Unbalanced Wind Load on Hip Roof next to adjacent building

[rharting]

I have an addition to an existing building. The existing building is approximately 150' long with a relatively flat roof. The new building is a hip roof with 25' eave to ridge distance and a slope of 3 on 12.

I believe i should be treating this like a hip roof (with an unbalanced load) with the lu equal to the full build up of snow = 152'+25' (similar to a high roof). Using a 35 psf snow load, my hd would be approximately 4.45' and thus making my unbalanced load of ~42 psf with a width of almost 24'(using figure 7-9). If i consider lu to be only 25' (7.6.1 states that the lu is equal to the eave to ridge distance) this decreases my unbalanced load to 16.1 psf with a width of 9'.

I believe the latter neglects the full buildup of the snow which was not the intent. I also considered using the full buildup on the side opposite the exsiting building and the 25 for the side adjacent (considering there is no 'build-up'), but this would then neglect buildup on the windward side. So while it seems ultra-conservative, i think i need to design both sides for the full buildup of snow which takes my snow load from a balanced 35 psf to an unbalanced 77 psf.

Any agreements or thoughts?

 

[ishvaaag]

http://eurocodes.jrc.ec.europa.eu/doc/WS2008/EN1991_3_Formichi.pdf

This powerpoint presentation for eurocodes, not US practice, says clearly there's some amount of indefinition respect treatment of, between other things, drift effects. Hence for any situation where significants amounts of snow can be expected I would try if something to err on the conservative side.

http://www.crrel.usace.army.mil/library/conferencepapers/MP-01-5663.pdf

This last paper covers some practical cases as well

http://cemsig.ct.upt.ro/astratan/didactic/bef/c09-bef.pdf

And to end how snow ("nieve") loads are handled at current spanish code CTE. Paragraphs 3.5.3 and 3.5.4 address the drift issue.