Built-Up Wood Posts larger than 5ply (CSA O86:19)

[EngDM]

Hello all, happy 2025!

I am developing a simple spreadsheet to calculate the compressive resistance (Pr) of a built-up wood post according to CSA O86:19. Everything up to 5ply's makes sense, and my calculations match the tabulated values provided in the 2020 Wood Design Manual for built-up columns (the unbraced ones).

I am trying to determine an appropriate way to calculate 6-8ply posts that is code compliant, but I'm running into a bit of a wall.

Any code references herein will be for O86:19, older editions may have different numbered clauses. Any calculations mentioned below have been performed assuming dry service conditions, SPF No 1/2.

6.5.5.4 deals with built-up compression members, and 6.5.5.4.1 outlines that for 2 to 5ply built-up members you can use the following procedures, and take the greatest resistance:

• 6.5.5.4.1 - Use the clauses below, or the combined factored resistance of the individual pieces taken as independant members.
• 6.5.5.4.2 - Nailed built-up compression members
• 6.5.5.4.3 - Bolted built-up compression members
• 6.5.5.4.4 - Split-ring-connected built-up compression members

When the post is within a sheathed or blocked wall, taking individual members and summing up their resistances appears to be the governing in most cases.
If the post is free-standing, then proceeding with 6.5.5.4.2-6.5.5.4.4 governs.

Now, for a 6ply post, it looks like I am to use 6.5.5.4.5 - Built-up compression members as simple compression members. For a free-standing post this poses a large issue, whereby the capacity of a 3m long 6ply 2x6 is 1.92kN, and thus the overall strength is 11.55kN. For reference, a 4ply post calculated in accordance with 6.5.5.4.2 is good for 94kN.

Further to this, 6.5.5.4.5 says that the calculations are to be performed considering all ply's as independant; the slenderness ratio of the weak direction quickly exceeds 50 when we do not rely on the other ply's for bracing. So my 3m 6ply 2x6 mentioned above technically doesn't pass the slenderness ratio < 50.

I'm curious as to what the members of this lovely forum have considered as reasonable when dealing with these larger built-up posts.

 

[W460x68]

I have not encountered this issue before but, say, for a 6-ply 2x6 post could you calculate the capacity of two 3-ply 2x6 posts and add those together?

 

[ChorasDen]

I don't know CSA, but typically for U.S. code, I would check the NDS equation 15.4-1 and the supplemental equations. CSA code is not available online to my knowledge, but you can access the NDS code for free from the AWC website. Believe there is an adjustment of 0.6 depending on the direction the load is checked (this adjustment is typically for the interaction of plies sliding overtop one another).

 

[EngDM]

I have not encountered this issue before but, say, for a 6-ply 2x6 post could you calculate the capacity of two 3-ply 2x6 posts and add those together?

I'm not sure if this is code compliant, the wording of 6.5.5.4.5 is "...the factored compressive resistance of built-up compression members not meeting the requirements of clause 6.5.5.4.1 to 6.5.5.4.4 shall be taken as the combined factored compressive strength of the individual pieces considered as independent members".

I'm not sure if I could clasify (2) 3ply posts as "individual pieces".

To add, Forte does not check any built-up free standing posts, and for posts in a wall they only go to 4ply.

 

[EngDM]

I don't know CSA, but typically for U.S. code, I would check the NDS equation 15.4-1 and the supplemental equations. CSA code is not available online to my knowledge, but you can access the NDS code for free from the AWC website. Believe there is an adjustment of 0.6 depending on the direction the load is checked (this adjustment is typically for the interaction of plies sliding overtop one another).

Clause 6.5.5.4.2 discussed in my original post is the Canadian one with the 0.6 modification factor to buckling in the direction perpendicular to the ply's. For the strong direction of the individual ply's there is no 0.6 reduction.

I will give the NDS a read through and see what they have to say for bigger than 5ply posts. Perhaps I can justify it with the "alternate solutions" clause in O86.