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RE: Snow Load + Seismic

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The 2003 IBC uses "20 percent of flat roof snow load where flat snow
load exceeds 30 psf", in the effective seismic weight.  The Commentary
to the 2003 IBC states that "freshly fallen snow will not be firmly
attached to the structure, unlike ice buildup that will be attached" and
it refers to "the contents that might reasonably be expected to be
attached to the structure at the time the design earthquake occurs".
Thus the combined effect of the lower snow load limit and the 20 percent
weight appears to account for both probability and effective attachment
of the snow. 


Bill Sherman
CH2M HILL / DEN 
720-286-2792 

-----Original Message-----
From: Reza Dashti Asl [mailto:rezadashti(--nospam--at)hotmail.com] 
Sent: Tuesday, July 25, 2006 11:31 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Snow Load + Seismic

Neil,

It should not be just a question of probability. Even though the
probability of presence of the live load in a noraml building during the
earthquake is quite high ( furniture, people,etc...) you don't use that
live load in calculation of the seismic mass. I think it also has
something to do with the fact that these masses are not really connected
to the floor (or roof in the case of snow) and therefore when the
building starts to shake, they start to accept the force corresponding
to their masses and move (kinda) freely. So the question is what
percentage of the mass can move freely and how much of it will rest on
the structure to add to the W.

My 2 cents
Reza Dashti P.Eng
Vancouver, BC


>From: Neil Moore <nma(--nospam--at)omsoft.com>
>Reply-To: <seaint(--nospam--at)seaint.org>
>To: <seaint(--nospam--at)seaint.org>
>Subject: Re: Snow Load + Seismic
>Date: Tue, 25 Jul 2006 07:50:42 -0700
>
>Thor:
>
>I'm not sure that I would agree with the "statistical arithmetic" 
>concerning the reduction in snow loads.  We are aware of the snow loads

>and their duration at both Lake Tahoe and Mammoth areas;  big snow 
>loads and active seismic events.
>
>Last year, a State of Nevada county building official informed me that 
>we were to use 100 per cent of the snow weight during a seismic event, 
>and El Dorado County is 33 per cent
>
>http://www.co.el-dorado.ca.us/building/Snow_load.htm.
>
>Now Randy Volgelsand disagreed with me on the Nevada requirement, but I

>still used the 100% requirement on the particular project as it was 
>equipment on a roof with four enclosing walls that would not allow a 
>run off.
>
>I don't know what the criteria is at Kirkwood, but they do get some big

>depths - say 20 to 25 feet, heavy enough to crush sill plates on summer

>residences at Silver Lake.  That area stays closed for a number of 
>months and the area is rumbling almost every day with low level seismic
acitivity.
>
>
>
>Neil Moore, SE, SECB
>neil moore and associates
>structural engineers
>
>
>distressed structures investigations
>
>
>At 07:16 AM 7/25/2006, you wrote:
>>An article in Building Standards (ICBO's magazine, now Building Safety

>>under the ICC) several years ago addressed the 75 percent reduction.  
>>A statistician-type wizard analyzed snow loads in (I think) Stephens 
>>Pass in Washington state (400 psf or so snow load) and somewhere in 
>>the high desert (Utah, maybe) with 40 psf snow.  Conclusion was that 
>>in both cases the likelihood of an earthquake occuring simultaneously 
>>with full snow load was so small that the 75 percent reduction was 
>>actually conservative.  I was very happy when the local building 
>>official began allowing the reduction after I showed him the article.

>>It's possible I could find the article in my files, and at least give 
>>anyone who's interested the date it appeared.
>>
>>The likelihood that your lateral force resisting system will not be 
>>constructed as you designed is fairly high, unfortunately.
>>
>>Thor Matteson
>>www.shearwalls.com
>>
>>
>>
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