Metal Stud Box Header Unbraced Length
Metal Stud Box Header Unbraced Length
(OP)
I am looking at a typical metal stud boxed header with (2) studs vertical and a track on the top and bottom. What do you consider the unbraced length for bending? Do you assume the top track fully braces the compression flange of the studs? I have always considered this unbraced unless I add a kicker but all the header span tables I have seen state this is the capacity for fully braced. How do you brace a header in the wall over a window that is below the ceiling?
Thanks,
Thanks,






RE: Metal Stud Box Header Unbraced Length
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: Metal Stud Box Header Unbraced Length
RE: Metal Stud Box Header Unbraced Length
Globally, the tracks would either need to span horizontally to a bracing mechanism at the ends (multiple full height studs or kickers), or you add kickers at some spacing across the header to transfer the horizontal loads to the framing above. These two bracing types are independent of each other.
Regarding kickers when the ceiling is higher than the header, you can install the kickers above the ceiling and cantilever studs down to brace the header. (pinned at the roof, pinned at the brace and free at the header)
RE: Metal Stud Box Header Unbraced Length
My thinking was that the track wouldn't add all that much extra lateral stiffness to the top 1/3 or so of the box beam so long as the box beam were behaving compositely about the weak axis. Certainly, the track could brace the top flange if it possessed sufficient strength and stiffness. Given the modest improvement, however, I'm surprised that this is not something that would be evaluated numerically.
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: Metal Stud Box Header Unbraced Length
RE: Metal Stud Box Header Unbraced Length
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: Metal Stud Box Header Unbraced Length
RE: Metal Stud Box Header Unbraced Length
RE: Metal Stud Box Header Unbraced Length
And yes, the top and bottom track of the box header must resist wind load (cripple studs above, window or curtainwall below).
DaveAtkins
RE: Metal Stud Box Header Unbraced Length
1) It feels as though there would still be considerable potential for section warping.
2) Would it be common practice to ensure that the track is screwed down to both plies of the built up beam?
I'm curious, do we make the same assumption (braced/composite section) for headers in wood systems?
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: Metal Stud Box Header Unbraced Length
The out of plane stability for wood is higher than cold formed sections in my mind. And I think that's more to do with your point #1. Steel studs seem to susceptible to warping and the like, wood is more robust in that sense.
In regards to point #2, my steel stud lintels all have screws into each ply noted. at a fairly safe spacing no less so I'm comfortable assuming it acts compositely.
RE: Metal Stud Box Header Unbraced Length
Just to clarify, you're saying that you do consider wood headers in stud wall systems to be LTB braced, right?
Are you speaking of screws from one header ply into the neighboring ply or from the track into the tops of the headers? I've been referring to the latter and, in my opinion, there's a significant difference.
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: Metal Stud Box Header Unbraced Length
Yes (caveat, for most cases there are always going to be odd scenarios. I guess because the header is usually close to the top plates (and generally the spans aren't ridiculous) I don't see the ability for it to rotate significantly. The exterior wall sheathing and interior drywall I can almost envision acting in a force couple to resist the LTB.
Definitely the latter, rows of screws from the track into the top flanges of the header plys and also through the sides of the track into the top and bottom of the outer ply webs.
RE: Metal Stud Box Header Unbraced Length
DaveAtkins
RE: Metal Stud Box Header Unbraced Length