Wood Design Kh factor in CSA Standard O86

[ajk1]

In CSA Standard O86-01, Table 5.4.4, the "System Factor" Kh is given for "compression parallel to the grain" as 1.10 for Case 1, and 1.00 for Case 2 built-up beams. There is no listing for built-up columns. I am checkimg a built-up wind-column in the exterior wall of a cottage. It takes s small axial load due to roof dead load and snow, and a wind load which has a much greater effect than the axial load.

Questions:

a) Would I be correct to use Kh=1.00 for the resisting momnet calculation, and 1.10 for the compression strength of the column?
b) Why have they listed "built-up beams" but not listed "built-up columns" in the table?
c) Why have they listed built-up beams for CAse 2, but not for case 1?
d) Do other Standards have a similar table, and if so, what have they listed for Kh?
e) Has anyone used this table for VSL wood (i.e wood studs made up of vertical laminations), or is it not applicable?

I can attach a photographed copy of the table if that would help (I don't have a scanner at home).

 

[jayrod12]

a) Honestly when I run a built-up wind column I just use kh=1.0 because it's not really acting as a system of three or more parallel members.
b) Because a built-up wind column is the same as a built-up beam with additional compressive stress. That's why the Kh factor goes up for bending and down for axial load. The compression from axial load helps the moment resistance to a point.
c) When do you ever have identical built-up beams spaced at max 2 feet on centre?
d) don't know, I'm sure they must have something similar to account for load sharing across parallel members.
e) In canada vertically laminated posts are used but they're typically designed by the truss guys because they're laminated using truss plates. I'm sure they use portions of this standard but I'm not sure what else would apply to the design of them.

Oh and the most current edition of the CSA is O86-09.

 

[ajk1]

Hi Jayrod12;

Thanks for getting back so quickly .
I am aware that the most current edition is 09, but I am working from home yesterday and today and do not have the latest edition at home. I have the following follow-up questions:

a) ok

b) the Kh for columns is 1.10 and for built-up beams is 1.00. So Kh goes up for columns and down for built-up beams...it seems to be the opposite of what you say...or am I misunderstanding something?

c) If you interpret "members" to mean beams, then I see your point. But if you interpret members to mean the plies of a built-up beam (such a 4-2x6"s), then Case 1 would apply. No?

e) I was referring to something different. I meant something like "Versalam". My feeling is that O86 is not meant to be used for such proprietary products, because I am having difficulty understand how to select and apply some of the factors.

 

[jayrod12]

b) A built-up wind column would be considered a case 2 built-up beam. So bending would be 1.1 and compression would be 1.0. And in my tabe 5.4.4 the Kh for bending is at least 1.1 for all cases and uses.

c) Don't apply Kh to individual plies, apply it to your section (The total of all the plies). If you fasten them correctly they act as a single unit and that is how the strength of it should be calculated.

e) I don't see why you couldn't use the system factors for that type of material however as I said previously, when calculating the resistances by hand I generally just use a Kh=1.0 for both bending and compression unless the members are classified as case 1. All of my TrusJoist (Weyerhauser) stuff says they design according to the CSA O86 so I would think they would apply.

 

[ajk1]

ok, that is very helpful. I better get the latest edition ofthe Srandard...seems to be significantly different than the 2001 edition. Thanks again.

 

[ajk1]

Sorry -- I responded too fast. In my book too, the Kh is as you say. And what you say you would do for a wind column, is in fact excatly what I have done... although now I am wondering if it falls under even case 1 because the window each side comes right to the built-up column for part of its height, and in any event if you take "members" to exclude the studs that make up the built-up column, then it does not fall under case 1. So maybe I should use Kh=1. as you say that you generally do.

 

[jayrod12]

Case 1 is when you have typical members at a consistent spacing to allow for load sharing, i.e roof and floor joists, wall studs etc. It is not meant for one off members like wind columns. My thought for new construction is if you need the 10% increase on the moment capacity to make your column work then just add another ply. If you are analyzing existing construction for conformance then maybe you'd need the 10%

 

[ajk1]

I notice that the resisting moment given in the built-up beam selection table seems to be an integer multiple of the resisting moment for the same size joist plus 10%. This seems to indicate that they have used kh=1.10 for built up beams...even a single isolated built up beam.

 

[jayrod12]

Then possibly you've figured out the reasoning for the 1.1 Kh for all bending stresses