Composite Odd-shaped Built-up Column Strength

[jacktbg]

Hey guys, I'm new to this forum, so bear with me. Any forum advice would also be appreciated. I'm trying to find a way to properly gauge the strength of a unique built up column to be placed inside of a 2x4 sized wall. The idea is to build up a column that essentially consists of 2 2x6's (or 2x4's) glued face to face, potentially with some filler in between them to space them out, and on the edge side of that structure build up the column with the face sides of a pair of 2x4's. The final shape and dimensions would be a rectangle, potentially with filler in the middle, that is 5.5" long and 3.5" wide. How would one go about building something like that and figuring out an axial capacity, while following the guidelines set by NDS 2015? If anyone has any thoughts on the best way to tackle this problem, I would appreciate it!

This column would be loaded along the entire top area of the column. Therefore, I assume, if you assume the beam above sits flush with the column, each 2x6 would take up roughly 30% of the total load, and each 2x4 would take up about 20% of the load. I assume you'd look at each member in the built-up section as if it were a standalone column, continuously braced along the sides that are "braced" by the sides they are connected to?

Would you essentially calculate the axial capacity of a 2x6 fully braced along its strong axis and braced or partially braced between the other 2x6 and the wall, according to the NDS, then do a similar calculation for each 2x4 and see what fails first, with the percentage of the load that would be applied to each face applied to each member?

There has to be a way to somehow combine the added strength of the 2x4's with the 2x6's and come up with a reliable axial capacity (including windload.)

Depending on the wall, could you consider a built up column inside a wall as being partially or fully braced along the face that sits against the wall? I think I've read somewhere that you cannot assume an interior wall provides bracing for a column inside the wall.

Sorry if my thoughts are a little jumbled, it seems like there's a million ways to look at this problem and I'm not exactly sure where to start. If I can clarify my problem further please let me know. And thank you so much in advance for any advice you can give me!

 

[jacktbg]

Shoot, I keep missing updates to this thread before I finish my post. Sorry, hadn't read the previous responses, I'll take those into account.

 

[jacktbg]

Oops, hadnn't considered the fill in the center. That's a really good point.

 

[jacktbg]

Yeah, I have to rethink a LOT of things, some horrible assumptions have been made. Thanks for helping point some of them out. I'm going to look into this further and get back to this a little later. Thanks for the advice and constructive criticism, it's been extremely helpful!

 

[KootK]

No sweat. We've all quested for the structural equivalent of the perpetual motion machine at some point in time. I once spent three months thinking that I could fill a piston topped round HSS with baby oil and somehow create a buckle-free column. And nobody in the 30 person structural firm that I worked for was able to refute the theory (disturbing in retrospect). I had to eventually read about my own madness in Zdenek Bazant's stability text.

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.

 

[jacktbg]

Haha okay, I'm convinced that maybe a stud pack would be superior to my composite column, under realistic conditions. If any of you, or someone reading this thread, however, thinks of a way I could reliably check the axial capacity of my built-up member, I'd still love to hear your thoughts on how it could be done. It would at least be nice to be able to look at some hard mathematics, because I'm still just curious how strong you can get a column built that way to be.

 

[KootK]

If any of you, or someone reading this thread, however, thinks of a way I could reliably check the axial capacity of my built-up member, I'd still love to hear your thoughts on how it could be done.

You did notice that I provided such a method in my original response?

1) Check weak axis composite buckling.

2) Calc nail spacing for a shear flow value associated with a 0.04 X P lateral load applied at the column mid-height.

3) Use a nail spacing of the minimum of (#2, NDS prescriptive nailing for laminated studs, 12" o/c)

4) Swap out the nails for screws and some glue.

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.

