Bracing of Built-up Wood Columns 1

[phamENG]

Poll question: When designing built up wood columns (3+ 2x plies), do you consider them braced by drywall attachment?

I know it's common to consider it for stud walls, but for single point loads with little to no redundancy, I'd hate for a plumbing leak to dissolve the gypsum and quadruple my unbraced length...

 

[XR250]

If nailed together properly, that may end up being the strong axis...

 

[phamENG]

For 2x4, yes...but (3) 2x6 it's not. I don't usually consider it braced...just curious if I'm being over conservative with that. I don't think I am...if you look at standard load tables for glulams, PSLs, and other EWP columns they typically ignore bracing...of course that could just be a matter of simplicity and the importance of being conservative in a generic table.

 

[XR250]

I generally don't worry about it. There is so much redundancy and an actual lack of live load that it is unlikely to ever be an issue. I suppose you could add some blocking at third points or so.

 

[dik]

Do you not use drywall for providing weak axis bracing for a conventional stud wall?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

Check out this thread: Link. In it, I linked to a forensics article that very much agrees with your opinion. The link is dead but I did screen capture the conclusions. And some googlefu would surely turn it up.

I can't seem to scare it up easily just now but I know that there's at least one thread out there where a bunch of folks chimed in with respect to their "number" of studs that they're willing to consider sheathing braced in a pack. I tend to vacillate between two and three studs as my limit. I'm also situationally dependent. If it's two jacks and two kings, I'll usually let that go if the jacks are designed for the vertical and the kings for the lateral. Conversely, if it's four full height studs under a point load, I won't rely on the sheathing for that.

For what it's worth, I always fasten the plies together which I think is pretty typical.

 

[phamENG]

dik - conventional stud wall, yes. Degradation of the gypsum is likely to be concentrated at one or two studs and shouldn't spread to the surrounding studs before it's been repaired...or at least one can hope. So if you lose bracing on a few studs in a wall, it shouldn't bee the end of the world. But if you have a nice built up column supporting a large tributary area and it looses its bracing....well there's not as much distribution.

KootK - thanks. Here's the paper you mentioned. Good to know that others have considered the same and, if I am being a bit conservative, I'm not the only one. Not sure how I missed that thread in my search. Fastening the plies together per NDS chapter 15 is certainly a must.

 

[Aesur]

Typically we consider gyp as bracing the built-up column in the in plane direction. If you started designing for every possible "degradation" due to improper building maintenance in a building you will be out of business fast as your buildings are too expensive to build.

 

[KootK]

The one that always concerns me is one-sided drywall stud bracing at partition walls. E

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but I do it anyhow.

 

[phamENG]

Aesur - I agree with you completely. While we need to make sure our structures are designed to be constructable, we have to let the builders build it and trust them to do their job. We also have to trust people to take care of the buildings once they're built. That said, for critical and non-redundant components I'm not altogether comfortable trusting them to a material as unresilient as interior 1/2" drywall.

 

[dhengr]

PhamENG:
Sounds like a very good point to me, particularly when in the one case you are talking about a distributed load and support system, with some adjacent redundancy, but in the other you are talking about a much larger concentrated load on only one member, with no redundancy. However, I suppose the laminating improves the general member strength and stiffness a little when done well and when the ends and member length are cut well to assure good unform bearing. I’ve seen built-up col. members cobbled together such that only about half of the verts. were end bearing until something crushed 1/8th or 3/16ths of an inch or so. It seems that builders think that the number of plies is the only important issue, not how the load gets to all of them, and the rest of the details can be cobbled. Look at how the NDS, some good texts, and various research treat individual studs, and repetitive stud groups, and/or built-up members both solid and intermittently blocked, how they develop their approach, and rationalize it, explain it. That should give you some insight. Otherwise, I agree with your thinking that sht. rk. bracing a single 2x is quite different than the same sht. rk. bracing a huskier col. member with a large concentrated loading. Even dry sht. rk. nailing just won’t take some percentage (what % that is?) of that larger vert. load as a bracing load component.

 

[phamENG]

Thanks, dhengr. I was considering the percentage as well. Column bracing is typically 1% of the axial load (NDS doesn't specify), so for 2x4 and 2x6 built up columns you could be looking at anything from 50 (I could see that...) to 150 lbs (unlikely?). I've looked for capacity of a single wall board nail, but haven't had any success.