Roof Framing - Alternate Load Path Through Diaphragm

[RFreund]

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!



EIT

http://www.HowToEngineer.com

 

[msquared48]

Depending on the aspect ration of the diaphragms and the availability of the shear walls below to transfer to, I don't see why not. Remember to check your diaphragm deflections though.

Mike McCann
MMC Engineering

 

[AELLC]

Is this what you mean? I would be afraid that the diaphragms would bow outward and the rafters would collapse.

 

[msquared48]

In order to further stiffen the diaphragms, although very difficult to calculate for deflection, you could install "K" brace compression blocking in the plane of the 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

 

[SteelPE]

Why I am not a "wood guy" I don't understand why this couldn't be done. In fact, isn't something similar to this done at gable ends when you have a contractor who platform frames the building and you need to provide support for the hinge the forms between the gable truss and the wall below?

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.

 

[RFreund]

Thanks for the responses, somehow I missed my notification email.

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

http://www.HowToEngineer.com

 

[SteelPE]

You don't necessarily have to post down, you could use a "bent" W8 steel beam. I've seen it done a few times when the architect gets a little crazy.

 

[AELLC]

How are you going to detail the rafter to diaphragm to transfer the thrust load from the rafter to the diaphragm?

 

[RFreund]

SteelPE: Steel ? lets not get too carried away. What I actually meant was to create a triangular truss. Basically put wall up to the rafters on top of the beam supporting the ceiling joists at the opening.

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

http://www.HowToEngineer.com

 

[AELLC]

Is the ceiling diaphragm fastened to the wall double top plate?

 

[RFreund]

The ceiling joists and the rafters would be nailed to the top plate. I guess (and this may be what you are getting at) the issue is that normally you would have a rim joist (or band joist some may call it) around the perimeter which you would nail the edge of the sheathing to. However this is normally for transferring shear from the diaphragm to the shear wall, so this would be important in the perpendicular direction (at the ends of the diaphragm I am considering). Although in my case I don't really have a compression/tension flange directly connected to the sheathing. Unless I can argue that the compression/tension force transfers through the ceiling joist and into the top plates or you could add blocking/strapping at the end of the sheathing.

Maybe I am still missing your point?

EIT

http://www.HowToEngineer.com

 

[AELLC]

Well, the chord force in the diaphragm (which I assume is plywood) is tending to tear the panels apart at the joint.

See attached detail

 

[RFreund]

AELLC - right, good point. Good sketch. I prolly shoulda posted a sketch, but I think the Turkey is making me lazy. I was also trying to think if it could be done with sheathing the top of the ceiling joists...

EIT

http://www.HowToEngineer.com

 

[msquared48]

If stick framed, an attic diaphragm is placed on top of the attic joists with blocking at the ends to nail off the diaphragm, with the joists resting on a continuous single top plate across the top of the plywood, on a short stem wall, or directly on the plywood itself, and notched for bearing showing rafter tails smaller than the rafter itself. Depending on the situation, either solid blocking or bird blocking is placed between the rafters to nail off the diaphragm. The attic floor diaphragm may or may not need to be structural if the roof can function as a 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

 

[RFreund]

Mike, Thanks. That makes sense. Quick question - what is "bird blocking"? or how is it different than just regular blocking between the joists?

Thanks

EIT

http://www.HowToEngineer.com

 

[msquared48]

Bird Blocking refers to the blocking with screened holes through the blocking for attic venting.

Mike McCann
MMC Engineering

 

[RFreund]

Thanks.

EIT

http://www.HowToEngineer.com

 

[m1208]

Use a metal strap (i.e. Simpson H16) with longer legs to be tiled to the floor diaphragm. Strap wraps around the rafter (same as U hanger) and then is extended into the floor diaphragm to transfer the lateral/draft load to the floor.