The Development of the Map of Extreme Ice Loads for ASCE Manual 74

In this paper we discuss the development of the map of extreme ice loads with concurrent wind speeds that is in the latest revision of ASCE Manual 74 and in ASCE Standard 7. This map is based on historical weather data from hundreds of weather stations run by the National Weather Service, Air Force, and Federal Aviation Administration. Equivalent uniform radial ice thicknesses on wires perpendicular to the wind direction in past freezing rain storms were determined from the data at each weather station using ice accretion models. Qualitative damage information was obtained for the storms that appeared to be severe enough to damage trees and power lines. This information was used both to check the modeling algorithms and to group the stations into superstations for the extreme value analysis. Ice thicknesses for long return periods were determined by fitting the generalized Pareto distribution to the sample of largest ice thicknesses for each superstation. Wind speeds concurrent with the extreme ice thicknesses were also calculated. In the West, from the Rocky Mountains to the Pacific, ice thickness zones were extrapolated using qualitative information because extreme ice thicknesses have not yet been calculated from the weather data in this region. For application to overhead electrical wires, the mapped ice thicknesses are adjusted for return period, height above ground, topography, and possibly wire orientation.