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RE: Snow load in combination with seismic.

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The following reference should help.  It shows the basis of the load combinations in the 2000 IBC.
 
Author: Ellingwood, B.; Rosowsky, D.
Title: Combining snow and earthquake loads for limit states design
Call No: 600/A47
Citation: Journal of Structural Engineering, 122, 11, Nov. 1996, pages 1364-1368
Abstract: ASCE Standard 7, "Minimum Design Loads for Buildings and Other Structures," and the "NEHRP/BSSC Recommended Provisions for Seismic Regulations for Buildings" both contain load requirements for earthquake-resistant structural design. Many of the NEHRP provisions are being implemented in building codes and are the basis for the seismic load provisions in Chapter 9 of ASCE 7. However, the treatment of combinations of earthquake and snow loads differs in the ASCE 7 and NEHRP documents. Concerns regarding these differences were raised during the balloting of both the ASCE 7-93 and ASCE 7-95 provisions. To address these concerns, the load combination requirements involving dead, snow and earthquake load in these two resource documents were examined using modern probabilistic load models and load combination techniques.
 
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Michael Valley, P.E., S.E.                       E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201
 
-----Original Message-----
From: Bill Marczewski [mailto:bmarczewski(--nospam--at)martind.com]
Sent: Wednesday, June 06, 2001 2:30 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