Determining ice loads for tower structure design

Ice load is a major design criterion of tall towers. International Standard ISO 12494 gives a method to assess ice loads and combined ice and wind loads on complex structures by Ice Classes. The method has not been directly verified, however. Here, we present an analysis of the applicability of the ISO method based on field data on rime icing. The data include ice amounts simultaneously measured on the ISO Standard ice collector, a 7.5 m tall self-supported lattice structure and a 127 m tall guyed lattice TV-tower. We compare ice masses on these objects within specific Ice Classes and calculate the ice masses calculated for the structures based on the ISO method. The results show that ISO Ice Classes are a useful tool in assessing rime ice loads on structures, but that systematic errors arise. These errors tend to be on the safe side in regard to structural design and are, at least partly, related to poorly known ice shedding mechanisms.