Snow Loads: Explained in 1-minute

Snow Loading is a very important consideration in cold climates where significant accumulation on rooftops can be a significant cause for concern. The amount of snow that is considered for structural design is usually referred to as the design snow load.

Typicaly in most international codes, the design Snow Load for a given roof shape is derived from the ground snow load for a given location and the roof shape coefficient. In Japanese Building Law, the roof snow depth is calculated direction depending on the region and then adjusted based on the roof pitch, size and more.

Japan Snow Load Calculation

The snow load in N/m2 can be calculated as:

Where:
d is the vertical snow depth (cm)
ρ is the bulk density of snow (N/cm/m2)
A is the rooftop projected area (m2). 

Coming soon, but in the meantime, reference may be made to the Japanese JFE Steel Blue Book for more information.

Eurocode Approach to Snow Loading

The snow load can be calculated as:

Where sk is the ground snow load, μ is the roof shape coefficient, ce is the exposure coefficient, and ct is the thermal coefficient.

What is the Ground Snow Load?

Provided that there is sufficient meteorogical data, engineers can review historic snowfall data and perform an extreme value analysis to evaluate the probabilities of a certain amount of ground snow falling again.

Ground snow loads are specified in building codes and are normally defined as a 50-year return period ground snow load (snowfall quantity that has a 2% chance of occurring in any single year).

The ground snow load is expressed in units of kN/m2, kPa or N/m2.

What are the Rooftop Shape Coefficients?

The final snow accumulation that takes shape on the roof of building structures depends very heavily on the rooftop geometry. The ground snow load is multiplied by a roof shape coefficient to get the design snow load.

These roof shape coefficients are determined through experiments and can easily be found in building codes.

※ What are the Rooftop Shape Coefficients for Snow? Explained in 1-minute

For very complicated rooftop geometries, engineers may undertake special water flume experiments to establish rooftop shape coefficients and design Snow Loads.

Snow Drifting

Another point to consider is snow drifting. In strong winds, snow gets carried to areas of poor wind circulation on roofs and may accumulate there in very significant quantities compared to if no snow drifting were to be considered. Furthermore, snow drifting may lead to large unbalanced loads.

Other Considerations

Fresh snow is much less dense than water, having about 20% the density typically. However, when snow has settled for a long time and compacted by more snow falling onto it, the density of snow changes significantly. Eurocode provides some suggestions for different snow densities: