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Re: Snow load in combination with seismic.

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The percentage of the snow load currently used to determine seismic forces, in most of counties around here,
have been in effect for many years, pre-dating most studies.  Most of the time there is no reason behind the use of 1/3, 1/2, 100% (as opposed to 1/4), other than, "if 1/4 is the minimum then we should use 1/3 because it snows a lot up here" .
 
See the article that appeared in the April 1998 issue of Building Standards by Joseph D. Scholze, P.E., S.E.  The author did a statistical study of seismic and snow accumulation probabilities and determined that the ¼ snow load was conservative.  This agrees with other similar studies that consider joint probabilities. 
 
O'Roarke and others in the ASCE Journal of Structural Engineering, Sept 92, p2338, recommend using 20% of the 50-yr. ground snow load as the roof snow-load for seismic-design calculations.  They analyzed snow records at four sites, including one at Stampede Pass, WA which had a 50-yr. Pg of 511 psf. 
 
Also see "Combining Snow and Earthquake Loads for Limit States Design" by Bruce Ellingwood and David Rosowsky that was published in The Journal of Structural Engr. Nov 1996. 

I have not seen any scientific studies that suggest that there is reason to use more than 25% of the snow load as seismic dead loads.  If anyone has more information on this subject please post it. 
 
Randy Vogelgesang S.E.
South Lake Tahoe
----- Original Message -----
Sent: Wednesday, June 06, 2001 2:29 PM
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