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Re: Snow Load + Seismic

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• To: seaint(--nospam--at)seaint.org
• Subject: Re: Snow Load + Seismic
• From: "Reza Dashti Asl" <rezadashti(--nospam--at)hotmail.com>
• Date: Tue, 25 Jul 2006 10:31:14 -0700

```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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