Diaphragm without Sheathing? 1

[medeek]

So here is an interesting question, one which I have never encountered before. I've got an existing steel building in which an occupant would like to frame up some internal partitions and internal ceiling inside (ie. 2x6 @ 16" o/c, 2x4 @ 16" o/c and TJI 110 @ 24" o/c), insulate etc... The steel buildings are quite tall so the one story wood framing would leave quite a large attic space above. In order to prevent the occupant from trying to turn this added space into additional storage the building dept. will not allow the top side of the ceiling /(internal) roof to be sheathed with 7/16 OSB. Granted this "internal" roof will never see any wind or snow loads, however the vertical dead and live roof loads and particularly the lateral seismic loads now become the governing factors.

The manufacturers specs on TJU's suggest lateral bracing approx. 4' o/c (ie. 2x4 running perp. to the ceiling joists) however this doesn't really constitute a diaphragm in my opinion. Some sort of diagonal lateral bracing combined with the perp. lateral bracing might get me a little closer.

 

[KootK]

Can you put drywall on the underside of the joists?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Why not design for the storage loads? Not always that difficult, nor expensive, if you are already building lined partitions, ceilings, etc. Much of the cost is labour, which will only change slightly for the differences between the solutions.

I've actually refused to *not* consider the new internal ceiling as a storage load design. It will get used as storage. It will. If not this owner, then the next. It will. Design for storage.

 

[medeek]

Yes, all of the interior walls and ceiling with be 5/8" drywall.

The problem with designing for additional storage loads is that it would probably required tearing up the existing slab and re-pouring new footings/foundations for the internal structural. The original foundation plans exist and on paper there is a 6" slab with #4 rebar 24" o/c, actually a pretty decent slab in my opinion and more than adequate for the single story structure as shown below. To be conservative I've calculated the load on each lineal foot of wall with a 10 psf wall load and a roof load of DL 15 psf + L_r 20 psf. Based on these numbers and the following publication (Table 3-2):

UFC 3-320-06A

The slab should be adequate for the loads however additional storage loads or a floor load of say 30-40 psf would probably put me over the allowables and hence the decision to try and avoid this. Part of the problem is the original structure was erected about 20 years ago so the bldg. dept. records on the foundation are non existent and I think they are nervous about what is really inside that slab or what its depth really is. The directive from them is "one story only", sheathing the "interior roof" would constitute two stories in their eyes.

Note, the thickened edge footings shown are actually pad footings at eight places (20' o/c) so the exterior wood framed walls are resting on the 6" slab for about 60-70% of their length.

 

[KootK]

Well, if you've got a drywall ceiling, then you've got your diaphragm sheathing! By the look of it, you could probably also tie your ceiling deck to the steel structure laterally.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

The thought had occurred to me to tie the wood to the steel but the added lateral load on the steel might make it not pass current seismic code since it was designed per seismic code in the mid 1990's. So the idea is to keep the two systems separate as much as possible.

So with a gyp. board ceiling you feel like I have a diaphragm? I won't argue with that but it worries me that there is not adequate bracing on the compression side of the TJIs where they will be prone to lateral torsional buckling.

 

[KootK]

Yeah, I would consider it a diaphragm with the gyp. It works for shear walls. You may need to give it some extra detailing attention however. Interior walls normally break the plane of drywall ceilings.

You're right about the LTB though. You'll need bridging/blocking.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[wannabeSE]

Chapter 25 of the IBC has shear values for gyp board diaphragms. They aren't much, but it is something.

 

[medeek]

For 1/2" gyp. board at ceiling joists @ 24" o/c I get 70 plf from table 2508.5, since this is seismic zone D I reduce this value by 50% per footnote (b), which brings me to 35 plf. I'm not sure how this is going to get me to the shear capacity I need. I'm going to finish running all of the seismic lateral calcs and then show how much I deficient. The question I have is, will a gyp. board ceiling combined with full depth blocking and diagonal bracing of the tops of the TJI joists be enough and then how do I rationally prove it?

 

[KootK]

Foundation issues aside, CEL's strategy is sounding better all the time.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

As for possible bracing I was thinking something along these lines:

However, sheathing it with 7/16 OSB is the strongest and most sensible solution. This time I disagree with the bldg. officials but I see there point as well. I think the thing to do is to present a couple options and then let the bldg. dept. and the owner duke it out over the details.

 

[KootK]

I was thinking 1x4 strapping at 4' o/c with intermittent 1x4 diagonal bracing at some generous interval. Depends on the LTB bracing requirements for the TJI.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[XR250]

Use that cardboard crap that tract builders put on their houses to brace them. Won't support any storage per se, but might work in this case.

http://forums.finehomebuilding.com/breaktime/general-discussion/energy-brace-exterior-sheathing

 

[XR250]

...I would probably go with the blocking and GWB ceiling as others have suggested, however.

 

[medeek]

I like the 4' o/c spacing for the laterals. Went back into the software for the TJI and the LTB bracing requirement varies between 5-1/2 feet to 3-1/2 feet so 4' o/c is about right. Then I'll specify a diagonal brace that runs every 8', so o/c spacing is approx. 4 x 1.41 = 5-1/2'. 1x4 seems adequate enough, no need to overkill it. Still not sure how to run any numbers on this and it probably doesn't even come close to the shear capacity of OSB sheathing but it does seem pretty substantial and better than just relying on the gyp. board for diaphragm action.

 

[KootK]

I wouldn't consider the topside bracing that we've been discussing the "diaphragm" from a seismic perspective. Just LTB restraint. If the drywall doesn't get the job done, I suppose that you could sheath the ceiling with OSB with the drywall below. That'll cost a few pennies though.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

I see where you are going with this. I just finished running the lateral seismic loads and the diaphragm shear is 48 plf in the longitudinal direction and 51 plf in the transverse direction. Granted I ran the seismic with a rho of 1.3 which was conservative and I could have probably gotten away with rho = 1.0 which means my loads are conservative. Also the shear value in the IBC is for 1/2" Gypsum, unfortunately I don't see a value for 5/8" Gypsum but comparing the values for gyp. board in the SDPWS 2008 it appears that an increase of 1.3 to 1.5 is typical in moving from 1/2" to 5/8" (shearwall tables). So my adjusted allowable shear capacity would be 70 plf * 1.3 * (1/2) = 45.5 plf, so I'm pretty darn close.

 

[KootK]

The drywall diaphragm really isn't doing much more here than resisting it's own seismic weight. I'd be pretty disappointed (and skeptical) if it couldn't be made to work somehow.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[SteelPE]

I have always take the IBC diaphragm loads very literally. Now, I have never tried to use gyp to resist diaphragm loads, but I don't ever recall seeing a chart in the IBC that gives diaphragm values for a gyp diaphragm. I have see values for a gyp wall to resist loads. I may be wrong but I would not try to use gyp to resist any diaphragm loads.

http://www.eng-tips.com/viewthread.cfm?qid=240778

 

[KootK]

There are no tables for horizontal gyp diaphragms. I've always assumed that's because standard interior wall/ceiling detailing doesn't jive with it as I mentioned above. Is it actually prohibited somewhere? Might be a good question for your local building official.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.