Specification of the design snow loads for gable roofs based on a consistent probabilistic approach

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One of the basic tasks of codification in regard to climatologic actions is the area-wide specification of the design values based on observations for single stations. The statistical stability of the estimated values greatly depends on the ensemble size. This is true for the estimates of each single station as well as for the geostatistical interpolation, i.e. the more observations are sampled for the single station ensembles and the more stations are used as input for the geostatistical interpolaten the higher is the statistical stability of the results. For 224 stations in Germany, extreme value statistics can be performed directly based on the observed water equivalents, which are available on average for 37 years. The developed probabilistically equivalent snow density model in combination with the new approach of using independent events instead of yearly extremes leads to considerable larger ensemble sizes with 130 entries on average. The translation of the single station model to regional models allows analysing a further 525 stations which provide only observations of the height of the snow cover. A further improvement, i.e. a reduction of the statistical uncertainties, is obtained by introducing super-stations which combine observations from single stations following a consistent probabilistic strategy. Then, the type of distribution can be identified with a sufficient statistical stability. Aim of the project is the utilization of the observed snow height data in the scope of the extreme value statistics of the snow loads and the statistical optimal specification of the corresponding design values. This requires a consistent strategy for the detection of outliers in terms of very rare events. The actual strategy used in the European snow load research is less appropriate, since too many events are not detected and a large number of events are erroneously classified as outlier. Aim of the project is the development and application of an improved strategy, which especially considers unbiased estimators like BLUE. Thus, it becomes possible to analyse true outliers in a probabilistic consistent way. The accumulation and spatial distribution of snow on roofs greatly depends on the wind speed. The snow-wind climate of Germany can be sub-divided into three basic types. Type I shows for large snow loads virtually deterministic wind speed values. Then, the shape parameters can be specified on the load model which has been developed in the first period of the project. Type II features scattering wind speeds, which requires extensions of the basic convolution integrals. Only for type III storm and snow occur simultaneously. Then, consistent combination rules are required for these two climatologic actions.