ISO 4355:2013 pdf download – Bases for design of structures – Determination of snow loads on roofs
1 Scope
This International Standard specifies methods for the determination of snow load on roofs. It can serve as a basis for the development of national codes for the determination of snow load on roofs. National codes should supply statistical data of the snow load on ground in the form of zone maps, tables, or formulae. The shape coefficients presented in this International Standard are prepared for design application, and can thus be directly adopted for use in national codes, unless justification for other values is available. For determining the snow loads on roofs of unusual shapes or shapes not covered by this International Standard or in national standards, it is advised that special studies be undertaken. These can include testing of scale models in a wind tunnel or water flume, especially equipped for reproducing accumulation phenomena, and should include methods of accounting for the local meteorological statistics. Examples of numerical methods, scale model studies, and accompanying statistical analysis methods are described in Annex G. The annexes describing methods for determining the characteristic snow load on the ground, exposure coefficient, thermal coefficient, and loads on snow fences are for information only as a consequence of the limited amount of documentation and available scientific results. In some regions, single winters with unusual weather conditions can cause severe load conditions not taken into account by this International Standard. Specification of standard procedures and instrumentation for measurements is not dealt with in this International Standard.
6.2 Thermal coefficient
The thermal coefficient, C t (see 3.10), is introduced to account for effect of thermal transmittance of the roof. The snow load is reduced on roofs with high thermal transmittance because of melting caused by heat loss through the roof. For such cases and for glass-covered roofs in particular, C t , can take values less than unity. For buildings where the internal temperature is intentionally kept below 0 °C (e.g. freezer buildings, ice skating arenas), C t , can be taken as 1,2. For all other cases, C t = 1,0 applies. Bases for the determination of C t are the thermal transmittance of the roof, U, and the lowest temperature, θ, to be expected for the space under the roof, and the snow load on the ground, s 0 . Methods for the determination of C t for roofs with high thermal transmittance are described in Annex D. NOTE The intensity of snowfall for short periods, approximately 1 d to 5 d, is often a more relevant parameter than s 0 for roofs with considerable heat loss, since the melting is too rapid to allow accumulation throughout the winter. Since only s 0 , however, is available, it has been used with the modifications given in Annex D.