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

 

[KootK]

I don't follow. Both the wind on your supporting walls and and the horizontal components of the wind on your roof will create shear demand in the diaphragm 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]

there is no shear in the plane of the diaphragm if the wind load is defined as perpendicular to the plane of the sheathing and the rafters/trusses sit on top of a wall.

You take the horizontal component, which is at an angle to the diaphragm. What about the vertical component? This will also be at an angle, which cancels out and load in the
plane of the diaphragm perpendicular to the ridge. Do you understand what I'm saying? I don't see how the diaphragm sees any load for a gable roof when the force is perpendicular to the plane
of the diaphragm.

 

[Jerehmy]

any*, plane of the diaphragm* (minus the ridge part).

 

[KootK]

Do you understand what I'm saying?

I do think that I understand what you're saying. For a good while, I really struggled with envisioning how a pitched diaphragm resists horizontal wind shear. Try this, assuming trusses, wind from the east, and a roof ridge that runs north to south:

1) When wind blows/sucks on your east and west walls, some of that wind pushes the walls against your trusses and tries to move them to the west.

2) When the blows/sucks on your two roof planes, there is a horizontal component of those pressures that also tries to move your trusses to the west. Not much to debate so far.

3) So what keeps the trusses from moving to the west? Their connection to the diaphragm which is, in turn, restrained by the gable end shear walls. At the end of the load path, it winds up being the trusses that induce shear in the diaphragm.

4) Diaphragms can only really resist in plane movement. As such, only the horizontal component of the in plane diaphragm shear is effective in preventing westward movement of the trusses. This is why, when we design pitched diaphragms, we normally treat them as their horizontal projections. That method automatically takes care of the need to multiply the diaphragm strength by the cosine of the roof pitch.

What about the vertical component? This will also be at an angle, which cancels out and load in the
plane of the diaphragm perpendicular to the ridge.

They do cancel with respect to the force applied directly to the diaphragm. However, they do not cancel with respect to the aggregate force applied to the structure as a whole. The vertical component makes its way out to the gravity supporting structure, just like dead loads and snow loads.

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 see what you are saying, and that makes sense when you have that thrust at the bottom chord.

I was assuming the attic floor has a diaphragm. If it does, I don't see any in plane loads make it to the sheathing. I'm thinking of a rafter + ceiling joist assembly.
I guess you could have sheathing on the top of the bottom chords for trusses.

 

[KootK]

Ah, I see. With rafters and a finished attic floor, I would expect both the roof and attic floor diaphragms to share in resisting the wind shear in proportion to their relative stiffnesses. Really, virtually all trusses have a permutation of this indeterminacy as a drywalled ceiling is a diaphragm too. When using pre-engineered attic trusses you gotta be careful about assuming that the attic floor sheathing is a competent diaphragm. Usually the detailing ends up such that you don't have chords.

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]

Yeah when you draw up the FBD and look at the load path, the wind never has to "use" the roof diaphragm in that direction. Plus the fact that diaphragm is discontinuous makes
me just want to ignore it all together in that direction. I've just come full circle to my OP, ugh. Long day, I've just talked myself in a circle heh.

Back on topic which is what to do when you DON'T have a floor diaphragm, which is I guess full depth blocking with strapping (for wind parallel to the truss span). If you have no floor diaphragm, the attic isn't
going to be finished. Therefore there will be no sheathing on the bottom of the trusses, so full depth blocking won't disrupt the airflow. In the direction perpendicular to the
truss span we depend on the truss plates for shear transfer across the discontinuity I guess :/.

If it is finished, you can just transmit the loads into the floor diaphragm in that direction and I guess depend on the truss plates again in the opposite direction. I don't
like using them.

I've never heard of them failing at the ridge like that due to wind though, have you?

 

[KootK]

Nope, never. And I used to work for the wood truss council of America so there would have been a pretty good chance that I would have heard about it if it were a common problem. The official line with heel blocking is that it's not needed for heel heights up to 6". In my opinion' that represents a more serious discontinuity anyhow. I feel that lateral design in light frame wood frame construction should be prescriptive for all buildings. We blow our brains out trying to sort things out rigorously and the detailing still ends up riddled with load path holes.

