Roof diaphragm ratio for non-structural penthouse

[Kin1234]

I need to design a roof penthouse for a stair. The penthouse is small with 21 ft long and 3.75 wide. I think this penthouse can be designed as a non-structural component based on the CBC code. Do I still need meet the allowable diaphragm ratio 4:1? If so, what can I do to meet the diaphragm 4:1 ratio besides shorten the length?
Thanks for your help!

 

[KootK]

I wouldn't sweat the diaphragm ratio limit for something this small no matter what the code says. Despite appearances, structural engineering is still very much a game dominated by judgment and reasonableness in my opinion. If a cranky code reviewer calls you out on this, I suppose that you could just install a discrete, steel strap bracing system under the roof deck to make it a truss rather than a deck diaphragm. If your walls are concrete or block coming up from below, you might also be able to just make those cantilever.

 

[Kin1234]

Thanks for quick reply. The plan checker actually calls me out for the diaphragm ratio limit. My building is wood-framed building. I will add a bracing system under the roof deck as you have suggested. Thanks.

 

[KootK]

You're welcome. I guess 4' is pretty darn narrow.

If it's wood, I suppose that you could prefab or site build a flat truss to lay horizontal under or over the framing. It won't be working too hard.

 

[KootK]

Another thought. You're roof deck could easily be constructed with very few plywood sheathing joints (as few as two). If you placed framing members beneath those joints and supplied chords/flanges etc, I feel that you could claim that your roof beam is really a plywood webbed, custom beam rather than a diaphragm as conceived of in the usual sense. I couldn't say if this would fly with your code reviewer or not.

 

[jayrod12]

I've done KootK's alternate method and provided a horizontal plywood box beam. The AHJ in our area was leery, but ultimately accepted it.

 

[KootK]

Do you actually have an AHJ that checks stuff in Manitoba? If so, I'm jealous. In Alberta, we have no such thing.

 

[jayrod12]

Depends on the day.

 

[Kin1234]

All right, thanks guys! I think I need to call the plan checker up and present him the box beam idea. If that doesn't work, I will just tell the architect to widening the penthouse.