Bases du calcul des constructions — Determination de Ia charge de neige sur les toitures.
For buildings where the internal temperature is intentionally kept below 0 °C (e.g. freezer buildings. ice skating arenas), C, can be taken as 1,2. For all other cases, C = 1.0 applies.
Bases for the determination of C are the thermal transmittance of the roof, U, and the lowest temperature, 0, to be expected for the space under the roof, and the snow load on the ground, s.
Methods for the determination of C for roofs with high thermal transmittance are described in Annex D.
NOTE The intensity of snowfall for short periods, approximately 1 d to Sd. Is often a more relevant parameter than s for roofs with considerable heat loss, since the melting is too rapid to allow accumulation throughout the winter. Since only so, however, is available, it has been used with the modifications given in Annex ft
6.3 Surface material coefficient
The amount of snow which slides off the roof will, to some extent, depend on the surface material of the roofing; see 42.
The surface material coefficient, Cm (see 3.11), is defined to vary between unity and 1,333, and takes the following fixed values:
— L’m = 1,333 for slippery, unobstructed surfaces for which the thermal coefficient <0,9 (e.g. glass roofs);
— Cm = 1,2 for slippery, unobstructed surfaces for which the thermal coefficient C >0,9 (e.g. glass roofs over partially climatic conditioned space, metal roofs, etc.);
— Cm = 1,0 corresponds to all other surfaces.
NOTE Cm 1,2 could also be applied for C O,9 if this is assumed to be more reasonable.
6.4 Shape coefficients
6.4.1 General principles
The shape coefficients define distribution of the snow load over a cross section of the building complex and depend primarily on the geometrical properties of the roof.
For buildings of rectangular plan form, the distribution of the snow load in the direction parallel to the eaves is assumed to be uniform, corresponding to an assumed wind direction normal to the eaves.
The shape coefficients presented for selected types of roof (see Annex B), are illustrated for one specific wind direction. Since prevailing wind directions can not correspond to the wind directions during heavy snow falls, the condition that the wind during snow fall can have any direction with reference to the roof location should be considered when designing roofs.ISO 4355-2013 pdf free download.