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Re: Unbalanced snow on 12 in 12 roof

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That looks like the article in Structure, which I remember. The inconsistency doesn't occur until you get a roof pitch greater than 8 in 12 (lower on slippery roofs).  Above there, the sloped roof load and the flat roof load can differ by quite a bit.  In the past, a "shortcut" to quick checks was to just apply the ground load (0.7*1.5 = 1.0, rounding to even).  For larger slopes and slippery surfaces this no longer works because the unbalanced load is now based on the ground load instead of the roof load.

Actually, while reading the article originally, I must admit I found myself asking "where are the bodies".  Also, since I live/practice in a relatively low snow-load area (25-30psf), the snow load never affects the wall design, and has only a minor impact on the headers and beams, but the pattern loads are a pita to input into analysis programs and can't be done by hand in a production environment, nor can they be used with span tables.

Which brings up the another point - do the span tables produced by the lumber organizations (engineered and solid sawn) for snow load (LL @ 1.15 duration) account for unbalanced load profiles?  I just checked my IBC2003, chapter 23 table 2308.10.3(3) for rafter spans at (10DL/30lb ground snow load) and a 2x6 rafter should span 14'-5".  Using  10DL/30SL, 7 in12 pitch with woodworks sizer, I get a fb/Fb of 1.10 and a live deflection of L/170(!). Running the same rafter at 10DL/21SL (using the 0.7 factor to get the flat roof load) results in fb/Fb of 0.87 and a live deflection of L/243.  Are all of the tables going to be updated to match the "new" requirements (these rafters don't currently meet ASCE 7-02 loads), or will the "Engineered" solutions simple become more and more conservative than the code prescriptive requirements?

AWC Info wrote:
Unblaanced snow on 12 in 12 roof


I didn't think there was that much difference between 7-02 and 7-05. We did a comparison of 7-05 to 7-98 and didn't even see the magnitude of differences that you note below. Here's a link to our paper:

Take a look at that and see if it helps. If not, give me a few more specifics about your particular situation (e.g. span, ground snow load, etc.) and I'll see if I can help.


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