Roof Top Snow Loads

[KarlT]

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??

 

[connect2]

Users Guide NBC 1995, Structural Commentaries (Part 4), Commentary H, Snow Loads, 39. 'Areas Adjacent to Obstructions', (Figure H6), should do it.

 

[KarlT]

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?

 

[Ron]

KarlT...I would consider the worst case, though I agree your drift width of 24 feet seems a bit out of range.