Physical-based model for exposure coefficient and its validation towards the second generation of Eurocode EN 1991-1-3 for roof snow loads

In standards for structural design, models for snow loads on flat roofs generally provide the basis for determination of snow loads on any roof shapes. They are typically derived from ground snow loads and the predicted effects of exposure of flat roofs primarily due to wind, often conveyed through the exposure coefficient. The paper provides background information on the revision of the model for exposure coefficient in the second generation of the Eurocode on snow loads on structures, prEN 1991-1-3:2020. With the aim to provide an easy-to-use procedure with a minimum amount of input data, but to cover the main physical phenomena, a new formulation is proposed in prEN reflecting up-to-date scientific findings and the model provided in ISO 4355:2013. The prEN model corrects the main deficiency of the model in EN 1991-1-3:2003 where no account for wind velocities expected at the site during a snow season is taken. The prEN formulation additionally requires information on mean wind velocity for the coldest month at the site, that is commonly easily accessible. The contribution starts off by introducing the prEN model. To validate the new model, a parametric study is presented, considering plausible ranges of the main parameters influencing the determination of exposure coefficient in Europe. The outcomes of the new simplified prEN formulation are then compared with an advanced theoretical model. The prEN model is further applied considering detailed meteorological data representative for various climates. The results obtained by the prEN provisions are critically compared with those based on the advanced theoretical model and meteorological data, and with the estimates of the models in ISO 4355:2013 and in the JCSS Probabilistic Model Code. The presented validation demonstrates that the prEN model reflects well typical European snow and wind climates. The model captures a clear decreasing tendency of exposure coefficient with an increasing mean wind velocity. Mean wind velocity in the coldest month of 4.5 m/s appears to provide a reasonable threshold to make distinction between normal and exposed conditions.