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Re: 25% snow load is Conservative

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Considering joint probabilities, O'Roarke and others in ASCE J. of 
S.E., Sept 92, p2338... recommend using 20% of the 50-yr ground snow 
load as the roof snow-load for seismic-design calcs.  They analyzed 
snow records at four sites, including one at Stampede Pass, WA which 
had a 50-yr Pg of 511 psf.

Dave Evans, P.E.

> From:          BVeit <BVeit(--nospam--at)>
> Date:          Thu, 16 Apr 1998 13:52:30 EDT
> To:            seaoc(--nospam--at)
> Subject:       25% snow load is Conservative
> Reply-to:      seaoc(--nospam--at)

> I know not many people deal with lateral forces due to large snow loads, but
> those of who do might find this interesting.
> You can imagine the enormous lateral loads if you were to count even part of a
> 400psf snow load toward seismic.
> Compounding this, the code is unclear on how much snow load to count toward
> seismic in heavy snow country.  It says, at the discretion of the building
> official, the amount of snow counted toward seismic may be reduced to 25%.
> Some counties have chosen to go big, such as Placer Co @ 1/3rd and Amador Co
> (kirkwood) @ 1/2!  In addition, the snow loads in these areas are routinely
> above 300 psf.
> As someone has said previously (Lew Midlam?) Where are the bodies?
> A recent article in ICBO Bld. Standards, Mar-April '98, clearly, rationally
> articulates why 25% of snow is CONSERVATIVE.  It's all Stats 101.
> So if there are any code authors out there, how about addressing this in 2000?
> Brian Veit, P.E.