You're right, of course, John.
My eyes misread those tiny subscripted 3s as 2s and I thought you were
giving figures of mass divided by area.
I apologize for misreading your posting.
Jim
On 2011-02-09 1621, John M. Steele wrote:
Jim,
It is a density, and based on a specific condition of snow, but not on
depth. It needs to be multiplied by depth.
Like any density, it is dependent on the material meeting the condition.
It is based on fully saturated but draining snow (basically it needs to
be melting). However, I would assert that is the general condition when
roofs are at risk. If you have icicles hanging off the roof, at least
some of the snowpack is melting. The condition also applies if the
snowpack has been rained on, but on a sloped roof, is able to drain.
On a flat roof, there is a significant risk of appraoching the slush
condition. I don't know whether the hanger had a sloped or flat roof, as
I have not seen a picture.
--- On *Wed, 2/9/11, James R. Frysinger /<[email protected]>/* wrote:
From: James R. Frysinger <[email protected]>
Subject: Re: [USMA:49829] MM93-Item 3, Aircraft Hanger
To: [email protected]
Cc: "U.S. Metric Association" <[email protected]>
Date: Wednesday, February 9, 2011, 3:32 PM
> If anybody is worried about their roofs, please use at least 320
kg/m³ for wet snow on sloped roofs. If you have a flat roof,
determine whether the drain is working, it will be the difference
between 320 kg/m³ wet snow and 960 kg/m³ slush.
I believe that those figures assume an accumulation of 80 cm of
"snow", whether it be light and fluffy, wet and dense, slushy, or
icy. If 80 cm of snow falls, then, due to insolation (not
insulation!) and warming by air or by conduction from below, the
depth will no longer be 80 cm.
And of course, not all snow accumulations are 80 cm in depth.
Rather than recommending a load estimate figure (that is based on 80
cm of accumulation of whatever sort -- snow, slush, ice), it might
be preferable to teach the method, which then can be adapted to any
given precipitation amount.
I collect and report daily precipitation data for CoCoRaHS
http://www.cocorahs.org <http://www.cocorahs.org/>
including snowfalls. For snow I measure the depth of the
accumulation, collect snow from an area of known size, and weigh it
to determine the "rainfall equivalent". Working backwards for this
winter for some of our snowfalls, I observed that the snow:rain
ratio might be better stated as
1 cm:0.7 mm
1 cm:0.75 mm
1 cm:1.2 mm (followed rain and freezing rain)
1 cm:0.26 mm (notes indicate unusually light & fluffy snow
with large "flakes")
As you can see, there is quite a bit of variation in those ratios
for my location. Generally, I would tend to characterize our snows
as averaging 0.7 mm to 0.8 mm rainfall equivalent in 1 cm of snow
from what I have seen the last 4 years. As John suggests, other
areas might typically see snow of a different average density.
This variation is exactly why meteorologists melt (or weigh) fallen
and accumulated snow to determine its actual water content. If one
is concerned and capable enough to estimate roof loading, they
probably should do likewise.
By the way, some architects might show maximum snow loadings on the
plans for the structures built from those plans.
Jim
On 2011-02-09 1208, John M. Steele wrote:
> I just received Metrication Matters 93, and saw the aircraft hanger
> example again. As it is snow season in the US, and people need to
worry
> about their roofs, I have to point out two huge errors in the
example as
> I don't believe anyone should rely on that example.
> 80 cm of snow != 8 mm of rain
> Even if the 10% rule were true, it would imply 80 cm of snow is
80 mm of
> rain. Doing some Googling on snow load and roof designs, I find the
> density of wet, heavy snow is more like 32-33% water density, 320
- 330
> kg/m³. The Washington (DC) area is not noted for light, fluffy
powder,
> and light fluffy powder isn't what collapses roofs. Using the 320
kg/m³
> x 0.8 m, the actual roof load was more like 256 kg/m³ if the drainage
> system was still working, not the 8 kg/m² of the worked example.
> Flat roofs are a particular problem as snow tends to clog
drainage and
> then you get slush, a mixture of ice and water. Not surprisingly, the
> density of slush lies between 920 kg/m³ (ice) and 1000 kg/m³ (water).
> The figure above of 320 kg/m³ is for drained (but wet) snow - imagine
> snow on a screen so any water melt can drip out.
> I can't find the spec for flat roofs, a lot of local codes in the
> northern US are 35-40 lb/ft² for sloped roofs. That converts to 170
> kg/m². Some extreme snow areas are higher, and I would expect
flat roofs
> to be higher.
> If anybody is worried about their roofs, please use at least 320
kg/m³
> for wet snow on sloped roofs. If you have a flat roof, determine
whether
> the drain is working, it will be the difference between 320 kg/m³ wet
> snow and 960 kg/m³ slush.
>
-- James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
