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Re: Snow Load + Seismic

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Yep, in 2001 CBC, section 1630.1.1 says the same thing as the ASD load combo "may be reduced up to 75% ..." (i.e. 25%S is included in the seismic mass) when snow load is greater than 40psf. You can ignore snow load at less than 30 psf....

looks like I'm adding 10psf to my seismic roof mass...

Thanks, shoulda looked at that one....been a while...


On 7/20/06, William.Sherman(--nospam--at) <William.Sherman(--nospam--at)> wrote:
Look at the definition of W for the seismic lateral load equations.  ASCE 7-05 defines W to include 20 percent of the snow load in the mass - not sure about 2001 CBC.
Bill Sherman

From: Gerard Madden, SE [mailto:gmse4603(--nospam--at)]
Sent: Thursday, July 20, 2006 8:03 PM
To: seaint(--nospam--at)
Subject: Snow Load + Seismic

2001 CBC section 1612.2.1 (LRFD)

  • Load Combo 1.2D+1.0E+(f_1L+f_2S)
    • where f_2 = 0.2 for roofs that shed snow.
Section 1612.3.2 (ASD Load)

  • Load Combo D+L+S+E/1.4
    • Exception 2 states that for Snow Loads 30 psf of less need not be combined with seismic loads.

I have a project with a 40 psf snow load. Therefore, the S value must be combined with the E/1.4 in the ASD value and I may reduce it up to 75%.

What I am not clear about is if the mass of the snow is included in the E value. I read this as Snow is not a part of Seismic Dead Load.

I have done projects in Alaska under IBC a few years back where I had to include 50% of the snow load as seismic mass, but the building department I'm dealing with does not state that in their webpage of code requirements.

what do you think? The structure is a residence with a simple 6/12 gable roof.