Design Loads for Transmission Towers under Skewed Wind Loading

Transmission tower networks, consisting of lattice support towers and conductor cables, play an important role in the infrastructure system of many countries throughout the world. Lattice support towers are typically comprised of two different components: the vertical tower body and the horizontal conductor support cross-arms. Much research has been directed towards the evaluation of aerodynamic coefficients of the compact lattice form typical of the vertical tower body sections of transmission towers, which now can often be calculated (based on solidity ratio) to an acceptable accuracy. However, the horizontal lattice form of the conductor support cross-arms have not been studied in depth. In this paper, results from wind tunnel tests on a typical conductor support cross-arm are used to show that the aerodynamics of these types of horizontal lattice sections are not as easily predicted as those for vertical lattice sections. The results from the wind tunnel tests are compared to the values calculated through the use of ASCE Manual No. 74: Guidelines for Electrical Transmission Line Structural Loading, and reasons for these discrepancies are discussed. This comparison is used to illustrate two examples where Manual No. 74 may or may not provide a conservative design estimate for tower and line loading at skewed wind directions, depending on the relative contributions of the cross-arm and tower body sections to the total wind loads. This is attributed to the underestimation of the wind loads on the cross-arms at skewed wind directions. Examples are used to show that the ratio of cross-arm area to total structure area is important for the consideration of the design wind direction. In light of the emphasis on efficiency in the world of transmission tower design, recommendations towards an improved evaluation of cross-arm force coefficients are provided.