Sheathing Requirements

[medeek]

I've got a pre-fab steel building, fully enclosed with a 6" slab foundation. Inside this building my client wants to frame up an interior space and insulate it however he wants to spend as little money as possible and does not want to apply sheathing to the interior walls (12' height) or to the TJI joists of the ceiling. Of course there will be some shearwalls for the seismic requirement, but other than that is there some code that requires full sheathing of the perimeter walls or can he just get away with putting up some fire retardent plastic over top of the insulation (I know doesn't sound like much of a structure to engineer). With the ceiling joists I'm basically of the opinion the whole diaphragm needs sheathing to get the appropriate diaphragm action out of it.

I had previously posted some related questions to this project but now I'm back to the drawing board since the client feels strongly that I've completely over engineered the (interior) structure.

A confused student is a good student.

 

[msquared48]

If the client refuses to install sheet rock or plywood to the walls and ceiling, considering both structural and fire requirements, in short, walk away from his stupidity. He obviously thinks he knows more than you. Let him deal with the building department.

Mike McCann, PE, SE (WA)

 

[medeek]

Honestly I am just about to that point.

A confused student is a good student.

 

[Ron]

I agree it sounds like you have a cheap, ignorant client. However, the way the building looks on the inside is irrelevant unless there is a structural or fire separation need.

You mention diaphragm action. If the interior demising walls are not a part of a necessary diaphragm or shear wall consideration, there is no requirement for their materials or finish, so long as they don't change the fire rating of the structure.

If the use of the building requires storage or activity separations that would involve fire ratings for the interior walls, then sheathing and special finishing would be required.

I suppose the bottom line is that if the building is OK structurally without any interior walls and there is no fire rating requirement, your ignorant client can do whatever he pleases with the interior walls.

 

[CELinOttawa]

..

your ignorant client can do whatever he pleases with the interior walls.

On his own time, with his own money, and not under your seal. Just my two cents (which I couldn't give you even if I wanted to, being in Canada).

 

[medeek]

Perhaps it is partly my fault. I kind of assumed from the get go that he would at least want to sheath the exterior of the interior wood structure to fully enclose the envelope in a neat and tidy fashion. I just wish this conversation had happened a lot earlier on in the design phase. The buildings will be classified as "U" or agricultural storage. There is no activity separation and no special fire walls or fire separation barriers as nearly as I could tell from examining the requirements of the IBC and the building dimensions and classification. Single story, 12' wall height (interior), no mixed use.

I guess I'll try to trim the fat. 24" o/c studs. Only enough OSB on any of the walls to suffice for the seismic lateral loads and ensure the holdown forces do not exceed the capacity of a TitenHD embedded to 4". I do worry about the 12' studs wanting to buckle without any sheathing on one side or the other though at the heaviest loaded bearing walls (TJI ceiling above). Even with that the client has stated he would rather throw some diagonal bracing at it than sheath it with OSB. I'm not sure why the strong aversion to OSB sheathing, with a nail gun and studs at 24" o/c this should literally fly together.

Quality is not the factor here, absolute bare minimum is. Avoid such projects in the future.

A confused student is a good student.

 

[KootK]

Sheathing is expensive relatively speaking. Strap bracing is a good idea. Are your interior walls vertically separated from your ceiling joists such that they will truly see no axial load? If not, they are load bearing to some degree. There's also interior wind loading to consider. It's just 5 psf in a normal building. It might me more in your case if wind will gust in through large openings.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

In a PEMB, which this is, it is wise not to connect the new interior partitions to the PEMB structure as you would have to show it could withstand the forces. That would be difficult at best for many reasons. That leads to the necessity for shear walls on all four sides (three sides if the rooms are triangular). Hence the need for either sheet rock or strapping. Hold downs will most likely not be needed.

Mike McCann, PE, SE (WA)

 

[manstrom]

1x4 let in bracing or Simpson T bracing. Or at the least gyp at the corners.

Gyp sheathing on the ceiling and or 3/8" plywood decking.

Engineering something down to the bare minimum is a challenge in itself. You don't need osb shear walls do you?



