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Sheathing Requirements

Sheathing Requirements

Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

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)


RE: Sheathing Requirements

(OP)
Honestly I am just about to that point.

A confused student is a good student.

RE: Sheathing Requirements

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.

RE: Sheathing Requirements

..

Quote:

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).

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

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.

RE: Sheathing Requirements

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)


RE: Sheathing Requirements

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

RE: Sheathing Requirements

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

RE: Sheathing Requirements

(OP)
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...

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.

RE: Sheathing Requirements

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
www.HowToEngineer.com

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

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.

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

You can surely get 70 psf from gyproc no?

RE: Sheathing Requirements

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.

RE: Sheathing Requirements

(OP)
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.

RE: Sheathing Requirements

(OP)
For the chord element do I need a 1x4 under the blocking and on top of the blocking or can I get away with the 1x4 on top of the blocking since I will be putting it up there any way for the LTB.

Seems like a lot of messing around just to save a few hundred bucks on sheathing.

A confused student is a good student.

RE: Sheathing Requirements

(OP)
The more I think about it the more I'm convinced that if I were to install 1x4 diagonal bracing (both directions) at fairly regular intervals on the tops of these TJI's that would not only take care of the LTB but also it should give me enough diaphragm action. In essence these diagonal braces combined with the longitudinal TJI joists would be forming a type of lattice truss:



The problem is proving the numbers on this might be a bit messy. One could create a model of the whole ceiling arrangement in STAAD or RISA and apply the diaphragm load in both directions. The thing to check would be the forces in the diagonal 1x4 members and then the shear capacity of the nails connecting the bracing to the TJI joists.

Has anyone ever given this a try or am I thinking too far outside of the box?

A confused student is a good student.

RE: Sheathing Requirements

The post-frame building manuals have some values for strapped and light gauge metal diaphragms. Might be a place to start.

In theory using 1x4 as cross bracing would work. However, there's still a fair chunk of looseness at each crossing connection because they likely wouldn't put in more than 1 or 2 nails. And as the nails are worked the holes will slightly enlarge in the 1x4. But I think it could work.

How is he planning on finishing the walls, he must be putting something either on the inside or out.

And your drywall is expensive, 4x8 1/2" is 8 bucks a sheet at home depot here. OSB is the same.


RE: Sheathing Requirements

Strapping coupled with some cross bracing on top is about as cost effective as you can get. Some light gauge tin that does not qualify for mezz loads makes for easy cleanup, but I doubt he wants that. As for the analysis, unless you can prove it with simple hand calcs it is not feasible unless your client is prepared to pay for you to invest the time. If he baulks, explain that you can save him money, but it will cost $x in fees. It does not take a rocket scientist to figure it out then. This is the type of job where it must come out of a simple table as I doubt you are getting paid well enough.

You need to work on your client interview process. This is another project of yours where it seems to me you are paying out of your own pocket for the luxury to do the work. I know all about ag clients. Some are good, but others have deep seated issues when it comes to spending money.

RE: Sheathing Requirements

(OP)
He just wants stud walls with some insulation and then covered in plastic, no sheathing, drywall or any other fancy accoutrements. I think this client was my 2nd or 3rd client, I'm a little more careful now in accepting jobs.

The real problem with the lattice truss concept is that it would be a very weak truss in reality because of the 1 or 2 nail connections as stated by Jayrod12.

A confused student is a good student.

RE: Sheathing Requirements

I never thought that I'd see "drywall" and "fancy accouterment" in the same sentence...

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.

RE: Sheathing Requirements

(OP)
If the client really insists on eliminating some of the ceiling sheathing I think the annulus method holds the most promise. Some quick numbers show that with a 8' perimeter (transverse loading) the sheathing should not require upgraded nailing per Terry Malone's book or any special collector analysis at the openings. See below:



According to his text the panels around the openings need to comply with the aspect ratio requirements of Table 4.2.4 (4:1 for WSP blocked). I'm exceeding that slightly with an aspect ratio of 4.25 but it is within about 6% so I think it could get a pass.

This method would eliminate approx. 2,400 sqft of ceiling sheething which is equivalent to 75 sheets of OSB, at an average of $10 per sheet we are talking a savings of $750.00.

A confused student is a good student.

RE: Sheathing Requirements

(OP)
Of course you also need to factor in the labor savings from not having to install those additional 75 sheets, so I guess it does add up.

A confused student is a good student.

RE: Sheathing Requirements

(OP)
Then again if you factor in the time to block this modified diaphragm there may not be any signficant labor savings.

With an 8' perimeter this is how it will look. The building dept. is going to think I'm crazy when they see this detail.



A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

Am I missing something - you stated that the owner didn't want to sheath the stud walls and they are 12 ft. tall. How does 1 1/2" wide 2x material serve as a compression wall element without lateral bracing?

Are they at least sheathing one side? If so, what do you do about compression on the non-sheathed edges when the interior 5 psf goes that way? Do you assume the studs are braced against twist from the sheathing on one side and thus are OK?

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RE: Sheathing Requirements

(OP)
I haven't yet run the stud wall calc again for a stud wall without any sheathing, just getting back to this job after a small hiatus. Obviously it will need something, I will try to the run the numbers with mid-span blocking at 6' if this doesn't work then at 4' and 8' (two rows of blocking). If this does not work then sheathing is probably their only option. I'm not a fan of metal straps but this may be an option as well. As I mentioned in an earlier post Simpson's website/catalog stated they were not for lateral bracing but maybe they are okay for buckling or LTB, I guess I've just never used them before so any thoughts in this regard would be helpful.

