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.

 

[jayrod12]

Why not sheath the underside of the ceiling with osb prior to installing the gypsum?

 

[medeek]

Take a look at at Section 2508.5:

IBC 2012: 2508.5

 

[medeek]

I was about to call this one "mission accomplished" but then I was looking at my seismic worksheet and noticed the response modification coefficient for gypsum as 2 and not the 6.5 for WSP. Table 12.2-1 of the ASCE 7-10 (item 17) simply states light frame wall with shear panels of other materials. Granted a diaphragm is not a shear wall in the strict definition but it does deal with shear loading in a similar manner. If so then my R should be 2 which significantly drives up my seismic lateral loads and I'm back to sheathing the ceiling with 7/16 OSB. Am I interpreting this wrong?

 

[medeek]

I guess I will throw in the towel on this one and go with GWB on top of OSB for the ceiling diaphragm. Reading some more online and digging through every resource I could find I can't really find anyone of the opinion that using GWB as a diaphragm materal is a good idea or recommended. I'm still not sure on the response modification coefficient but I'm not going to take the chance.

 

[XR250]

How much could you seismic loads possibly be? There ain't a whole lot of mass up there.

 

[woodman88]

Per the ASCE 7-05 and 7-10

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.

 

[medeek]

I've discarded the idea of a GWB diaphragm however the client is not happy that we now have to sheath the ceiling with OSB and then gypsum. The building dept. will not allow the sheathing on the top in order to prevent unwanted storage loads and a single story becoming a "two story structure". It's hard to win on this one, but rather safe than sorry.

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.

 

[KootK]

I believe that the philosophy is for the diaphragm to remain elastic and the bulk of the energy dissipation to occur in the walls. That would lead me to believe that the correct technical answer is to base your force reduction on the wall construction. Of course, basing it on the drywall is conservative. And conservative in very tempting in exotic situations.

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

 

[woodman88]

As the R relates to the vertical elements, I would use the R for the shearwall. So a WSP would have R=6.5.

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.