Double 3-Sided Roof Diaphragm 7

[StrEng007]

I have a building that features s pop up roof section. Please see the image below.

Long story short, I have a hole right in the middle of my roof diaphragm. Based on the shear wall layout, there is no vertical element to support the edges of the roof diaphragm shown as #1 and #2.
Would this be a candidate for each diaphragm to be treated as an "open front diaphragm"?

In all the design examples I've seen, the open front end of the diaphragm didn't have an additional "point load" in the transverse direction.

Additionally, what I'm not showing is the fact this roof is a gable. I've shown it as being flat because that helps me first get an understanding of the approach I want to take. My second concern here is being able to get a 3 sided diaphragm to work as a gable roof with a steep slope.

 

[KootK]

Yeah, we should keep this moving for the sake of any popcorn eaters following along.

@StrEng007: can you process your thoughts on the last installment so that we can proceed?

 

[StrEng007]

Yes, I'm ready for the next installment.

 

[KootK]

Excellent. So if you buy into all of the above, then it's really just down to what is the "skyhook" in the sketch below.

Some options:

1) A truss would be ideal because the webbing supporting the peak would do fine job of being the skyhook. Very stiff along the length of the ridge.

2) A ridge beam would be my second choice. Less good than trusses because it would tend to be less vertically stiff away from its supports.

3) A ridge board would suck for this purpose. This makes me question whether or not these systems really do load share to their diaphragms.

So that's it. The major takeaways, as I see it, are:

a) The presence of vertical restraint at the pitch breaks is often necessary for diaphragm load distribution and;

b) The quality (stiffness) of the vertical restraint matters.

It's interesting to me that lateral load cases will induce vertical load into ridge beams, both downwards and upwards, in addition to the loads that they are normally designed for. I Imagine that the scale of the loads is small enough that ignoring them is sensible.

 

[KootK]

@StrEng007 : most importantly, does all of this serve to answer your burning question? I want it to.

 

[lexpatrie]

I'll have to come back to this, KootK really stepped up the scribbles on this thread, though, especially at the beginning.

I think the treat it as a flat diaphragm is a necessary step and once you're solid on that, then the gable can be addressed as an incremental change. I'm not completely sold on the "this part of the gable doesn't do anything" reasoning. It's forced to move by the direct action of wind on the other side and by virtue of the movement, given there's a finite stiffness, this engages the diaphragm on the far side, i.e. both sides, and I seem to recall seeing some "treat it as a flat diaphragm" from an APA presentation or two (Lockyear?)

This isn't quite germane, but the diagrams sure look nice.

U Michigan Diaphragm Notes, Arch 544 Peter Von Bulow.

Say, as a side note, does anybody have ATC 7 or the Diekmann paper?

Interesting web site regarding trusses - https://www.housebuild.com/construction/building-guidelines/roof-level/trusses

 

[Celt83]

@lexpatrie
you can get ATC 7 here they print it on demand I got my copy in a couple days: https://store.atcouncil.org/index.php?dispatch=products.view&product_id=11

The Diekmann article can be found in this book:

Wood: Engineering Design Concepts; Volume IV (Clark C. Heritage Memorial Series on Wood): Near Fine Hardcover (1986) | Jackson Street Booksellers

Hardcover - Materials Education Council - 1986 - Condition: Near Fine - Near fine copy in orange-brown cloth with gilt titles. 8vo. - Wood: Engineering Design Concepts; Volume IV (Clark C. Heritage Memorial Series on Wood)

www.abebooks.com

Diekmann also authored the diaphragm chapter in the 1989 wood handbook:

Wood engineering and construction handbook by Keith F. Faherty: Very Good (1989) | Hawking Books

Free Shipping - ISBN: 9780070198951 - 2nd Edition - Hardcover - McGraw-Hill Inc.,US - 1989 - Condition: Very Good - Very Good Condition. Five star seller - Buy with confidence! - Wood engineering and construction handbook

www.abebooks.com

 

[KootK]

I think the treat it as a flat diaphragm is a necessary step and once you're solid on that, then the gable can be addressed as an incremental change.

