Integrated Buildings & Structures

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Snow Drifts for Lower Roofs

  • 1.  Snow Drifts for Lower Roofs

    Posted 07-29-2021 12:45 PM
    Hi there, first time posting but I did not see this question elsewhere.

    In ASCE7-16 Section 7.7, the balanced snow load calculations are based on the sloped roof load, ps.  So the base snow height, hb, is based on the sloped roof snow load but the drift load pd, is based on Ground Snow Load (both gamma and hd are calculated with pg).

    I find this very inconsistent and reading the commentary didn't give me any answers, when these depths are added together seems conservative because the drift load is usually higher depth and is based on ground snow load.  It also seems inconsistent.

    My question is can this pd load be factored per equation 7.3-1 & 7.4-1 to get it to sloped roof load so that when I add it to the base snow load they are both the same?

    Trying to space my trusses in this area is difficult with 1 depth based on pg and the other on ps, the trusses are designed to ps, so how am I supposed to do this?

    Thanks,

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    Steven C. Dame P.E., M.ASCE
    Director of Engineering
    Champion Home Builders, Inc.
    Troy MI
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  • 2.  RE: Snow Drifts for Lower Roofs

    Posted 08-11-2021 02:17 PM
    Hi Steven,

    I've struggled with this in the past.  What I come back to is that balanced snow is an equilibrium amount that is able to stay on a continuous plane surface, whereas drift is the snow that gets stuck and has nowhere else to go, either because it backs up against a vertical surface (windward) or because it falls straight down from a horizontal one (leeward).  Perhaps the real drift behavior falls between sloped and flat snow loads, but it would still be unconservative to apply the slope factor to drift.  Not sure about this, though.  If anyone on the code committee for chapter 7 could weigh in, that would certainly be helpful.

    I would be interested to know whether any of the testing for the drift provisions used sloped surfaces.  A quick survey of papers referenced in the commentary by Speck, Tayler, and O'Rourke (Including "Proposed Code Provisions for Drifted Snow") did not turn up any discussion of sloped roofs.  If ASCE's research partners can come up with 15 different GCp tables for C&C roof wind pressures, surely they could stand to test out sensitivity of snow drift to roof slope.

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    Christian Parker EIT, A.M.ASCE
    Structural Project Engineer
    Washington DC
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