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RE: Snow load in combination with seismic.

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Is this building in an area which is likely to have snow on the roof for much of the year or rather infrequently? How often are the "high snow loads" likely to occur and to remain on the roof? There are no easy answers, but the percentage of snow used with seismic should be conceptually related to the probability that snow will be on the roof at the same time as an earthquake may occur. For example, in the mountains in Colorado snow can remain on roofs for a larger percentage of the year, than in Denver where snow rarely remains for long. I would use a lower percentage in Denver than in the mountains. If the building were in Alaska, I would also use a higher percentage. But it all comes down to judgment and relative risk.
-----Original Message-----
From: Bill Marczewski [mailto:bmarczewski(--nospam--at)martind.com]
Sent: Wednesday, June 06, 2001 3:30 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: Snow load in combination with seismic.

I have a multi-story building with high snow loads located in a very low seismic region (zone 1).  The building official is unable to provide information regarding the percentage of snow load in excess of 30psf that should be used in combination with the seismic load.  My preliminary calculations consisted of using 100% of the snow in the seismic dead load, and I feel this is a significant penalty.  I have previously been able to use only 25% of the snow load in seismic zone 3 in Jackson, Wyoming.  On the other hand, Steamboat Springs, Colorado (zone 1) has required 100% of the snow load to be used in combination with the seismic load.  The 1997 UBC states that snow loads beyond 30psf must be considered in seismic design, but may be reduced up to 75% when approved by the local building official.  What should I do, or does anyone have any recommendation on what percentage of snow to use when the building official cannot provide guidance on this issue?  I think the use of 100% snow load in combination with seismic is really to harsh.  Comments would be great.  Thank you.
 

Bill S. Marczewski
Martin Design, Inc.
1360 S. Clarkson St.
Denver, CO 80210
bmarczewski(--nospam--at)martind.com