Roof Framing - Alternate Load Path Through Diaphragm
Roof Framing - Alternate Load Path Through Diaphragm
(OP)
What if you discontinued your ceiling joists, so you had no ties at the bottom of the rafters. You also did not have a structural ridge. Could you resist the trust at the end of the rafter by creating a diaphragm in the attic, then transfer this force to the outside/shear walls?
Probably a bit of a stretch but I'd like to hear some comments if you'd be willing?
Thanks!
Probably a bit of a stretch but I'd like to hear some comments if you'd be willing?
Thanks!





RE: Roof Framing - Alternate Load Path Through Diaphragm
Mike McCann
MMC Engineering
RE: Roof Framing - Alternate Load Path Through Diaphragm
RE: Roof Framing - Alternate Load Path Through Diaphragm
I think still that this is very doable, depending on the constraints I listed in my post above. Just watch your connections, particularly at the rafter tails.
Mike McCann
MMC Engineering
RE: Roof Framing - Alternate Load Path Through Diaphragm
While I agree that it could be done, why couldn't you just use a ridge beam to support your roof and eliminate the "kick" from the rafters? Certainly it would be easier to construct and you would probably feel better about it when you were done.
RE: Roof Framing - Alternate Load Path Through Diaphragm
RE AELLC: Yes that is what I am getting at. I think your concerns are justified but I think Mike's Idea would help.
RE Mike: Thanks and I agree with your recommendations.
Re SteelPE: That is an option but the end of the ridge beam falls in the middle of a room so posting down gets complicated. Regarding the gable end situation - really that is for the wind load on the wall. Which is really just like any other floor or diaphragm, meaning it transfers lateral load to the shear walls. Having said that I suppose the only difference is that in this case the lateral load is from the kick (or thrust) from the rafters. I suppose the diaphragm does not know the difference, so this method should work assuming you follow a load path and detail the connections.
There are some other alternatives however, this is just a thought I had while thinking through some ideas.
Thanks for the comments!
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm
RE: Roof Framing - Alternate Load Path Through Diaphragm
RE: Roof Framing - Alternate Load Path Through Diaphragm
AELLC: The rafters are nailed to the CJ's (along side) and then the sheathing is nailed to the CJ's.
If you say there was a leeward wind pressure on the roof and you used your ceiling as a diaphragm, I imagine this would be the same load path.
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm
RE: Roof Framing - Alternate Load Path Through Diaphragm
Maybe I am still missing your point?
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm
See attached detail
RE: Roof Framing - Alternate Load Path Through Diaphragm
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm
In the case of trusses, then the truss bears on the wall double top plate, and the roof diaphragm usually becomes the primary diaphragm. I usually never rely on the sheet rock of a ceiling diaphragm to function as a structural element, although others regularly do.
Mike McCann
MMC Engineering
RE: Roof Framing - Alternate Load Path Through Diaphragm
Thanks
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm
Mike McCann
MMC Engineering
RE: Roof Framing - Alternate Load Path Through Diaphragm
EIT
www.HowToEngineer.com
RE: Roof Framing - Alternate Load Path Through Diaphragm