Cantilever Steel Deck Diaghragm

[canwesteng]

I'm designing a small attachment (single level, ~900 sf) to an existing steel building To save on bracing, I'd like to design the roof diaphragm as a cantilever (see attached sketch - traditional method on left, proposed method on right). I can see no good reason this wouldn't work, and it would basically eliminate the need for bracing this addition, but it's also something I've never done. Some background - wind loads will control, seismic loads are an afterthought where I am and will be much smaller than the wind loads on this structure. No significant gravity loads on the roof outside of snow.

 

[KootK]

I vote AOK. Two things to check:

1) Deflection of the diaphragm at the free end.
2) Diaphragm aspect ratios limits expressed in the governing code.

Expect many of our cohorts to dislike this from a seismic design perspective.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Well, you can't ignore the added wind load to the existing structure. You probably will have to reinforce the existing building to take this in at least one direction.

Mike McCann, PE, SE (WA)

 

[Lion06]

How do you intend to tie it into the existing diaphragm to take out the force couple? How will you verify the capacity of the existing diaphragm to take this additional shear force. Maybe the force is so small it doesn't matter for the diaphragm shear, but maybe not.

 

[canwesteng]

@kootk - agreed, regarding seismic I think as compared to RC/masonry/timber buildings, steel has plenty of ductility, and also seismic loads are marginal for this.
@ms - I'll certainly be checking it, but it frames into a 180' tall 80x80 braced frame building. So unlikely to need any reinforcement
@lion- tying into an existing braced building, not a diaphragm. Otherwise it would be a whole different ball game

 

[Once20036]

I assume that the existing building is at the left side of your sketches, and the question is about the bracing along the right side?

Assuming a rigid diaphragm, I don't think that I'd have any concerns.
With the more typical metal deck/flexible diaphragm, you need to keep a close eye on deflections along the right hand side - including both shear and flexural deformations.