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RE: Wind Load on Deep Snow Profile

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Paul,

 

That’s the first I’ve ever heard of the concept of increasing wind loads due to built-up snow. Although I will grant most of my experience is mid-Atlantic US, 20-30psf ground snow loads.

 

I can’t see it making much of a difference. It doesn’t seem to me, even with heavy snow buildup, that you’d increase the mean roof height much more than 5 feet or so. That translates (glancing at the height adjustment tables) to about a 5% increase in the wind pressure. But the wind load is nowhere near that accurate to begin with. For starters, if you’re using the low-rise method, you’re locked to the design pressures for 30 feet mean roof height even if you only have a 15 or 20 foot mean roof height. Then if you’re in Exposure B, you’re actually designing to almost an Exposure C surface roughness; the roughness values picked for Exposure B are at the lower bound (i.e. nearly open terrain).

 

Plus, if you’ve got that much snow it’s probably building up on the ground as well as on the roof too. So “grade plane” is higher too…

 

Regards,

Gary

Gary J. Ehrlich, PE
Program Manager, Structural Codes & Standards
National Association of Home Builders (NAHB)
1201 15th Street, NW, Washington, DC 20005
ph: 202-266-8545  or 800-368-5242 x8545
fax: 202-266-8369
gehrlich(--nospam--at)nahb.com


From: Paul Blomberg [mailto:paul.blomberg(--nospam--at)gmail.com]
Sent: Tuesday, June 10, 2008 2:44 PM
To: seaint
Subject: Wind Load on Deep Snow Profile

 

I'm putting together design criteria and am curious how you handle / take into consideration the added projected wind surface area due to built up snow on the roof of a building?

 

I have a new metal building going into northwestern Montana and the ground snow load is over 120 psf.  That is a relatively deep pile of snow.  When you put together your load case that combines snow plus wind, do you increase the projected profile of the building to reflect the height of the snow on the roof and the associated wind load? 

 

I normally include this additional snow profile to calc the wind loads but this my first heavy snow load situation where the increased load is significant.

 

Paul.

Phoenix, AZ