Cold Formed Steel - gap under bearing walls?
Cold Formed Steel - gap under bearing walls?
(OP)
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?
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?






RE: Cold Formed Steel - gap under bearing walls?
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.
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.
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.
RE: Cold Formed Steel - gap under bearing walls?
Thanks!
RE: Cold Formed Steel - gap under bearing walls?
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: Cold Formed Steel - gap under bearing walls?
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.
RE: Cold Formed Steel - gap under bearing walls?
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.
RE: Cold Formed Steel - gap under bearing walls?
What do they use for lateral stability?
RE: Cold Formed Steel - gap under bearing walls?
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: Cold Formed Steel - gap under bearing walls?
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.
RE: Cold Formed Steel - gap under bearing walls?
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!
RE: Cold Formed Steel - gap under bearing walls?
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: Cold Formed Steel - gap under bearing walls?
RE: Cold Formed Steel - gap under bearing walls?
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: Cold Formed Steel - gap under bearing walls?
RE: Cold Formed Steel - gap under bearing walls?
RE: Cold Formed Steel - gap under bearing walls?
RE: Cold Formed Steel - gap under bearing walls?
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.
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.