include wind loading, high-intensity winds (HIWs), and ice and wind loading. The wind force calculation is based on the selection of appropriate values of wind speed, wind pressure exposure coefficient, gust response factor, and force coefficient. The gust response factor accounts for the dynamic effects, as well as the correlation of gusts with the wind response of transmission line components. The wind speeds that transmission line structures experience can be significantly influenced by topography. Analyses of line failures in several countries have identified HIW events as the leading cause of transmission line failures. In addition to imposing substantial vertical loads on the structural system, the ice buildup on the wires and conductors presents a greater projected area exposed to wind.

This chapter addresses additional structure and wire system loads resulting from such causes as unbalanced wind and ice, broken conductors, and construction and maintenance. These loads apply to both new installations and modifications to existing facilities. Specific focus is on longitudinal loads, line security, and failure containment; construction and maintenance loads; worker access and fall protection loads; wind-induced structure vibration; wire galloping loads; and earthquake loads. Transmission line structures are subjected to longitudinal loading owing to an imbalance in wire tension or a failed component. Construction loads are imposed on structures during assembly and erection, as well as installation of ground wires, insulators, conductors, and line hardware. Maintenance loads are those loads applied to the structures that result from scheduled or emergency inspection and/or replacement of all or part of a structure, ground wire, insulator, conductor, or conductor hardware system.

Wire tensions temporarily decrease as wire temperatures increase and thermally expand (elastically lengthen) the wire. Conversely, tensions increase with decreased temperatures. This chapter discusses how the wire system is affected, along with the assumptions that may be used for determining the loads at the structure attachment points. Particular focus is on tension sections, wire conditions, and wire tension limits. Wire tension can be calculated by the ruling span method and by using structural analyses of a single tension section and multiple tension sections. In 3D models that incorporate lateral and longitudinal movements for conductors, support hardware, and structures, the computational results are more accurate than those of 2D models. This chapter also discusses loads at wire attachment points, focusing on wire unit loads and considering the wind factor and weight span. The most simplistic calculation of weight span occurs when three consecutive structures support the wire at the same elevation.

This chapter presents worked numerical examples of electrical line transmission loads. The first example concerns latticed suspension tower loads and includes calculation of wind pressure on wires, calculation of shield wire loads and conductor loads, and calculation of wind on transmission tower structures. A specific case of load calculation, with the wind at 30 degrees yaw angle, is also included. The second example given in the chapter compares weight spans with and without wind for the center tower. Comparison of weight spans using the traditional catenary constant is also demonstrated, with the traditional catenary constant based on the resultant unit weight.

This section contains references for this Manual of Practice along with Additional Resources.