Attic Truss - Floor Sheathing Flat Diaphragm

[StrEng007]

Hello, everyone. Seems like all my posts have been about diaphragm action lately. Here's another one I'm working through on a current job.

I have attic trusses with "floor" sheathing and would like to make use of that sheathing as I diaphragm. One of my concerns is that I know there will be a vertical web member that will interrupt the wood structural panels. One option is to notch around these vertical webs but is see that being problematic in the field (i.e. nice on paper but seems like it would cause the GC a headache).

Now, I realize that I could sheath the top and bottom of the truss chord (1 for floor sheathing and 1 as a ceiling diaphragm). This would be my last resort with this sort of option since I don't want to double the cost of the sheathing.

Moving forward, my concerns/questions are as following:

1. Without a ceiling diaphragm, do I need to be concerned about the mechanism by which lateral wind load will go from the top plate into the floor diaphragm? My thinking is there is some local transfer of axial force through the bottom chord that gets picked up by the diaphragm.

2. Is it necessary to have the sub diaphragm at each end as shown in the image below? Or would the truss bottom chord (behaving like a strut) be good enough to get the load into the "Main Diaphragm"? Obviously, it'll still be sheathed due to function of the attic, but I'd just ignore it's contribution. On the other hand, the truss manufacturer might require some additional webs at each end to make it work.

Sheathing and Blocking for discontinuous panels:

Additional Truss webs where Sub1/Sub2 no longer seems like an option:

 

[Craig_H]

I seem to remember asking the same question a while back. Recommended solution was to sheath the vertical webs and provide blocking at the web-to-top-chord joint. See this:

https://www.eng-tips.com/viewthread.cfm?qid=475689,

and KootK's excellent diagram copied below for convenience:

 

[KootK]

Why do you not want to use the roof sheathing as your diaphragm? That's the conventional approach.

With respect to the sketch that Craig posted, those vertical "shear walls" are. at best. usually a combination of:

1) Gypsum wall board and;

2) diagonal bracing on the outside of the vertical web.

 

[StrEng007]

Why do you not want to use the roof sheathing as your diaphragm? That's the conventional approach.

I typically would and there will be roof sheathing. So in real terms, there will be 2 diaphragm systems available because of the roof and the attic floor.

I suppose I've been met with the idea to use the ceiling framing so many times here, that I thought this would be an excellent way to do it.

So to answer your question, I'm not fully decided either way. I'm testing the water so to speak.

 

[KootK]

I'm concerned that you might wind up shot if you try to sheath the top of the bottom chord outside the occupied space. Material cost aside, it would be pretty tedious to try to get into the heel area to nail the sheathing.

 

[StrEng007]

Agreed. For the sake of completing the now theoretical question (it's unlikely I'm going the attic floor route anymore), could one argue that the truss bottom chord is "good enough" to get load into the actual diaphragm portion of the attic floor? I realize this is a function of localized axial force into the bottom chord first bay, something the truss manufacturer probably doesn't consider.

KootK,
I'm not ignoring the other post. Taking me an extra minute to digest the last one.

 

[KootK]

I'm not ignoring the other post. Taking me an extra minute to digest the last one.

No rush at all. Seriously. Take weeks if you like. In some respects, slower is better for me so long as we eventually finish.

For the sake of completing the now theoretical question (it's unlikely I'm going the attic floor route anymore), could one argue that the truss bottom chord is "good enough" to get load into the actual diaphragm portion of the attic floor?

Yeah, I think it would be fine. If the bottom chords would be unsheathed below, which seems highly unlikely, one might want some intermittent blocking for axial.

 

[lexpatrie]

So there is a roof diaphragm?

 

[StrEng007]

There is a roof diaphragm, yes. There always was. The idea was there may have been diaphragm more willing to participate closer to the load source. As KootK pointed out, the detailing and construction of this option is not the best option.

 

[lexpatrie]

Well stiffness draws force but if it fails out, the roof is there to take over, and if strong enough, the roof can serve as the entire diaphragm from the looks of it, so your design should be conservative.

 

[XR250]

So there is no sheetrock on the underside of the truss bottom chords? Why not use it if it exists?

 

[KootK]

Why not use it if it exists?

1) To my knowledge, there is no code sanctioned, horizontal diaphragm option in gypsum.

2) The detailing of ceiling gypsum at walls is often done in such a way that:

a) it permits some verical movment so as to prevent cracking and;

b) it makes chord / drag detailing all but impossible.

I'm not saying that horizontal gyp diaphragms can't participate. I'm sure that they do. They're just ugly to evaluate and slip past the gaze of enquiring eyes.

 

[KootK]

 

[phuduhudu]

What is wrong with just using the central portion of the attic floor as a diaphragm? There are regular trusses to transfer the load from both the wall plate and the roof into the diaphragm. The problem I can see is that there isn't a continuous edge chord for your diaphragm and only blocking between the truss bottom chords. You could fix a continuous timber over the diaphragm as a chord against the truss uprights.

 

[KootK]

What is wrong with just using the central portion of the attic floor as a diaphragm?

Mostly that it would only be viable in one direction (load parallel to the trusses).

 

[XR250]

1) To my knowledge, there is no code sanctioned, horizontal diaphragm option in gypsum.

2) The detailing of ceiling gypsum at walls is often done in such a way that:

a) it permits some verical movment so as to prevent cracking and;

b) it makes chord / drag detailing all but impossible.

I'm not saying that horizontal gyp diaphragms can't participate. I'm sure that they do. They're just ugly to evaluate and slip past the gaze of enquiring eyes.

I would venture to say that over 80% or all houses in the US use a sheetrock ceiling diaphragm. The IRC requires balloon framing of gable ends in high wind areas but can be hinged at the ceiling level in other cases.

 

[phamENG]

1) To my knowledge, there is no code sanctioned, horizontal diaphragm option in gypsum.

2508.6 Horizontal gypsum board or gypsum panel produce diaphragm ceilings.

Gypsum board or gypsum panel products shall be permited to be sued on wood joists to create a horizontal diaphragm ceiling in accordance with table 2508.6.

 

[KootK]

Sweet. Live and learn.

Still:

1) The detailing of many gyp diaphragms will be incomplete.

2) Where a gyp diaphragm has to compete with a same sized wood panel diaphragm on the same load path, I would expect the wood panel diaphragm to dominate.

 

[XR250]

The detailing of many gyp diaphragms will be incomplete.

Certainly. Not much oversight in Sheetrock Installation.
Have not personally witnessed a failure in ANY house though even after a hurricane passed through. Probably a bunch of failures down at the coast though.

 

[KootK]

Have not personally witnessed a failure in ANY house though even after a hurricane passed through.

Let's not rehash this for the umpteenth time XR. You know full well that I agree that:

1) Gypsum diaphragm's are doing constructive work in many buildings.

2) There is not rash of gypsum diaphragm failures taking out daycares etc.

Where it's a problem, it's primarily a problem of justification, not performance.