Cold Formed Steel - Box C-Section

[VirL]

Hello,
I have a box section consisting of 2 c-section connected with bolts.

I have to designing according to AISI code. Should i do every section individually or consider it as a box section? Is there any examples to look up for reference (any code is welcome)?
Thank you in advance

 

[EZBuilding]

How is anyone going to install those bolts!?!?

The more common box section utilized is a stud nested inside of a track.

 

[phamENG]

How are the bolts installed?

Leaving that aside, let's assume horizontal (as drawn) is the x axis and the vertical (between the two sections) is the y. Loads in the y axis causing bending about the x will be resisted equally by the two (as long as they're both loaded together - if it's side loaded it's a different story), so it won't matter. For loads in the x causing bending in the y, you'll need to do a shear flow calculation. If the "bolts" can handle the shear then it can be designed as a single section. If not, they're two separate sections.

(I would suggest either shop welding the sections together or putting a track top and bottom and using self drilling TEK screws to fasten them to the stud sections.)

 

[phamENG]

You beat me to it, EZ.

 

[VirL]

Hello, yes! my bad.. it's a welded section. Can i designed it with the box formulas?

 

[phamENG]

I'd say no - design it as a built up section. Light gauge steel has some peculiar behaviors due to it's especially slender shapes. They're not always intuitive if you haven't studied it in detail. I try to avoid making assumptions with light gauge unless I've thoroughly researched it and understand what I'm giving up/risking by making the simplification.

 

[phamENG]

It would probably be worth doing the analysis for both. That way you can compare the two and get a better feel for it. If they are roughly equal, feel free to come back and call me a dumby. And then we'll all know that it's at least a semi-valid assumption.

 

[VirL]

Thank you all!

 

[r13]

In general, I would think it is a box if the welds are continuous, otherwise you have a composite section, and should be designed as thus.

 

[skeletron]

The critical element for CFS is almost always some sort of local buckling or crippling. Nest or back-to-back the sections to give you effectively thicker local elements. As the sketch shows, I think your section is going to be rather floppy in at least one direction as a narrow tube.

 

[WARose]

I'm not wild about this arrangement because I'm not sure I've seen this before. The more common set up is to have the webs back to back. (And bolt through.) There are a number of design examples with it done that way. (See 'Cold-Formed Steel Design', 3rd ed., by Yu.)

At least about one axis.....you'd have the same moment of inertia anyway. (Not sure what you need however.)

 

[phamENG]

WARose - this is the most common arrangement where I am, though it's usually nested inside a pair of tracks to facilitate screwing it together in the field. It's commonly used for headers. Be careful, though - insulation has to be installed while it's being built up, but often times the insulation hasn't arrived on the job site yet. It requires special coordination by the GC.

 

[geoffdale]

We do this all the time. Straps on the outside flanges screwed together.

 

[Agent666]

If you're welding the sections together then I believe the correct approach for the torsion and warping constants is to simply add them.

If you're connecting them in any other way then you just analyse it as two separate members that act as two separate members.

I went through and semi proved this logic a while back for another post of back to back channels when someone on EngTips was querying how to work out the section properties.

Whatever you do don't use random formulas and hope for the best, at best they are a good fit, at worst completely wrong.