Roof deck tripping snow drift requirements, per reviewer

[gcs3pe]

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?

 

[dik]

It's the height above the roof that contributes to the snow accumulation. I don't know how your code addresses that. What is the height of the new roof above the existing?

Dik

 

[gcs3pe]

If I hang the deck joists flush between the beams, the finished deck elevation (and the tops of the beams) looks to be about 2' off the deck in back and 3' off the deck in front (roof slopes from back to front).

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'.

 

[SteelPE]

What code does your jurisdiction follow?

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.

 

[gcs3pe]

The code is the IBC. Does that provision apply to snow loading? I always thought of that in terms of changes in use.

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.

 

[mijowe]

Is the Pg 30psf or is the Pf 30 psf? If the local code has a Pf of 30 psf min, you may be able to justify using the actual Pg and calculating a drift that may be lower that the 30psf.

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.

 

[SteelPE]

From what I can tell, you can't look at the roof as a whole, you have to break it down to it's individual elements. I tend to look at this as a allowance for the building owner to make small modifications w/o having to consult a structural engineer. With out the limit, every time the owner wants to make a change he would need to hire an engineer.

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.

 

[gcs3pe]

I misspoke earlier - the reviewer is okay with the 15' limit for the deck area and railing. The beams, however, will be 30' wide because I cannot load the center of the building economically.

Pg is 30, Pf calcs to 21, but the local code is Pf = 30 min.

 

[PMR06]

So the beam counts as higher surface creating drift... then wouldn't Lu be like 6" or something? Let me guess, the reviewer will say "Lu = 6" which is less than 25ft so use 25ft?" I guess windward drift could govern? This seems excessive.

 

[gcs3pe]

Exactly, on both counts. Lu = 12" so use 20' (although I will have two beams 15' apart, so I can see how it might cause some type of drifting).

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.

 

[shobroco]

If the height of the beams is less than the depth of snow at your required snowload, there is no additional drift load. There is no snow load on the roof at all under your deck, so analyze the existing joists without any snowload under the deck, that should gain you some capacity. I've seen someone else design something akin to this simply to avoid reinforcing a roof for new snow drifting: a higher addition on the end of a plaza suspended a deck off the new structure out over the existing roof, not touching it but close enough to catch any snow and transfer the load to the new structure.