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.

 

[JAE]

Is there vertical load on the studs? Or just cladding?
Is the h/t issue also at the top of stud? (I'd assume so right?)

So is your problem with h/t an issue due to shear in the stud (from your wind) trying to get into the track?
What happens to your shear in your sketch is that some load is transmitted by bearing into the leeward track leg while some other portion of load is transmitted into the windward track leg via the screws in tension.

So with that shared load condition is there really a significant web crippling issue?
I could see web crippling with a "REACTION" if all load was getting compressed into one "support" as your code reference states, but here you have a pull and a push.
Just a thought.

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[DaveAtkins]

I don't have the AISI code handy, but I would use the web crippling formula to determine a capacity. There may be something in there about the formulas not applying when h/t > 200, I don't remember. The formulas do take into account whether the stud is screwed to the track, whether it has lips, etc.

Once you get a capacity, then you have to use your engineering judgment on whether or not to ignore the h/t requirement for your given situation.

DaveAtkins

 

[KootK]

Thank you very much for your input gentlemen. I'll tackle JAE's stuff first.

Is there vertical load on the studs? Or just cladding?

Just cladding.

Is the h/t issue also at the top of stud? (I'd assume so right?)

Yesir. I haven't seen it yet but, presumably, slip track.

So is your problem with h/t an issue due to shear in the stud (from your wind) trying to get into the track?

Exactly.

So with that shared load condition is there really a significant web crippling issue?

I don't know. It assuages my anxiety somewhat to hear that you're apparently less concerned than I am. These are the things that are causing me angst:

1) I agree that there's front/back load sharing. But I don't know the distribution and I'd suspect that that the lion's share would go to the back / leeward side.

2) Whatever load goes to the back as bearing against the track leg, I see that as inducing crippling. I almost see this as worse than a conventional bearing reaction because I'd expect flex in the track to concentrate the reaction over a shorter distance than, say, bearing on a block wall or something.

3) The tabular clip above is from a CFM catalog where they are clearly considering the condition where the stud is bearing against track. To me, this suggests that the Steel Stud Manufacturer's association shares this concern.

 

[KootK]

Thanks for your input as well Dave.

I don't have the AISI code handy, but I would use the web crippling formula to determine a capacity. There may be something in there about the formulas not applying when h/t > 200, I don't remember.

There is something in AISI about the formula only applying when h/t < 200. I posted that at the bottom of the OP although I didn't make the source clear there.

Would you be comfortable extending the crippling equation past AISI limit based on judgement. That is, of course, one of my options here.

 

[XR250]

KootK,

I have rationalized the opposite screw providing some measure of web crippling resistance when I was in bind.
i know other local consultants do it as well. 27 mil studs are pretty flimsy, however.

 

[JAE]

KootK,
So if you do get web crippling, and the contact/compressive/bearing pressure into the leeward track leg diminishes as a result, you then have 100% (theoretically) of the load being resisted by the windward screw and track leg.

The web crippling, as a phase in the behavior, might possibly allow some slippage or shift in the studs.

So you may have adequate strength but somewhat crappy serviceability. In an analysis of an existing condition like this, perhaps that is the best you can do.
If the Owner/Client understands the risk, then perhaps that is a solution that is acceptable.

All this presuming the windward screw is adequate of course.

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[KootK]

Thanks XR. Specialist to the rescue.

I have rationalized the opposite screw providing some measure of web crippling resistance when I was in bind.

How do you actually do the rationalizing though? The h/t limit seems to be a black and white thing. If you're under you have full capacity; if you're over, you have none. How does one:

1) Estimate how much load goes where and;

2) Estimate the capacity on the crippling side?

If the answer is that you just wish it away without any rigorous evaluation, that's useful to me. There are some documents out there that discuss the particular nature of stud to track connections (Link). They just always seem to be research rather than straight design guidance. Moreover, it's always research based on more reasonably proportioned studs which makes it difficult for me to point to the research and claim similitude.

One interesting aspect of the testing is that these connections exhibit a great deal of ductility prior to failure. You're not likely to rupture anything. You'll just contort the snot out of it and see some movement as a result.

