stick built roof diaphram
stick built roof diaphram
(OP)
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://t imber.ce.w su.edu/Sup plements/S hearWall/d efault.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.
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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.






RE: stick built roof diaphram
DaveAtkins
RE: stick built roof diaphram
Mike McCann
McCann Engineering
RE: stick built roof diaphram
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.
RE: stick built roof diaphram
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
RE: stick built roof diaphram
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.