snow drift on very small lower roof

huangyhg

snow drift on very small lower roof
what is your opinion on a very long lu and very tall upper roof, against a very small lower roof and in which the calculated drift width w ends up being a lot larger than the lower roof width?
would you then use the drift width w the same as the lower roof itself or would use a truncated snow drift shape? (trapezoidal shape) doing that would increase the snow drift load by a factor 6 or 7.

i would consider impact!
mike mccann
mmc engineering
per code you must use the truncated shape.
daveatkins
yes, asce 7 is pretty clear on this.  you must use the truncated shape.
i just find it hard to justify the truncated loading pattern. please take a look at the illustration, and you are suggesting i use option 1...what about option 2?
the code says you're stuck with the truncated shape.  seems silly to me, though.  i've never seen six feet of snow on a four foot wide roof.  perhaps this is a place for engineering judgement to overrule the code.
we see this situation a lot on warehouse type buildings with small canopies and you'd be amazed how the snow actually does pile up on these small roofs.
use the truncated drift.
i don't know why you find it so hard to imagine 6' of snow on a 4' wide roof.  additionally, if you ignored it and it ended up failing you will have a lot to answer for.
use the truncated shape.  asce is very clear on this and i don't believe it's open to much interpretation.
i agree.. but i think asce needs to improve it somehow.  lets say you have a skyrise and you have a small canopy on the 1st floor, i dont see how snow from the penthouse can drift to the lower roof.  only leeward drift makes sense.
never, but never question engineer's judgement
i've lived for 58 years in new england, and i've never once seen it.  it's something that i look for, as i once designed a canopy to attach to a parking garage.  i used the truncated load to design the support.  the city put up some cheap, flimsy canopy instead. twenty years later, and it's still there.  it's never had even close to the snow drift i had designed for.
for such short