stick built roof diaphram 1

[AlpineEngineer]

Thanks so much for all your input on my last question about the overturning moment on a shear wall; it all makes sense now, I wasn't fully understanding how the roof diaphram forces transfer to the shear walls. FYI I found a great online resource for us slow learners:

http://timber.ce.wsu.edu/Supplements/ShearWall/default.htm

Now, my question is: If I have a stick built roof system with a ridge beam does this diaphram behave like a webbed truss diaphram? It would seem to me there should be lateral loading consideration on the ridge beam and its connections, but then it is starting to look like a post/beam frame that takes lateral loads (a moment frame, right?). Is there a way to build this ridge connection so that there is no lateral load transferred to the ridge beam (maybe blocking between the joists at the ridge beam and nailing into the blocking?) I've looked in Breyer's book and I don't see any considerations for stick built roof diaphrams and this ridge beam consideration.

Thanks a ton guys.

 

[DaveAtkins]

The roof sheathing (plywood or OSB, for example) is the diaphragm, not the roof framing. You shouldn't need to count on portal frame action.

DaveAtkins

 

[msquared48]

Similar to a built-up steel girder with web stiffeners, the rafters, purlins and beams are the stiffeners (with the required nailing per the IBC) for the plywood, the roof diaphragm.

Mike McCann
McCann Engineering

 

[AlpineEngineer]

Dave,
I understand what your saying, that the OSB itself is the diaphragm, but, correct me if im wrong, the shear leaves the diaphragm at the chords (sides) which are parallel to the applied force, if my diaphragm is broken at the ridge beam (as it is in a stick built roof) it seems to me that the shear will act out of that chord and end up in my ridge beam unless I do some sort of strapping over the top of the ridge to connect the 2 halfs of the diaphragm together. So lets say my ridge beam runs east/west and I have a windward gable facing west, the wind on that west gable goes up into the diaphragm and exits out of the diaphragm on the east/west chords of the diaphragm. The east/west chords rest on a top plate (at the bottom of the rafters) and a ridge beam at the top of the rafters (unless I strap the rafters together at the ridge to allow the two halfs to act together like a webbed truss).

And, as I thought about it last night, with forces acting the other direction on this diapragm (north/south), the forces will have to exit out the north/south rake walls, and in that case it seems critical that the end rafters resting on the rake walls are connected properly to the rake walls in order to transfer this force into the rake wall.

Does it make sense or I am missing the boat here?
Thanks for your time.

 

[DaveAtkins]

OK--first a little terminology. A diaphragm chord is the member at the windward (or leeward) edge of the diaphragm that takes tension or compression, analagous to the top or bottom flange of a steel beam. The rafters are not chords.

Now--to answer your first question. If the OSB on both sides of the roof is nailed to the ridge beam (that is, there is no ridge vent), then you need not worry about diaphragm shears. The entire gable roof acts as a single diaphragm. If there is a ridge vent, then I would treat the roof as two separate three-sided diaphragms (racking the diaphragm loads to the shear walls and resisting the induced torsion at the rake walls).

And your second question--yes, those wood outriggers (or rakes, as they are sometimes called) do lift the OSB above the rake wall, and if the diaphragm forces are large enough, blocking should be put between the outriggers.

DaveAtkins

 

[AlpineEngineer]

ahhh, it all makes sense, in my situation the ridge beam is dropped and exposed inside for a timber frame look, hence the rafters sit on top of the ridge beam and there is no OSB nailing at all into the ridge beam. If it was a typical ridge beam situation where the rafters hanger off the ridge beam and the ridge beam was buried in the ceiling then I see it wouldn't be a concern (unless there was venting as you say).

Also, in my situation this dropped ridge beam cantilevers outside (exterior) (as well there are cantilevered beams at the top plates, at the bottom of the rafters) thus the eve (rather large at 4') is rafter built and not built with outriggers and blocking. So the only connection this roof diaphragm has to the rake wall (I think & correct me if Im wrong) is a rafter (esentially my blocking) that sits on top of the rake wall and i doubt toenails into the raked plate are enough to transfer this force into the rake wall, I think it will require some A35's or something. I found a great guide for design of shear rake walls (Wood framed shear wall construction by Thor Mattenson,

http://www.shearwalls.com/)

Thanks so much for your patience and advice.