I feel that comment to be inappropriately dismissive here. Consider:

1) Pretty much every step of a carefully set out logical argument is, by definition, "incremental". But, just as I've done here, a bunch of incremental steps can get you from zero to a solid, fundamental understanding of something complex if you follow it through. In fact, I would argue that logical baby steps is precisely how most difficult conceptual problems come to be resolved.

2) I believe that reducing an articulated diaphragm to a faux flat diaphragm is precisely the logical leap that most people struggle with. It is, after all, pretending that something is something other than what it physically is. So, to gloss over this aspect as a minor step is, I feel, to fundamentally miss the point.

So how have you, gotten yourself on the necessity of treating articulated diaphragms as flat diaphragms without following reasoning similar to mine?

For what it's worth, I do not believe that treating articulated diaphragms as flat diaphragms is "necessary". Rather, I feel that it is simply an expeditious convenience. Moreover, there are some aspects of diaphragm behavior, such as deflection, for which the faux flat diaphragm simplification will not be accurate.

 

[KootK]

...It's forced to move by the direct action of wind on the other side...

I believe that statement to be fundamentally in error.

Yes, any diaphragm plane may well be encouraged to movement by the action of any wind pressure that it receives directly. However, that is not sufficient to support the supposition that we commonly employ in design that all components of an articulated diaphragm share load evenly.

For load sharing consistent with our design practices, I believe that it must be demonstrated that all diaphragms share all loads, regardless of where within the system those loads originate. Otherwise, it's just a bunch of separate diaphragms each doing "something", but doing it more or less independently.

The above speaks to the concept of superposition here which I mentioned at the beginning.

More complex situations can be handled by way of superposition. It is just easier to elucidate important points by paring the situation down to only that which is truly necessary.

Those logical baby steps, one increment at a time...

 

[lexpatrie]

Anyway, I'll just ignore that for now. I know full well I'm not going to convince KootK of anything. When it's a flat roof there's no ridge so there's no need to explain how both sides of the ridge are active and provide resistance via deflecting.

Here's my concept on this (flat roof).



At one point I saw a master's thesis somewhere that was going through various diaphragms and showing the breakdown of the deflection components by term, for the more normal diaphragms, the primary deflection source was shear, versus flexure, chord elongation/slip, or nail bending. I don't have the research at hand anymore.

 

[KootK]

Anyway, I'll just ignore that for now. I know full well I'm not going to convince KootK of anything.

Frankly, I feel that it would be gentlemanly of you to at least try. You tend to repeat a pattern that I feel is a bit unfair to me with regard to my time and effort. It goes like this:

1) You freely admit that you haven't actually read my stuff.

2) Despite not reading my stuff, you assume the prerogative to challenge parts of it.

3) I respond to your challenges. You know, that whole "discussion" thing.

4) You high tail it.

If you're going to bother to jump in, jump in for goodness sake.

 

[KootK]

@kissymoose @MJB315 @phuduhudu + any other popcorn munchers:

Now that I'm done with my swing at this, I would welcome any feedback on my theory of how this stuff works. I've not seen any of this any any book, and I have most of them. I'm very interested in vetting the theory if there are alternate theories and/or points of disagreement.

 

[StrEng007]

KootK,
I'm ALMOST good with you approach. Before I say what alternative I'm thinking, I need another minute to make sure that what I put on the table makes sense. I just did a job with some fine attic trusses, major lack of webbing. So my version of the skyhook at this moment is incomplete.

 

[KootK]

I just did a job with some fine attic trusses, major lack of webbing.

Right. I can't resist taking a swing at that in advance.

I would say that a truss does not actually require webbing in order to have a skyhook in the sense that we've been discussing it. Take all of the webbing away and the skyhook effectively becomes just the axial resistance of the top chords.