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[CELinOttawa]

Use the metal bracing... Let in doesn't really work without sheathing atop.

 

[medeek]

I am not attaching any of the interior wood frame structure to the steel building. I probably should have posted a picture of the general floorplan for reference here is the bearing wall drawing:

My shearwalls are at the wall lines indicated. I definitely think I can cut back some of the sheathing (segmental shearwall length) on Walls 1,2,3,4 and B while still maintaining acceptable holdown forces (TitenHD into 6" slab on grade), however I still have to use some OSB. The SDPWS-2008 does not give any numbers for using metal straps for shearwalls and gyp. board and lumber are less effective and probably more expensive. Simpson's wall bracing products explicitly state that they are for resisting racking during construction and not a substitute for structural panels:

http://www.strongtie.com/ftp/catalogs/c-c-2015/C-C-2015-p225.pdf

As for the TJI ceiling, the Bldg. dept. will not let me sheath the top since that would allow the occupants to potentially turn the "roof" into a second story. So my next option is to sheath the underside of the TJI ceiling joists with OSB. I've still got to brace the topsides of TJI joists for lateral torsional buckling so I've call out some 1x4 diagonal bracing on the top. However, the question is how do I eliminate some or all of the OSB sheathing on the ceiling. The client just wants insulation, not gypsum or anything else fancy.

I was thinking the diagonal 1x4 bracing will provide some diaphragm action but I have no idea how to calc. that out and prove that it is sufficient given the reactions from the shearwalls.

My seismic data:

SDS = .997
SD1 = .721

Shearwall Loads due to Seismic:

SWLA 8761 lbs
SWLB 8806 lbs
SWL1 2823 lbs
SWL2 5232 lbs
SWL3 4877 lbs
SWL4 2949 lbs

I was also thinking maybe I could just sheath the ceiling around the perimeter in the highest shear areas, but then one would need collectors installed (blocking) at the opening edge so maybe that method just entails more labor.


A confused student is a good student.

 

[RFreund]

A few questions:

1. Is this a gable roof?
2. What is the issue with just using the perimeter walls for shear walls? Sheath as much as the perimeter wall as you need to.
3. The roof is sheathed, correct? I would say you only need to sheath part of the ceiling near the gable ends (or create some sort of truss). Maybe I'm missing something though.

EIT

http://www.HowToEngineer.com

 

[medeek]

This is a metal building with an interior wood frame structure (walls with TJI ceiling joists).

The interior wood frame walls indicated will be used as shearwalls, as I mentioned above they will not need to be entirely sheathed what will govern how much I need to sheath and the length of these shearwalls is the amount of holdown force I can accept given I have no footings under these walls just a prexisting 6" slab with minimal reinforcing. If the shearwalls are too short the holdown forces will be too great and I then would be forced to cut up the slab and install footings underneath which I would like to do at all costs.



A confused student is a good student.

 

[medeek]

Correction, I would like to avoid at all costs. I really wish this forum had a way to edit posts.


A confused student is a good student.

 

[KootK]

I like the concept of sheathing an annulus around the perimeter. Clever. Quite like discrete bracing used in heavy industrial buildings. Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

The diaphragm loading in the longitudinal direction (east-west) is 307 plf. If all the ceiling is sheathed then my unit shear is a measly 70 plf. So my thinking is maybe there is a way to partially sheathing the ceiling without making things to complicated, eliminate some of the OSB yet still make the numbers work.

A confused student is a good student.

 

[medeek]

Something along these line perhaps:

My only concern with this method would be that the edges of the sheathing in the north-south direction would need to be blocked and a continuous strap in order to create a chord at these locations. In the other direction the edge of the panel will catch the 2nd TJI in.

A confused student is a good student.

 

[jayrod12]

You can surely get 70 psf from gyproc no?

 

[KootK]

You were going to need blocking for LTB anyhow. If you want to forgo straps, just run 1x4 under the blocking for your chord element. Take advantage of the non-ceiling!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

If my client is going to balk at OSB ($8.55/4x8 sheet) he will surely have a problem with Gyp. Board ($10.55/4x8 sheet). Note, these are current Home Depot prices.



A confused student is a good student.