In-place Wood Column - Improve Slenderness 3

[DCEngr1]

I have a nominal 6x6 sawn-wood column with a 14' unbraced length. The column supports an eccentric load from 16' span & 8' beam spans (supporting roof trusses) on opposing sides of the column. The bottom of the column has been cast in concrete - literally; as an interior column for a pole barn application. My calculations for the beam design portion including Le show that Cs is 56.2; which is > 50. So, according to my calculations, I need a minimum of a 7x7 column for the slenderness. Can I construct an 8.5x 8.5 built-up column using 2x's glued and fastened to the sides to improve the slenderness? What type glue is appropriate? Is there any design information on line from one of the wood industry institutes or organizations?

Thanks,

DCEngr1

 

[SHOchris]

i dont know for sure if you can do that, but i would think bolting it say every 2 or 3 feet would be better than gluing....

 

[Lion06]

I would try gluing and lag bolting. I would not rely solely on glue for strength, but it will help with slip to achieve a more composite section. There is a factor for built up columns that takes this into account, but it wouldn't hurt to have the factor and still glue it.

 

[DCEngr1]

Are you saying that I can improve slenderness by adding the 2x's? Basically, I need to design a built-up column "around" this in-place 6x6 to work with what I have. The structure is not built yet; only the columns are in place. The information that I have regarding built-up column design indicates that only the core is considered for slenderness. If there is some more recent built-up design procedures that would be applicable for this case, that information would be helpful.

 

[Lion06]

I'll check my NDS at lunch, but I am positive there is a provision to use the entire built up section, but for sections attached with fasteners there is a reduction factor to account for the slip of the members due to the mechanical fastening.
You can minimize this slip by using the glue you talked about, but I wouldn't count on it in the calcs, just a belt and suspenders kind of thing.
That being said, I might check it with a pinned base since the composite section will not be present where it is fixed.
Well, let me clarify..... calc the allowable compression stress assuming pinned.

 

[csd72]

How about bolting 2 5x5 angles to it at diagonal opposite corners. This would definately improve the slenderness ratio.

 

[DCEngr1]

The compression and bending are checking, it is just the slenderness of 56 that is out. If you have some design procedure for built-up that accounts for slenderness, I could use that. The feeling is that adding the 2x's would get me there, but I want a design procedure that gives me some numbers. I can provide an e-mail address if someone can provide design procedure.

I have given some thought to adding some steel to this, but if I can, I would like to stick with wood.

 

[Lion06]

I don't think there will be a design procedure other than VQ/I for bending. That will likely be satisfied by the prescriptive requirements for attachment of built up column members. Check the NDS.......... if you don't find it by lunch I'll look it up and let you know where it's at.

 

[Lion06]

And actually, if it already checks out and all you need is the slenderness<50, then you only need to follow the prescriptive requirements.
Is there any way you can cut down the span? Are you taking the L to the centerline of the trusses or to the bottom face?
Try to justify a smaller L if possible.

 

[DCEngr1]

I do not have ready access to NDS, so I do not know what the prescriptive requirements are. I figured the column height from the slab to the underside of the beam that supports the trusses.

 

[msquared48]

Unless it is a rated system or a standard, tested product such as a glulam, you cannot rely on the glue in the connection. You would be relegated to the bolts for the connection, with the glue "doing no harm" so to say.

Personally, I would just use a glulam column and be done with the mess.

Mike McCann
MMC Engineering

 

[DCEngr1]

I could have used an 8x8, but the owner had already poured the footing with the column in place. At this point, I am trying to beef this up instead of having to replace the column.

 

[Lion06]

Per NDS 15.3.3 (for nailed built-up columns):
a) adjacent nails are driven from opposite sides of the column
b) all nails penetrate at least 3/4 of the thickness of the last lamination
c) 15D<=end distance <= 18D
d) 20 <= spacing between adjacent nails in a row <= 6tmin
e) 10D <= spacing between rows of nails <= 20D
f) 5D <= edge distance <= 20D
g) 2 or more longitudinal rows of nails are provieded when d> 3tmin

15.3.4 (for bolted built-up columns)
a) a metal plate or washer is provided between the wood and the bolt head, and between the wood and nut
b) nuts are tightened to insure that faces of adjacent laminations are in contact
c) for softwodds: 7D <= end distance <= 8.4D
d) 4D <= spacing between adjacent bolts in a ros <= 6tmin
e) 1.5D <= spacing between rows of bolts <= 10D
f) 1.5D <= edge distance <= 10D
g) 2 or more longitudinal rows of bolts rae provided when d > 3tmin


D = diameter of fastener
d = depth (face width) of individual lamination
tmin= thickness of thinnest lamination

I hope that helps. I would also make sure the additional plies are the same species and grade as the core.

 

[ronster]

Why do you need a Cs<50? KLe/d needs to be less than 50. You have a KLe/d=30.66 < 50. Cs is the slenderness factor used to determined the allowable bending stress for a beam-column.