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.

 

[KootK]

I would take 6.5.5.4.5 with a grain of salt. Code writing is tough and code writers imperfect, especially in Canada.

I believe that the intent of all of this stuff is just to acknowledge that it may not be practical to try to achieve composite buckling behavior of a stud pack with more than five studs in it for buckling in the plane of the wall. Shear slip and all of that jazz. I don't think that CSA O86 really means what it says with "treat every ply as an individual member". That would be silly, as you can see from the result.

Rolling with that, I would recommend something similar to what @W460x68 suggested:

1) Design the post as whatever combination of sub-stud-pack assemblies (<5) gives you the most capacity.

2) Don't sweat the slenderness limit on individual plies so long as everything is fastened together robustly.

3) Do give some thought as to whether your axial load will be delivered in a manner that can plausibly engage all six of those studs.

#3 is the "engineering" in my opinion. Since you're building a spreadsheet, this might take the form as a warning to the user.

 

[EngDM]

I would take 6.5.5.4.5 with a grain of salt. Code writing is tough and code writers imperfect, especially in Canada.

I believe that the intent of all of this stuff is just to acknowledge that it may not be practical to try to achieve composite buckling behavior of a stud pack with more than five studs in it for buckling in the plane of the wall. Shear slip and all of that jazz. I don't think that CSA O86 really means what it says with "treat every ply as an individual member". That would be silly, as you can see from the result.

Rolling with that, I would recommend something similar to what @W460x68 suggested:

1) Design the post as whatever combination of sub-stud-pack assemblies (<5) gives you the most capacity.

2) Don't sweat the slenderness limit on individual plies so long as everything is fastened together robustly.

3) Do give some thought as to whether your axial load will be delivered in a manner that can plausibly engage all six of those studs.

#3 is the "engineering" in my opinion. Since you're building a spreadsheet, this might take the form as a warning to the user.

Typically in these cases the beam would be flush and full bearing, so maybe some localized stress at the inner most ply as the beam deflects, but it's not like I'm trying to drag 3jack studs into 4 king studs per se. I think in this case I am more worried with the uplift at the end of the beam; Forte warns of uplift once the support gets large and the uplift gets LARGE.

What I'm gathering from this is to essentially stick with 6.5.5.4.2 even for larger than 5 ply's, since that is the one that is all nailed up.

At the end of the day the application for this is largely for multistorey wood framed construction on an interior bearing line, so little to no wind load aside from internals and the post is only like 8-10ft tall.

 

[skeletron]

Are you nailing each ply to the adjacent pack? Or are you bolting/screwing through?

If you are nailing ply-to-pack, I would rely on 6.5.5.4.2 and maybe consider similar to what @W460x68 is proposing since beyond 3 or 4 studs your nails won't help the whole thing act compositely.

If you are bolting or screwing through the whole pack, I would rely on 6.5.5.4.3 provided that the fastening is defined.

It seems that you are using these in a wall scenario. Using built-up stud packs makes sense. But in a free-standing column or anything with substantial load, I wonder if a solid post (D.Fir or PSL) would make more sense.

 

[EngDM]

Are you nailing each ply to the adjacent pack? Or are you bolting/screwing through?

If you are nailing ply-to-pack, I would rely on 6.5.5.4.2 and maybe consider similar to what @W460x68 is proposing since beyond 3 or 4 studs your nails won't help the whole thing act compositely.

If you are bolting or screwing through the whole pack, I would rely on 6.5.5.4.3 provided that the fastening is defined.

It seems that you are using these in a wall scenario. Using built-up stud packs makes sense. But in a free-standing column or anything with substantial load, I wonder if a solid post (D.Fir or PSL) would make more sense.

Solid posting would for sure make more sense, it's more to allow the contractor to use what they are using for wall studs, and since it might be an intermediate post that is not necessarily in a wall; for posts within a wall I am not worried whatsoever in the plys weak direction since I will have blocking. Saves me an RFI later.

I believe they nail each play on one at a time. I could certainly specify GRK's through the whole thing; we do this for larger flush beams already and have had no complaints.

I just want a reasonable approach for 6ply+ since the code is not clear for us cold folk.

 

[EngDM]

As an update, I ran some posts in the RISA-3D demo and it appears they just use the methods prescribed by the code even for built-up members of more than 5 ply's. I'm probably just going to do that.

Doing that has very minor % difference between taking 2*4ply post capacity and just doing the calculation with an 8ply (5% difference for 2x6).

 

[jayrod12]

Keep in mind the nailing patterns and required lengths etc. they expect that you'll use 6" spikes from each side for a 4 ply built-up post. See figure 3.2 in the 2020 WDM. 6.5.5.4.2.b requires penetration at least 3/4 of the way through the last ply. Therefore for a nailed built-up column of 8 plies, it would require spikes that are 11 5/8" long. I've never seen those on site before.

 

[EngDM]

Keep in mind the nailing patterns and required lengths etc. they expect that you'll use 6" spikes from each side for a 4 ply built-up post. See figure 3.2 in the 2020 WDM. 6.5.5.4.2.b requires penetration at least 3/4 of the way through the last ply. Therefore for a nailed built-up column of 8 plies, it would require spikes that are 11 5/8" long. I've never seen those on site before.

Interesting. I wonder if nailing on the additional ply's one at a time with say 6" spikes would suffice as an alternate.

If I were to just spec (2)-4ply posts beside eachother then I'd only need 6" spikes for each of those.