Metal Stud Box Header Unbraced Length

[cfox142]

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,

 

[KootK]

Like you, I generally consider beams to be unbraced for LTB in these situations.

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]

It is common practice to consider the track as adequate to brace the flanges. However, The track needs to be checked for out-of-plane wind wind loads from the cripples above. If it can take that, it can typically brace the flanges.

 

[hawkaz]

There are two different types of bracing involved here- First, locally, the tracks will brace the compression flange of the vertical stud members in the header (like a tube).

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)

 

[KootK]

Huh. It seems as though I am in the wrong on this one. Or, at the very least, in the minority.

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.

 

[cfox142]

Upon further investigation today in the code and comparing different calculation software it looks as if when you compute Lu per AISI this "built up" section is treated like a tube shape. For (2) 800S162-68 boxed studs with track top and bottom I got an Lu of 675.5 in. So it looks like if the unbraced length is less than about 56' lateral torsional bucking isn't checked. With this I used the fully braced condition to calculate the header capacity of 8.3 k-ft. I guess that is why all the header tables assume fully braced.

 

[KootK]

Good to know. Is it the connections to the track that allow the multiple members to be considered composite for torsion? It must only work when you're flange against flange, right?

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.

 

[cfox142]

I looked at in in RAM Elements. It looks like the shape they have for boxed studs is stitch welded. I am still in the process of justifying a top and bottom track will make the members act similar to stitch welding. I also looked at in AISIWIN8 but it isn't clear how they attach the members together.

 

[wannabeSE]

Doesn't the cripple wall above prevent the header from rotating and brace the header?

 

[DaveAtkins]

I think this is one of those "it works but perhaps it cannot be proved numerically" situations. I do a fair amount of cold formed steel design, and I always assume box headers do not need to be braced, because they are tubes. You COULD try to work out the screw spacing or stitch weld spacing to create a truly composite section, using shear flow principles, but I never do--I just assume it works.

And yes, the top and bottom track of the box header must resist wind load (cripple studs above, window or curtainwall below).

DaveAtkins

 

[KootK]

When you are making the shear flow connection to ensure composite section buckling rather than resist a particular applied torsion, the demand is next to nothing. The demand analogous to the stitch welding problem in built up columns and is actually pretty tricky to estimate. Certainly, I would consider any reasonable stitch weld pattern to be sufficient. I'm not so sure about a screw connection via the track however. Two concerns:

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.

 

[jayrod12]

Wood is a different animal if you ask me.

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.

 

[KootK]

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.

Just to clarify, you're saying that you do consider wood headers in stud wall systems to be LTB braced, right?

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.

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.

 

[jayrod12]

Just to clarify, you're saying that you do consider wood headers in stud wall systems to be LTB braced, right?

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.

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.

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.

 

[DaveAtkins]

One other comment about CFS box headers--I assume they are composite for LTB, but not for the actual design. That is, I design the two side members (the C sections) for the gravity load, and the top and bottom members (the tracks) for lateral loads.

DaveAtkins

 

[jayrod12]

Agreed with Dave on that one as well.