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!
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
why dont you spec a 4 ply 2x4 stud pack and call it a day?
RE: Composite Odd-shaped Built-up Column Strength
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RE: Composite Odd-shaped Built-up Column Strength
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
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RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
Your sketch shows a section 3.5 x 8.5. Has that been revised?
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:
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).
I believe that you are wrong and failing to think about this the right way.
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
You did notice that I provided such a method in my original response?
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
Thanks!
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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.
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.
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
RE: Composite Odd-shaped Built-up Column Strength
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
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
RE: Composite Odd-shaped Built-up Column Strength
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.