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.

 

[medeek]

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.

 

[medeek]

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.

 

[jayrod12]

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.

 

[Brad805]

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.

 

[medeek]

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.

 

[KootK]

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.

 

[medeek]

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.

 

[medeek]

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.

 

[medeek]

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

http://www.medeek.com

 

[JAE]

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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[medeek]

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

http://www.medeek.com

 

[medeek]

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 (C[sub]D[/sub] = 1.6)

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[medeek]

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

http://www.medeek.com

 

[jayrod12]

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.

 

[woodman88]

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.

 

[medeek]

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 L[sub]R[/sub]L: 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

http://www.medeek.com

 

[rn14]

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.

 

[medeek]

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

http://www.medeek.com