Wood Gable Roof Diaphragm 6

[Jerehmy]

In my Breyer book they don't go into any detail about the discontinuities of a roof diaphragm at the ridge due to ventilation requirements. In the Breyer ridge detail (Page 9.28 Figure 9.10e of the 6th ed.) they show the sheathing as continuous. Not sure how that's realistic.

For the job I'm currently doing, I have a gable roof. The attic is finished so I was just going to use the rafters to transmit the lateral wind load into the attic floor diaphragm. But what if the attic is unfinished, how have some of you resolved this? Treat each roof face as two separate diaphragms?

You could put blocking at the sheathing edge near the gap at he ridge, but this would disrupt air flow from the eave is the ceiling is finished. But I guess you have a finished ceiling, you'd have an attic floor to use as a diaphragm. So is that how you guys do it?

Curious as to what others do. Breyer advises to use "ATC Guidelines for Design of Horizontal Wood Diaphragms 1981" for steep roof diaphragms, I might have to pick it up. Anyone else us it?

Thanks

 

[ChiefInspectorJeff]

As an Inspector I can count on one hand the number of roof diaphragms with ridge vents designed per code protocols.
Yes, there have been roof diaphragm failures in hurricanes and I remember some online photos. But what about the loads perpendicular to the truss plates especially at a gable.
The whole system is questionable. If parts of the walls were designed as cantilevered post or box beams these could take spot OM loads from SWS above and support diaphragm edges. The trouble began when houses were forced to become engineered products within the same budget as the old way: no way.

 

[RFreund]

I plan to go to a woodworks seminar this August with the fearless leader in diaphragms, Terry Malone. I'd like to compile a list of our gripes to see if he can address them. If anyone cares to add to the list, that'd be great, but I'll have this subject on there, as well as as the gable end wall bracing and interior shearwall / roof diaphragm transfer discussion.

Gable End Wall -

http://www.eng-tips.com/viewthread.cfm?qid=384330

Collector force diagragm -

http://www.eng-tips.com/viewthread.cfm?qid=384620

Interior Shear wall -

http://www.eng-tips.com/viewthread.cfm?qid=376604

EIT

http://www.HowToEngineer.com

 

[medeek]

I would be very interested to see what Malone's take is on the ridge vent issue. Thanks for doing this RFreund.

Please also add this discussion on portal frames and the method used to analyze them to your list:

http://www.eng-tips.com/viewthread.cfm?qid=374730

I haven't programmed the portal frame calculator yet but I am still interested in creating that app/spreadsheet and making it available to anyone who might be interested.

Calculating the deflection of a portal frame is also something that I have not yet nailed down either. My thinking is a starting point would be the deflections equations/methods from the SDPWS for shearwalls and then modify them as needed to deal with the particulars of the portal frame.



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

http://www.medeek.com

 

[medeek]

I really feel that we need a better method to quantify the performance of portal frames. Their sheathing, nailing, straps and holdowns should be "engineered" for each application. Using a generic "PFH" detail from the IBC or IRC seems like a cop-out.

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

http://www.medeek.com

 

[XR250]

I really feel that we need a better method to quantify the performance of portal frames. Their sheathing, nailing, straps and holdowns should be "engineered" for each application. Using a generic "PFH" detail from the IBC or IRC seems like a cop-out

Are you nuts. Why?
I would rather make the same money spending 5 minutes pulling it out of the code than spending 2 hours detailing one.

 

[medeek]

I'm an engineer, I like to engineer things. Up to a certain limit a prescriptive approach works but about 50% of the time I find that I either need to beef up the PFH or go with another option (ie. Simpson StrongWall etc...) I also feel that most of the prescriptive footings beneath these portal frames are probably inadequate and don't calculate out.

What about deflections? The SDPWS, ASCE7-10 and IBC spend alot of ink on shearwall and diaphragm deflections, what about portal frames, do they get a pass?

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

http://www.medeek.com

 

[XR250]

No kidding. A bunch of stuff in the code does not calc out (portal frames are one of them IMHO). But if that is what my competition is doing, I will lose business if I charge for a custom portal frame for each job. I actually do not count on them very much anyway as I usually try to incorporate alternate load paths. On uber expensive houses, I will use custom steel moment frames if needed. While we are on the subject, try to see if prescriptive brick veneer support on wood framing calcs out. This is one area that I do for each job as they fail all the time in my area.

 

[KootK]

This is one area that I do for each job as they fail all the time in my area.

As in they fail in the real world and not just on paper??

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.

 

[XR250]

Real world. Not talking about collapse, but enough movement to cause fairly significant cracking.