Lateral Buckling Restraint - Steel Members

[Gousey]

Hi,

I am currently designing a partition wall system with cold formed steel PFCs that contain horizontally stacked AAC (Autoclaved Aerated Concrete) panels.

My question is in regards to whether my fixing arrangement allows for the PFC flanges to be laterally restrained so that I can take the full section capacity (for flexure) of the PFCs as opposed to the member capacity, which considers the buckling effect of the member.

The panels fit snugly into the PFCs and my proposal is to fix screws through the web of the PFC that fixes the AAC panels to the PFCs.
My theoretical understanding and experience tells me that this connection (Connection A) is sufficient to restrain the flanges - which will allow me to design using the full section capacity.
I cannot see how these PFC members can buckle using connection A - however I just need to confirm this for my detail.

I am hoping to not use connection B as this involves double the amount of the screw fixtures.

Would I be able to use connection A and design the PFC to contain the full section capacity??

Thanks.

 

[KootK]

Connection A definitely restrains lateral torsional buckling. I stil see potential for local bucking of the compression flange about the fastener line however. Unfortunately, I don't know my light gauge well enough to recommend a simple way to evaluate that.

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.

 

[BAretired]

Connection B is much more positive than Connection A and is the preferred connection.

BA

 

[BAretired]

As a check on Connection A, determine the required spacing 'S' of fasteners for Connection B, then determine whether the web with fasteners spaced at S is adequate in bending to act as a lateral brace for the flange using the code criteria for lateral bracing of a compression member.

If Connection A requires a closer spacing of fasteners than Connection B, there may not be much saving in using A.

BA

 

[BAretired]

While I find the alternate method much more appealing to my intuition, I'd be inclined to go with the newer method. I can't see it taking more than 30 min to sort out. Especially if you're just going to be coding it into a spreadsheet anyhow. I doubt that either method is particularly accurate.

I don't understand, KootK; Could you expand on that?

BA

 

[KootK]

I don't understand, KootK; Could you expand on that?

Doh. That response was intended for an altogether different thread. My bad. Egg nog on the brain.

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]

Doh. That response was intended for an altogether different thread. My bad. Egg nog on the brain.

Did you make that mistake in two separate threads? Must've been a good night's worth of egg nog

 

[KootK]

Nice jayrod... couldn't let me escape with a modicum of dignity intact? You like to hide keys from Alzheimer's sufferers too? This incident will be reflected in the quality of yout KootK Christmas card.

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]

couldn't let me escape with a modicum of dignity intact?

You've left me feeling inadequate more than once, can't let you off that easy.

 

[Tomfh]

If it's harder for the flange to buckle laterally (e.g. by folding along the screw line, or pulling the screws out) than to not buckle then it won't buckle.

So I agree with BAretired assessment, check if the fasteners and web stiffness qualify as a compression flange brace. AAC is pretty weak remember. You can crush in between your fingers. The screw pullout wont be great.

 

[KootK]

A good solution might be to stagger the fasteners within the web such that every second fastener ends up close to whichever flange is in compression. My gut feel is that, with an appropriate fastener spacing, this would get you to a fully braced condition where your capacity would be governed either by the cross sectional strength or the usual local buckling checks.

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.

 

[BAretired]

Yes, staggering the fasteners as suggested by KootK might be a good idea but I expect the number of fasteners would end up approximately the same as in Connection B.

The fastening of Connection B has an added advantage; it tends to prevent local buckling of the flange by pinning it to the AAC at the midpoint of the flange.

I'm not familiar with AAC and if it can be crushed easily by finger pressure as mentioned by Tomfh, both pullout and shear value of the screw may be questionable. This should be assessed before relying on screws for resistance.

BA

 

[Gousey]

Thanks for the responses everyone. Great idea to stagger the screws. I have since calculated the lateral buckling of the compression member considering every second screw will restrain the compression flange. These screws will be fixed at a skewed angle to ensure the screw is properly fixed into the AAC without risk of spalling the face the screw is close to.
Also for your information the AAC cannot be crushed between your fingers - I have tried....... We also have testing data for the pull-out screw capacity over a number of specimens with values ranging from 0.7kN to 1.4kN.

 

[BAretired]

Upon further consideration, I don't see how staggering the screws is helpful. Why not simply use two screws through the web, one near each flange, spaced at S. Whether or not they are staggered, the number of screws is the same as required in Connection B. My vote goes to Connection B.

BA

 

[Gousey]

At this stage I have put through two options for the builder to consider. One is with staggered screws along the web, and the other is screws through the flanges. As the builder wants a clean finish along the wall face - he will be more inclined to used screws through the web as opposed to at the flanges.

In regards to the stagger, I agree that it is effectively the same thing using 2 screws at the same level and keeping the spacing the same. However there is an element of nice aesthetics of the staggered screws I feel

 

[BAretired]

I won't quarrel with the aesthetics of staggering the screws, but it was stated that they would be installed at a skewed angle to lower the risk of spalling the near face of AAC. The exposed screw heads would have a serious aesthetic effect, it seems to me. Perhaps it would be preferable to install them normal to the web a little further from the face of AAC.

BA

 

[Tomfh]

The exposed screw heads would have a serious aesthetic effect, it seems to me.

Yes it would look bad.

I agree just put in enough normal screws. 1kN per screw will stop a flange buckling.