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Composite Odd-shaped Built-up Column Strength

Composite Odd-shaped Built-up Column Strength

Composite Odd-shaped Built-up Column Strength

(OP)
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!

RE: Composite Odd-shaped Built-up Column Strength

unless I'm reading wrong, you're making this much more complicated than it needs to be.

why dont you spec a 4 ply 2x4 stud pack and call it a day?

RE: Composite Odd-shaped Built-up Column Strength

(OP)
That's an awesome resource, thanks! But it's not a spaced column that I'm describing. I guess just imagine a 2 ply 2x6 column with 2x4's attached on the ply edges of it, with all 4 boards running vertically. I want to see if this custom column could be safely used in place of a PSL column, given proper bracing. My hunch says that it can, but I need to back it up with some Engineering. My plan now is to consider a 2 ply 2x6 with 1/3 point bracing about its weak axis, and also consider the 2x4 axial capacity with some degree of bracing and find out what it takes to get this column to outperform a similar PSL. I'll just use the NDS 2015 stuff. I might be overthinking all of this, but if I could get this to work it would be awesome because it would save having to order a custom PSL and allow an option to use readily available materials (extra sheathing, basic sawn lumber).

The issue with using a 4 ply 2x4 stud pack is that it won't perform the way a PSL would (I think) And a spaced column isn't exactly what I'm looking at; yes there is a small space in the center as the (2)2x6 would only be 3" wide and the 2x4's on either side would be 3.5", so there needs to be some sort of spacing or filler to get this to work, but the issue isn't so much the spacing as the added 2x4's. Hope that helps clarify my problem. Thanks for the speedy responses guys!

RE: Composite Odd-shaped Built-up Column Strength

for typical 8' to 10' posts in a platformed framed wall, the governing mode of failure is usually in top plate or sill plate crushing.

if not 4 pack of 2x4s, why not a 5 or six pack? as long as they are well nailed together, I never consider weak axis bending, only strong axis. if you start forming a weird column shape like you are describing, weak axis bending will become an issue I would imagine.

apply the KISS principle

RE: Composite Odd-shaped Built-up Column Strength

(OP)
Okay! I've attached a (poorly) drawn image of the shape of the column I have in mind.

RE: Composite Odd-shaped Built-up Column Strength

I agree with the multiple 2x4s stud pack rather than the proposed column. If you struggle with capacity, width of (2)LVLs make up the depth of a 2x4, and could get a lot of capacity. Bearing at the plates will likely 'control' although that is more a serviceability thing than a strength thing.

RE: Composite Odd-shaped Built-up Column Strength

Agree with the KISS suggestion. I'd be doing stud pack. Usually your sheathing braces the post in the plane of the wall so composite action there is of little benefit. For buckling out of the plane of the wall, your arrangement is actually worse than a stud pack because you've disrupted the shear flow capacity between "flanges" with the gap.

Quote (jackbtg)

he issue with using a 4 ply 2x4 stud pack is that it won't perform the way a PSL would

Why-ever not?

Given uniform axial loading, Doing what you're proposing requires evaluating the following:

1) Weak axis buckling check on the composite section.

2) Check the nailing to ensure that it is strong enough to resist the inter-planar shear forces associated with composite buckling action.

3) Check the nailing to ensure that it is stiff enough to resist the inter-planar shear forces associated with composite buckling action.

#2 is hard. #3 is pretty much intractable given the realities of nail slip etc.

Were I attempting this, my path would be this:

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
Multiple 2x4 stud packs get expensive, is what I am finding. A 5 1/4 x 3 1/2 PSL gets 18,000lb roughly, unbraced. I'm not worrying about plate bearing for now. Just focused on getting a column that can do as close to what a PSL column can do as I can. According to NDS 2015, a (2)2x6 column alone, unbraced on the strong axis, and 1/4 braced on the weak axis, can get 9000 lb, but it fails in strong axis bending. there is room on either side of the strong axis within that wall to reinforce the strong axis to some degree using a pair of 2x4's, and if that could get me to a 13,000 lb axial capacity I'd be happy, because it would mean that instead of going straight from a 3-1/2x3-1/2 psl to a 5 1/4 psl, I could toss in a custom column instead. Not sure if a stud pack can do that for me cost efficiently. I do like the idea, but don't think it's going to work for me because it kind of defeats the purpose when it ends up costing as much as the psl would. I need to find a way to figure out what putting 2 2x4's on the sides can do for the 2x6 column, if anything.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
Didn't see your post Koot. I'll give the stud packs some thought as well, but on the off-chance anyone would know how to tackle my other solution I'd love to hear your thoughts!

