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Vaulted Ceiling with Knee Wall - Bracing Option

Vaulted Ceiling with Knee Wall - Bracing Option

(OP)
Interestingly framed vaulted roof -

The roof is a vaulted roof that sits on knee walls (aka stub walls), so the bottom of the rafters are 3' above the floor. There are no ceiling joists or collar ties as the room has a vaulted ceiling. There is another wall (parallel to the outside wall) inset 16" from the outside knee wall and between these walls there is full sheathing. Basically this creates a moment type connection that actually seems to work when modeled.

See attached.

What are your thoughts on this?

Also - this roof happens to have a hip on one end and a dormer on the other and it only acts over a small portion of roof. In the bigger picture this is probably helping things but I'm not inclined to rely on this or try to analyze it that way.

EIT
www.HowToEngineer.com

RE: Vaulted Ceiling with Knee Wall - Bracing Option

If it works, then it works. Good. Perhaps these could be placed every 4 feet or so if the framing can span that far, and depending on the thrust seen.

The only thing extra you need is metal strapping to the rafters and plates at the ends of the wall to take the uplift.

As an alternate to the plywood, you could also use a diagonal member, but it seems that the plywood is simpler.

Mike McCann
MMC Engineering

RE: Vaulted Ceiling with Knee Wall - Bracing Option

What is the spacing on the 2x12's and the floor joists. I assume they are 16" on center. Then what is the spacing of the OSB diaphragm. Is it just in one location or does it repeat along the walls at a certain spacing. Is the OSB just on one side or both sides of the studs.

RE: Vaulted Ceiling with Knee Wall - Bracing Option

(OP)
Thanks for the input.

These are at every rafter which are 16" oc. They are on 1 side but another side could be added.

Interesting notes:
If I argue that the there is no "holddown" or reaction capacity of the lag screws in withdrawl, the analysis still seams to work as there is a moment between the floor wall and rafter due to the restraint of the plywood. However the difficult part is how to accurately analyze the moment transfer from the sheathing to the rafter and wall.

If I say that the the restraint of the lag screws is valid (which it should be) then the behavior is similar to a brace as Mike notes.

EIT
www.HowToEngineer.com

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