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  • 1.  Snow Drift Loading

    Posted 04-08-2019 02:53 PM
    Edited by Tirza Austin 04-08-2019 02:52 PM
      |   view attached

    Per the attached figure, when calculating snow drift, I typically assume the wind blows in a direction diagonal to the main building footprint giving an upwind fetch distance of the square root sum of the squares of the building width and length.  I have seen other engineers take the upwind fetch distance as the perpendicular distance from the edge of the building to the parapet (B in the attached sketch).  Typically, the first approach which may be slightly conservative still gives reasonable numbers.  However, when the building is very long compared with the width (as might be the case for an industrial building), using the square root sum of the squares gives an extremely large drift compared with only using the perpendicular distance.  Is there any justification in ASCE 7 for using the perpendicular distance?



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    Caleb Drummer
    Carpenter Wright Engineers
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    Attachment(s)

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    Snow Loading.pdf   513 KB 1 version


  • 2.  RE: Snow Drift Loading

    Posted 04-09-2019 09:50 AM
    Edited by Tirza Austin 04-09-2019 09:50 AM
    ​I'm thinking that using the diagonal distance is conservative.  That long diagonal distance is only accurate for a very small length of the roof step and drops off proportionally as you move along the step (the diagonal distance feeding the drift decreases).  You might make a case for increasing the drift load slightly at the corners or ends of a step, but I can think of other mitigating arguments against even that.

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    Greg Thein, PE
    Cleveland, OH
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  • 3.  RE: Snow Drift Loading

    Posted 04-09-2019 09:51 AM
    Edited by Tirza Austin 04-09-2019 09:50 AM
    ​Colin,
    Figure 7-8 (ASCE 7-10) shows the fetch perpendicular to the parapet/change in roof height.  Ll is not marked, but Lu is defined as a perpendicular distance in the figure.

    Regards,


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    Stacey Brown M.ASCE
    Stantec Consulting Inc
    Berkley MI
    (734)213-6091
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  • 4.  RE: Snow Drift Loading

    Posted 04-09-2019 12:14 PM
    Edited by Tirza Austin 04-09-2019 12:14 PM
    I would suggest that this method isn't only conservative but incorrect. ​To know for sure one would need to dig deep in the research and talk with snow load committee members. 

    From my past experience following Oregon's ASCE Chapter developing the Oregon snow loading manual, and recollection from seminars, etc., the loading equations are based at least in part on observations of real drifts and measurements of the fetch distance perpendicular to the object (parapet etc.). Probably some lab work as well. The wind during storms almost certainly came from a variety of directions for all the various observations that went into the data.

    I would also suggest that using the perpendicular distance is what engineers do (as far as I know, I may be wrong). I'm not aware that there is any substantial history of failure of properly designed roof structures using the perpendicular distance.

    I wouldn't read into the code more complication than is required by the text.

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    Brett King M.ASCE
    GHD Inc.
    Lake Oswego OR
    (971)925-3743
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  • 5.  RE: Snow Drift Loading

    Posted 04-09-2019 01:17 PM
    Edited by Tirza Austin 04-09-2019 06:01 PM
    ​Comment by Mike O'Rourke - former Chair of the ASCE 7 Snow & Rain Load Subcommittee.

    Since the mid-1980's the drift size in ASCE 7 has been a function of both the ground snow load and a fetch distance. The relationship was based upon empirical observations from insurance company files. In the insurance company files, the fetch distance was quantified as the  perpendicular distance. Using the diagonal distance as the fetch adds a bit of conservatism to the calculated drift load.

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    Michael O'Rourke M.ASCE
    Rensselaer Polytechnic Institute
    Clifton Park NY
    (518) 276-6933
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  • 6.  RE: Snow Drift Loading

    Posted 04-09-2019 01:43 PM
    Edited by Tirza Austin 04-09-2019 01:43 PM
    ​Thank you to everyone for the responses.  I really appreciate your help!

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    Caleb Drummer EI
    Carpenter Wright Engineers
    Knoxville TN
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    Original Message


  • 7.  RE: Snow Drift Loading

    Posted 04-09-2019 10:41 PM
    Edited by Tirza Austin 04-09-2019 10:41 PM
    A good tool for snow loading is the "SBCA Load Guide", a free download from the Structural Building Components Association (but you must provide a valid email address).

    This tool has spread sheets to help with a number of loading conditions including snow loading 


    SBCA Load Guide | Structural Building Components Association
    Sbcindustry remove preview
    SBCA Load Guide | Structural Building Components Association
    In spring of 2016, SBCA updated The Load Guide to the 2015 International Building Code (IBC), the 2015 International Residential Code (IRC), 2015 NDS and the 2014 TPI 1. Commentary shows the provisions of ASCE 7-10 and how the changes to ASCE 7-10 have evolved from ASCE 7-05.
    View this on Sbcindustry >


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    Michael Richmond
    Summerville SC
    (843) 851-1782
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    Original Message


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