Height of 25 psf snow? 4

[WWTEng]

In our area the ground snow load is 25 psf and we typically design roofs for 25 or 30 (schools) psf of snow. What is the density of fresh snow? I am trying to figure out how many inches of fresh snow adds up to 25 psf.

Thanks,

 

[ToadJones]

I believe there is a simple equation in ASCE 7 in the snow load chapter

 

[burningDiesel]

In my country it's done by wet snow, not dry.
Density of wet snow is +- 500kg/m³, dry about 100kg/m³.
So 25 psf is about 10 inches of wet snow, or 50 of dry (fresh) snow. But you better grab a calculator 'cause I've surely messed up somewhere with your empirical stuff.

 

[bpstruct]

g = 0.13pg + 14 (in pcf) but no more than 30 pcf

 

[emmgjld]

As a practical side, low snow weights (than 40 psf) are on wet, compacted snow. Fresh fallen snow is often very lose, compacted to snow even an hour old.

In the high snow areas, an additional problem occurs, in that after several snow falls (with periods of partial melting) and/or heat loss through the roof, the layer against the roof may become ice. And you have a very good idea what that weighs.

 

[MiketheEngineer]

That is really impossible to answer - obviously dependent on density. I have see 6'' of snow I could hardly shovel and 24'' that was no problem.

 

[WWTEng]

I understand that it all depends on how compacted the snow is. I guess I should have indicated where my question was coming from, I was thinking about the Feb 2011 billzard that dumped 21.2" snow in Chicago. I figured most of that snow could be considered fresh and was wondering if that snow height was covered within the 25 psf.

 

[EngJP]

For fresh snow, see ASCE 7-10 equation 7.7-1. The density works out to be 17.9 pcf for 30 psf snow. To get the height, take 30 psf / 17.9 pcf to give you 1.67 ft or 20".

 

[chris457]

The NBCC (National Building Code of Canada) Commentary notes that they use an average density of 3kN/m[sup]3[/sup] if I remember correctly (that's about 19 pcf, sounds like it's in line with other responses), but I believe the commentary does note that that value is conservative in dry areas and could be unconservative in very wet areas.

 

[CarlB]

Apparently the 21 inches weighed no more than 10 Psf, based on its water equivalent of 1.57 inches.

http://articles.chicagotribune.com/...n-gauge-annual-precipitation-water-equivalent

 

[dcarr82775]

For a very wet heavy snow that was rapidly melting I measured about 12 pcf. I think I once measured something heavier in a drift that had been created over a full winter so it was packed hard. Typical snowfall is less than that

 

[ajk1]

As alreasy noted, freshly fallen snow can vary dramatically in density. Of interest perhaps, but not directly relevant to your question, is that about 5 years ago I took samples of wet snow that had been ploughed up into a pile on a plaza in Toronto, and found the density to be about 40 pounds per cubic foot, to the best of my recollection. From this we were able to make an assessment of the height to which they could safely plough the snow. Nothing to do with your question, but thought it might be of interest.

 

[WWTEng]

ajk, thanks. That is very informative.

 

[dcarr82775]

Came across an AASHTO table yesterday quite by accident. They give:

Freshly Fallen 6 pcf
Accumulated 19 pcf
Compacted 31 pcf
Rain or Snow 31 pcf

 

[ellas1]

Agree with dccarr82775. We use 20 pcf which is slightly higher than the accumulated AASHTO.