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Wood Gable Roof Diaphragm
5

Wood Gable Roof Diaphragm

Wood Gable Roof Diaphragm

(OP)
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

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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 lol

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?

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

Quote (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.)


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.

RE: Wood Gable Roof Diaphragm

This is kind of what I have in mind:

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

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

(OP)
I meant like angular, not triangular.

RE: Wood Gable Roof Diaphragm

Quote (jerehmy)

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.

RE: Wood Gable Roof Diaphragm

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

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

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

RE: Wood Gable Roof Diaphragm

Quote (Jeremhy)

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.

Quote (Jeremhy)

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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?

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

(OP)
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 :/.

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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

RE: Wood Gable Roof Diaphragm

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

RE: Wood Gable Roof Diaphragm

Quote (ChiefInspectorJeff)

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.

Quote (ChiefIspectorJeff)

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.

RE: Wood Gable Roof Diaphragm

The problem is the wood butchers trying to build a precision engineering product! A trim carpenter would be a better choice.
Most inspectors do not look at the top plate nailing so if that is defective you have no load path anyway.

RE: Wood Gable Roof Diaphragm

Our typical ridge detail involves a structural ridge (homes in my area are generally very custom / vaulted where trusses dont make sense). We drop the ridge down 1.5" and put in a 16" long flat 2x block on top of the ridge. The diaphragm nailing is clustered to the block and there is roughly 6" of airflow space available at each rafter bay.

RE: Wood Gable Roof Diaphragm

I don't quite get your detail jdengineer. I'd like to though. Can you elaborate or post a sketch?

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.

RE: Wood Gable Roof Diaphragm

FYI
Woodworks has some workshops entitled "Advanced Diaphragm Analysis – A Technical Presentation of Complex Diaphragms" with Terry Malone.
They are reasonably priced. For locations, see http://www.woodworks.org/events-calendar/upcoming/

RE: Wood Gable Roof Diaphragm

This is a great thread. It's one of the reasons that I pretty much don't do wood design. I would love to do it, but getting wood to be designed correctly is difficult enough. It seems as though getting a complete load path is next to impossible with the way wood buildings are typically framed and detailed (by that I mean non-structural components). That, and contractors seem to think you're crazy if you do detail a complete load path.

My experience has been that some of the other engineers in my area are ignorant of complete load paths and I can't compete with them.

Anyway, to the thread subject. one thing to consider is that if the building is symmetric, for the wind load parallel to the ridge, the shear should be close to zero at the ridge (IE the midspan). Obviously that's not the case for wind load perpendicular to the ridge. The only major wood building i designed that had gable roofs had spray on insulation on the roof sheathing, and therefore didn't have ridge vents.

As for considering the roof as two separate diaphragms, personally, I wouldn't do that. The diaphragm really needs to be rigid or close to it in my opinion to consider that.

With all that considered, we structural engineers aren't building a swiss watch, but it's easy to get bogged down into the details and stress the small (IE small gaps) details

RE: Wood Gable Roof Diaphragm

this may have already been mentioned:

There is APA literature called "Introduction to Lateral Design" which states that "A continuous ridge vent can be used with an "Unblocked Diaphragm". An unblocked diaphragm has no requirement for blocking between unsupported edges of adjacent panels. It also states you could stagger the blocking in a "blocked diaphragm" allowing vents every other truss bay. You then provide double the nailing requirement at the blocked bays. So you have two choices: design the diaphragm as unblocked or stagger your vents. I don't know your ventilation requirement, so that could dictate which one you use.

I stole this from a previous thread: http://www.eng-tips.com/viewthread.cfm?qid=184481

EIT
www.HowToEngineer.com

RE: Wood Gable Roof Diaphragm

Nicely done RFreund. Sadly, I already have that document but never got around to reading it.

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.

RE: Wood Gable Roof Diaphragm

When you really think about it an unblocked diaphragm could just as well have a row of vents along every unblocked seam (perp. to rafters/trusses) and still be qualified as a diaphragm. I think we have been thinking about this all wrong. The ridge vent seam is then no different from any other seam other than it is a little more opened up and it occurs at the center of the diaphragm. At some point though this gap width is going to impact the overall performance of the diaphragm. My question is when does it start affect the performance, strength and deflection. (Note: I am specifically talking about unblocked diaphragms here.)

