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Re: 25% snow load is Conservative

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Dan Vines wrote:
> Please forgive my ignorance (before you mountain dwellers jump on me), but,
> being both a young engineer and someone who has lived his whole life in the
> southeastern US (where 1 inch of snow shuts down a city), I would like to
> know where snow loads like these can occur.  400 psf is like 6-1/2 feet of
> WATER.  What are the depths and densities of snow that can produce loads of
> this magnitude?  Where do they occur?
> Thanks
> Dan Vines, EIT
> -----Original Message-----
> From: BVeit <BVeit(--nospam--at)>
> To: seaoc(--nospam--at) <seaoc(--nospam--at)>
> Date: Thursday, April 16, 1998 2:01 PM
> Subject: 25% snow load is Conservative
> >I know not many people deal with lateral forces due to large snow loads,
> but
> >those of who do might find this interesting.
> >
> >You can imagine the enormous lateral loads if you were to count even part
> of a
> >400psf snow load toward seismic.
> >
> >Compounding this, the code is unclear on how much snow load to count toward
> >seismic in heavy snow country.  It says, at the discretion of the building
> >official, the amount of snow counted toward seismic may be reduced to 25%.
> >
> >Some counties have chosen to go big, such as Placer Co @ 1/3rd and Amador
> Co
> >(kirkwood) @ 1/2!  In addition, the snow loads in these areas are routinely
> >above 300 psf.
> >
> >As someone has said previously (Lew Midlam?) Where are the bodies?
> >
> >A recent article in ICBO Bld. Standards, Mar-April '98, clearly, rationally
> >articulates why 25% of snow is CONSERVATIVE.  It's all Stats 101.
> >
> >So if there are any code authors out there, how about addressing this in
> 2000?
> >
> >Brian Veit, P.E.
> >
> >
I live in Montreal, and we do have lots of snow here. We design flat
roofs for 45 pounds per square feet. If the roof has different levels,
this value may be multiplied up to a factor of 3.

This winter, we had a catastrophic ice storm (recurrence in excess of
100 years) that lasted one full week. Hundreds of electric pylons have
failed. Roofs of commercial and industrial buildings have held very
well. There might have been 2 or 3 failures, one of which being an
inflatable roof for a golfing dome.

I don't design buildings anymore, but I recall roof structures for steel
buildings made out of light open web steel joists about 16 inches deep
spaced at 5-6 feet on center. 

I would be curious to know what type of structure is required to resist
snow loads of 300 psf.

Bruno Côté