12:12 Pitch Roof Diaphragm Question

[medeek]

If you have a 12:12 pitch roof the two sides of the roof diaphragm are 90 degrees to each other, similar high pitched roofs are close to 90 degrees. How can you consider two diaphragms that are 90 degrees to each other a continuous roof diaphragm? Does this change anyone's analysis of this type of roof?

Usually when I have a high pitched roof I'm dealing with attic trusses and hence an attic floor diaphragm which can partially transfer the shear loads in conjunction with the roof diaphragm. However, there are times where I'm dealing with an actual vaulted (ridge beam) roof and the roof diaphragm is really two planes at an almost 90 degree angle to each other.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[DaveAtkins]

I think it can act as a single diaphragm, as long as you check the shear in the sheathing and the chord forces.

DaveAtkins

 

[oldestguy]

Wishful thinking. Maybe beam action of a few upper sheathing panels in the weak direction.

 

[medeek]

I would like to see a thorough treatment of this type of high pitched roof.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[MacGruber22]

To me, the severity of the diaphragm irregularity disqualifies the whole roof as acting predictably like a simple diaphragm. I would try to treat it as two separate diaphragms in the same way you would treat offset roof planes with clerestory windows. Hopefully you can live with the 1/4 diaphragm stiffness, 4x chord forces, and shorter shear transfer length.

The other thought that comes to mind is utilizing a load path similar to pre-engineered buildings with wall girts. If you can create lateral resisting lines close enough together, maybe you can abandon the simple diaphragm model for these high pitched roofs.



"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

Fundamentally, I don't see how a 12:12 roof is different from a 4:12 roof. In both cases, you need to adjust for the projection onto the horizontal plane and put up with a discontinuity at the ridge where ridge venting precludes blocking.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BadgerPE]

I have to agree with Koot on this one. After all, when wind loads are parallel to the ridge, there is no shear at the ridge (assuming a equal distance from ridge to eave), correct? When wind loads are perpendicular to the ridge there are a series of small "rigid links" (trusses) that transfer the forces across the ridge and in my opinion, make the diaphragm act as 1 continuous unit. One side can't deflect independent of the other so it should be assumed to act as one. However, I don't know where I would limit the roof slope, but I seem to think 12:12 is getting pretty close to what I would be comfortable with.

Maybe a good master's thesis topic for the poster asking for ideas this week??

 

[medeek]

I have considered going back to school to get a MS or PhD, probably make more money doing that than this residential stuff. I have some interest in ground snow loads and how those are derived, that would probably be my topic, but you never know. My other big item of interest is creating a shearwall program and gravity program for residential engineering that runs inside of SketchUp, which would provide a very nice visual and intuitive interface and hopefully automate a lot of what I do as well as make the load propagation down through the structure fully automated. Sorry for getting so far off topic. I might try some FEA in RISA to try and look at the diaphragm as the roof pitch increases, this should be interesting.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[calvinandhobbes10]

I'd check it two ways - one as two separate diaphragms, one as combined. I usually find roof diaphragms to be lightly stressed, and diaphragm problems in the floors, with their large foyer openings, catwalks, high-bay greatroom windows, and other open-floor complexities of many high-end residences.

 

[XR250]

I don't think I have ever checked a roof diaphragm on a residential structure other than when trying to make something 3-sided work out. We are not in a high wind or seismic region however.