CFM Stud Web Crippling Slenderness Limit - Any Recourse? 1

[KootK]

Short Story

I've got some existing studs that require web stiffening in order to slide past the code requirement for h/t < 200. Is there any way around that requirement? Obviously, such studs have some capacity. Is there any defensible way to put a number to that capacity? If I looked at a K=1.0, 1" wide strip of web as a 6" long column might that represent a lower bound on crippling capacity? Any other ideas?

Long Story

- Existing stud work being evaluated as part of an envelope upgrade.
- A previous consultant flagged the studs as being inadequate and requiring replacement.
- It seems that an interior grade of stud was used in an exterior application. Oops.
- Surprisingly, I can make the bending and deflection numbers work for the existing stud.
- I cannot make web crippling work for the existing stud because of the h/t < 200 limitation.
- The existing studs could be reinforced locally but that's a lot of work taken over an entire mid-rise tower.
- Studs with h/t > 200 obviously have some web crippling capacity. If I could determine it defensibly, I could likely eliminate a lot of unnecessary retrofit.
- I'm hesitant to get into FEM or DSM given the effort required but would consider it if I could identify a clear path forward for investigation.
- Some things I've read suggest that the condition with track and screws improves matters. I don't know how to quantify that though.

 

[KootK]

Some updates:

1) It turns out that my h/t for these studs is 204. That, compared to the 200 limit makes me completely willing to let it ride and calculate crippling capacity in the usual way.

2) It turns out that the AISI doc on wall studs actually has a recommendation for track to stud capacity as shown below. Since it specifically mentions the stud being connected to both legs of the track, that's clearly being accounted for in some fashion and I feel as though I'd be remiss to claim any capacity above and beyond that. Luckily, the resulting capacity isn't too bad. Although the numbers appear to be higher than the SSMA numbers so I'll need to reconcile that somehow.

 

[XR250]

KootK,

i am pretty sure the free AISIWIN has this function built in..

CFS Design Software - SCAFCO Steel Stud Company

www.scafco.com

 

[KootK]

Nice. Thanks XR, I'll check those out. The trouble that I've had will all software on this so far is that they won't let you choose S600125-27 precisely because it's a stupid choice for exterior curtain wall. It's a pretty annoying catch 22 for a poor sap trying to make a go of it with that designation.

 

[XR250]

Yup, you are correct. 33 mil is the best it is gonna do for you.

 

[KootK]

For reasons that I'm not at liberty to explain, my client may prefer that the studs NOT work. Doh.

 

[Ceinostuv]

2) It turns out that the AISI doc on wall studs actually has a recommendation for track to stud capacity as shown below. Since it specifically mentions the stud being connected to both legs of the track, that's clearly being accounted for in some fashion and I feel as though I'd be remiss to claim any capacity above and beyond that. Luckily, the resulting capacity isn't too bad. Although the numbers appear to be higher than the SSMA numbers so I'll need to reconcile that somehow.

In case you were still looking into this/curious - I believe the SSMA tables are based on the same equation you show, but they use AISI S100 factors which are slightly different than the S211 factors (the wall stud doc). Just an FYI if you ever run into this again.

Judgement-In-Training

 

[KootK]

In case you were still looking into this/curious

Thanks for that. I have to present my findings to a strata council soon and it would be nice for me to be able to speak to the discrepancy.

but they use AISI S100 factors which are slightly different than the S211 factors (the wall stud doc).

Does that make sense to you as thing for SSMA to do? It seems to me that the wall stud doc factors would be the most applicable to...wall studs? I would have thought SSMA would be trying to squeeze every last credible drop out of the systems. Perhaps a better way question to ask is this: can you think of any reason why using the AISI factors would be more appropriate than using the S211 factors?

 

[Ceinostuv]

Does that make sense to you as thing for SSMA to do? It seems to me that the wall stud doc factors would be the most applicable to...wall studs? I would have thought SSMA would be trying to squeeze every last credible drop out of the systems. Perhaps a better way question to ask is this: can you think of any reason why using the AISI factors would be more appropriate than using the S211 factors?

Looks like you may already know the answer to your question.

Since it specifically mentions the stud being connected to both legs of the track, that's clearly being accounted for in some fashion and I feel as though I'd be remiss to claim any capacity above and beyond that.

The S100 equations are formulated to reflect pure web crippling while the S211 equations are supposed to be more consistent with the behavior of an actual stud-to-track connection; essentially accounting for the contribution from the screw connections and the track. The S211 commentary talks about this in more detail in case you need a reference.

The SSMA tables are just more universally applicable and conservative.

Hope that helps!

Judgement-In-Training

 

[Coty]

Hey KootK, AISI S211 Equation B2.2-1 includes a footnote stating the equation is only applicable for studs 0.0346 - 0.077 inches. If your stud is only 27 mil, then I would be want to check if it could still be applicable. Since you said the problem likely occurs at the top of the stud as well, then the design thickness for the studs is even greater for slip tracks. Did you look into this?