Roof deck tripping snow drift requirements, per reviewer
Roof deck tripping snow drift requirements, per reviewer
(OP)
Looking to put a 15'x20' roof deck atop an existing, older, 30'x40' building. The existing structure of the nearly flat roof does not meet today's building code. In order to leave the roof undisturbed, we planned to span the roof from side wall to side wall with small W shaped beams (the side walls can take the load), only high enough above the roof to allow for maintenance below the beams. Contemplating using hangers to keep the roof decking flush with the tops of the beams to reduce its overall profile.
Reviewer says beams hovering above existing roof trigger the snow drift provisions, increase the local loading on the roof, and require that we bring the entire roof up to code, including design for drift loads (and probably tripling the project cost).
I offered to reduce the deck to 15'x15' in order to avoid having to trip the drift codes, and I argued that we'd be taking a large bit of the snow load off the roof via the deck - no dice, he says.
Is he right? If he is, does anyone know of any design changes I can make to swing his code interpretation (that we're "increasing the roof load"), allowing me to leave the existing roof as is?
Reviewer says beams hovering above existing roof trigger the snow drift provisions, increase the local loading on the roof, and require that we bring the entire roof up to code, including design for drift loads (and probably tripling the project cost).
I offered to reduce the deck to 15'x15' in order to avoid having to trip the drift codes, and I argued that we'd be taking a large bit of the snow load off the roof via the deck - no dice, he says.
Is he right? If he is, does anyone know of any design changes I can make to swing his code interpretation (that we're "increasing the roof load"), allowing me to leave the existing roof as is?






RE: Roof deck tripping snow drift requirements, per reviewer
Dik
RE: Roof deck tripping snow drift requirements, per reviewer
The deck will have handrails all the way around it, though, but I might be able to ignore those if the deck is 15' x 15'.
RE: Roof deck tripping snow drift requirements, per reviewer
If it is some form of the IBC you may want to take a look at chapter 34. I thought they allowed a 5% increase in gravity loads before the whole structure needs to be brought up to code (I don't have my codes with me at the moment). I also thought that the 5% was based off the current requirements of the code, that is, you would use the snow loads found in your code today not what they were 20 years ago.
Depending on your conditions, I would look at any way possible to avoid an increase greater than 5%.
I can't see a 30'x40' building creating that much snow drift.... but the reviewer is correct with the fact that it needs to be considered.
RE: Roof deck tripping snow drift requirements, per reviewer
If it does, and if the roof was be considered a single element, than I could probably keep the net increase to 5%, but breaking the roof down into beams and joists, any drifting at all would seem to increase the local 30psf design load to 31.5psf.
RE: Roof deck tripping snow drift requirements, per reviewer
Maybe raising the deck would be better, if you could justify that it is high enough that the snow will not drift.
If he is not letting you use the 15' wide justification you may be fighting a losing battle.
RE: Roof deck tripping snow drift requirements, per reviewer
Usually when we add RTU's to existing buildings we reinforce the joits for the weight of the RTU + snow and usually the girders and columns are within the 5% limit.
Mijowe brings up a good point that I forgot as well, if your element is less than 15' wide you should end up with no snow..... of course your will probably end up putting concentrated loads in the middle of your 30x40 building but that is something you are going to have to look into.
RE: Roof deck tripping snow drift requirements, per reviewer
Pg is 30, Pf calcs to 21, but the local code is Pf = 30 min.
RE: Roof deck tripping snow drift requirements, per reviewer
RE: Roof deck tripping snow drift requirements, per reviewer
I'm now trying to convince the client to live with a 15' x 15' deck and trying to convince the reviewer that if I keep the beams hovering right above the roof, within the height of the design snow load, that I'm not impacting the roof.
I know there's a way to raise the structure to eliminate drifting (per O'Rourke), but that's not going to work in this case.
RE: Roof deck tripping snow drift requirements, per reviewer