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!

 

[NorthCivil]

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?

 

[SlideRuleEra]

What you describe is a "spaced column". For information on how to design, see pages 205 through 207 of the American Wood Council (AWC) publication "Wood Structural Design Data" 1986 Edition With 1992 Revisions.

http://www.SlideRuleEra.net

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[jacktbg]

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!

 

[JAE]

Why don't you post a sketch of your column?

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[NorthCivil]

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

 

[jacktbg]

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

 

[jacktbg]

Wait, maybe I haven't... one sec

 

[structSU10]

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.

 

[KootK]

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.

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.

 

[jacktbg]

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.

 

[jacktbg]

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!

 

[KootK]

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.

 

[jacktbg]

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.

 

[jayrod12]

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.

 

[KootK]

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.

 

[jacktbg]

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!

 

[jacktbg]

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

 

[KootK]

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?

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:

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).

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.

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.

 

[jacktbg]

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.