Built-Up Sawn Lumber Column
Built-Up Sawn Lumber Column
(OP)
I am trying to determine if I use a built-up column in a wall to support a beam load do I need to fasten the individual plys together per NDS chapter 15.3. I am trying to determine if the 1/2" plywood sheathing on either side of the built-up member that has been noted to be fastened to each ply with nails at 12" o.c. (staggered) precludes me from having to follow the the built up fastening in NDS. The specific requirement I reference is side nailing of built up the built up member with nails that penetrate 3/4 of the thickness of the last lamination.
Any help would be appreciated, thanks.
Any help would be appreciated, thanks.






RE: Built-Up Sawn Lumber Column
1) The NDS provision is intended to ensure that all of the studs act compositely when the column attempts to buckle in the plane perpendicular to the laminations. Since your column, and all of its laminations, will be braced against that kind of buckling by attachment to the sheathing, I'm not even sure that any fastening is technically required (I provide it anyhow for good measure and some degree of load share transfer capacity).
2) In concept, connecting all of the plies to the sheathing should accomplish the same thing as fastening the plies together: composite buckling behavior. The trouble with this would be in assessing it. NDS assumes efficiency factors for different kinds of fasteners (60 & 75% respectively for nails and bolts). I'm not sure how you'd go about assessing the fastener efficiency in your case.
You may find this useful: Link
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RE: Built-Up Sawn Lumber Column
RE: Built-Up Sawn Lumber Column
I also have the same opinion that the sheathing should preclude you from having to do the built-up fastening even though this is contradicting to what jayrod12 as noted. As for determining the efficiency that is a good point. Can it be assumed to be 100% as the compression member is supported throughout is length by the sheathing to prevent lateral displacement.
RE: Built-Up Sawn Lumber Column
I would think not. Efficiency is about preventing slip between the interfaces between plies, not about bracing. That said, my argument above was that you can essentially treat the built up column as several, individual, non-composite columns each braced against weak axis buckling.
This forensics article adresses a very similar case and disagrees with me: Link. Their argument essentially amounts to built up columns being too structurally important to rely on the quality control and potential impermanence issues associate with using sheathing to brace columns.
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: Built-Up Sawn Lumber Column
That being said, Koot is a bright guy and he makes a good argument about treating it as individual members all braced against weak axis buckling. I've been persuaded before. I just can't see how spending this much time determining if they are required is cheaper and easier than just putting the nails in.
RE: Built-Up Sawn Lumber Column
However, the concern always is whether the sheathing nailing encompases the outer plies of the built-up column. If they only nail to one ply there is the chance that an outer column ply could buckle away from the column independently within the sheathed wall.
We simply assume the full column works (with the adequate sheathing nailing on outer plies) and do not use the Kf factor for built-up columns either.
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RE: Built-Up Sawn Lumber Column
Another interesting facet of this would be the ability of the the sheathing itself to brace the stud pack. To my knowledge, this isn't something that we check explicitly for wood studs. And to be frank, I'm not even sure how I'd go about checking it were I to try. I can say with some certainty, however, that a large stud pack would require a good deal more bracing stiffness and strength than a single stud.
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: Built-Up Sawn Lumber Column
axial design capacity of about 29,700 lbs. 2% of that is 594 lbs., perhaps a rough (sledge hammer) estimate of the lateral bracing force required.
That's about 4 10d common nails each side with 1/2" plywood sheathing.
For a more moderate (2) 2x6 studpack you'd have an axial of about 5950 lbs x 0.02 = 119 total lbs. - two 10d nails work fine.
(Assumes concentric axial load on the column)
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RE: Built-Up Sawn Lumber Column
Admittedly, required bracing loads don't get all that huge.
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: Built-Up Sawn Lumber Column
The problem with this situation is that section 15.3 of NDS can't even be followed as it limits the number of lamination to 5 plys.
RE: Built-Up Sawn Lumber Column
Yes you should nail the individual studs together to make up the column (stud pack), so they do act in unison, and brace each other. Exactly how many nails is not that important, various codes give some guidance on this nailing. One of the most important issues that I don’t think has been touched on, for all the discussion about lateral bracing, is that the studs should be cut and fit-up, true to length, so that all of the studs are truly end bearing t&b. I’ve seen these thrown together so that only two out of four studs provided bearing, and maybe both on one side or the other, not even symmetrical.
RE: Built-Up Sawn Lumber Column
Eh, if you can make it work with five ply, I'd not sweat the extra ply being thrown in there for fun. I wouldn't even bother to check the fictionly five for eccentrically applied load.
Excellent point. I've always found it odd that built up stud columns don't seem to have a fastening requirement for load distribution at the ends. If this were two steel channels fastened to a wide flange column, you'd throw some end termination welds in there to move the load around a bit at the entry and exit points. At least I would.
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.