RE: Composite Odd-shaped Built-up Column Strength

Quote (OP)

Multiple 2x4 stud packs get expensive, is what I am finding

Then somebody's feeding you bad information. I'd expect a PSL and the built-up to be more costly.

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
It's actually stronger than the calculations I just did, just realized those calculations are for spruce-pine-fir. I'm interested in using southern pine. So my custom column may yet work in replacing the lower end use of the 5 1/4 psl.

RE: Composite Odd-shaped Built-up Column Strength

2x4s depending on location are maybe 5 bucks for an 8 ft length. You can buy a lot of those for the cost of 1 PSL column.

RE: Composite Odd-shaped Built-up Column Strength

For axial + bending out of the plane of the wall, I challenge you to tell us a single way in which this custom column would be better than a 6-2x4 stud pack. And not cost since all six studs are probably worth less than 15 minutes of your billable time.

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
The rectangular cross section I have made is 3 x 5 1/2. The equivalent size roughly of that would be a 3 or 4 ply stud pack, which is weaker than the 2x6's alone. Right? So already a 2x6 column is cheaper for what you're getting. So here's my thoughts on the stud pack. In order for even a 5 ply stud pack to be superior (to a (2)2x6), if my math is correct, the weak axis needs to be considered unbraced. How would a 4 or 5 ply stud pack solve my problem better than a pair of 2x6's would? I've done the equations for both, and checked both using woodworks. I'm considering a 2 ply 2x6, unbraced on the strong and fully braced on the weak axis. It beats out any stud pack as is, and with the additional strength the 2x4's provide won't it beat out a stud pack every time? Please correct me if I'm wrong, or not thinking about this the right way. Also, thanks for your input guys!

RE: Composite Odd-shaped Built-up Column Strength

(OP)
What does a 6 2x4 stud pack get, for those conditions? Southern pine, unbraced. I guess I haven't checked. one sec.

RE: Composite Odd-shaped Built-up Column Strength

Quote (jack)

The rectangular cross section I have made is 3 x 5 1/2.

Your sketch shows a section 3.5 x 8.5. Has that been revised?

Quote (jack)

The equivalent size roughly of that would be a 3 or 4 ply stud pack, which is weaker than the 2x6's alone. Right?

Are you attempting to utilize your 2x6 strong axis capacity here? If so, that's incorrect. Among other things, your capacity will be limited by step number one of my recommendation above:

Quote (jack)

1) Weak axis buckling check on the composite section.

Have you done that? Rationally, given the same material and proper lamination, there's no way that a solid built up section (your box) is going to have more capacity than a section of the same size with material removed from the middle (stud pack of equal dimensions).

Quote (jack)

Please correct me if I'm wrong, or not thinking about this the right way.

I believe that you are wrong and failing to think about this the right way.

Quote (jack)

I'm considering a 2 ply 2x6, unbraced on the strong and fully braced on the weak axis.

You are not able to mobilize the unbraced strong axis capacity of the 2x6's in this situation. Buckling will take about an axis parallel to the weak axis of the 2x6's.

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
A #2 grade southern pine column, (5)2x4, with < 19% Moisture Content, Ct=1, Ci=1, Ke=1, Height=8ft, Unbraced on the strong axis, continuous on the weak axis, with SDS screws in both sides, with 0 wind applied gets 9255lb axial capacity as per a 2015 NDS calculation. A 2(2x6) for those same conditions and bracing gets 11075lb. Unless I have miscalculated something. So a 2x6 alone for my desired conditions already outperforms a much bigger stud pack, assuming I have done my calculations correctly. Someone please correct me if I have not.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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.

RE: Composite Odd-shaped Built-up Column Strength

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

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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!

RE: Composite Odd-shaped Built-up Column Strength

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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.

RE: Composite Odd-shaped Built-up Column Strength

Quote (OP)

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?

Quote (KootK)

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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!

RE: Composite Odd-shaped Built-up Column Strength

(OP)
Oh, and it would be screwed, not nailed.*

RE: Composite Odd-shaped Built-up Column Strength

Quote (jack)

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.

Quote (jack)

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.

Quote (jack)

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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.

RE: Composite Odd-shaped Built-up Column Strength

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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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.

RE: Composite Odd-shaped Built-up Column Strength

(OP)
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.

RE: Composite Odd-shaped Built-up Column Strength

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.

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