Rafter design with intermediate support
Rafter design with intermediate support
(OP)
A builder here is putting up a 38' deep house is using 2x12 rafters (span = 19') and a non-structural ridge board. The frame will be tied together at the eaves with the appropriate thrust resisting connections. In my eyes, that's all good.
Changes get made in the field and now we've got two bearing walls in the attic, 5' on either side of and parallel to the ridge. The builder plans to rest the rafters on the bearing walls (birdsmouths), changing the primary span to 14' and a second span of 5' to the n/s ridge board. Again, no problem, I'm thinking.
I get an email today - he wants to use 2x10s (or 2x8s if possible) for the rafters. My first thought is 2x10s should be plenty, but then I start thinking about the moment on the cantilevered portions of the rafters at the top. Now I'm sitting here, stumped on how to model this and not sure how to answer.
FYI: snow 30psf, pitch 8/12.
Changes get made in the field and now we've got two bearing walls in the attic, 5' on either side of and parallel to the ridge. The builder plans to rest the rafters on the bearing walls (birdsmouths), changing the primary span to 14' and a second span of 5' to the n/s ridge board. Again, no problem, I'm thinking.
I get an email today - he wants to use 2x10s (or 2x8s if possible) for the rafters. My first thought is 2x10s should be plenty, but then I start thinking about the moment on the cantilevered portions of the rafters at the top. Now I'm sitting here, stumped on how to model this and not sure how to answer.
FYI: snow 30psf, pitch 8/12.





RE: Rafter design with intermediate support
I would also be worried about the framing below the new interior bearing wall(s) that have been added and the capacity of that framing, clear to the foundation. That, too, needs to be verified.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Rafter design with intermediate support
RE: Rafter design with intermediate support
RE: Rafter design with intermediate support
Model the ridge board as a support, huh? That's simple enough. I thought it would be more complicated than that because they don't meet at 90 degrees, and the whole set up is effectively a truss. Actually, I was concerned the ridge board may act more like a hinge, and my recollection of how to treat a hinge is sketchy.
RE: Rafter design with intermediate support
Model the ridge board as a support, huh? That's simple enough. I thought it would be more complicated than that because they don't meet at 90 degrees, and the whole set up is effectively a truss. Actually, I was concerned the ridge board may act more like a hinge, and my recollection of how to treat a hinge is cloudy at best.
RE: Rafter design with intermediate support
Wait until the shingles are loaded on the roof...you'll get your answer rather quickly!!
I agree with Mike's approach and share his concerns.
RE: Rafter design with intermediate support
So, I'll model the opposing rafter as a third support (same diagram as modeling the ridge board as a support, but I understand the need to clarify). 2x10s braced along the bottom are all good.
Of course, now the thrust calcs are off....
Thanks.
RE: Rafter design with intermediate support
RE: Rafter design with intermediate support
RE: Rafter design with intermediate support
Thanks.
RE: Rafter design with intermediate support
I want to preface this paragraph by saying that I'm referring to 4thorns first sketch, not the second - i.e. the sketch that does not have a collar tie at the top of the intermediate wall. Now, when you introduce this intermediate support, you get a vertical spring support, not a pin. This makes a huge difference. Your model is essentially a sloped beam with three supports. A pin at the base, a vertical spring at the intermediate support, and a vertical roller at the upper support. The stiffness of the verical spring at the intermediate support (assuming the load i transferred out at these joists and the wall doesn't go directly to ground) is not very stiff. I think what you'll find is that it's not very effective, both in reducing the lateral thrust at the base or changing the moment diagram of the rafter).