Built-Up Sawn Lumber Column

[StructENG82]

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.

 

[KootK]

I'm assuming that your column is loaded from the top with all plies assumed to be loaded equally. My thoughts:

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



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.

 

[jayrod12]

No it does not preclude the built-up member fastening. The sheathing fastening is for the sheathing, the built-up fastening is to ensure it acts as a single structural member.

 

[StructENG82]

KootK thanks for the post I have actually gone to calling out the Simpson SDW screws for knee braces and built up columns where sheathing is not present.

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.

 

[KootK]

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.

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.

 

[jayrod12]

My answer does not prohibit you. It was more my gut feeling, the cost of nails is extremely cheap so they should just provide them. Also, the built-up member nailing requires that the nails fully penetrate all plies (at least in the CSA O86) and I doubt that the nails used for sheathing will be long enough.

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.

 

[JAE]

jayrod - we assume the same thing that KootK suggests.

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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[KootK]

I had assumed that this was an existing condition with some desperation at play. If this is new-build, I definitely advocate using the typical built up column fastening arrangement. Like they say in that article that I posted, QC is probably a nightmare for this.

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.

 

[JAE]

Well if the design axial compression is limited to 900 psi (in the range for a 4 layer stud pack with SPF No. 2 at about 8 ft) and you take a (4) 2x6 studs as a column you would have an
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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[KootK]

My main concern is for the strength and stiffness of the stuff beyond the fasteners. Is the stud pack in the center of a 20' wall? Or at the end of a 3' wall? What's the flexural and shear stiffness of that stuff? We may assess that stuff for shear walls but not for the bracing function in my experience.

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.

 

[StructENG82]

To give a little background into a situation where this has come up at our office in the past is when you have a 8 3/4" wide glulam beam being supported at the center of a continuous wall. The designs I have seen call for (6) 2x6 built up column framed into the wall to support the beam. This is more than required for capacity but provides a 9" width so side plates can be extended up to fasten to the beam.

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.

 

[dhengr]

StructENG82:
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.

 

[KootK]

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.

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.

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.

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.