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]

Yeah everyone says Breyer is the best book for wood design, but it isn't very technical. Almost done with chapter 6, trying to read through the whole thing.
I guess that's reflective of how wood structures are designed. Definitely an excellent book though, so many little tidbits of information hiding in the pages.

 

[KootK]

Have you read Irregular Diaphragms by Malone? It's much more technical wrt lateral design. So much so, in fact, that it might ruin any chance you might have at doing wood frame construction profitably.

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]

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http://

http://www.woodworks.org/wp-content...zbGwr2JgXJOJ87MdGRiwpKQ&bvm=bv.93990622,d.aWw

this?

 

[KootK]

Reference four at the bottom of that document. Same guy, entire book on the subject. Whatever you do, don't attempt to read it on a Kindle/Kobo. The frequent references to complex diagrams placed a ways away from the referencing text was brutal for me.

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.

 

[mike20793]

I think it's cheapest to buy it through the ICC if I recall correctly. I've spoken with him several times in the past year regarding projects and he always sends me sketches and calculations to help with his explanations. His experience as a plan checker is invaluable in my opinion since he has seen so many drawings by different firms. As Koot said, there are many times when you will have to flip back and forth to see his diagrams or refer back to the original layouts. It's a very good reference.

 

[Jerehmy]

I can only find it on Amazon for 130$ hardcover. I doubt I can convince my boss to buy it . We don't do very many new buildings. Half our business is
repairs, the other half is insurance claim reports. And he LOVES the insurance claim reports since you don't have to worry about getting paid, it pays well,
and you don't have to deal with clients as much :/.

 

[BadgerPE]

As low as $80 here... The Analysis of Irregular Shaped Structures: Diaphragms and Shear Walls

 

[medeek]

So what was the final consensus on the detail I posted above? Does flat blocking at both side of the ridge gap make sense? I've actually never seen this done on any residential roofs. My brother used to run a roofing company in Utah and I remember him sheathing the roof and then running his circular saw along the ridge opening up the air passage for the ridge vent. No thought was ever given to structural problems with this configuration.

If you look at a simple rectangle structure with four exterior shearwalls and then consider a lateral force parallel to the ridge the shear in the diaphragm is technically zero at the center point.

In the other direction with the lateral load perp. to the ridge is probably more problematic. One could consider the diaphragm as two separate diaphragms however what to do about the absent chords at the ridge line for both diaphragms (one in tension the other in compression)?



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

http://www.medeek.com

 

[KootK]

My contribution to the consensus is as follows:

- I think that the blocking helps a good deal.
- I've no idea how to quantify the help provided by the blocking.
- Even with the blocking, I think it best to consider the entire diaphragm as a single unit across the discontinuity.
- I'm going to omit blocking and using only unblocked diaphragm values for fear of getting molested on site.

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]

Badger - thanks. I guess I didn't look hard enough at the ICC store.

 

[ChiefInspectorJeff]

I have discussed this problem with some PhD engineers and they seemed surprised at the problem.
The diaphragm is cut in half by most ridge vents leaving the edge unsupported and with no path to a shear wall. I suppose you could block the cut sheathing diaphragm
together at an interval .You need boundary members to work, otherwise I assume it would rotate:: there's the answer! Design the 3 supporting walls as a channel or for wood diap. in rotation.
Possible? Something is keeping the roofs on in this situation in storms so maybe this can be tested.
As an Inspector for 20 years I can tell you that there are many discontinuities in second floor
diaphragms not designed for. I have seen a 3 story building with no shear walls supporting the top 2 floors and no provision for overturning in those walls.
Only testing and real life situations can tell us if it is really needed or is it just a theory that won't work with wood materials. I have never seen a wood post
on a house porch designed for a fixed base either.
Jeff

 

[KootK]

there's the answer! Design the 3 supporting walls as a channel or for wood diap. in rotation.

Two cantilevered diaphragms! That may actually have some merit.

I have never seen a wood post on a house porch designed for a fixed base either.

Yeah. In my area, I see that and a lot of decks that are 8ish feet in the air with no moment connections top or bottom.

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.