 

[jacktbg]

Could you elaborate on (2)? I know how to check weak axis buckling, it's pretty clear in NDS. I've been running a lot of calculations on a basic 2 ply 2x6 assuming that it can be braced on its strong axis (ply axis) completely, and assuming that the 2x4's can provide some degree of bracing equivalent to weak axis bracing (say 1/2?). Not sure what can be safely assumed about the bracing the 2x4's would provide. That all, I'm sure, depends on the nailing scheme. But how do I do a calculation for nail spacing for a shear flow value associated with a .04 x P lateral load? I'm assuming, for now, it won't be exposed to wind. What's the basis for this check, and how do I find more info on how to perform it?

Thanks!

 

[jacktbg]

Oh, and it would be screwed, not nailed.*

 

[KootK]

I know how to check weak axis buckling, it's pretty clear in NDS.

And that's weak axis buckling for the entire, composite section (4 pc) rather than the individual pieces right? So you'd need to calculate a composite moment inertial etc? This step alone will get you to the point where the stud pack option becomes the clear winner.

Not sure what can be safely assumed about the bracing the 2x4's would provide. That all, I'm sure, depends on the nailing scheme

This is the part that the 0.04P lateral load is checking. Essentially, you're saying that the 2x4's brace the 2x6's at mid-span. We commonly assume that a minimum bracing force of 2%P can brace a column. I'm recommending 4% because of nail slip and all the other unknowns here. Basically, you just look at the composite column as a beam with a weak axis point load of 0.04P applied to it at mid-span. Then do the usual VQ/It calc to get the shear flow and required nail spacing.

What's the basis for this check, and how do I find more info on how to perform it?

The basis for the check is as I've described above. I doubt that you'd be able to find any additional information on it other than just general, mechanics of materials stuff. What you're trying to do here pretty atypical. And my recommendation comes from fundamentals rather than prescriptive design requirements.

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.

 

[jacktbg]

Well, no. I'm not talking about the 4 pc composite weak axis buckling... I'm talking about the buckling along the weak axis for a simple (2)2x6 column, because I know the composite column will get at least that. But that depends completely on what I find in (2). If I find that the nailed 2x4's can't be counted on as providing the 2x6's with some degree of weak axis bracing, then it's almost pathetic what the structure gets. But if it can be assumed to be equivalent to 1/2 point weak axis bracing, then the axial capacity, including buckling about the weak axis, goes up from roughly 4000 to roughly 12000... for my conditions. If it can be assumed to be even more braced than 1/2, which doesn't sound that far out at first glance, then it goes up even higher. So essentially I am just, for now, considering using a (2)2x6 column, and trying to find out whether it can be "braced" somewhat by nailing/screwing 2x4's to the sides. So my first step is to explore (2), if I'm not mistaken.

 

[KootK]

If you're looking at it as 2-2x6 laminated and buckling about the weak axis of the composite, 3.3 x 5.5 section that creates, then I think that you can just skip #2 and use the code mandated lamination between plies. In that context, I think that the 2x4 on the sides should be considered to do very little bracing in that context. Rationally, it makes little sense to "brace" something (laminated 2x6) with something else (2x4) that is actually less stiff in the direction of buckling than the thing being braced.

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.

 

[jacktbg]

Would the 2x4's not be more stiff in their strong axis direction than the (2)2x8 would be in its weak direction? The strong axis capacity of 2 2x4's I believe is greater than the weak axis capacity of 2 2x6's

 

[jacktbg]

Oh. I am incorrect. A 1 ply 2x4, for example, gets 2641 lb axially if the strong axis is controlling (if it's braced along its weak axis.) And I don't think you can just multily the value by 2 for 2 boards. A 2ply 2x6 failing about its weak axis gets 4400 lb.

 

[jacktbg]

How does a 5 pack of 2x4's compare? If they're in a 2x4 wall, you can only support them about their weak axis. A 5-pack of 2x4's will therefore get a maximum of 9200lbs... That's not very impressive either. They both sound like terrible solutions. PSL it is, I guess, for anything over 10,000lb.

 

[KootK]

That sounds about right. The benefit of PSL doesn't come from its shape so much as its material properties. Otherwise, it probably wouldn't exist as you could always use a stud pack as a work around. That whole no free lunch thing.


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.