 

[JAE]

Also...I perused through our copy of our AISI Specification and commentary (AISI S100-2007) and there is no reference to what happens with h/t > 200 other than dammit - get stiffeners.

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[KootK]

@JAE: wow, I basically simul-posted a photo of exactly what you described. You're like the Oracle of Omaha but presumably not a billionaire.

 

[JAE]

That photo shows a complete loss of support - and significant movement as it appears to be more of a gravity member where the "windward" track leg certainly can't support the load.
In your stud/wind case perhaps the track can.

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[JAE]

Oh, I am working on the first billion - got a ways to go, though.

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[Celt83]

First thought: 27 mil.....exterior.....slap some new studs in there back to back with the original and be done with it.

Taking a closer run thru reading thru the commentary for the AISI it comes down to a limit of the testing that was done so far all the research goes up to h/t<200.
one such study:

https://pureportal.strath.ac.uk/fil...o_Flange_Loading_Conditions_Part_I_Tests_.pdf

Would it work as a cold-from angle with the leg half the depth of the stud?



Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

Would it work as a cold-from angle with the leg half the depth of the stud?

Well now there's something creative. Off of the top of your head, do you know if an established method for checking crippling on angle shapes exists? I'm ashamed that I'm asking you rather than going digging myself. Unfortunately, it's one of those days. I've barely got time to service this thread and I've just had a frozen diet Dr.Pepper explode all over my desk...

 

[TehMightyEngineer]

I've just had a frozen diet Dr.Pepper explode all over my desk...

Sound like it's time to switch to something stronger.

I'm not experienced with cold-formed steel; but I'm wondering if there's a cheap way you can strengthen it. Probably crazy and not feasible, but what about deliberately deforming the web to stiffen the stud? Kind of like these studs:

https://encrypted-tbn0.gstatic.com/...XGwkLTein6XgZmb6rvMAHQpZaYPQHxsYjQJdkOly65el3

Probably not feasible but it seems like it would be relatively easy to make some quick tool out of scrap steel and have a guy give the ends of each stud a couple of tapy-tap-taps with the tool and a hammer? No idea what the cost of this labor vs the reinforcement via a backing stud would be...

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[XR250]

How do you actually do the rationalizing though? The h/t limit seems to be a black and white thing. If you're under you have full capacity;

Good question without a good answer from me. It usually came down to checking the load tables for web crippling capacity, see how much I was over, check the pullout of the screw, add that to the allowable and pray. I have never seen any crippling failures in wall studs - including tearing into a 25 ga exterior wall next to the taxi area of an airport that was supposed to be designed for 130 mph winds. I believe the sheetrock provides some uncalculable rotational restraint of the flanges which could lower your h by 50%
It would not be hard to reinforce most of the studs - maybe that is easier than losing sleep.

 

[Celt83]

Off of the top of your head, do you know if an established method for checking crippling on angle shapes exists?

I don't off the top of my head and quick leaf thru the AISI and commentary don't produce anything of note on the subject. Failing to find a method to check the unreinforced web installing one of the SteelNetwork StiffClips/DriftClips at the base/top may be enough to satisfy the web stiffener criteria.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Ceinostuv]

You may have already tried this... but you may want to check the composite stud tables and back calculate an allowable reaction based on the loading. The composite stud tables bypass the h/t ratio (based on testing, I believe) and I don't imagine web crippling would be affected by the stud's composite/non-composite status...

Judgement-In-Training

 

[DaveAtkins]

Would you be comfortable extending the crippling equation past AISI limit based on judgement. That is, of course, one of my options here.

Depends. If it was something I designed, then found the mistake after it was built, I would "pull out all the stops" to avoid having a contractor correct the problem. And I would think long and hard about what could possibly happen over the next several decades (I like to sleep at night).

But you are not in that situation. Like you, I try not to require a contractor to put in a lot of time and expense just because "the numbers don't work." But in your case, I am leaning toward sistering new, exterior studs to the inferior drywall studs.

DaveAtkins

 

[KootK]

You may have already tried this... but you may want to check the composite stud tables and back calculate an allowable reaction based on the loading.

I haven't tried that. It's a creative solution and I'll look into.