Diaphragm without Sheathing?
Diaphragm without Sheathing?
(OP)
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.
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.






RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
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.
RE: Diaphragm without Sheathing?
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 + Lr 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.
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
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.
RE: Diaphragm without Sheathing?
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.
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
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.
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
http://forums.finehomebuilding.com/breaktime/gener...
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
http://www.eng-tips.com/viewthread.cfm?qid=240778
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
IBC 2012: 2508.5
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
RE: Diaphragm without Sheathing?
12.2.1 Selection and Limitations
"... Each type is subdivided by the types of vertical elements used to resist lateral seismic forces."
GWB as a diaphragm material is okay for very low force conditions. Personally, I use have the value allowed by the table.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Diaphragm without Sheathing?
However, my one still burning question is what is my appropriate response modification coefficient if I have WSP shearwalls yet a GWB diaphragm, am I still okay to use 6.5 or does it go to 2 because of the GWB involved?
In my case because of the local design criteria usually wind governs over seismic for lateral loads so my experience and familiarity with all the nuances of seismic design is limited.
RE: Diaphragm without Sheathing?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Diaphragm without Sheathing?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.