The client keeps complaining that the structure should be treated like a greenhouse and then I have to explain that there is a few thousand pounds of joists, sheathing, bracing, lights and other potentially unaccounted for loads that are sitting above their heads.

With the shearwalls I've basically decided to cut back on the length while maintaining a holdown force of no more than approximately 2200 lbs. The seismic is governing the design. The idea here is to try and eliminate some of the shearwall sheathing (7/16 osb) but make sure my compression loads at the shearwall chords are still okay to bear on the pre-existing 6" slab. I'm going to use TitenHD anchor bolts for the holdowns and technically my embedment cannot exceed 4" (2" clearance above the soil).

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

(OP)
I'm rewriting my stud calculator to take into account the lack of lateral bracing as well as the min. 5 psf interior partion load. My question is what load duration factor is appropriate for this live load? I'm assuming 1.0, the same as for any standard floor live load, however my thinking is most live loads to the walls would probably be of a lesser duration more similar to wind and seismic (CD = 1.6)

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

(OP)
Interestingly enough mid span blocking at 6 ft, is about 6% above capacity, so for the heaviest loaded bearing walls we can either sheath them one side with OSB or mid-span blocking at 4' and 8':



A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

(OP)
When you think about it a 12 foot stud wall is freaking high, that is taller than many garages ceilings. An unbraced 2x6 stud at this height is nothing more than a spaghetti noodle along its weak axis.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

(OP)
I'm also requiring them to mid-span block all of the other non-bearing walls (at 6' height) to eliminate any other buckling and twisting problems regardless of their sheathing options.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

12' is extremely tall. That's why I would never let it go without some sort of sheathing. I don't care what the owner wants. I would be showing some form of sheathing on my final drawings, whether he installs it or not is his choice. I guess in theory cross bracing would suffice, but I'm still not comfortable with it.

Most framers that I have talked to say that if you account for the labour costs, sheathing without blocking is cheaper than blocking and cross bracing. Material cost of sheathing may be higher but takes a tenth of the time to install.

RE: Sheathing Requirements

What building code is this project under?
From your wall stud calculation I am getting a 24 psf live and dead loading for the ceiling, is this correct?

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: Sheathing Requirements

(OP)
I arrived at my bearing wall loads by applying at 15 psf dead load + 20 psf roof live load to the TJI ceiling joists. Instead of simply assigning loading to the walls by trib. area I ran the loads into Forte to check the TJI's and also grab the reactions at the supporting walls. As I suspected with a two span situation the middle supports reaction was considerably higher. Due to a little uncertainty with what the client was going to do with the walls themselves I then added a 10 psf dead load for the walls self weight, hence my highest loaded wall is:

TJI DL: 363 plf
TJI LRL: 484 plf
Wall DL: 120 plf

TL: 967 plf

Obviously there is no snow load, however I figured I would treat the ceiling similar to a roof in as much as there would be a construction load (ie. people crawling around on it). The 15 psf dead load is probably a bit conservative but then again the client will be mounting some equipment to the ceiling (distributed) so I feel pretty good about that number.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

Just to add a data point... Just the other day I was doing some calcs on a shop with 12' and 14' exterior walls. I ended up going with 2x6 with
blocking at 1/3 points even with sheathing on one side. I also wind to contend with though.

RE: Sheathing Requirements

(OP)
Which brings up a good point. You can see from the calculator output above that I am assuming some unbraced length about the y axis for the stud wall. Now if for example you have a garage wall that does not have drywall on its interior surface and you consider the worst case (zone 5 C&C) in suction or negative pressure. Let's assume that the 12' high stud wall has no mid-span blocking just the exterior siding and sheathing.

Will the ext. sheathing count as any sort of bracing at all, I'm assuming not and hence the need for mid-span blocking. I guess I kind of answered my own question there but what constitutes adequate bracing/blocking?

Just from my own experience working with wood and framing a few structures on the farm I know the sideways force caused by LTB is quite small compared with the main axial and bending forces applied to the member.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Sheathing Requirements

In the case I mentioned there was only exterior sheathing. With the negative wind pressures the compression face is not braced so I assumed unbraced
for the entire length. That didn't work so I tried blocking at half then thirds. As far as what constitutes sufficient bracing, I know the steel manual
has a section (look up stability bracing) on it where they actually quantify strength and stiffness requirements but I'm not sure that I have seen
anything like it for timber.

RE: Sheathing Requirements

Wood is a trick one for actually determining what is adequate blocking. It's not difficult to determine how often you need the blocking (i.e. 1/2 height, 1/3 heights etc.) but more difficult to know if the blocking will actually provide that lateral stability or will all the studs fail via buckling at the same time.

I say it is highly unlikely that each stud will buckle at the same time, and just as unlikely (or more so) that they'll all buckle the same direction.

RE: Sheathing Requirements

Jayrod,take a look at TB-125. It is really short but they do mention global buckling of TJIs due to wind uplift forces.

RE: Sheathing Requirements

Thanks for that rn14,

IT appears as though they say the TJIs could buckle as a unit when only provided with bottom chord bracing, and that they should provide full depth blocking at an intermittent spacing to transfer these loads to the roof diaphragm.

Wall blocking is full depth and as such I would tend to think meets the requirements of transferring the buckling load on the inside of the studs to the exterior diaphragm.

All that being said, medeek's case is unusual where he won't have any diaphragm to resolve the buckling forces into.

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