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Capacity of Sandwich Beam (AAMA TIR A8) - Capacity Depends on Load?

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KootK

Structural
Joined
Oct 16, 2001
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CA
I need a sanity check on this in the worst way. Don't worry if you don't know your way around composite split mullion design. Apparently that makes two (or more) of us.

I've got a system similar to the one shown below and my task is to work out its capacity. You know, like Kawneer wind load charts. How hard can that be? Pretty hard it seems. Two pieces of aluminum and some shear soft crud in the middle.

I've worked my way through the AAMA TIR A8 guide which is the standard for this kind of thing. And I've made a functioning spreadsheet of it which checks out with the example problems. So far so good. However, now I need to generate data. Span + spacing = wind pressure capacity. When I get into this though, it turns out that the load carrying capacity is a function of the load itself. It seems to be recursive in a way that isn't much fun to deal with for my particular problem. At first I thought that I'd made an error but, when you look at how the effective section moduli are calculated below, I think that it checks out. There's an S_eff dependency on M which doesn't cancel out. I think.

Does this make sense? Interestingly, I_eff does not seem to depend on load. It does cancel out. And no, sandwich thing mechanics is not my particular forte.

c01_tjovhr.jpg

c02_izydp0.jpg

c03_helrny.jpg

c04_auwovz.jpg


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.
 
These are thin walled sections right? I mean, cold form sections effective axial area is load dependent and iterrative.
 
Excellent point. They are very thin walled (~1/8").

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.
 
Yeah I made a spreadsheet to calculate the section properties of any size cold form channel with/without lips since most contractors don't use SSMA studs and stuff, and it was a pain in the butt. Took my many hours recomputing everything by parts.

I couldn't imagine doing a section like this with all the little stiffeners and slots.
 
Kootk

If you're doing this for a single project, I would neglect the entire exterior portion of the section, left of and including the thermal break on your drawing. The remaining closed section on the interior is all I would consider. Maybe nip off the little curved portions that the glazing stops clip into. Calculate the moment of that remaining section, do a simple moment resistance. The closed section should deform mostly elastically, and if not fully, will get lots of help from all the bits you've neglected.

I wouldn't bother getting into all the fancy flange buckling, web crippling,etc calcs in the code, it's not going to help your case very much. Storefront isn't made to span very far.

You could even check the glass itself in two way bending, and maybe it can span further than the vertical section under the tributary load you're giving it.

If you're making wind load charts for kawneer, I would follow the same approach, don't go under conservative. that way for the next 20 years we don't have architects playing engineer specifying storefront that spans 18 ft. a competent glass guy could pull the DWGs if he needs and analyze the hell out of it for a particular case if need be.

Just my 2¢ (and that's all they're worth)
 
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