snow drift on canopies

huangyhg

snow drift on canopies
i have a canopy that projects out 5 feet from the building.  the roof of the building is flat and is 30 above the canopy.  i considered windward drift which puts me at a maximum drift of 35psf.  i don't think i should have to design for leeward drift, but i am looking for some hard evidence that states i do not.  i don't see snow blowing off a roof 30 feet above and drifting onto a 5ft canopy.  (leeward drift calculation gives 70psf of snow max).  code is very limited on canopy loads.  is there any documentation that if the roof elevation change is x feet or greater you do not have to consider leeward drift?
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canopy drift happens - even with high roof-to-roof distances - see it every winter.
the code is silent in terms of a high vertical differential serving as a cutoff - if i re  
jae is correct. i've checked this numerous times, looking for a way out..don't know of one.  have had to design some beefy canopy structures as a result.
35 psf appears to be on low side. don't know about area, but for bigger roof area, you are expecting it sometime above 100 psf.
even if you were to ignore drifting, i believe the code requires a minimum 75 psf live load on canopies.
daveatkins
daveatkins...can you give a citation for that one?  hope that's not the case.
allow for the leeward drift, but for a 5 foot canopy the code has a clause to allow for the maximum angle of build up from the front of the canopy. it ends up more but not a massive amount more.
ibc table 1607.1, item 25, "marquees."
how about the rest of you?  do you think a canopy qualifies as a "marquee"?
the code defines a marquee as "a permanent roofed structure attached to and supported by the building and that projects into the public right-of-way."
daveatkins
daveatkins:
interesting. that definition of 'marquee' would seem to fit a canopy if it extended out over a sidewalk or other public right-of-way.
i have always used ibc table 1607.1, item 30, 'awnings and canopies' for the minimum, non-snow, load on an entry canopy. but i can see your point if the canopy is over a sidewalk.   
we always design our canopies for 75psf.
the other day i saw some workers using a canopy as a stage area to set up their window washing platforms, etc. they had a few sheets of plywood protecting the canopy, the platform, a whole bunch of workers, and other equipment up there. i went back and asked the engineer who designed it, if he used 75psf and told him what i saw.
we do however reduce the load for a deflection check based on engineering judgement and material on the canopy.
  
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

i base the canopy snow loads on a drift condition, but it usually doesn't get to 75 psf.  that's for canopies attached to or adjacent to a building.  for free standing walkway covers, usually with flat roofs, unless they are close to the building, i use the ground snow load.  these are typically rigid frame aluminum structures with extruded decking.
i can see the need for the marquee loading on something that has a parapet (like a marquee would have or its sign) over an entrance.  for flat roofs, i don't see the minimum 75 psf.  just compute the drift and apply it.