Cold Formed Steel - gap under bearing walls?

[Backcheckrage]

Hello Engineers:

AISI recommends a 1/8" gap (max) between the bottom of a bearing stud and it's track. What is the basis of this? So what happens if this bearing wall gets loaded with like 3 levels of floor load? wouldn't the stud-to-track screws then shear off in shear failure and therefore that is the max load that stud can take?

This concept would theoretically apply to a stud group for say a large hold down device of a shearwall.

Literature points to a document CF02 something but I can't seem to find it anywhere on the web. This seems like a critical load path issue, but no one seems to discuss it?

What's the deal?

 

[KootK]

My understanding:

What is the basis of this?

Tolerance. There is a bend radius in play that makes it rather hard to get full contact without applying force. Additionally, studs are never cut perfectly square.

Couldn't the stud-to-track screws then shear off in shear failure and therefore that is the max load that stud can take?

I would hope that you'd hit a more ductile failure mode like plowing prior to outright shearing of the screw. This is something that can be controlled by the designer.

Literature points to a document CF02 something but I can't seem to find it anywhere on the web.

Probably this: Link

There's a panelized CFM supplier near me that does buildings up to 14 stories in light gauge. They have a special process whereby they squeeze the 1/8" out of the wall assemblies on the table. It's the only way to keep from having 6" of settlement at the top. Well, I guess it's not that special. Just squeezing. I'm easily impressed.





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.

 

[Backcheckrage]

KootK: Thank you for the advice and link. 14 Stories in cold formed steel!? Pictures or it didn't happen! Are you able to tell me the name of the contractor/engineer?

Thanks!

 

[KootK]

I'm hesitant to divulge the contractor/project as a) they are a client and b) I expect that many here will consider such a structure to be a pretty terrible idea. Engineer was me -- or at least my team -- in one instance where the project ultimately got scrapped. Another innovation that makes this possible is plant precision in stud placement that yields near perfect stud alignment all the way up. I've seen conceptual drawings for a 28 story building put together by an industry organization. And yes, it is all a little unnerving in some respects. CFM was not the lateral system for thee buildings.


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.

 

[XR250]

And yes, it is all a little unnerving in some respects. CFM was not the lateral system for thee buildings.

Thank gawd. The panelizing makes me feel a little better about the structure, but still - 14 stories makes me cringe.
I watched a stick framed eight story hotel go up near me. I was not impressed with the quality of the framing. It also seemed that there was so much metal on the lower stories that it could not have possibly saved them money. Seems alot harder to insulate and run utilities that it would if they just had a bunch of 20ga in-fill.

 

[KootK]

Perhaps I've neglected to mention the most important innovation. The walls are prefabbed icluding the building envelope (outboard insulation), exterior finish, and some aspects of the servicing. They fab the washrooms separately as nearly complete entities. So there's very little interference with the relatively dense/heavy stud work.

On a purely cost basis, there's really just no competing with this system for stacked residential. The worst part of the design is just that it's impossibly tedious. Imagine load take down, header and post design, and drafting on a 14 story wood frame building.

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.

 

[XR250]

The worst part of the design is just that it's impossibly tedious. Imagine load take down, header and post design, and drafting on a 14 story wood frame building.

I'll pass - thanks. I have enough problems with three stories :>

What do they use for lateral stability?

 

[KootK]

Mine was steel braced central core. Others were concrete core and even light gauge shear walls. All with cold formed floor joists. In a CFM building that tall, I struggle with having flexible diaphragms and, in my opinion, no very accurate way to assess drift for P-delta stability. I also don't love the balcony details but that's unrelated to height I suppose.

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.

 

[dhengr]

Backcheckrage:
Search Roger LaBoube, PhD, PE, Missouri Univ. of Science & Technology, Rolla, MO.
He’s a good part of the brain trust behind the cold-formed steel industry, AISI, and their codes and stds. I believe you will find that he (his group, uni., students) did some testing on your issue, to see how that gap changed the strength and settlement of stud walls. They settled on an allowable gap which was o.k. strength wise and settlement wise, and was something which could be reasonably achieved in the field. I don’t remember the exact numbers, but your 1/8" sounds about right. They did look at the failure modes, screw shear, plowing, etc., and this testing pretty much set the code stds. for this issue.

KootK:
“ balcony details... unrelated to height” you say. You’re right, when they go bad, it’s only the first few stories that really count, after that, the next 11 stories, the ride is about the same, and the stopping is a real bitch in either case.

 

[BSVBD]

I would hope that you'd hit a more ductile failure mode like plowing prior to outright shearing of the screw. This is something that can be controlled by the designer.

Koot... dhengr... others... Can you please explain "plowing"? I've never heard of this concept. I frequently deal with CFS, but, NOT 14 stories!

Also, after the explanation, if necessary and expedient, please briefly explain what control the designer could apply.

Thank you!

 

[KootK]

Plowing is the concept of the fastener pushing through the surrounding sheet steel. Much like a bearing failure with conventional steel bolts. The designer could exert control over the connection failure by selecting the faster and/or substrate thickness such that a ductile failure mode like plowing would occur prior to a shear failure in the fastener. I doubt it's a problem for most combinations.

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.

 

[sticksandtriangles]

The AISI two failure mechanisms for self drilling screws is tilting and bearing right? Is bearing different than this term plowing? Never heard of it before.

 

[KootK]

Bearing = plowing

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.

 

[sticksandtriangles]

got it haha

 

[Backcheckrage]

Ktook: Just FYI I found the background information to my query on the stud-track gap. There is a good explanation within AISI S200 Commentary to section C2.4.4 where they get into detail regarding strategies to minimize this gap, and more importantly to me - the story at this connection when the building gets loaded. It's actually a huge issue now that I chew on this. Precompression of the panels in-shop is also discussed. However in the region I am working in (Australasia) they tend not to stick frame, but use the FrameCAD panelization system... this system is more automated and difficult to incorporate "special" activities like precompressing a wall panel.

 

[XR250]

Seems like it would be easy to hit the corners of the cut studs with an angle grinder to get rid of the interference with the track bend radius.

 

[KootK]

Ktook: Just FYI I found the background information to my query on the stud-track gap.

Thanks for reporting back to deliver the update. Interesting that, for steel thicknesses in excess of 1.37 mm, screw shearing does actually happen if the gap isn't taken down to 1/16". It's more of an issue than I'd anticipated it seems.

Seems like it would be easy to hit the corners of the cut studs with an angle grinder to get rid of the interference with the track bend radius.

This raises an interesting point on the flip side of the question. For many purposes, I'd think that you'd want full contact at the flange and the corners of the stud. Certainly, that contact is reflected in our bearing calculations at the foundation. Additionally, where axial load were delivered to only the web of the studs, you'd think that one would have to account for some weak axis eccentricity in the stud design. Of course, if it is the flange that makes contact first, then you've got the same problem the other way I suppose.

I'd kind of like to just transfer the load through the screws and say to heck with the direct contact. I guess that wouldn't be a very practical outcome however. Must have faith in testing... must have faith in testing...


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.