What is the definition of the ground snow load rating?

[leeStruct]

Does anyone know the exact definition of ground snow load rating? Say, for a 40psf ground snow load area, is it means that a whole year's snow fall accumulation is 40psf, or is means that just one snow fall will reach 40 psf (that will means over 24" snow depth at one snow fall)?

Many thanks for your answer.

 

[leeStruct]

I read more about the ASCE7-05 Figure 7-1, seems like the mapped ground snow is the maximum snow fall during 50 years period. In this sense, for a 40 psf ground snow, it means at this area, one maximum snow fall during 50 years is 40psf. Am I right?

Thanks.

 

[BAretired]

I think it means 40 psf is the total snow load on the ground with a 1 in 50 probability of exceedance, based on measured depths and densities.

BA

 

[jimstructures]

The only note I'd like to add to "BAretired" is "40 psf is the total snow load on the ground with a 1 in 50 probability of excedance in any given year, based on measured depths and densities." Note the any given year addition.

Jim

 

[BAretired]

I agree with jimstructures amendment.

BA

 

[NITTANYRAY]

The ground snow load is the maximum total weight of snow on the ground. It could be the result of several snows and possibly mixed with rain.

A 12" snow will over time consolidate under its own weight.
Freezing and thawing could also lessen the measured depth without actually reducing the weight. Even in the same snow fall the density of the snow can vary with depth due to temperature changes during the snow event.

After a storm last week where we received 12" of snow I weighed a 12" cube of snow. The bottom of the cube was very dense while the upper portion was much lighter. The snow started out very wet but as the temperature dropped toward the end of the storm the snow was dryer.

The cube weighed 8.6 pounds. I estimate that if the whole snow event had been the wet variety the cube would probably weighed closer to 10-12 pounds. Both of these figures are less than the density prescribed in the code>
(16.8 for a 20 PSF ground snow and 17.25 for a 25 PSF ground snow.

We have had several snow falls since the original (8+4+9).
The measured snow depth is only about 15" total. It is hard to determine how much melting, evaporation and consolidation has occurred but obviously the whole is less than the sum of its parts in this example.

I haven't redone my snow weight sample but I am sure the density is higher than when I first measured it. This is probably why snow densities per code are much higher than the typical random sample.

This much snow is not common in central Ohio. I have seen about 4 or 5 similar combined events in the 40 years I have lived here.

A lot of pre-engineered buildings were designed for less than 20 PSF in the past. The fact that they are usually not well insulated has probably saved many buildings from collapse.

 

[jike]

I suspect that the ground snow load is from observation of what exists on the ground at any one time. This would then consist of multiple snows, freezing and thawing, rain, etc.
In Northern Wisconsin, we expect to consistently have a January thaw sometime in that time period which reduces our ground snow.