The APA should do some ridgevent/diaphragm testing to better quantify this behavior. My suspicion is that the minor discontinuity caused by the ridgevent air gap is so slight that it does not affect the performance in a measurable amount when the stresses are low (ie. unblocked diaphragm, typical residential roof).

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

RE: Wood Gable Roof Diaphragm

I proposed the unblocked diaphragm strategy at the top. In my mind, there is a fundamental difference however. For a normal unblocked diaphragm, you transfer shear across a continuous framing member that spans the sheets of plywood. At a ridge, you're transferring that same shear across a joint between framing members that likely has questionable weak axis moment/shear transfer capacity (truss plate). From what I've seen, there is a lack of testing on pitched diaphragms in general, for both plywood and steel deck.

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.

RE: Wood Gable Roof Diaphragm

The ridge vent seems to be the primary (though not necessarily the only) problem in this discussion. Why not omit it, and get your ventilation elsewhere... like down at the eave? Aside from the diaphragm issues discussed above, ridge vents eventually leak at the poorly (non-existent) sealed fasteners (nails) that eventually withdraw (or completely come out), or are at least susceptible to infiltration of wind-driven rain at higher wind speeds (if you doubt this, just Google "ridge vent leak"). Common sense states that the more penetrations you have in your roof, the more likely you are to have leaks. Problem avoided = problem solved (at least in this instance)

RE: Wood Gable Roof Diaphragm

(OP)
Because there already is eave ventilation. You need top and bottom side ventilation so the air can flow. Having only ventilation at the eave wouldnt do much of anything. Gable end vents count as top side IIRC so thats out too.

RE: Wood Gable Roof Diaphragm

Jerehmy,

As far as I know, there is nothing in the code that typically requires "top" ventilation, as you refer to it. "Cross ventilation" is required, but this can be accomplished by having ventilation at the eaves on either side of the roof, as I previously described. If 40% - 50% of the ventilation is not located in the upper portion of the attic, the "minimum net free ventilating area shall be 1/150 of area of the vented space", as opposed to 1/300 (http://publicecodes.cyberregs.com/icod/irc/2012/ic...). Essentially, there is a "penalty" applied due to having slightly less than desirable ventilating conditions; however, my point was, if you're already providing ventilation down at the eave, why not just increase it, avoid the possibility of leaks through a ridge vent, and remove most of the concern about analysis and diaphragm force transfer at the opening created by the ridge vent?

I would further, maybe naively, ask the question... if you must have a ridge vent, and we assume the building is rectangular in shape, with the ridge vent running parallel to the long direction, and being at the center of the roof (midspan of the structural roof members), and with lateral (i.e. wind/seismic) forces being applied perpendicular to the long direction, exactly what forces are you really concerned about? Visualizing the horizontal diaphragm as a I-beam (http://nisee.berkeley.edu/lessons/a_figure26.jpg), tension/compression forces are resisted by the flanges (i.e. the diaphragm chords down at the exterior walls/eaves), with the bending stress being lowest at the neutral axis (i.e. the ridge)... correct (http://www.expertsmind.com/CMSImages/761_Determine...)? So then, what you have left to be concerned about are the shear forces in the diaphragm, which are highest at the ends of the "beam" (http://www.awc.org/calculators/connections/images/...). Restrict placement of the ridge vent to within the middle third or "middle half" of the long direction, where the shear forces are lowest. Have you seen these beams before (http://quick-calcs.net/designs/images/holes.jpg)? Those are some awfully big "ridge vents" in the web of the beam.

I personally think we're making a mountain out of a molehill here. Where are the widespread structural failures of wood-framed, single-family residential roofs, at the ridge vent, due to wind/seismic forces?

RE: Wood Gable Roof Diaphragm

As has been pointed out a few times above, the peak shear demand across the ridge would come from loads acting perpendicular to the ridge. Additionally, even for loads parallel to the ridge, it`s hardly reasonable to assume perfect symmetry in loading.

I myself mentioned the lack of evidence for real world diaphragm failures above. However, this becomes quite problematic for designers. All the codes, articles, books, and seminars make it plane that the expectation is for wood diaphragms to be rigorously designed for lateral loads. When we choose not to do this, for reasons that seem vary pragmatic, it puts us in a precarious position ethically.

