Nonbuilding and Special Structures
The effects of wind directionality are commonly considered for the design of low-rise and high-rise buildings, as well as bridges and other structures. This is commonly carried out by calculating the highest wind load for a critical direction, and providing a reduction to recognize that the design-level wind speed may not originate from the direction resulting in the highest load. The effects of the joint speed and directional characteristics of wind are not commonly considered or applied in transmission line design. This is due to a number of reasons, including the large geographic extent of some transmission line systems and the possibility that the load factor used in design may not be as high as for other structures. As a result, design wind loads are generally calculated for wind directions perpendicular to the line, which in the majority of cases yields the highest wind load. However, the wind loads on transmission lines are very dependent on wind direction compared to many structures.
The presentation will detail a study in which wind loads were calculated based on the recommendations in ASCE Manual of Practice No. 74: Guidelines for Electrical Transmission Line Structural Loading. The resulting wind loads are combined with a number of statistical wind climates from around the United States, which provide a probabilistic representation of wind speed and direction. Through combination of the calculated wind loads for each wind direction and the statistical wind climate, a measure of the reduction that may be expected due to directionality effects for each location are provided. These location-specific wind loads are then compared to what the design loads would have been without consideration of wind directionality. For each location a measure of the resulting reductions in the expected wind loads are developed, and the overall trends discussed. The reductions are also discussed in the context of the typical load factors which may be used in transmission line design.
The level of load factor used in transmission line design is not as consistent as for structures designed using ASCE 7. This is partially attributable to transmission lines not being occupied structures, however, due to the increasing demands of society on electricity (and the importance of its reliability) there is currently debate as to the classification of transmission lines as critical structures. However, the wind loads are generally calculated in a similar fashion among practitioners, which involves no reduction for wind direction on structures which are quite directionally-dependent. The work discussed in this presentation is intended to begin a discussion on the effects of wind direction on design loads for transmission lines and how this is related to the current design methodology in terms of load factors. Practitioners in the transmission or utility industries may find the topic of interest, as would practitioners in the field of wind engineering.