Tornado risk model for transmission line design

Abstract

A tornado risk model for transmission line design has been developed. The risk model is based on the McDonald model for point structures and has been developed such that it can be simplified for applications in codes of practice and design recommendations. The model is expanded in terms of damage length of the tornado and the length of transmission line as opposed to the damage area of the tornado which is applicable to point-like structures. The model does not take into account the orientation of transmission line relative to the tornado path, or the point-wise nature of the individual transmission line towers. The design of transmission line structures is typically governed by wind loading, and in some parts of the world, wind loading is in combination with ice loading. The wind load used in most codes of practice and design recommendations for transmission line design have until recently been based almost exclusively on large-scale wind storms, which may include severe storms such as hurricanes and typhoons. Relevant statistics from South Africa are presented to illustrate the application of the model together with tornado risk areas for South Africa. Of interest to note is that in the highest risk areas in South Africa the occurrence of tornadoes per square kilometre is comparable with the lower risk regions in the mid-western USA