Light Gage Steel Track

[XR250]

I have an interior metal stud wall supporting floor joists. The contractor wants the joists 24" o.c. and the studs 16" O.C. As such, my top track has to act as a beam spanning 16" The positive moment capacity of the track is approx. 4 in-k (legs in tension) while the negative moment capacity is only 1.5 in-k (legs in compression). In the past , I have always designed for the lesser negative moment capacity. Is the situation self limiting, however? If it begins to fail in negative moment, won't the positive moment capacity take over (i.e. acting as a series of simple span beams)?

 

[BSVBD]

I simply do not allow CFS joists to bear on a track without a CFS stud immediately below.

Cold-Formed Steel (CFS) tracks are NOT designed for load-bearing.

 

[XR250]

Cold-Formed Steel (CFS) tracks are NOT designed for load-bearing.

No offense, BSVBD, but why do you say that?

They can be is they are heavy enough. A 12ga track works for this situation for both +/- moments. I am just trying to lighten it up a bit.

 

[BSVBD]

XR250, i say that for simplicity. Please forgive me - On one hand, i answered you as though i was speaking with the client. I often like to establish simple "rules of thumb" to keep designs and construction consistent and simple. I understand this is not always possible. But, in general, i still believe that it was not the original intent that CFS track was intended to be a load-bearing, flexural element. Without the lip of the Cee, acting as a flange, i consider the vertical element of the horizontal track to be unstable.

I DO agree with you (XR250), if the track is heavy enough and the load is light enough, it WILL work and i accept that. As options, in the distant past, i've used hot-rolled channel and even angle in place of the CFS track. Both of these were unique situations of which i do not recall the details of why.

I typically deal with equipment and light storage mezzanines of 125 PSF. These often do not allow for horizontal track alone, without a stud immediately below. In my case, several years ago, similar to my above statement, i felt i had to establish a standard of simplicity with my typical general contractor client, to make all of our lives simpler when i have to deal with a half-dozen different expediters and thus personalities and opinions. For the past two decades, with the same GC, i've been able to keep THIS venue "simple".

 

[KootK]

If it begins to fail in negative moment, won't the positive moment capacity take over

I'm torn. Or perhaps oil canned. On the the one hand, I agree that the positive moment capacity should remain available after track leg buckling has neutered the negative moment capacity. But, then, I wonder what simple span shear capacity looks like once your track legs are flared out like a newborn calf post negative bending local buckling.

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.

 

[BSVBD]

XR250, please forgive me; in my rebuttal, i didn't address your question(s).

I haven't run any numbers, but, I believe the negative moment will occur over the top of the stud that does not have a supported joist immediately above it? Therefore, that negative, over-stressing moment, would then buckle or cripple the vertical legs, in compression, at or near that location. Since the situation is NOT a series of simple span beams, rather, a continuous scenario, the positive tensile, "reckoning" force shouldn't exist where the negative occurs.

This is "reminiscent" of the time when i had to insist that the GC had to provide a triple top plate rather than the traditional double top plate because they wanted to support 50' span roof trusses at 24" oc on studs at 16" oc. I was forced to proved, with calcs, that the double top plate ("that we've been using forever") even in a multi-span condition, no longer supports the new era of long-span design.

 

[DaveAtkins]

flared out like a newborn calf

I must have skipped class when this failure mode was explained

DaveAtkins

 

[KootK]

I tend to stick with native north american stock but, perhaps, a baby pachyderm would have been more apt. This is what comes to mind when I think of buckled track. Imagine studs providing vertical support under both the front and hid legs. Some good positive flexural capacity around the rib cage though.

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]

Thanks for the all the help. I like that analogy and it is why I have always designed for the negative moment previously. This case is pretty lightly loaded as it is sleeping lofts with 14' spans. I can get a 12ga top track to work.

 

[DaveAtkins]

Engineers who don't work with light gauge much may not understand how hefty a 12 GA track really is. It is 1/8" (almost) thick steel, 50 ksi.

DaveAtkins

 

[XR250]

I know how heavy it is as I keep samples in my office just to get a feel for things. Have had to go 10ga on some members before.
I am glad i am not the one with a screwgun :?

 

[jwilki]

Why don't you provide intermediate blocking between the studs (size equal to the lighter of stud/top plate)... fabricator will quickly see the benefit of studs at 24"

 

[XR250]

I think they are required to have the studs 16" O.C. by their contract.