Large scale wood residential projects are notorious for ending up in court. I shudder to think what might happen if some savvy lawyer gets wind of the fact that most of our wood diaphragm designs are full of load path holes that we`ve knowingly turned a blind eye to. I imagine my day in court unfolding like this:

Prosecutor: This building leaked! Now you owe $1B KootK.

Me: No, I`m a good engineer!

Prosecutor: If you`re such a good engineer, why did you omit ridge diaphragm blocking that has been clearly documented in industry publications as a requirement for diaphragm load transfer?

Me: *tubmle weeds rolling bye*

Me: Because I`m SMARTER than the codes damnit! And a bunch of randoms on the internet agree with me!

Prosecutor: put the check in the mail.

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.

RE: Wood Gable Roof Diaphragm

(OP)
Nothing requires it, but in the experience in our office, it's never a good idea just to have eave vents. A good portion of the claims we
do is with attic ventilation issues. And most of them are on newly built structures. Just having eave ventilation doesn't seem to do the trick. So yes it's allowed by code,
but with all the issues we see with attic ventilation, we wouldn't do ventilation that way. At least not in our area where it can get pretty cold making ventilation more of an issue.

I don't see how the air would flow with just eave ventilation anyways. The cold outside air isn't going to push the warm inside air out. The warm air
rises and sucks the cold air in out the ridge. I'm not an expert in thermodynamics so maybe there are other mechanisms at work. I'm just going based off
our offices stance on attic ventilation. We also don't like those fibrous ridge vents. They always seem to get caked with snow. I've actually seen a few with mold in them.

RE: Wood Gable Roof Diaphragm

(OP)
Warm air rises out the ridge and sucks the cold air in the eave*

RE: Wood Gable Roof Diaphragm

No time to draw a picture. Saw a condo being built here in SoCal with conventional double 2x12 ridge beam, roof sheathing both sides boundary nailed to ridge beam, 3" holes drilled through roof sheathing just outside the ridge beam on both sides and spaced so only a percentage of the diaphragm was penetrated, then a wide ridge cap was placed covering the holes. The composition roof tiles were held back to expose the holes but were covered by the vent.

RE: Wood Gable Roof Diaphragm

@KootK

You're absolutely right, at the ridge or any other pitch break not only is the diaphragm discontinuous (unblocked diaphragm but the rafters or trusses are also "broken" at these locations. In the case of MPC wood trusses all of this load will be shouldered by the truss connector plates and the wood-to-wood contact of the chord members.

I'm sure there is some way to model this behavior in RISA or some other method.

My thinking is that this location (center) of the diaphragm is not critical so that is why we don't see any failures in the news from ridge vents.

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

RE: Wood Gable Roof Diaphragm

Quote (Medeek)

My thinking is that this location (center) of the diaphragm is not critical so that is why we don't see any failures in the news from ridge vents.

Not so. For lateral load perpendicular to the ridge, the shear at the ridge, and adjacent to the gable end, will be the maximum value of the entire diaphragm for load in that direction. And since gable building trusses generally span the short direction, that often makes the shear demand at the ridge the critical value for either direction.

If you really want to give yourself a headache, contemplate shear transfer in the sheathing across hip roof ridges where there is no blocking along the ridges which is, of course, always the case.

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.

RE: Wood Gable Roof Diaphragm

Has anyone ever even seen a wood frame roof diaphragm failure?

RE: Wood Gable Roof Diaphragm

Here is the new 2012 IRC roof ventilation requirements:
R806.2 Minimum vent area. The minimum net free ventilation area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:

In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
At least 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space… with the balance of the required ventilation provided by eave or cornice vents.

RE: Wood Gable Roof Diaphragm

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.

RE: Wood Gable Roof Diaphragm

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
www.HowToEngineer.com

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm

Quote:

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.

RE: Wood Gable Roof Diaphragm

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
www.medeek.com

RE: Wood Gable Roof Diaphragm


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.

RE: Wood Gable Roof Diaphragm

Quote (xr250)

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.

RE: Wood Gable Roof Diaphragm

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

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