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Load Bearing CFMF Eccentricity Question

Load Bearing CFMF Eccentricity Question

Load Bearing CFMF Eccentricity Question

(OP)
I've got a condition where I have vertical load being applied eccentrically to cold form metal studs below.

As you can see, the one of the flanges is not fully engaged from the element applying the load (left generic on purpose but lets call it a 6" wide concrete beam for fun). When I designed the load bearing studs below, I simply applied a point moment to the studs below which = the vertical load times the 1 1/2" of eccentricity and came up with a reasonable design.

I have a cold form designer telling me that in cases like this, where one flange is not directly engaged by the element delivering the load (the 6" wide concrete beam), they take a a 50% reduction on allowable strength. They mention that this is not a code provision but something that they do. Is this justified? The flange to the side of the offset concrete beam will go into compression and the other flange will go into tension inherently even though it is not directly under the concrete beam.

Is there something I am missing that is so concerning about this that it warrants a 50% reduction in design strength? Let me know if I am crazy for thinking this should be easier than what it appears.

Thanks
S&T

RE: Load Bearing CFMF Eccentricity Question

At the very top of the stud, the load is resisted by one flange, one lip, and a portion of the web. By the time the load reached mid-height of the stud, the entire area of the stud will resist the load, but you should check the stud for the axial load PLUS the moment caused by the axial load.

In my experience, the moment caused by the eccentricity of the load will be small compared to the moment in the stud caused by wind load.

DaveAtkins

RE: Load Bearing CFMF Eccentricity Question

I'm with you on this one SNT. Both legs need to travel vertically together in unison -- more or less -- in order to use the full capacity of the section. To make that happen here, the flexural stiffness and strength of the track web should be such that it can drag the outer flange along for the ride. That's easily verifiable and I doubt that it would be a problem at the proportions you've proposed.

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: Load Bearing CFMF Eccentricity Question

(OP)
Thanks Dave,

I did check the stud for the axial and the moment caused by the axial load.
Do you see any reason for the 50% in design strength mentioned by the CFS designer? Maybe to account for the load only being resisted by one flange and a portion of the web? Still feels like a hefty hit on allowable strength.

RE: Load Bearing CFMF Eccentricity Question

Crap, thought we were worried about the track. Agree with Dave, it's just an eccentrically loaded stud design. At worst, I guess just check the directly loaded elements of the stud to make sure that they can take the load delivered without yielding or buckling locally near the point of load application.

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: Load Bearing CFMF Eccentricity Question

(OP)
Thanks for your thoughts KootK.

I do not even see the need of the track to drag the studs other flange along for the ride (although I can see it helping). Like I said, I feel that inherently the outside flange will automatically "move up" due to the applied point moment.

RE: Load Bearing CFMF Eccentricity Question

For fun, would you be able to reduce the thickness of the element applying the load 1 1/2" so the eccentricity is only 3/4"?

I also agree that their 50% reduction seems a little steep. It may require a heavier gauge track to ensure the legs travel vertically in unison, but it's not impossible.

RE: Load Bearing CFMF Eccentricity Question

(OP)
The element applying the load has it's dimensions fixed unfortunately jayrod.

Quote (jayrod12)

It may require a heavier gauge track to ensure the legs travel vertically in unison, but it's not impossible.
Again, as in my above post, I feel that track does not even need to help in this behavior as it is inherent to the applied loading.

RE: Load Bearing CFMF Eccentricity Question

Is it possible they're taking a 50% reduction in axial load capacity, and then ignoring the eccentricity in the calculations?

RE: Load Bearing CFMF Eccentricity Question

(OP)
I just talked with an engineer at the cold form company. They seem to be evaluating their stance based on the conversation I had with them.

RE: Load Bearing CFMF Eccentricity Question

Quote (jayrod)

Is it possible they're taking a 50% reduction in axial load capacity, and then ignoring the eccentricity in the calculations?

My spidy senses were tingling about the exact same thing. In part, it's hard to imagine a CFM industry acolyte taking such a seemingly conservative approach to a product within their wheelhouse.

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: Load Bearing CFMF Eccentricity Question

I just take the point moment as you said and run with it. Never heard about the 50% rule. How much load are you talkin about?

RE: Load Bearing CFMF Eccentricity Question

Quote (Koot)

My spidy senses were tingling about the exact same thing.
It makes my day better when one of the heavy hitters here agrees with me. Even if it's just in part and not in whole.

RE: Load Bearing CFMF Eccentricity Question

(OP)
Figured I would close the loop on this conversation.

I reviewed the calculations sent over by CFS designer and was able to gain insight to their "50% reduction" in strength.

Essentially this eccentric condition makes equation C5.2.1-1 (AISI Spec 2012) the controlling equation for combined bending and axial load. This equation makes you consider moment magnification and makes you use a reduced nominal strength in your stud design. In a tradiation concentrically loaded stud this equation does not govern. After running this calculation with equation C5.2.1-1, I had about a 15% reduced strength in my stud design. I had not previously utilized this equation, but still a 15% reduction is not nearly as bad as the 50% they told me about.

Thanks,
S&T

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