gable roof with dormer diaphragm

[structSU10]

I have seen discussion to treat a sloped diaphragm the same as its flat projection for analysis purposes. I have existing gable roofs with proposed dormers to let in light. With this concept it would seem that cutting the existing diaphragm and adding the dormer would not modify the diaphragm as I would have the same projected surface but it seems to simple. Do I just provide connection at the new diaphragm edge that can handle the shear at that location and be done with it?

 

[DaveAtkins]

Most of the time I ignore the effect of dormers. But since you are adding dormers, I recommend you check the diaphragm shear at the dormers to make sure it will still be acceptable.

I would be more concerned about reframing around the dormers. Are you cutting out trusses or rafters?

DaveAtkins

 

[structSU10]

That is another part of it - in one scenario I have rafters to a ridge beam that is then supported on trusses, with the modification happening between trusses. Another condition is trusses at 16" O.C. which will be much more difficult. There is a potential for these to be fake dormers if needed but the goal is to get better light into the space.

 

[KootK]

I have seen discussion to treat a sloped diaphragm the same as its flat projection for analysis purposes.

Note that there is no conservatism built into that procedure. The underestimate of the length of the resisting diaphragm is offset by the fact that, often, only the horizontal component of the diaphragm resistance is effective in resisting the applied lateral load. Cosines abound, signifying nothing.

I'm reluctant to treat dormers as anything other than outright diaphragm openings. My reasoning:

1) It takes some good detailing to move diaphragm shear in and out of a dormer robustly. Got hold downs on those pony walls? confident in shear transfer where the dormer sheathing meets the main roof sheathing? This stuff is even more challenging when it's renovation as opposed to new build.

2) Dormer diaphragms are really three sided, cantilever diaphragms. As such, they tend to be much more flexible that the main roof diaphragms that surround them. This makes their concurrent participation with the main diaphragms dubious.