huangyhg

roof top snow loads
quick question. i have an existing building in canada (built in 1931), upon which the owners are considering putting up a monstrous air handling unit (length = 38ft, width = 13ft, height = 12ft, weight = 35,000 lbs). now i pretty much already know without calculating anything that it won't work, but my main question was about the snow loads surrounding the unit.
if i treat it as a rooftop obstruction, i get a snow drift of 75 psf max down to 33 psf, extending for a distance of 24 ft (twice the height of the unit). if i input it as a small upper roof with a step height of 12 ft, i get 75 psf down to 33 psf, over a distance of 11 ft +/-. would i need to consider the worst of the two conditions, or would i need to superimpose them together? seems to me i would only need to consider the worst of the two conditions, however a 24 ft long snow drift seems kind of ridiculous??
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users guide nbc 1995, structural commentaries (part 4), commentary h, snow loads, 39. 'areas adjacent to obstructions', (figure h6), should do it.
actually, fig. h-6 is what i used for the second case i ran up above. however, the plan area of the top of the unit is large enough to also generate built-up snow loads around the perimeter of the unit from the top of the mech unit. (based on fig. h-4(a) of nbcc commentary)
my ? was more about whether i would take the worst case of the two conditions or would somehow superimpose the results?
karlt...i would consider the worst case, though i agree your drift width of 24 feet seems a bit out of range.

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