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  • 1.  Friction Support

    Posted 02-04-2020 08:39 AM
    Edited by Chad Morrison 02-04-2020 08:47 AM
    I am designing a roof top stair on an existing building.  I am prohibited from anchoring to the roof slab because it would penetrate the roof rubber membrane.  I can tie the stair to the structural dunnage steel to resist some lateral loads. 

    Without anchorage, is it possible to count on any horizontal support at the bottom of the post?  It seems like friction might seem to be enough to resist the loads present.  I would only need to know 2 things to calculate the available resistance: the reaction of the post and the material of the roof.  All I would need is the coefficient of friction between the steel and rubber interface.  I have never used this approach before, is it permissible?  Where could I find an acceptable coefficient?

    Thank you,

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    Chad Morrison P.E., M.ASCE
    Professional Engineer
    Greenville RI
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  • 2.  RE: Friction Support

    Posted 02-05-2020 12:08 PM
    Edited by Tirza Austin 02-05-2020 04:02 PM
    Hi Chad.  I always like to see creative problem solving, but including the roofing membrane in the horizontal load path will likely get you in trouble.  If the horizontal loads are transient (wind or seismic) they will likely be accompanied by a vertical component that would reduce the normal forces and render the friction force unreliable just when you need it the most. On the other hand, if these are static forces, depending on the actual roofing material, the continuously applied forces will likely induce creep, stretching and eventual rupture of the roof membrane.  You could try to prevent this by adding a protective layers or a slip sheet, but that would itself negate the friction resistance.  I'm thinking the best solution here is to take a closer look a the structural dunnage steel to figure out how it can handle 100% of the horizontal forces.

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    Peter McConaughy PE, ASCE
    Structural Engineer
    McCon Engineering, Inc.
    Kingsville, MD 21087
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    Original Message


  • 3.  RE: Friction Support

    Posted 02-05-2020 12:09 PM
    Edited by Tirza Austin 02-05-2020 12:09 PM
    Solar panels are routinely anchored to a flat roof by ballasting them down and relying on friction.  Be aware that in moderate or high seismic areas, you can't rely on friction from gravity loads to resist the seismic loads.  While the owner may not want to breach the roof membrane, it is often done with a round pipe penetrating vertically through the roof membrane and supporting the stair stringer several steps up.  The round pipe is easiest to flash and seal the roof membrane and there needs to be enough clearance above the membrane to flash the pipe so the support is several steps up.

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    Paul Blomberg P.E., M.ASCE
    Phoenix, AZ, United States
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    Original Message


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