Discussion: Combined wind and ice loading on antenna towers

Abstract

The paper Wind and loading on antenna by Fig. Dl. In-cloud on a lattice telecommunication mast at Wahba et al. included a note from the Editor welcoming Yllas, Finland. The top of the mast is 750 m above sea level. written discussion. Accordingly, some comments are given in the following. The analysis by Wahba et al. concerns loads due to freezing precipitation only. This and the fact that in-cloud icing could produce greater values of accretion on towers were clearly stated by the authors. Nevertheless, the title of the paper, graphs called ice maps, and the discussion on combined wind and loads are potentially misleading when applied to tower design in regions with significant in-cloud icing. The approach used by Wahba et al. implicitly assumes that most of Canada does not belong to such a region. This assumption is certainly correct as far as Canadian weather stations are concerned, but highly suspect for antenna towers, which often are located on high terrain and typically extend to heights of 5001500 m above sea level. In-cloud loads generally increase exponentially with elevation (Lomilina 1977). Sundin and Mulherin (1993) have analyzed icing-related tower failures in the U.S.A., Sweden, and Finland. In-cloud icing was a major factor in 14 of the 16 tower collapses. The situation in Canada may be different to some extent, but is unlikely to warrant the use of maps based on freezing precipitation only. One should note that there are sites where the correlation between the measured accretion and precipitation is not only low but negative (Ahti and Makkonen 1982). Site-specific tower load estimates should always be made for both freezing precipitation and in-cloud icing. For loads due to freezing precipitation, methods more sophisticated than that by Chain6 and Castonguay (1974) might be worth considering (Mitten et al. 1988; Haldar et al. 1988). One should also note that the scale effect discussed by Wahba et al. for freezing precipitation is different for in-cloud icing (Makkonen 1984). Methods to do comprehensive mapping for towers based on meteorological data have been proposed by Mihel (1971) and more recently by Makkonen and Ahti (1994). These methods, in contrast to that suggested by Wahba et al., include the essential effects of temperature, elevation, and tower height. The points raised above also apply to the wind pressure