Wall strapping instead of sheathing

[Woody1515]

Hello again everyone,

I have a contractor who is building a large shed, using typical frost walls for the foundation, slab on grade for the floor and wood studded walls (14’ tall) and trusses. They are trying to build this thing as cheap as possible...

For the walls, they are pushing for not using any sheathing. Instead, they are arguing that 2X4 strapping @ 24” O/C that they will use to attach the tin siding to will act as the lateral support system. They said they have “done this multiple times before” and have had no problems, but I have some doubts. The walls would simply be studs, 2X4 strapping on the exterior and then the tin siding. No interior drywall or other finishes. In your opinion, will this strapping be able to adequately replace the sheathing? Is the strapping able to prevent the stud from buckling about the weak axis, the same way sheathing would?

They said they could add some 2X4 diagonal bracing on the inside of the walls if required. I can see that helping the lateral stability of the entire building (although minimally), but I don’t think that solves the problem of ensuring the studs are adequately braced against buckling.

Essentially, I am looking for any potential alternatives to plywood or osb sheathing that will a) prevent weak axis lateral buckling if studs and b) lateral stability of entire building.

Thanks in advance!

 

[Canuck65]

This is done all the time for farm structures, sheds, etc... No exterior sheathing. Siding attached to strapping. There still needs to be some bracing for lateral stability and it is common to see it on the inside of the stud walls. Trusses also need bracing. There are some Canadian farm building designs floating around that you may be able to find as examples.

 

[Woody1515]

Thanks Canuck65. Would you say the exterior strapping is adequate for prevent stud buckling about the weak axis?

 

[GC_Hopi]

Jumping in cold but this sounds similar to a post frame building. Photos below show what I envision.

 

[gte447f]

The horizontal 2x4 members you are referring to are usually called wall purlins or girts. They can serve the purpose of bracing the wall studs by reducing the weak axis buckling length. However, in order to serve this purpose, the purlins need to be anchored to some sort of bracing element with strength and stiffness in the plane of the wall. Each stud cannot simply be braced by the stud adjacent to it, because all of the studs can theoretically buckle simultaneously in the same direction. That is why the purlins need to be anchored to something like a diagonal brace at intervals along the wall. In cold formed steel stud construction, flat strap diagonal x-braces are often used for this purpose. For more information about this topic, look up bracing of axially loaded cold formed steel studs and anchorage of brace forces for cold formed steel studs. The principles are equally applicable to wood studs. With wood studs, it is generally acceptable to brace only one face of the stud.

As Canuck65 pointed out, you also need lateral bracing for the global stability of the entire building to resist lateral loads from wind or earthquake. This lateral bracing could be diagonals installed on the inside face of the walls as you have suggested, as long as this method provides the required amount of lateral resistance (i.e. check the capacity of the diagonal braces and connections against the lateral load demand).

 

[Woody1515]

Thanks gte447f. That is what I was thinking, the studs could theoretically buckle at the same time in the same direction. The same way people say adding additional blocking between studs in a tall wall helps, when it really would not.

With that being said, how is attaching the studs to normal sheathing any different? The sheathing is just attached to the studs, it is not attached to a separate lateral resistant system. The sheathing must have additional lateral load resisting capabilities?

 

[gte447f]

Woody1515, sheathing panels act as a diaphragm, which has in-plane stiffness and can resist in-plane bending, whereas blocking between studs or purlins fastened to studs just act as a link between studs that can resist axial load only.

 

[dauwerda]

I assume the "tin" siding is actually 29 (or 26) gauge steel siding. As GC_Hopi mentioned above, this sounds similar to how post frame buildings are constructed. The NFBA has done testing on this type of diaphragm/shear wall and has published capacities for them that can be found in their Post-Frame Building Design Manual, available here:

https://associationdatabase.com/aws/NFBA/pt/sd/product/14881/_PARENT/layout_details/false

Rather than depending on the diaphragm action of the steel siding, you could design discrete bracing as noted, however (at least for post frames) it is often very difficult to get the connections to work out.

As for the buckling issue, the steel siding would certainly provide the shear capacity needed to brace the weak axis.

 

[Woody1515]

Would it be any different if the 2X4 strapping was placed so that there was no spacing? Obviously that isn’t practical, but I’m not sure if that would mimic the sheathing.

 

[dauwerda]

there was actually a recent post asking about something similar in the structures forum,

https://www.eng-tips.com/viewthread.cfm?qid=475916

The NDS does provide design capacities for "lumber shear walls" (table 4.3D) (no where near what you get with plywood sheathing but slightly better than you get with the values given for metal clad walls per the NFBA mentioned above (depending on details usually around 100 plf).

 

[Woody1515]

Thanks for the input everyone. I’m still have troubles getting over how the sheathing resists the in plane bending while the strapping cannot. The sheathing is only attached to the studs, similar to how the strapping would be? Is there another way of thinking of this? I must be missing something.

Even if the strapping was also attached to a diagonal brace at intervals along the wall, the brace is only attached to the studs as well. I’m struggling to grasp how that would work.

 

[SwinnyGG]

I’m still have troubles getting over how the sheathing resists the in plane bending while the strapping cannot

In-plane bending of the wall elements tries to move the header laterally relative to the footer. Sheathing is continuous from header to footer, and resists the lateral movement of the header through its own internal shear strain. If you try to lozenge the panel, it is going to fight back. This works regardless of the connections between sheathing and stud because the sheathing is continuous, and can take up loading in shear. In other words, you don't have to make every sheathing/stud connection a moment connection in order for the sheathing to provide lateral resistance.

The strapping is not continuous from header to footer. If the header tries to move east, the strapping adjacent to the header can't provide any meaningful resistance because it has no ability to absorb anything other than axial load. The only way the strapping could provide any meaningful lateral resistance is if every strap/stud connection was a full moment connection; this would allow bending force in a stud to be resisted by bending force in the strapping.

 

[Dvdt18]

IRS Section R602.10.4 may Assist with your questions

https://codes.iccsafe.org/content/IRC2015/chapter-6-wall-construction

Personally, I wouldn't count on the strapping adequately transferring the lateral loads of the structure as a whole to the foundation. Yes the strapping will brace the studs in their weak direction, but I wouldn't consider them to be effective at transferring the lateral loads without cumbersome detailing and trust that your contractor will carry out it out properly. Perhaps Let in bracing (see IRS Section R602.10.4) may help with the lateral forces at the end walls allowing the strapping to be used.

 

[Mike Mike]

Weak axis buckling of your 2x6 studs: Horizontal 2x4s could be sufficient to prevent weak axis buckling so long as they are connected to an element that spans between foundation and roof diaphragm. As gte noted, x-braces could do the trick. Another idea I had off the top of my head is a 6x6 post each corner of the shed.

Wall panel ability to act as a shear wall: dauwerda's idea sounds cheapest and easiest, but I wasn't able to view the NFBA manual because there was a fee. dauwerda can you please give us some idea of what their published capacities are? Another idea I had off the top of my head is to carry out a finite element analysis and figure out how much in-plane shear your wall can take with your horizontal 2x4s at a given spacing. I was thinking just disregard any help from the tin siding, but that's only because I don't know anything about tin siding. My thought is if the horizontal 2x4 has at least 2 nails in the narrow face of the 2x6 studs, there is a small moment capacity associated, and the system could resist in-plane loads through frame action. Admittedly this idea is probably more time consuming than desired unless you're designing a thousand sheds.