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

 

[PengStruct]

I've ran into this issue as well but did not have an attic floor. At the eave and ridge blocking I called out for the blocking to have holes drilled through it to allow for ventilation to flow in the soffit vents, through the eave blocking via the drilled holes, up to the ridge through the rafter bays and again through the holes drilled in the ridge blocking and then out through the ridge vents.

This roof had a vaulted ceiling space at the interior.

 

[KootK]

Thanks for asking this question. I've been wondering about this for months, ever since I generated the wacky detail below to deal with a similar situation. It seems to me that, if you don't have ridge blocking, they you have an unblocked diaphragm at best. You would also need similar blocking down the ridge lines of hip roofs as well.

I've been conducting an informal poll the last few months on this issue. Every time that I drive past a trussed roof under construction, I look for ridge blocking and record the result. Let's just say that I'm in no danger of exhausting my pencil leads. And that makes me have little to faith in blocked, pitched diaphragms -- at least not in my area.

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.

 

[Jerehmy]

heck of a detail you have there Kootk. Contractors where I live wouldn't like doing that too much and/or they'd screw it up

Either of you try doing each half of the gable as an independent diaphragm assembly? I guess you could use each gable end as a shear wall for both. If the loads aren't too high, it should work no? Thoughts?

 

[KootK]

Yeah, I don't have high hopes for the detail. Any minute now, I fully expect that XR250 will show up and mock my attempts at theoretically correct light frame wood engineering. The "Medeek Zone" as it's come to be known.

I've thought about the split diaphragm. However, the two split diaphragms would each need their own chords at the ridge (loads parallel to ridge). That would require blocking and strapping which would seem to be quite terrible as well.

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.

 

[Jerehmy]

is it possible to use a continuous piece of steel sheet metal, like a long simpson strap, as the chord in lieu of wood. you could have this shingle side near the ridge, low enough so the shingles can be attached. and lap it where needed.

 

[KootK]

As evidenced above, I've done stranger things. Sure, metal straps!

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.

 

[KootK]

Well, not so fast. The chords would need to work in compression too. You'd need to argue that, in compression:

A) load gets transferred between plywood sheets not in direct contact via the straps.
B) The plywood is the compression element that prevents chord buckling between trusses.

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.

 

[Jerehmy]

prolly just put blocking in for compression, and if you're gonna put blocking in for compression might as well strap intermittently instead of the whole thing. Back to square one. Idk im just thinking out loud in between working.

 

[medeek]

That is one heck of a detail KootK, I'm assuming this is for a commercial job or a very large custom residence.


I haven't given the ridge vent question much thought up until now. I recently did a garage design and engineering with a continuous ridge vent running the full length of the ridge. There is a 2" sheathing gap at the ridge to allow for airflow. Without a doubt the roof diaphragm is bisected along this line. See my typical (non-structural and overly simplistic) ridgevent detail below:

The first problem with this detail is that the dashed line indicating the #30 felt is not clearly visible, but that aside there is no consideration given to the discontinuity in the roof diaphragm. Granted all of my typical residential diaphragms are unblocked (low unit shear) diaphragms.



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

http://www.medeek.com

 

[KootK]

My detail was for the ridge truss spanning across the chapel of a church. It was one of those frustrating wood situations that I always seem to find myself in where the numbers steer me towards a) a ridiculous looking detail or b) turning a blind eye to the numbers.

Maybe our next wood connector patent should be for a ridge vent that can transfer diaphragm shear. It's clever marketing to solve problems that clients don't yet know that they have. Unfortunately, in our world, we don't just solve the problems, we create them. Clients like that a little less.

That's okay, this silly little problem probably only affects 80% percent of north america's building stock.


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.

 

[medeek]

In figure 9.10e Breyer shows a shaped blocked along the ridge with cont. nailing at the panel joint along the ridge. He also notes that the trusses probably have sufficient strength and stiffness to maintain diaphragm continuity at the ridge.

With a ridge vent the blocking at the ridge is a problem as shown in Breyer's book. Would blocking at each side of the ridge work? Maybe oriented flatwise? This still does not really fix the discontinuity in my opinion but it does give a blocked edge.

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

http://www.medeek.com

 

[Jerehmy]

the only problem with blocking is if the attic is finished. Then you're disrupting the air flow from the eave.
I guess if you had 2x8 rafters and 2x4 blocking it might be ok. I've seen way too many moist, moldy attics.

 

[KootK]

While I know of no way to formally quantify the improvement, I feel that this would help a great deal. As discussed above, the blocking can't serve as chord without strapping for tension. However, even without strapping, the blocking would serve well as a collector to better deliver the shear to the truss top chords.

I've been using the blocking detail shown below which is a ripoff of a similar detail that I found in the TJI catalog.

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.

 

[medeek]

This is kind of what I have in mind:

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

http://www.medeek.com

 

[KootK]

That's it. Now that I see the detail in real life, I have to question the capacity of the truss to transfer the diaphragm shear from one side of the ridge to the other. What's the reliable capacity of a truss plate loaded in withdrawal?

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.

 

[Jerehmy]

I was just thinking, gable roofs can only have shear perpendicular to the plane of the sheathing, thats how the wind loads are defined. so there is really only diaphragm action when the wind is perpendicular to the gable end wall, right?

In that detail mdeek just posted, I keep thinking of putting something like sheathing or a strongback at the bottom of the top chord but it'd have to be triangular otherwise we'd still have the discontinuity. I'm not sure what the pullout capacity on the truss plates is but I don't know of another way to tranfer the shesr thats reasonable.

 

[Jerehmy]

I meant like angular, not triangular.

 

[KootK]

so there is really only diaphragm action when the wind is perpendicular to the gable end wall, right?

Your diaphragms are working for you for wind in both directions. The shear demand across the ridge is actually largest for wind perpendicular to the ridge.

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.

 

[Jerehmy]

What I'm saying is, I'm not seeing how there is shear in that direction. The forces are perpendicular to the diaphragm.