 

[lexpatrie]

Not accurate, KootK. Not accurate. I'm bypassing you because at this point, what I posted isn't relevant to your theory. I haven't found the documentation I "know" is out there on the subject so I'm not engaging in that aspect of the discussion. That's more or less what I said several times here, i.e. "go ahead, I'll wait". No need to flip out with the ad hominem.

I think I found that document I mentioned earlier.
Deflection of Light Frame Wood Diaphragms, Curtis Earl, Washington State University, 2016.

 

[KootK]

No need to flip out with the ad hominem.

I'm not flipping out. I'm simply trying to encourage you towards what I feel is a reasonable degree of etiquette when it comes to technical discourse. You make a lot of fine contributions here and are clearly very knowledgeable. I would very much like to avail myself of that and engage you in interesting debate when the opportunity arises. Unfortunately, your critiques of my work often feel more like drive by shootings than productive discourse. I feel that you are quick to critique work that you haven't properly digested and then reluctant to defend your critiques.

Not accurate, KootK. Not accurate.

Completely accurate. And it would be a simple matter for me to quote a handful of your comments in this thread to demonstrate that. Perhaps twice.

I'm bypassing you because at this point, what I posted isn't relevant to your theory.

Then perhaps you ought to have bypassed me before offering your critiques of my theory. All I ask is that, if you critique my work, please at least do me the courtesy of continuing the conversation long enough that we might resolve our differences. Since this is the internet, you will of course do as you wish.

 

[kissymoose]

3) A ridge board would suck for this purpose. This makes me question whether or not these systems really do load share to their diaphragms.

I would say that a truss does not actually require webbing in order to have a skyhook in the sense that we've been discussing it. Take all of the webbing away and the skyhook effectively becomes just the axial resistance of the top chords.

Ridge board design will typically come with tension ties, aka truss without webbing. Why the doubt in ridge boards to provide vertical restraint?

Regardless...
I agree with post #39, if the boundary conditions shown below were present, both diaphragms would engage. I don't understand the jump to post #43:

a) The presence of vertical restraint at the pitch breaks is often necessary for diaphragm load distribution

Wouldn't two rollers & no restraint @ ridge still engage both diaphragms from lateral displacement? Meaning sky hooked be damned, as long as the diaphragms are pinned, they're both engaging.

 

[KootK]

Why the doubt in ridge boards to provide vertical restraint?

Thanks for coming back to play.

1) Because they are not designed to provide vertical restraint, not even for gravity.

2) The size of most ridge boards would provide very little vertical stiffness. Heck, some aren't even flexurally continuous.

3) Many ridge boards are not vertically restrained themselves. No end supports / tie downs.

 

[KootK]

Wouldn't two rollers & no restraint @ ridge still engage both diaphragms from lateral displacement?

No, I don't think so.

I started my diatribe with this simple assertion: what matters for load sharing is that both diaphragms experience compatible, in plane displacement under lateral load.

Are you in agreement with that? Because, if you are, the answer to your question is right there in your sketch where you have shown the windward diaphragm moving much more in plane than the leeward diaphragm. And I love that because it means that, on an intuitive level, you feel this.

Of course, everything is a matter of degree. If the leeward diaphragm experiences some amount of movement, it will pick up some amount of load. But I would argue that could me much less load than we normally assume when we do conventional diaphragm design.

 

[KootK]

@Celt83 has brought something to my attention that may be relevant.

The stuff below is from a Mesozoic era Diekmann publication. I actually disagree that there is a vertical component to the chord forces. The chord forces are just axial in my view. That said, it maybe just a perspective thing. Regardless I think that this stuff tracks in a similar vein to my skyhook concept.

I'm going to go give Celt83 a random star someplace in appreciation for this...

 

[SE2607]

Qualitatively, I believe deflections will be excessive.